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2014-07-13

Severe Weather Event of 13 July 2014,Severe weather affected the eastern United States (Fig. 1) from northwestern Connecticut southwestwards into Ohio and Kentucky. Over Pennsylvania, there were two distinct semi-linear damage paths. One was over western Pennsylvania and one was over central Pennsylvania. It will be shown that each of these observable patterns was associated with persistent convective features containing bow echoes.An examination of the storm reports and radar over Pennsylvania and New York (Fig. 3) showed several distinct quasi-linear bands of severe weather. The three most linear damage patterns were located near Pittsburgh, over the Maryland-West Virginia panhandles, and across the southern tier of New York. Though not as clear, a distinct bow echo produced most of the damage in central Pennsylvania and another mini bow-like feature produced the linear damage swath north of the Maryland border

2014-07-08

Severe Weather Event of 08 July 2014 damage focused with persistent features Severe weather affected the eastern United States (Fig. 1) from northern New England to the Ohio Valley. Two distinct northeast to southwest lines of severe weather are apparent in the storm reports. One extends from southern New York southwestwards to West Virginia and the other from Northern New England into northeast Ohio. Pennsylvania was affected by both systems and there was a less distinct area of convection to include several tornadoes in central and northeastern Pennsylvania. The severe weather was observed ahead of a deep 500 hPa trough (Fig. 2) which moved into the Great Lakes on 9 July 2014. The 500 hPa height anomalies were -2 to -2 below normal with this trough. The trough brought a plume of deep moisture and high precipitable water into the eastern United States (Fig.3) and a fast-moving low-level jet (Fig. 4). The strong jet peaked over western Pennsylvania around 1800 UTC 8 July (Fig. 4d) and rapidly moved into New England. The strong shear with the western linear convective damage swath was closely aligned with an area of 1200 to 1800JKg-1 (Fig. 5d) surface based CAPE. By 0000 UTC 9 July the axis of high CAPE had shifted to the east and then off the coast by 0600 UTC 9 July 2014. The following sections examine the set-up for the convection and several of the features which produced a significant percentage of the severe weather and the confirmed tornadoes in Pennsylvania. It will be shown that most of the damage and tornadoes were associated with long-lived supercell thunderstorms and a distinct bow echo. In southern Pennsylvania and Maryland a quasi-linear convective system (QLCS) was effective in downing trees and thus aiding in reports of severe weather. Tragically, a falling tree resulted in a fatality in Maryland (AP 2014). New accounts suggested over 300,000 customers lost power in Pennsylvania due to the storms.

2014-06-19

Deep moisture moving over the top of a ridge, a strong low-level jet and instability produced a severe weather event over western and central Pennsylvania. To larger QLCS systems developed one moved out of northeastern Ohio tracking to the east-southeast. Despite how long the QLSCS was, most of the damage was focused near and long the path of an embedded spearhead echo. Most of the damage with this system was in the form of wind damage with 2 reports of hail early in the event. The feature across eastern Ohio and southwestern Pennsylvania was not examined. However, a slow moving storm ahead of the line in central Pennsylvania evolved into a mini-bow echo and move rapidly to the east. This feature produced golf ball size hail and damaging winds near Belleville, PA.

2014-06-18

An MCS moved from northwestern Pennsylvania southeastward across the State during the overnight hours of 17-18 June 2014. The MCS produced locally heavy rainfall (Fig. 1) and several reports of wind damage. Power outages were reported in west-central Pennsylvania with to areas in Clarion and Jefferson County Pennsylvania were around 2000 customers lost power. Other areas reported sporadic power outages due to down trees and wires. The axis of higher rainfall and severe reports were relatively well aligned with the heaviest rainfall, over 32 mm in western Pennsylvania where most of the wind damage and downed trees were reported.

