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2013-06-17 | Lancaster Severe Thunderstorm 17 June 2013: A cluster of showers and thunderstorms developed shortly before 1900 UTC in the Harrisburg area of southeastern Pennsylvania. From this cluster one large and relatively long-lived storm traversed Lancaster County from northwest to southeast. Along its path is produced hail stones reported to be 0.70 to 1.00 inches in diameter. The storm likely produced several wet microbursts along its path which led to reports of sporadic areas of downed trees, and wires. Strong winds from a wet microburst blew down a tobacco barn in Paradise, PA.
This storm developed in a modest low-level equivalent potential ridge, on the warm side of a weak boundary The CAPE supported large updrafts as forecast and analyzed CAPE was in the 1400 to 2200JKg-1 range. The -20 level was near 25kft and this storm produced 50 dBZ cores to around 25kft as it traversed Lancaster County. Few 50 dBZ cores reached much above 25kft. Most of the hail was associated with TBSS signature on KCCX radar.
The large scale pattern over the region (Fig. 6) was relatively quiet with weak winds, weak vertical shear and lacking a source of deep moisture. The analyzed CAPE in the 13km RAP showed some potential for CAPE values to reach as high as 2400 JKg-1 in southeastern Pennsylvania in close proximity to where the isolated storms developed.
Several images depicting the type of damage observed are included in Figures 8-10.
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2013-06-13 | Eastern Derecho 12-13 June 2013 A strong mid-tropospheric short-wave coming over a sharp 500 hPa ridge produced a series of mesoscale convective complexes on 12-13 June 2013. One MCS classified as a derecho producing widespread wind damage from the western Great Lakes into southeastern Pennsylvania. The severe weather in Pennsylvania was observed mainly between 0600 and 1200 UTC 13 June 2013 with the long-lived derecho producing MCS.
This system included a deep surface cyclone for so late in the season and NCEP models produced significant amounts of QPF with the system. Overall, most forecast systems grossly over forecast the QPF relative to observed rainfall data. These over predictions of QPF lead to the issuance of flood watches and concerns about flooding that did not materialize due to the general lack of heavy rainfall. Probabilistic QPF threshold values from the NCEP SREF showed high end rainfall amounts were a low probability outcome.
This paper shows the pattern in which the derecho producing MCS developed. The pattern and thus the pattern of the precipitation was relatively well predicted, though as stated earlier the NCEP models grossly over predicted the rainfall. |
2013-06-13 | Heavy Rainfall Event of 10-11 June 2013: A fast moving short-wave (Fig. 1) produced rainfall and areas of over 48mm of heavy rainfall from Maryland, across Pennsylvania and southern New England (Fig. 2). The heaviest rain, over 64mm, fell over portions of New Jersey and southern New York. A significant portion of the rainfall and the areas of heavier rainfall over Pennsylvania fell late in the event (Fig. 6) mainly between about 2300 UTC 10 June and 0300 UTC 11 June 2013. This led to some local flooding in central Pennsylvania where over a small area rainfall exceeded 75 mm (3 inches).
The pattern was a pattern associated with precipitation and most of the higher precipitation amounts fell in the region affected by the 1-2.5 above normal precipitable water plume (Fig. 2c). The PW field also showed a more north-south boundary from south-central Pennsylvania into the Carolinas which lined up well with the more north-south band of convection which affected northern Maryland and Pennsylvania between about 10/2200 and 11/0300 UTC. This sharper line with limited instability in Pennsylvania but 1200 to 2400 JKg-1 to the south produced severe weather and tornadoes from Delaware southward into the Carolinas (Fig. 16). The area affected by the severe weather was also experienced the passage of a strong low-level jet with +3 850 hPa v-wind anomalies (Fig. 7).
The rainfall, after the main area of rain moved to the east (not shown) over central Pennsylvania, locally exceed 75mm. The SREF, GFS, and NAM clearly showed that after 2100 UTC 10 June the area where this rain fell was not an area the model atmosphere’s expected heavy rainfall. Not a single SREF member had predicted much more and 25mm in the affected region. The shorter range RAP showed some clues for increased rainfall potential.
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2013-05-25 | Memorial Day Weekend 2013: Snow and Cold
A slow moving 500 hPa low and associated unseasonably cold air in the lower troposphere brought unseasonably cold weather and a record late season fall to the higher elevations of northern New York and New England. Over 30 inches of snow fell on the slopes of Whiteface Mountain in northern New York. Many of the higher elevation locations, where ski resorts are often located had heavy snow. Trace amounts of snow were reported as far south as Binghamton, New York and Scranton, Pennsylvania.
