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Mid Atlantic Heavy rainfall event With an inertial gravity wave?:An unseasonably strong 500-hPa upper-level wave (Fig.1) moved into the eastern United States on 5 May 2017. The 500 hPa heights were -3 to -4 below normal. In the implied deep southerly flow on the east side of the 500 hPa trough there was a surge of deep moisture. The precipitable water (PWS: Fig.2) was +2 to +3 along the Mid-Atlantic and southern New England coastal zone. The deep moist flow produced a significant rainfall event as shown in Figure 3. Broad areas of central Pennsylvania had 25 to 50 mm of rainfall and over 50 mm of rainfall was observed over New Jersey. The subtle streak of 24 to 48 mm of rainfall in the Susquehanna Valley into southern New York will be addressed later in the paper. This feature, in addition to producing a brief period of heavy rainfall, was associated with rapid pressure fall and rise couplet of 5-7 hPa (Fig. 4). From the mid-Susquehanna Valley and into southern New York close to the time of the pressure fall there were strong and damaging winds. Behind the wave as the pressure rose, the precipitation ceased (Fig. 4) to the south. The feature which moved through Pennsylvania and New York had 5-minute pressure traces which showed sharp 5-7 hPa pressure falls then similar rises as the wave moved to the north-northeast. These values are in line with the gravity wave diagnosed by Bosart and Seimon (MWR 1988) and Bosart et al. (1998). The event also occurred on the cool side of a surface boundary (Fig. 3 Bosart and Seimon 1988) between the downstream trough and the upstream ridge. The synoptic settings for gravity wave evolution are summarized in Uccellini and Koch (1987: See their Fig. 1). This short paper will summarize the Mid-Atlantic heavy rainfall event of 5 May 2017. The focus is on the pattern and the feature which contributed to local QPE maximum and the focused area of high wind and wind damage.


Mid-Mississippi Heavy rainfall and flooding Double whammy rain events late April and early May:A persistent ridge over the western Atlantic and a deep trough over the central United States (Fig. 1a) during the period of 26 April through 5 May 2017 produced two heavy rain events in the MMV. The trough and ridge were stronger during the first event which led to more rainfall and more extensive flooding than the second event. The western Atlantic ridge was considerably weaker during the second event and the rainfall totals were significantly lower in the MMV. Both events occurred in a well-known pattern for heavy rainfall, including a deep trough to the west and the ridge to the east... This general pattern was found to be critical for heavy rains in the mountains of California (Junker et al. 2008); Junker et al. (1999) showed the importance of deepsoutherly flow and high PW air in the Great Midwest floods of 1993. The pattern during this event was similar to the pattern and composites which accompanied the Mid-Mississippi Valley floods of April 2011 (Fig.4: Grumm 2011). The heavy rainfall of 29-30 April was a highly successful forecast of a synoptic scale extreme rainfall event. However, from a spatial perspective, the GEFS had the highest QPF amounts displaced east of the verifying location and the GEFS under forecast the extreme amounts of in excess of 150 to 200 mm of QPE observed. Relative to the event of 4-5 May 2017 the stronger trough ridge pattern and the more anomalous plume of high PW led to more rainfall in the earlier event. The GEFS was able to produce higher QPFs which reflected the stronger signal in the first event. Thus, the QPFs indicated that the GEFS can distinguish significant from extreme rainfall events.