2014-06-16

Central Plains Severe and heavy rainfall Event of 16-17 June 2014 Overview A significant severe weather event affected the central Plains on 16-17 June 2014 (Fig. 1). In the afternoon hours of 16 June a supercell developed in eastern Nebraska, south of the main line of storms. This storm produced deadly and destructive “twin tornadoes” near Pilger, Nebraska. The storms developed in an environment with high convective available potential energy (CAPE: Fig. 2) and with a second surge of above normal precipitable water (Fig. 3). The second surge of high CAPE around 1800 UTC 16 June was accompanied by and enhanced 850 hPa jet (Fig. 4). The 850 hPa winds were +2 to +3s above normal at 1800 UTC 16 June peaking at over +4s at both 0000 and 0600 UTC on 17 June 2014 (Fig. 4e-f). Hourly RAP13km data showed the strong and persistent 850 hPa jet as it evolved from 1700 through 2200 UTC 17 June 2014 (Fig. 5). The strong low-level jet was played a role in the convective evolution and the resulting Mesoscale Convective System which evolved over the course of the period from about 2200 UTC 16 June through 1200 UTC 17 June 2014. As in many convective events and heavy rainfall events, a large ridge was present south and east of the affected area and a trough was located upstream of the same region (Fig. 6). The larger scale synoptic pattern had a “return flow” appearance allowing moisture from the Gulf to move around the western flanks of the larger scale ridge. This “ring-of-fire” pattern is often a pattern know to favor both severe weather and heavy rainfall (Bodner et al. 2011; Junker et al 1999). The 24-hour rainfall (Fig. 7) showed the heavy rainfall from eastern Nebraska and across Iowa and portions of adjacent States. This short note will document the severe and heavy rainfall event of 16-17 June 2014. The pattern has been shown to be one associated with heavy rainfall. The following sections focus on some of the severe weather and features which contributed to both the heavy rainfall and the severe weather.

2014-06-11

Western Pennsylvania Derecho and Snake Springs Tornado 11 June 2014: The combination moist air (Fig. 1), a strong low-level jet (Fig. 2), and instability (Fig. 3) combined to produce severe weather (Fig. 4) over western Pennsylvania during the late afternoon and early evening hours of 11 June 2014. The clustering of wind damage reports in southwestern Pennsylvania was associated with a mini-derecho which moved southwest to northeast across the region. The tornado and hail reports were associated with a supercell thunderstorm which developed well ahead of the mini-derecho.

2014-06-03

A quasi-linear convective system (QLCS) developed along the shores of Lake Erie and propagated southeastward across Pennsylvania. The system produced a few reports of severe weather (Fig. 1) and an enhanced axis of rainfall (Fig. 2) from northwest to southeastern Pennsylvania as the QLCS moved southeastward. In central Pennsylvania, impacts were all in the form of wind damage based on trees. One storm, in the Harrisburg area had a three body scattering signature (TBSS). Rain and gusty winds were the main impacts of the storms

2014-05-22

A series of rotating thunderstorms produced severe weather in the eastern United States (Fig. 1) from east-central New York southward to Virginia. Most of the storms in New York and Pennsylvania produced atypically large hail (Table 1) and a rare EF3 tornado was observed in the Albany Metropolitan area. Farther south wind damage and several tornadoes dominated the severe weather reports. As shown several storms developed strong signals in both ZH and V and in the dual-pol fields which may have aided in identifying the potential for large hail In central Pennsylvania some of the hail reports, 2-2.5 inches (Table 1), represented a relatively high end to extreme hail event. There were 59 reports of hail along the boundary on 22 May 2014. The largest hail, 4 inches in diameter was associated with the Albany storm (Table 2) though hail stones in the 2 to 2.5 inch range were observed in central Pennsylvania.