The deep trough and cold air delayed the start of summer for a few days on the normally busy Memorial Day weekend. Despite the time of year, this storm shared many of the characteristics often associated with East Coast Winter Storms including a strong low-level northeasterly jet. The heaviest rainfall and heavy snow fell in and near the axis of this feature. The 850 hPa temperatures over the northeastern United States were below 0C during the precipitation and many locations were -2C, despite the deep cold air, accumulating snows were generally limited to elevations above about 2000ft.
The overall pattern and the potential for cold and rain was well predicted by the NCEP models and ensemble forecasts systems.
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2013-05-22 | Mid-Atlantic Severe Event of 22 May 2013:strong low-level and a surge of high CAPE produced a severe weather event over Pennsylvania during the late afternoon and evening hours of 22 May 2013. Most of the reports of severe weather were due to strong and damaging winds. In central Pennsylvania a long-lived bow echo developed which accounted for a significant portion of the wind damage reports from Westmoreland County northeastward to Luzerne County, Pennsylvania.
This was the second event in May of 2013 where a strong bow echo developed out of a cluster of storms. The bow echo then went on to produce a significant number of the reported severe weather. |
2013-05-21 | Multi-day severe event of 18-22 May 2013
A relatively slow moving Trough over the western United States and a ridge over the eastern
United States setup a relatively persistent pattern from 18-22 May 2013. This pattern produced a
period of enhanced severe weather over the United States from 18-22 May 2013. Relative to the
month of April 2013 this was an extremely active 4 day period which in fact produced more
severe weather reports than were reported during the entire month of April 2013.
The key features associated with the active severe weather of 18-22 May 2013 included a slow
moving Trough over the western United States and a ridge over the eastern United States. The
resulting enhanced southerly flow, the evolution of a strong LLJ, between these two systems
allowed warm moist air from the Gulf of Mexico to move into the central United States from
Texas to the Great Lakes. This led to increased values of CAPE in close proximity to strong
shear. The result was a multi-day period of enhanced severe weather with three successive days
of 300 or more reports of severe weather from 19-21 May 2013 (Table 1). These data show that
despite previous conditions, a rapid change in the pattern can rapidly produce significantly
different weather.
A very dry air mass over the southwestern United States showed up as a region of below normal
precipitable water from 18-22 May 2013. This dry air, as it moved off the higher terrain of New
Mexico and west Texas produced an elevated mixed layer and a loaded gun sounding over
portions of Oklahoma including the period of the deadly New Castle and Moore, Oklahoma
tornado. |
2013-05-15 | The quick warm up of 14-15 May 2013 A retreating late season cold front and an advancing early season warm air mass led to rapid temperature changes in the Midwest on 14 May and in the Mid-Atlantic region on 15 May 2013. The elevated mixed layer of very warm air mixed down as the cool air mass retreated producing 283 near or tied record high temperatures in the afternoon hours of 14 May 2013 after a relatively chilly period for early May.
High temperatures in the Midwest reached well into the 80s and 90s on 14 May. Several dozen station reached high temperatures of 100F or greater to include locations in the State of Minnesota.
In the Mid-Atlantic region, the rapid erosion of the low-level cold air during the later morning and afternoon hours of 15 May 2013 led to many rapid temperature rises in the Mid-Atlantic region. Despite the rapid temperature rises, few high temperature records were set in the Mid-Atlantic region.
This meteorologically curious event was also relatively well predicted.
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2013-05-13 | Multi-day severe event of 18-22 May 2013:A relatively slow moving Trough over the western United States and a ridge over the eastern United States setup a relatively persistent pattern from 18-22 May 2013. This pattern produced a period of enhanced severe weather over the United States from 18-22 May 2013. Relative to the month of April 2013 this was an extremely active 4 day period which in fact produced more severe weather reports than were reported during the entire month of April 2013.
The key features associated with the active severe weather of 18-22 May 2013 included a slow moving Trough over the western United States and a ridge over the eastern United States. The resulting enhanced southerly flow, the evolution of a strong LLJ, between these two systems allowed warm moist air from the Gulf of Mexico to move into the central United States from Texas to the Great Lakes. This led to increased values of CAPE in close proximity to strong shear. The result was a multi-day period of enhanced severe weather with three successive days of 300 or more reports of severe weather from 19-21 May 2013 (Table 1). These data show that despite previous conditions, a rapid change in the pattern can rapidly produce significantly different weather.
A very dry air mass over the southwestern United States showed up as a region of below normal precipitable water from 18-22 May 2013. This dry air, as it moved off the higher terrain of New Mexico and west Texas produced an elevated mixed layer and a loaded gun sounding over portions of Oklahoma including the period of the deadly New Castle and Moore, Oklahoma tornado.