Mid Atlantic Severe Event of 1 May 2017 Central Pennsylvania QLCS event: A strong upper-level wave (Fig.1) moving into a strong subtropical ridge over the western Atlantic produced a widespread severe weather event over the eastern United States on 1 May 2017 (Fig. 2). Most of the severe weather on 1 May was observed from southern Ohio northeastward into northeastern New York. It will be shown that as observed by radar this was a quasi-linear convective severe (QLCS) event. Thus most of the event was dominated by severe winds though there were some reports of hail and short-lived tornadoes. The deep southerly flow between the 500 hPa trough and ridge (Fig.1) brought a surge of moisture as shown by the precipitable water (PW: Fig. 3). The initial surge of high PW air was into the Mississippi Valley and southern Canada. Not surprisingly, there was a heavy rainfall event (Fig. 4) and flooding in portions of the Mid-Mississippi Valley (MMV). At the surface (Fig. 5) a slow moving cyclone was associated with the 500 hPa low (Fig. 1) and this system slowly lifted into the Great Lakes. There was a strong low-level jet (Fig. 6) in the warm air ahead of the cold front. This strong LLJ moved across eastern Ohio and into eastern Pennsylvania and New York between 1200 UTC 1 May and 0000 UTC 2 May 2017 (Figs. 6b-e). The CFSR showed modest CAPE in the eastern United States head of the warm surge (not shown). The HRRR analysis with 1-hourly spatial and 3km temporal resolution showed slightly more favorable CAPE (Fig.7). The layer CAPE was used as it was slightly more favorable than the surface base CAPE. Clearly, what CAPE the HRRR detected maximized in the 1800 to 2100 UTC timeframe (Figs. 7b-c) when CAPE values peaked at 1200 to 1600 JKg-1 over portions of central Pennsylvania. After 2100 UTC CAPE values were generally 600 to 800 JKg-1. This was a good setup for a low CAPE/high shear event and the shear somewhat favored a QLCS event with the potential for rotation. The low-level shear was well over 30kts from the southwest (240 degrees). The setup for a severe event was favorable. Section 2 shows some of the forecast guidance which favored a severe weather event and section 3 shows some of the stronger and more damaging features which affected central Pennsylvania.


Southeastern United States Heavy rainfall event A slow moving upper-level low and surge of subtropical moisture brought heavy rainfall to the southeastern United States. The heaviest rainfall was focused over North Carolina, despite forecasts of the heavier rainfall likely being in South Carolina and along the North Carolina and South Carolina border. The NCEP GEFS was used here to examine the QPF predictability. However, other models had similar spatial and temporal issues. As an example, the NCAR 3km EFS was used to examine the QPFs. The 0000 UTC 24 April cycle started prior to the onset of heavy rainfall and encompassed the entire event. Thus, it was used to show the QPF for the event. The probability matched mean and probability of 6 inches or more QPF showed that the NCAR EFS had too much heavy QPF along the coastal plain (Fig. 10) and more QPF than observed near the higher terrain of western North Carolina and South Carolina. The spatial and temporal forecasts were better than those provided by the NCEP GEFS.


The 5-7 April Spring Storm: Uncertainty with the QPF in the Mid-Atlantic region: A strong cyclone moved into the eastern United States on 5-6 April 2017 (Fig. 1). The strong storm system produced moderate to heavy rainfall in the Mid-Atlantic Region on 6 April (Fig. 2) and severe weather over portions of the southeastern and Mid-Atlantic regions of the United States (Fig. 3). The potential for severe weather and heavy rainfall were the main concerns as this storm system evolved in forecast guidance. Despite the confidence in the potential for heavy rain and severe weather, there was a lot of uncertainty related to this event. Many forecast cycles of the NCEP GEFS showed the potential for heavy rainfall in the Mid-Atlantic and northeastern United States. However, there were large run-to-run inconsistencies in these forecasts. The predictability horizon diagrams (PH) for Bradford and Lancaster, Pennsylvania illustrate this point (Fig. 4). The diagram for Bradford shows all GEFS forecasts from 24 hour prior to the onset of the rainfall and all forecasts to 144 hours in duration. The large spread between the members, illustrated by each point, suggested large uncertainty at longer ranges. An 84 hour forecast produced nearly 80 mm of QPF. Just prior to the rain the GEFS focused on an 18 to 41 mm rainfall event with a mean of around 30 mm. Widely varying forecasts such as these do not facilitate effective decision making with much lead-time. Forecasts of heavy rainfall proved difficult in this event with significant lead-time. Complicating the forecast of impacts included relatively high flow form the rain and snow event of 31 March to 01 April 2017 (Fig. 5). This left behind relatively moist soils, high flow in rivers and streams and snow water to melt and add to the run-off. This paper will document the spring storm of 5-7 April 2017. The focus is on the pattern, an overview of the key weather in the Mid-Atlantic region, and forecasts issues. They key forecast issues were the track of the cyclone related to the areas where heavier rainfall would likely fall.