2014-05-21

Ring-of-fire Mesoscale Convective Complexes Mesoscale heavy rain and floods in northwestern Pennsylvania : A strong ridge (Fig. 1) over the Gulf States set up a somewhat classic northwest flow ring of fire pattern over the eastern United States. A surge of moisture coming over the ridge in the strong northwesterly flow lead to the development of successive MCSs over several nights (Figs 3 & Fig 13). Two MCS developed over the Great Lakes late on 20 and early on 21 May 2014 and moved over Pennsylvania. These two MCSs produced locally heavy rainfall (Figs. 8 & 9) and flooding in northwestern Pennsylvania. The pattern was generally well forecast and is a pattern often recognized as having the potential to produce severe weather and heavy rainfall. Farther west, in the more unstable air severe weather was observed in the early stages of the MCS evolutions over the Midwest. In Pennsylvania the issue was locally heavy rainfall and flooding. Though the pattern was easily recognized and was well forecast by the NCEP models, the convective response realized in the atmosphere was not so well predicted. Though the 4 primary NCEP systems (GFS, NAM, SREF and GEFS) all forecast rain over Pennsylvania, they all fell short (GEFS not shown for brevity) of getting the location and the amounts. Most systems missed the higher end QPF by a factor of 4 or more. Due to the convective nature of this and similar MCS driven mesoscale rainfall events, there should be little expectation of these coarse, non-convective following systems in predicting events of this nature. Convective allowing models used in storm scale ensembles (SSE), with resolutions in the 3-4km range would be required to predicted events of this nature. The NAM 4km for this event (Fig. 14) had higher amounts than the non-convective allowing systems. But the 3 forecast cycles shown suggest the initial conditions cause considerable run-to-run variation as to where the heavy rain would fall. The model produced nearly twice the total QPF relative the GFS and some of the wetter 12km NAM runs. These Shorter range 4km NAM forecasts showed about 2 inches in regions where 4 inches were observed. The 4km NAM did relatively well with the location suggesting once the convection is evolving these high resolution data may improve situational awareness as to the general location but may grossly under forecast the rainfall amounts. They do show potential benefit to the user relative to the coarser guidance. It may be that SSEs with improved initial conditions may be required to help predict events similar to this. Ensembles of this nature are likely 5-10 years from being fully implemented in an operational setting. SSE’s may help produce confidence as to the regions where convection may develop and produce higher rainfall amounts, though the maximum rainfall may, in events like this imply 50 to 75% of the rainfall observed. On an observational note, the Stage-IV data did respectable verse observations in the regions where the heavy rainfall was observed. These data also show how fast the bulk of rain fell and the hourly data captured the distinct rain events from the successive MCSs.

2014-05-16

Eastern United States Synoptic Rain Event 15-16 May 2014 :heavy rain and flooding event..... A deep 500 hPa trough with -4s height anomalies over the central United States (Fig. 1) combined with a strong ridge with +2s height anomalies over eastern Canada, produced strong southerly flow. The deep southerly flow brought a plume of high precipitable water (PW: Fig. 2) air into the region along with strong 850 hPa southerly flow (Fig. 3) resulting in heavy rainfall (Fig. 4) and isolated severe weather (Fig. 5). The heavy rainfall produced flooding in the Mid-Atlantic region from Virginia northward into New York State. The pattern was well predicted by the NCEP models and ensemble forecasts systems. The NCEP SREF will be used to illustrate the successful prediction of the larger scale pattern. The precipitation forecasts showed some uncertainty as to the amounts and locations of the heavy rainfall though all forecasts systems correctly predicted the potential for heavy rainfall in the general region where the rainfall was observed. The paper will provide an overview of the pattern, the resulting weather, and forecasts of the rainfall. Local severe weather which included several mini-supercells and a bow echo are presented, focused on their impacts in central Pennsylvania.

2014-04-30

Eastern United States Wild Weather 27-30 April 2014 Significant quantitative precipitation bust: Over a 3-day period extreme weather was experienced across the United States including extremely heavy rainfall (Fig. 1) and flooding over western Florida (AP 2014) which produced over 20 inches of rainfall in portions of the Florida panhandle. The weather during the period was the result of a strong short-wave which at 500 hPa moved over the Pacific northwest on 26 April (Fig. 2a) and rapidly moved eastward. As the trough moved eastward a ridge developed over the western Atlantic allowing moisture from the Gulf of moisture to move into the central United States (Fig. 3). The high heights over eastern North America produced a cold air damming situation over the northeastern United States. The deep moisture (Fig. 3). and strong low-level winds (Fig. 4) produced a severe weather event over the eastern Plains and Mississippi Valleys on 27-28 April (Fig. 5). The severe weather diminished as the system moved eastward and encountered relatively cold low-level air. The boundary between the cold air and the warm moist air produced heavy rainfall and flooding from northern Virginia into southern New York (Fig. 6). The surge of high PW air and strong winds over the frontal boundary in the northeast had the appearance of a Maddox synoptic type event (Fig. 7 & 8). This paper will focus on the two areas of rainfall which produced flooding over the eastern United States. The focus is on the rainfall and how well it was predicted with the NCEP operational forecast systems around Pensacola and Philadelphia. The Stage-IV rainfall data is used to compare the rainfall timing and amounts relative to the forecasts systems