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2013-05-10 | Pennsylvania Severe Weather Event of 10 May 2013 The combination of instability and a frontal system brought severe weather to Pennsylvania and southwestern New York on 10 May 2013. Vertical profiles implied that there was a modest inverted-V sounding implying the potential for evaporative cooling and the production of cold pools should convection develop.
Most of the severe weather in western Pennsylvania was associated with a quasi-linear convective system. Over northwestern Pennsylvania (5) and southwestern New York (5) several modestly rotating storms developed which producing hail around 25mm (inch) to 37.5 mm in diameter. All other reports were associated with strong surface winds.
In central Pennsylvania the larger QLCS produced only 1 report of damage. All other wind reports came from a single storm which developed in Somerset County and raced northeastward producing damaging winds from Altoona to Lock Haven, Pennsylvania. This storm, in its early evolutions had dual-pol characteristics which implied that melting hail and large drops may have contributed to the initial evolution of the cold pool and strong outflow boundary which raced up Bald Eagle Valley.
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2013-05-09 | Early May Cut-off low and Mid-Atlantic rains:A deep 500 hPa cutoff developed in the southern Plains on 3 May 2013. It produced a prolonged period of unseasonably cold weather in the Plains to the Gulf States from 2-6 May 2013. There were some reports of late season snow from this system in places were spring time observations of snow are rare.
As this relatively long-lived cutoff moved northeastward, it produced bands of heavy rainfall in the Mid-Atlantic region and southern New York. The rain fell in the northeast quadrant of the cutoff as it slowly moved northward and merged with westerlies.
The initial rain bands in the northeast quadrant of the cutoff were relatively well predicted by the NCEP SREF. The second surge of heavy rainfall across New Jersey and New York, though predicted by the SREF was a relatively low probability outcome event.
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2013-04-30 | Impacts of the April 2013 Mean trough over central North America weather
The mean 500 hPa flow over North America featured a trough over the continent and ridges along the East and West Coasts. The mean trough was associated with several surges of cold dry air which penetrated into the Gulf of Mexico. This produced several cold episodes and a 10 day period where over 100 record low high-temperatures were set or tied. The relatively cool air led to 3 significant late season snow falls from the Rockies to the Great Lakes.
There were two surges of deep moisture and high PW air. The first surge produced 3 of the 6 severe events which had 100 more severe reports between 9-11 April 2013. The second surge around the 18th of April was associated with a second round of severe weather and a heavy rainfall event in the Mid-West which produced river flooding in Illinois and Michigan.
The mean trough limited the surges of high PW which in turn limited the severe weather and tornado activity in April 2013.
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2013-04-19 | A fast moving cold front with limited CAPE, strong low-level winds and strong shear produced a minor convective event across Pennsylvania during the afternoon hours of 19 April 2013. Due to the low CAPE storms had limited vertical extent. In Pennsylvania wind damage was the primary means to verify severe weather. No METAR sites observed winds over 45kts and in central Pennsylvania, only KUNV reported thunder. Most of the lightning with this event was south of the Mason-Dixon line.
Farther south higher CAPE over Virginia and the Carolinas produced deeper convection and stronger more persistent thunderstorms. This led to more widespread reports of severe weather in the Mid-Atlantic States south of the Mason-Dixon Line.
There were a few unique bowing segments in Pennsylvania that lined up well with some of the areas of wind damage.
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2013-04-18 | Mid-West Heavy rains 18 April 2013: The relatively wet conditions during the first 16 days of April 2013 set the stage for potential flooding over the Midwest. A strong frontal system and a strong ridge to the east pushed a plume of deep moisture and high values of precipitable water into the Mid-Mississippi Valley and Great Lakes region. This plume of moisture produced heavy rains, several areas received 50 to 100 mm in 12 hours and 75 to 175 mm in about 36 hours.
The heavy rains produced flooding and disrupted transportation in towns and cities. The intense rainfall disrupted autos, rail, and air transportation in Chicago. As the water flowed into rivers and streams, it produced flooding along many rivers. The Grand River in western Michigan and the Illinois river in Illinois both had major flood and near record flooding.
From a prediction perspective, both the pattern and the probability of heavy rainfall were relatively well predicted by the NCEP guidance systems. The GFS produced over 100m of QPF and the SREF showed a broad region to receive in excess of 100 mm of QPF.
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2013-04-10 | A large subtropical ridge over the western Atlantic and adjacent southern United States pumped deep moisture, strong winds, and instability into the Mississippi and Ohio Valleys then eastward across Ohio and Pennsylvania. This produced a series of early season MCS which brought strong and damaging winds to the region.