2014-04-08

Southern United States Heavy rain and flood event 6-8 April 2014: A HIWE event, in the form of heavy rain and associated flooding affected the southern United States from 6-8 April 2014. This event occurred in a pattern known to be conducive for heavy rainfall which included a deep trough to the west and a ridge to the east. As the trough moved eastward the strong flow between the trough and ridge produced a surge of deep moisture and strong low-level flow. The result was a widespread heavy rainfall event. The pattern was relatively well predicted by the NCEP GEFS (Fig. 6-8). Correctly predicting the synoptic pattern, the GEFS and GFS correctly predicted the potential for a heavy rainfall event in the southern United States with a 4-6 days lead-time. The exact areas to received heavy rainfall varied from run-to-run (Fig. 9-11). At shorter ranges, the SREF showed a similar focus for heavy rainfall in the southern United States. The SREF (16km), with a mix of model cores, model physics, and varied initial conditions is a more diverse system than the GEFS (55km). The SREF is actually run at a finer resolution, 16km than the 27 km GFS. The mean QPF from the SREF was not shown here. The focus was on the probabilistic data. Strength of the SREF in this event was showing a wider region and slightly different region which might experience heavy rainfall relative to the GFS-GEFS family of forecasts. All three systems correctly predicted heavy rain in the southern United States where heavy rain was observed.

2014-04-03

Severe Event of 3 April 2014:comparison to the two larger events of the winter of 2013-2014. A significant spring time severe weather event brought convectively driven high winds, hail and 13 confirmed tornadoes to the Mid-Mississippi Valley (MMV) on 3 April 2014 (Table 1 & Fig. 1). The severe weather was associated with a plume of deep moisture (Fig. 2) and a surge of high precipitable water air into the Mississippi and Ohio Valleys. The plume of deep moisture (Fig 2) likely contributed to the severe weather (Fig. 1) and the heavy rainfall (Fig. 3) over the MMV and Ohio Valley. The first significant high-CAPE severe event of the 2014 struck the MMV and Ohio Valleys on 3 April. Similar to the larger severe events of the winter of 2013-2014 this event had a deep trough to the west and a strong ridge to the east. This pattern produces strong southerly flow and provides surges of high PW air. All three significant events, with over 300 reports of severe weather had these common characteristics.

2014-03-12

The Winter Storm of 12 March 2014 Leveraging uncertainty and short predictability horizons A winter storm brought heavy snow and blizzard conditions across western New York State on 12 March 2014. South and east of the area of and despite earlier optimistic forecasts for heavy snow, rain was observed (Fig. 1) and only the northern edge of the precipitation was snow. The optimistic forecasts were likely related to early (Fig. 2) GEFS, EC, and GFS forecasts (GFS and EC not shown) tracking a cyclone over the Mid-Atlantic region and out over the coastal regions of New England. This track, combined with the observed trend in the 3 March 2014 storm, which tracked farther south and east of the forecast track (Grumm 2014) likely provided optimism that the storm would produce conditions supporting snow across most of interior New York and Pennsylvania. This paper will examine the winter storm of 12 March 2014. The overall pattern is presented, with a focus on where the precipitation fell and the pattern in the context of standardized anomalies. Forecasts presented focus on the uncertainty associated with forecasts of this storm. The focus is on the NCEP GEFS and SREF forecast systems. The term forecast systems will be used in this paper to denote models and EFS-- all of which had difficulty with the forecast evolution of this event. This case implies that basic uncertainty information is not being leveraged in the forecast process.

2014-03-03

Poorly Forecast Winter Storm of 2-3 March 2014 Who wouldve thought that non-linear chaotic systems are hard to predict A winter storm was forecast to impact much of the United States from the southern Plains to the Mid-Atlantic region from 2 to 3 March 2014. Forecasts from the GFS, EC, GEFS, and SREF converged on a potentially significant winter storm. Similar to the Post-Groundhog Day Storm and the March 2009 Megastorm (Stuart et al. 2013), despite a convergence of solutions of several models and ensemble forecast systems, the event proved to be difficult to forecast. The convergence of solutions and perhaps several relatively successfully forecasts led to high confidence in the storm of 2-3 March 2014. This paper will examine the winter storm of 2-3 March 2014. The overall pattern is presented, with a focus on where the precipitation fell and the pattern in the context of standardized anomalies. Forecasts are presented focused on the uncertainty associated with forecasts of this storm. The focus is on the EC, NCEP GEFS and SREF forecast systems. The term forecast systems will be used in this paper to denote models and EFS all of which had difficulty with the forecast evolution of this event.

2014-02-19

Freezing rain event of 19 February 2014: A freezing rain event affected central Pennsylvania during the morning of 19 February 2014. The precipitation was generally light (Fig. 1), with most locations receiving no more than a quarter of an inch of liquid. However, the timing of the event during the morning rush hour created numerous traffic accidents, including a 50-vehicle pile-up on Interstate 80 in Clearfield County, Pennsylvania, closing a 13-mile stretch of the interstate. A second, 7-mile stretch of Interstate 80 was closed about fifty miles to the east of the pileup in Clinton County, Pennsylvania due to icy conditions and jackknifed tractor trailers (AP 2014). Additionally, many school districts across the region were forced to delay or cancel classes for the day. After the freezing rain ended, temperatures quickly rose above freezing. This was a difficult event to forecast, as the precipitation was light and the system was weak and rather ill-defined. It has been suggested that weakly forced systems in fast flow have short forecast horizons relative to strongly forced systems (Grumm and Ross 2014). The models struggled to pick up on the event and, once they did, there were precipitation-type issues leading right up to the onset of the precipitation. The following sections summarize the data and methods used to examine this event, as well as the weather pattern and forecasts associated with the event.

2014-02-18

Clipper snow of 18 February 2014:A fast-moving clipper system produced a quick-hitting snow event across central Pennsylvania during the early to mid-morning hours of 18 February 2014. A swath of 4 to 8 inches of snow fell from Northern Ohio eastward into west-central Pennsylvania (Figure 1). The Lower Susquehanna and Lower Delaware Valleys received 1 to 3 inches of snow from this event. A secondary maximum of 4 to 6 inches of snow fell across inland portions of Southern New Jersey. A look at the accumulated liquid equivalent precipitation (Figure 2) also shows the two maximums of snowfall. Although this was generally a minor snow event for much of Pennsylvania, one of many to occur during the winter of 2013-14, the timing of the heaviest snowfall during the morning rush hour produced a significant impact for parts of Central Pennsylvania. The SREF did relatively well predicting the two locations of the most significant precipitation, one being from northeastern Ohio into west-central Pennsylvania and the other across Southern New Jersey. It has been suggested in another recent case study (Grumm and Ross 2014) that clipper systems in fast flow may have relatively short forecast horizons relative to more strongly forced systems. This appears to be the case with this event as well, as the ensembles did not hone in on accurate snowfall amounts until about 24 hours prior to the first snowflakes falling.

2014-02-15

Clipper snow of 15 February 2014: A fast moving clipper on the heels of the high impact winter storm of 12-14 February 2014 brought a modest 1-6 inch snowfall from the Ohio Valley to the Mid-Atlantic region on 15 February 2014. The Clipper developed a strong cyclone along the East Coast producing snow in southern New York and New England. The heaviest snow fell near and along the track of the 850 hPa cyclone. Forecasts of this event showed that in both NCEP versions of the SREF the event had a relatively short predictability horizon. Examination of both SREF and SREF PARA plumes diagrams for State College revealed that the event was in about 20% of the members by 0900 13 February increasing dramatically in the 1500 UTC forecast cyclone and then was consistent thereafter (not shown). Both systems had about 30 to 36 hours of lead-time showing the potential of the event being a high probability outcome and about 48 hours of lead-time using the lower probability forecasts as indicators for the potential for a snow event. Interesting the GEFS (Appendix-I) showed increased snow potential from the same wave from GEFS forecasts initialized at 1200 UTC 13 March. This case suggests that clipper systems in fast flow may have relatively short forecast horizons relative to strongly forced systems. Thus forecasts can change rapidly as these systems are better resolved by forecasts systems.