High CAPE was observed in the warm moist air over the ridge and south of a strong low-level frontal boundary. The conditions including the strong low westerly winds, deep moisture, and unseasonably high CAPE produced long lived quasi-linear convective systems. Embedded within these systems were bow echoes which accounted for most of the severe weather, from damaging winds, which affected the Mid-Atlantic region.
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2013-03-31 | A persistent high latitude block over northeastern North America produced relatively cold conditions over much of the eastern United States during March 2013. A series of short-waves moved beneath this block produced several late winter snow events in the eastern United States and one significant early spring Mid-Atlantic snow event on 25-26 March 2013. In the west, several large ridges developed keeping most of the southwestern United States relatively warm. Several surge of Pacific moisture moving over the ridge brought moisture and heavy precipitation to the Pacific Northwest (Fig. 2).
These daily data show contribution and the impact of the series of transient ridges in western North America and short-waves moving beneath the high latitude ridge which contributed significantly the monthly pattern. The only persistent feature though most of the month was the persistent high latitude ridge over northeastern North America. The high latitude ridge was associated with blocking and a period of strongly negative values of the AO.
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2013-03-18 | Late winter storm beneath high latitude block.Produced heavy snow in portions of the East. Over forecast snow and QPF in Pennsylvania. Rain and severe weather event in Gulf States. Focus is on the pattern and anomalies and the over prediction of snow in the Mid-Atlantic region. |
2013-03-12 | These data show that a fast moving short-wave brought a brief surge of above normal PW into
the Mid-Atlantic region. The PW values were only about 1σ above normal (Fig. 2) indicative
of a good but not extreme rainfall event. The pattern was a well-known pattern associated with
many moderate and heavy rainfall events, a Maddox-Synoptic pattern with a surge of strong
southerlies and moisture ahead of an advancing frontal system.
As shown here, the NCEP SREF correctly predicted the relative timing, orientation, and pattern
of the precipitation shield. In areas of heaviest rainfall it may have slightly underestimated the
precipitation, though at least 1 member did predict in excess of 50 mm of QPF in close proximity
to the close 48 mm contour in Figure 3. The SREF forecasts indicated the axis of heavier QPF
quite well and as forecast length decreased, the forecast region of heavier rainfall converged
toward where most of the heavier precipitation was observed.
The pattern and the SREF QPF probabilities correctly indicated that this was not likely a high
flood threat event. The two points in New York which reached flood stage likely had some
contribution due to snowmelt. Most locations remained well below flood levels. Without a
contribution from snow melt or frozen ground, it is often difficult to get serious flooding impacts
when rainfall is below 75mm over much of Pennsylvania. |
2013-03-11 | A strong mid-tropospheric wave moved across the central United States and produced heavy rainfall in the mid-Mississippi Valley (MMV) and a band of moderate to heavy snow across the central Plains. The NCEP GEFS correctly predicted the potential for 25 to 50 mm of rainfall in MMV with ~6 to 7 days lead-time. The forecasts of the precipitation band on the cold side of the wave had a predictability horizon on the order of ~1 day.
The NCEP SREF showed large uncertainty with the 500 hPa trough and surface pressure fields. As the forecast length decreased, uncertainty decreased and the SREF and GEFS converged on a band of precipitation north and west of the track a deeper cyclone than longer range forecasts had indicated. The uncertainty contributed to large differences in the character and evolution of the 250 hPa and 850 hPa upper and lower level jets respectively.
Despite the uncertainty with the trough and significantly different evolution of the system in shorter-range forecasts, the area of 25 to 50 mm of rainfall was relatively well predicted with ~3days lead-time in the SREF and about ~6-7 days lead-time in the GEFS. The predictability horizon of the band of precipitation, which produced heavy snow, was on the order of only ~1 day. This case shows that within a similar region and when affected by the same synoptic weather system, predictability horizons can vary considerably.
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2013-03-06 | A late season winter storm brought snow, rain, and strong winds to a broad swath of the eastern United States on 5-7 March 2013. The estimated precipitation field (Fig. 1) provides a good overview of the strong northern stream system (Fig. 2) which pulled moisture form the Gulf of Mexico, to produce the deep cyclone along the East Coast (Fig. 3). The Clipper-like (Thomas and Martin 2007). short-wave with -1 height anomalies (Figs. 2a-e) produced a swath of moderate to heavy snow (Fig. 4) as it moved across Wisconsin, Illinois, Indiana, Ohio, West Virginia, and western Pennsylvania. As the energy transferred to the coast system, beneath the deep 500 hPa low heavy snow fell in southern Pennsylvania, West Virginia, Virginia, and western Maryland.