2014-02-13

Eastern United States Winter storm of 12-14 February 2014 Dealing with divergent model and ensemble forecast systems A high impact winter storm brought snow, freezing rain, and rain from the Gulf Coast to Maine. The storm had many of the characteristics of previously studied East Coast winter storms including a coupled jet, strong easterly flow north of the surface and 850 hPa cyclone, and cold air damming along the coastal plain, in this case well into Georgia. There were forecast issues in the NCEP models related to the track of the cyclone and the western edge of the precipitation shield and how far west it would extend. The EC model produced the earliest solution of a storm tracking along the coast and forecasts issued on 9 February produced a significant area to be affected by 25mm or more of QPF (Fig. 10). Relative to observations, this was not a particularly skillful QPF. The higher amounts of precipitation were shifted to the east. Though the western extent of the QPF shield was generally farther west in EC forecasts relative to the GFS (not shown) and GEFS. The EC clearly picked up on the stronger cyclone close to the coast relative to the NCEP models and EFSs. This may be due both the higher resolution of the model and the asynchronous data assimilation methods employed. The NCEP SREF produced a more westward QPF shield than the GEFS and the SREFPARA indicated a potential for more QPF and snow farther west than the SREF. The SREFPARA also showed a larger spread in the QPF relative to the operational SREF. Likely uncertainty with the phasing wave issue which impacted all the modeling systems. This case seems to imply that the wave phasing issue and the development of the cut-off created considerable uncertainty in the forecasts. The coarser resolution models appeared to be slower to deal with the wave phasing issues and had more difficulty addressing it. This likely lead to the poorer GFS-GEFS QPF and cyclone tracks. The SREF did a bit better with this and the SREFPARA a bit better too. Clearly, the SREFPARA covered the spread relative to the other two NCEP EFSs. This case involved phasing waves and what proved to be a complex winter storm. There was considerable uncertainty with this storm and how to best forecast a storm in the face of conflicting guidance, even when one single model is generally viewed as superior, is a difficult chore. A poorman’s ensemble (PME) and a super-blend are likely the means to deal with events of this nature.

2014-02-05

Eastern United States Snow event of 5 February 2014 Dealing with uncertainty and varying predictability horizons: A significant winter storm brought snow from eastern Kansas and Missouri, across the Midwest into northern Pennsylvania and New York State on 4-5 February 2014. The overall pattern and the potential for a winter storm in the eastern United States was relatively well predicted with at least 8 days of lead-time in both the NCEP GEFS and US-Canadian 42 member NAEFS. The details as to where the heavier snow and higher precipitation amounts were not as well predicted as the overall pattern. The predictability horizon of the storm and region impacted was on the order of days. The details of the areas for heavy rains, heavy snow, and ice were not as well predicted. The ice forecasts in southeastern Pennsylvania had high confidence predictability horizons on the order of tens of hours. The pattern in which this storm developed is a pattern common with many winter storms and ice storms. The strong ridge of the western Atlantic played a critical role in the winter storm of 4-5 February 2014. The strong gradient between the trough and ridge (Fig. 1) allowed for a surge of warm air and above freezing temperatures as far north as the Mid-Atlantic region (Fig.4-5). The strong gradient on the northern edge of the ridge produced a strong jet streak (Fig. 7) and a thermally direct circulation which helped maintain the baroclinic zone and maintain the low-level cold air. Nearly ideal conditions for a winter storm with sub-freezing air maintained at the surface and a wedge of warm air in the layers above the surface from at least 850 to 700 hPa. The heavy snow was along the northern edge of the strong southerly flow (Fig. 6). Snow and heavy snow were also observed in the strong easterly flow north and west of the track of the 850 hPa. The anomalously strong 850 hPa winds indicated a strong cold conveyor in the cold air. These features implied a strong jet entrance region (Fig. 7) which is often present during winter storms and particularly in ice storms. The over GEFS forecasts correctly predicted the potential for both the storm and high QPF amounts with 7-8 days of lead-time. These forecasts of the storm and the Miller-B evolution were rather impressive. However the details remained elusive and there were at times signals for an East Coast Snowstorm which did not occur. Social media users employed forecasts from single models and began advertising the potential for a big snow storm over the Mid-Atlantic region 5-7 days in advance of this storm which ended up being a messy event with little snow south of the Mason-Dixon Line. The shorter term SREF forecasts did well with many aspects of the event, though the details with the rain-snow and freezing rain areas were slow to focus in on the higher threat in southeastern Pennsylvania. The SREF freezing rain forecasts (Fig. 18) are shorter in duration as the SREF emphasized the freezing rain farther west and the predictability horizon in southeastern and eastern Pennsylvania was quite short. In tight gradients, near boundaries, and in cold air damming situations, the details are difficult to ferret out and require vigilance. Despite how far in advance this event was predicted, the high impact weather, to include the devastating ice was not well predicted with more than 1-2 days of lead-time in eastern Pennsylvania. These data show that despite successful long-range forecasts, the important details are not as easily ferreted out.

2014-02-03

Eastern United States Snow event of 2-3 February 2014 Dealing with uncertainty and short predictability horizons A winter storm brought snow from the southern Plains to the Mid-Atlantic region on 2-3 February 2014. Initially, the weak system was forecast to pass to the south of the Mid-Atlantic as shown in the GEFS probability of 16mm or more of quantitative precipitation (QPF) from the 0000 UTC 30 January 2014 forecast cycle (Fig. 1a). As the forecast length decreased, there was a general trend toward higher QPF amounts, though amounts which might support heavy snow were only slowly approached by GEFS QPFs issues after 1200 UTC 1 February (Fig. 1e) and a high confidence in 16 mm (0.63 inches) or more QPF; which was sufficient to produce heavy snow using a 10:1 ratio; was forecast by the GEFS QPFs issued at 1200 UTC 2 February 2014. These QPFs show general trend toward higher QPF and a higher probability of higher QPF values. In these 6 images shown, the trends were consistent after the forecasts on 31 January which had lower QPF amounts (Fig. 1b). This change in the trends supports the research (Hamill 2003) which showed that trends are not always consistent and they can and they do change. This paper examines the winter storm of 2-3 February 2014. In the eastern United States, this event was not well predicted more much more than 36 to 48 hours in advance. The system ended up stronger and the precipitation shield shifted farther north than longer range forecasts implied. The uncertainty and changing forecasts associated with this event identify many of the issues which tend to limit forecasts of critical weather elements such as snow and the decisions organizations affected sensible weather elements, such as snow, must address.

2014-01-28

Southern United States Winter Storm of 28-29 January 2014 High impact snow on edge of forecast precipitation shield An arctic front pushed into the southern United States on 28 January 2014. Ahead of the front many locations were in the 50 and 60s within 24 hours of the onset of snow. The rapid temperature falls and onset of snow likely contributed to treacherous driving conditions in a region not accustomed to dealing with icy roads and snow. The rapid temperature changes may have contributed to icy road conditions. In addition to the rapid temperature falls, there were issue related to the onset time and area to be affected by snow and ice. The city of Birmingham, AL was perceived to be location which might escape the snow as it was on the northern edge of the precipitation shield. Despite low probabilities of 6.25 mm or more QPF, snow fell in and around the city leading to serious disruptions to human activities. This storm demonstrates that even within areas where the threat of wintry weather was well predicted, many municipalities in the Deep South are ill prepared to deal with snow and ice. From climatological and fiscal perspectives, it is probably unfeasible to prepare for these rare events. The costs of plows and salt spreaders, which could go unused for years is likely prohibitive. This combined with drivers not used to driving in snow and ice implies that with a high probability outcome of snow and ice event, the best strategy may be to keep key roads open and restrict driving activities until the roads or cleared or the ice and snow have melted. A serious issue arises in the lower probability zones, such as Birmingham, AL where several forecast cycles showed a 0 to 10% chance of 6.25 mm or more QPF, which would fall as snow. In areas where the low probability event could be an extremely high impact event, precautions may be required. The impact of the forecast low probability event in Birmingham was clearly evident with massive traffic accidents, 5 related traffic deaths, and stranded students. In climates were snow and ice are more frequent, a few inches of snow would likely have been a nuisance, snow and ice removal equipment could be easily deployed; drivers are more accustomed to driving in snow; and a percentage of drivers likely have tires specially designed to drive in snow. Keywords: Ensembles anomalies snow