This paper will document the 5-6 March 2013. The focus is on the pattern and standardized anomalies to show how strong the storm was and which features in the may have played a role in event. The paper also examines forecasts from the NCEP GEFS and SREF to provide some insights into predictability of this storm and perhaps aid in better prediction of similar storms in the future. In the Mid-Atlantic region, many lower elevations locations facing strong low-level easterly flow received rain despite below freezing temperatures above the PBL. This caused scenarios where snow fell with warm temperatures with little accumulation |
2013-02-26 | The second storm within a week brought heavy snow from the eastern slopes of the Rocky Mountains and southern plains into New England. Heavy snow and blizzard conditions affected portions of the New Mexico, Texas and Oklahoma panhandle where wind gusts reached 50 to 84 mph. Portions of Missouri and Iowa saw the second large snowfall in less than a week.
Similar to the storm of 21-22 February 2013, the heavy snow fell in a region of strong easterly winds. The 850 hPa u-wind anomalies during the storm reached -5 below normal.. The strong low-level winds were the result of the gradient between a modest anticyclone to the north and a deept surface cyclone to the south. Unlike the previous storm, this storm lacked a strong anticyclone which and had a deep cyclone, which may have limited the amount and extent of sleet and freezing rain.
The storm was relatively well predicted by the NCEP forecast systems to include both the Global Ensemble forecast system and the short-range ensemble forecast system. Both showed the potential for heavy snow, a deep cyclone, anomalous easterly flow on the cold side of the boundary and sufficient QPF to produce heavy snowfall. During this event model and ensemble precipitation types and quantitative precipitation amounts correctly highlighted the areas likely to receive heavy snowfall. There were some precipitation type issues in the transition zone from snow to rain.
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2013-02-21 | Central United States Winter Storm of 20-22 February 2013:
A strong winter storm brought heavy snow, sleet, and freezing rain to the central plains and lower Missouri Valley. Heavy snow affected Kansas, Nebraska, and Missouri with sleet and freezing rain along the southern edge of the snow shield across southern Kansas, Missouri and northern Arkansas. Snowfall amount between 12 and 20 inches were observed in Kansas to Nebraska and a more east-west band of heavy snow fell across Missouri.
The heavy snow fell in a region of strong easterly winds. The 850 hPa u-wind anomalies during the storm reached -4 below normal. The strong low-level easterly winds implied a strong frontal circulation which kept the low-level cold air in place and enhanced the lift. The strong low-level winds were the result of the gradient between a strong anchoring anticyclone to the north and a modest surface cyclone to the south. Not all major winter storms require a strong surface cyclone and often strong anticyclones are key players in mixed precipitation and heavy snow events.
The storm was relatively well predicted by the NCEP forecast systems to include both the Global Ensemble forecast system and the short-range ensemble forecast system. Both showed the potential for heavy snow, an anomalous anticyclone to the north, anomalous easterly flow on the cold side of the boundary and sufficient QPF to produce heavy snowfall. During this event model precipitation types and areas to be affected by heavy snow and mixed precipitation were relatively well forecast with 3-5 days lead-time.
Key words: ensembles standardized anomalies winter storms. |
2013-02-13 | A fast moving short-wave (Fig.1) and surface cyclone (Fig. 2) brought rain and snow from the Ohio Valley into the Mid-Atlantic region (Fig. 3). Despite what appeared to be marginal conditions for snow central Pennsylvania, most areas received snow. Snowfall in central Pennsylvania briefly fell at rates of up to 2” per hour with 2-4 inches reported in Altoona and State College communities. Short range model guidance struggled with the precipitation type forecasts, with the NCEP SREF and NAM precipitation types were a mix of rain and snow, primarily due to a warm boundary layer. The 850 hPa temperatures over most the region was -2 to -4C and was forecast toward cool to the wet-bulb temperature during the event.
The fast moving 500 hPa short-wave (Fig. 1) produced a strong area of lift well north of the surface cyclone (Fig. 2). The 500 hPa heights and IR satellite image (Fig. 4) show this enhanced area well north of the surface circulation. The central Pennsylvania radar (KCCX:Fig. 5-6) showed enhanced displaced south the coldest IR clouds tops, a northward moving band of snow. The maximum reflectivity in the band was in the 30 to 40dBZ range. The 500 hPa wave and the IR imagery show a similar structure to the banded snowfall conceptual model (Novak et. al 2004), though the wave in these case had an open wave structure.
This paper will document the event of 13 February 2013. Focus is on the pattern and use of short-term forecasts to forecast low predictability horizon events of similar nature in the future.