2014-01-21

East Coast Winter Storm of 21-22 January 2014 Short Predictability horizon event: A fast moving Clipper in strong northwesterly flow developed into a strong cyclone along the East Coast producing snow from Minnesota to Maine. The storm produced heavy snow from northern Maryland northeastward into southern New England. Though the Clipper was relatively well predicted, the strength of the system was poorly predicted with much more than 36-48 hours of lead time. The potential for heavy snow along the East Coast had a limited predictability horizon. Thus, the high confidence in a winter storm with heavy snow along the East Coast had a predictability horizon on the order of 1 to 2 days with this particular event The pattern (Fig. 3 & 4) showed a relatively well known pattern favoring the evolution of Clipper-like systems (Hutchinson 1995; Thomas and Martin 2007). This particular system produced more snow; north and west of the track of the cyclone; than most Clippers typically produce. Similar to most Clippers, areas north and west of the Clipper received the most snowfall. The strength of this system, with cyclone developing south of the Appalachians likely contributed to the higher than normal Clipper related snowfall. The ability to correctly initialize the short-wave which moved through the northerly flow likely played a critical role in predictability of this Clipper. Similar to the GEFS, the SREF showed the evolution of a stronger cyclone closer to the coast as forecast length decreased. SREF 850 hPa winds (Fig. 12) and QPF probabilities all trended toward more QPF; hence more snow; in the heavily populated coastal corridor. Forecasts of 12-30 hours in length provided useful guidance. It should be noted that the QPF values were not overly impressive, generally under 20mm and contours of probabilities in the 12.5 to 16mm ranger were used to capture a sense of where the higher QPF was forecast. Southern stream ECWS typically have 25 to 50mm of QPF, this Clipper system had significantly lower QPFs and in many locations heavy snow was based more on snow to water ratios. keywords: anomalies ensembles uncertainty

2014-01-06

The Arctic Outbreak of 4-8 January 2014: The first significant arctic outbreak of the 21st Century affected much of the eastern United States from 4-8 January 2014. For the first time in over a decade 850 hPa temperatures of -30C spread over portions of the eastern United States plunging surface temperatures to near or below previous record lows for the date. Many locations in the core of the arctic air struggled to get to zero Fahrenheit. The cold caused massive closing of schools, froze rivers and streams, and caused rapid formation and expansion of ice on portions the Great Lakes. The pattern which produced the cold included a deep polar vortex which moved into eastern North America and sub -30C air at 850 hPa, marker used to track some of the colder arctic air masses in North America. Intrusions of -30C are relatively rare and intrusions of the even rarer -40C are extremely rare. This event clearly saw a deep penetration of -30C air into the eastern United States. Several 20th Century arctic outbreaks were presented. Nearly all which were observed since 1979 were associated with -30C or lower 850 hPa temperatures. Events of this nature have dropped off considerably since the mid-1990s and the arctic outbreak of 1994 was one of the last significant outbreaks in recent memory. The outbreak of December 1983 was one of the more significant outbreaks in recent history. Keyword: Polar vortex, anomalies, arctic air, record cold.

2014-01-03

East Coast Snow event of 2-3 January 2014: The use and misuse of NWP and uncertainty information:An artic airmass pushing to the eastern United States and a shortwave produced a snow event along the East Coast on 2-3 January 2014 which was followed by a brief cold snap. A strong cyclone developed well offshore and most of the higher QPE amounts close to the cyclone remained well offshore. Despite this, strong flow between the arctic air and the cyclone produced several bands of snow. Due to the presence of the arctic air on the north side of the cyclone some areas received heavy snow, despite pretty anemic observed QPE. Snow ratios during the event were relatively high . The high snow to water ratios likely contributed to the relatively successful forecast of this event, despite the track of the surface cyclone well to the east.Overall, ensemble forecasts of the QPF shield and the cyclone were relatively good. The actual storm did in fact remain well offshore. Though not of concern here; this storm rapidly deepened as it moved along and up the coast of eastern Canada; and likely qualified as a “bomb”. Several deterministic model runs at times showed the potential for the storm to track closer to the coast and produce the threat of a significant nor’easter to impact much of the heavily populated corridor from southeastern Pennsylvania to New England. The EC model initialized 1200 UTC 28 December 2013 and 0000 UTC 30 December 2013 showed a strong storm close to the coast (Figs. 16 & 17) with strong implied easterly flow on the cold side of the storm. Strong easterly flow (Fig. 18) in the cold air produced high amounts of QPF (Fig. 19) which was forecast to fall as snow. These single model forecasts initiated concerns on social media and weather outlets about a major nor’easter.

eyewall.met.psu.edu web 1

 

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