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2013-02-10 | A fast moving short-wave raced across the southwestern United States and into the Mid-Mississippi Valley from 8-10 February 2013 (Fig. 4). This wave pulled a plume of deep moisture with PW anomalies of 2-4 (Fig. 5) above normal with strong low-level (Fig.6). The convection and severe weather developed in this plume of deep moisture and strong shear.
Radar imagery (Figs. 3 &4) and RAP simulated radar (Figs. 8-10) showed a strong line of convection moved across Texas then into the Gulf States. After 1800 UTC 10 February the line broke into more discrete storms and these discrete storms produced tornadoes and at least one EF4 tornado near Hattiesburg, MS. The storm damaged in excess of 800 homes and caused 10s of millions of dollars of damage to the University of Mississippi (Huffington Post 2013).
The strong frontal system and convection produced 25 to 50 mm of rainfall across western Texas and across the lower Mississippi Valley. Due to the strong forcing the NCEP SREF was able to predicted a line of enhanced rainfall moving across the region at about the correct time. The SREF also predicted over 25 mm of rainfall over the correct region. The NCEP 13km RAP did reasonably well showing the convective evolution of the system over the Gulf States. It lacked resolution and had both intensity and timing issues. However, these data show the emergent power of high resolution and rapidly updated forecasts to aid in anticipating the timing and mode of convection for strongly forced high-impact weather events.
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2013-02-09 | A high impact winter storm brought rain, wind, and record heavy snow to the eastern United States on 8-9 February 2013. The heavy snow from southeastern New York and Long Island, across Connecticut and into Maine was the result of a strong cyclone which tracked up the East Coast on 8 February before interacting with a northern stream wave. The two systems merged during the evening hours of 8 February 2013. During this merger period intense snowfall affect central Long Island and Connecticut producing areas of 30 to 40 inches of snowfall. From a historic perspective this storm was compared to the February 1978 Storm. This was a top 5 snowfall event in many locations across southern and eastern New England and for many sites in Long Island and Connecticut is the new snowfall of record.
The storm was relatively well predicted 1 to 3 days in advance and the European Centers high resolution model provide some insight into storm potential about 6 days in advance. As the storm approached and the forecast length decreased the models and ensemble prediction systems produced a deep cyclone with an anchoring intense anticyclone to the north, resulting in forecasts of 850 hPa winds -5 to 6 below normal, implying a near record if not historic event.
With the strong frontal forcing implied by the strong winds, both model and ensemble forecast system quantitative precipitation forecasts were on the order of 25 to 50 mm in areas where the predicted precipitation type was forecast to fall mainly as snow. Forecasts of 18 to 38 inches were common in SREF forecasts at least 48 hour prior to the onset of precipitation.
This paper documents the event of 8-9 February 2013 providing reanalysis of the event using standardized anomalies to but the event in a climatological context. Supporting data on the predictability of the event is provided focused on the European Centers longer range deterministic model forecasts and the National Centers for Environmental Prediction Centers forecasts of the event.
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2013-01-30 | A strong Pacific wave coming over a ridge produced a strong trough over western North America. This trough pulled a plume of deep moisture from the eastern Pacific into the southwestern United States and northern Mexico. Ahead of this plume of deep moisture and the developing wave, a large 500 hPa ridge developed over much of the eastern United States.
The large ridge produced a period of unseasonably warm weather from 27-30 January 2013. At the peak of the intrusion of warm air ahead the trough, over 300 high temperature records were set on 28 January. Many sites also set new all-time record high minimum temperature records. The warm air eventually flooded most of the eastern United States.
As the wave moved eastward a strong surface front developed and a strong surface cyclone formed along the front. In the plume of deep and anomalous moisture, where precipitable water anomalies peaked in the +3 to +6 above normal range a heavy rainfall and severe weather event developed. In the course of two days there were over 600 reports of severe weather and over 20 tornadoes. This was one of the largest and most widespread cold season severe events. Though it produced fewer tornadoes than the February 2008 “Super Tuesday” outbreak. It shared many of the characteristics of the 3 winter season severe outbreaks of January and February 2012.
In addition to the severe weather, the event produced heavy rainfall and flooding.
The overall pattern favoring heavy rainfall and severe weather was generally well predicted by the NCEP models and ensemble forecast systems.
Key words anomalies ensembles winter severe and tornadoes. |
2013-01-24 | The period of 15-27 January saw the incursion of arctic air into eastern North America and much of the north-central and northeastern United States. During the peak of the cold episode (Fig. 1) a deep trough with -2 to -3s height anomalies (Fig. 1a) and a pool of deep cold air (Figs. 1b&1c) were present over much of eastern Canada and the northern tier of the United States. Much of the North America was dry with large areas where the precipitable water (Fig. 1d) was near or below normal. The strong and persistent ridge over the southwestern Atlantic limited the penetration of the cold air into the southern and southeastern United States.
A sudden stratospheric warming (SSWE:Smith and Kushner 2012; Baldin et al. 2012) event was observed in long range forecasts in late December 2012 and early January 2013. SSWE events are typically monitored above 50 km and temperatures on model pressure surfaces of 70 to 10 hPa are often examined to monitor these events. Baldwin and Dunkerton (2001 hereafter BD2001) noted that stratospheric events often follow the arctic oscillation (AO). Observational studies suggest that SSWE events be used to predict changes in weather regimes. Large warm ups over the Polar Regions often lead to arctic outbreaks over North America. BD2001 entitled their paper “Stratospheric Harbingers of Anomalous weather Regimes” due to the apparent observational predictability component of such events. During most winters, in the January to February time frame there is typically 1 major stratospheric warming event (Kuttippuarth and Nikulin 2012)
During the onset of the SSWE event, conditions had been relatively warm over most the eastern United States. Through December through about 6 January a cold pocket was present over the pole at 10 hPa which was replaced by a ridge and above normal temperatures after 6 January 2013 (not shown). The strong ridge over the southwestern Atlantic (Fig. 1a) was dominant feature through first half of January 2013, producing generally warm weather over most the eastern United States. High temperature records were tied or broken during a prolonged period in the southeastern United States (Table 1) through 18 January 2013. A surge of warm air ahead of the first blast of cold air tied or broke over 100 daily maximum temperatures records from 12-13 January 2013 in the eastern United States. The warmth then emerged over the southwestern United States (Table 1).
This paper examines the pattern of January 2013 with a focus on the evolution of the big chill of mid-January 2013. The persistent ridge over the southwestern Atlantic precluded the intrusion of the cold air into the citrus growing regions of the southern United States. Forecasts of the event are presented showing that the event and pattern change were relatively well predicted. Finally, this event and its associated pattern are compared to the arctic outbreaks of January 1985 and 1994.
Key words: cold anomalies ensembles |
2013-01-18 | A large and persistent subtropical ridge (Fig. 1) provided unseasonal warm conditions to much
of Australia during early January 2013. The large subtropical ridge had +1 to +2σ above normal
250 hPa height anomalies associated with it. The ridge appeared to peak on 15 January 2013
when the 250 hPa height anomalies exceeded +2σ beneath the closed anticyclone. News
accounts indicated that the heat wave persisted most of the first 2 weeks of the month with high
temperatures over 48C common place and 49.6C reading at Moomba in southern Australia.
Beneath the ridge the 850 hPa temperatures were above normal over most of the continent (Fig.
2). The subsidence beneath the ridge over the eastern side of the continent maintained a hot dry
air mass (Fig. 4). Plume of high precipitable water air and deep moisture (Fig. 3) was present
on the southwestern edges and ocean regimes south of the continent. The strong flow about the
subtropical ridge blocked the region from the deep moisture plume.
Beneath the ridge surface temperatures were warm with extended heat on 7 and 8 January (Fig.
5) the heat beneath the ridge shown in Figure 1 is depicted in Figure 6. The ridge earlier in the
month which produced the extreme heat on 7-8 January was also associated with a strong ridge
(Fig. 7). The ridge earlier in the month was displaced farther south and the air was considerably
drier (not shown) over much of the continent beneath the strong ridge in early January.
This paper will document the pattern and standard anomalies associated with the eastern United
States precipitation event of 15-16 January 2013. The focus is on the standardized anomalies to
describe the pattern and on using guidance to include ensemble guidance to aid in the prediction
of this and similar events.
This case shows the value of standardized anomalies in characterizing high impact weather
events and the clear association of strong ridges with enduring heat episodes and droughts. |
2013-01-16 |
A weak wave and a strong anticyclone (Fig. 1a-e) produced a precipitation event from the Mid-Mississippi Valley into southern New England on 15-16 January 1996 (Fig. 1f). A strong frontal boundary was present over the region with cold air to the north associated with the surface anticyclone and warm air to the south in the region of the surface trough and to the south and east (Fig. 2). The 850 hPa temperatures were above normal on the warm side of the boundary. The heaviest precipitation fell over portions of the Ohio Valley and western Virginia, along and mainly on the warm side of the 850 hPa frontal boundary. A moderate snowfall was observed on the cold side of the storm. Central Pennsylvania received 1-4 inches of snowfall early on the 16th of January.
There was a surge of moisture ahead of the frontal boundary (Fig. 3) with precipitable water (PW) values in the 25 to 35mm range in the warm air. The anomalies were in the +2 to +3 range on the warm side of the southwest-to-northeast oriented frontal boundary. The 850 hPa winds were relatively weak and out of the south-southwest during the period of precipitation (Fig. 4). The southwesterly flow and modest 850 hPa winds produced higher values of moisture flux and at times 3 to 4s above normal moisture flux anomalies in the warm air, in close proximity to the region of the higher precipitation amounts (Fig. 5).
The larger scale pattern showed a deep trough over the western United States and strong ridge over both the eastern Pacific and western Atlantic (not shown). The flow between the deep trough and the western Atlantic ridge produced a strong 250 hPa jet (Fig. 6) with 250 hPa winds near 100kts in the jet axis going over the implied strong Atlantic ridge. The 250 hPa wind anomalies. Were +4 to +5s above normal near the ridge and implied a strong jet entrance region over the eastern United States with the strong jet core on the cold side of the 850 hPa boundary (Fig. 2).
The synoptic pattern suggested a strong frontal boundary and potential jet entrance moving over the region. There were indications in the 700-500 hPa layer of a short-wave which was part of the jet entrance circulation. The pattern was well suited to produce a precipitation event and had the potential, with the sub-zero C air to the north to produce some snow. The winds at lower levels were not indicative of a widespread high impact weather event (HIWE). Lacking strong forcing, the event was not well predicted with significant lead-time, especially the northern edge of the precipitation shield.
This paper will document the pattern and standard anomalies associated with the eastern United States precipitation event of 15-16 January 2013. The focus is on the standardized anomalies to describe the pattern and on using guidance to include ensemble guidance to aid in the prediction of this and similar events.
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2013-01-09 | A slow moving and deep 500 hPa cyclone moved over the eastern Mediterranean basin from 5-10 January 2013. As the low deepened the strong frontal system pulled warm moist air into the eastern Mediterranean leading to locally heavy rainfall on the 5th and 6th of January 2013. As the low deepened it both generated and advected cold air into the Middle East bringing with it sub-zero temperatures through a most of the atmospheric column. Once the cold air moved over the region, this supported snow inland and at higher elevations. Jerusalem received in excess of 20 cm of snow. The resulting multi-day event produced heavy precipitation, flooding, cold, damaging winds, and heavy snowfall.
The heavier rainfall in the event occurred as a surge of high PW air and strong winds moved into the eastern Mediterranean. This produced high moisture flux and moisture flux anomalies on the order of 6s above normal. The result was heavy rainfall which was relatively well predicted by the NCEP GEFS, likely due to the strong forcing within the model atmosphere.
The snow and cold phase of the event occurred when the deep 500 hPa cyclone moved into Israel, Lebanon, and Syria. Beneath this deep cold 500 hPa cyclone the 850 hPa temperatures were in the -2 to -5C range. This was sufficiently cold to allow the precipitation to fall as snow in the higher terrain and away from the warm boundary layer air from the Mediterranean to the west. The deep cyclone and cold air was relatively well predicted, with long lead-time by the NCEP GEFS.
Synoptically, a deep slow moving cyclone, with a close mid-level cyclone brought a surge of strong winds and above normal moisture into the eastern Mediterranean basin. The result was a multi-day precipitation event, with locally heavy rainfall, inland and elevation dependent snowfall, and strong winds. The precipitation resulted in regional flooding and some areas of heavy snow fall. The heavy snow portion of the event involved a deep 500 hPa closed cyclone and a core of cold air at 850 hPa. This produced the largest snowfall in Jerusalem since February 1992 when a similar deep cut-off low and deep cyclone moved over the region.
key words: Ensembles anomalies snow Israel January 2013 |
2013-01-01 | An examination was made of lake ice out data over the eastern United States. Ice out data was obtained from Minnesota to Maine. Every lake examined showed a general trend toward earlier ice out dates. In addition to ice out dates, a few lakes had both ice-in and ice-out data allowing the examination of the changes in total ice days. Similar to ice out dates, the length of time lakes are iced over shows a trend toward a later total time of ice cover. The few lakes examined showed a trend toward a later beginning of the ice season.
National Weather Service Cooperative Observing site data in close proximity to several of the lakes were examined. These data showed a basic trend toward warmer late winter and early season mean temperatures similar to the trend in earlier ice out dates. In general warm March and April weather was associated with the earlier ice out dates. Conversely cold late winter and early springs were associated with the later ice out dates, which often extended into late April and May during the coldest years. |
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