East Coast Cyclone of 31 March-1 April 2017:A cyclone moved through the eastern United States 31 March to 2 April 2017 (Fig. 1). The storm system produced moderate to heavy rainfall in the Mid-Atlantic Region on 31 March and in southern New England on 1 April 2017l (Fig. 2). There was heavy wet snow in interior portions of New York and New England on 1 April 2017 (Fig. 3). Farther south there was severe weather southern Virginia and North Carolina. This late season storm produced quite a breadth of weather. This paper will document the spring storm of 31 March to 2 April 2017. The focus is on the pattern, an overview of the key weather in the Mid-Atlantic region, and forecasts issues. The key forecast issues included the of track cyclone related to the areas where heavier precipitation. The snowfall north of the low center is not examined from a forecast perspective


The 14 March 2017 East Coast Winter storm Slushmageddon: A rapidly developing surface cyclone (Fig. 1) moved up the East Coast of the United States on 14 March 2017. This strong cyclone produced a large swath of heavy precipitation from the southeastern United States into New England (Fig. 2). The first guess snowfall plot from gridded snowfall reports is shown in Figure 3. These data show a wide swath of 16 inches or more of snow over central and northeastern Pennsylvania and over 32 inches of snow in northeastern Pennsylvania and the Catskill region of New York. Snowfall of 31 inches was observed in Binghamton, NY and over 40 inches was reported in West Winfield , New York State (Table 1). This event also occurred on the heels of an exceptionally warm February which did not produce significant snow over the Mid-Atlantic region. Despite the February warmth, the event produced record to near snow in many locations. However, snowfall amounts were generally lower along the coastal plain and along the heavily populated corridor from Washington-Baltimore to Philadelphia, New York City, and Boston. Unlike the Megalopolitan storm of 11-12 February 1983 (Sanders and Bosart 1985) and the Superstorm of March 1993 (Kocin et al. 1995), this storm spared the major cities from a crippling snow event. It will be shown that this storm was forecast with relatively long lead-times. However, the details related to the exact storm track, intensity, and precipitation type were not as skillfully forecast. There was considerable uncertainty in these varied forecasts. Thus, the precipitation type became a critical forecast aspect of this storm. Initial forecasts implied relatively high confidence in a significant snow event for much of the Megalopolitan corridor though the confidence in an all snow event diminished as the forecast length decreased. The convergence toward a mixed precipitation and rain event along the coastal plain evolved with 18 to 42 hours of lead-time. High resolution ensemble forecast systems such as the NCAR 3km ensemble (Schwartz et al. 2015) performed relatively well during the event as did the 3km HRRR. It will be shown that at forecasts lengths of 6 to 36 hours these mesoscale ensembles produced guidance of critical value relating to snow fall and precipitation type. This paper will document the winter storm of 13-14 March 2017. The pattern, the areas of snow and higher QPF are presented along with forecasts and issues related to forecast uncertainty.


Mid-Atlantic and southern New England Snow 10 March 2017:A well forecast short wave and frontal system ahead of a surge of arctic air produced 2-10 inches of snow across central Pennsylvania eastward across southern New England. The heaviest snow fall was in the mountains of west-central Pennsylvania where 3 to 10 inches of snow was observed. Higher snow amounts were confined to the higher elevations of central Pennsylvania. As shown both the NCEP GEFS and 3km HRRR did relatively well forecasting the snow, the timing of the snow and general snow amounts. The HRRR QPF was well aligned with the observed QPE. However, the HRRR produced too much QPF in the Poconos of northeastern Pennsylvania.


The 25 February 2017 severe weather and tornado event:A late winter severe weather event affected the Mid-Atlantic and the northeastern United States on 25 February 2017 (Fig. 1). There were around 231 reports of severe weather and 6 tornadoes. There were two tornadoes reported in both Pennsylvania and Massachusetts. The Massachusetts tornadoes were the first known February tornadoes recorded. Serendipitously there was a similar severe weather and tornado event 366 days earlier on 24 February 2016 (Grumm 2016) which produced the first tornadoes in Pennsylvania since 16 February 1990 which was the proceeded by the tornadoes of 11 February 1887 (MWR 1887). February tornadoes in the northeastern United States used to be a less common occurrence. This paper will document the pattern associated with the severe weather event of 25 February 2017. It should be noted that this event was associated with a strong cold front which ended the epic eastern United States warm episode of 20-25 February 2017 (Grumm and Ross 2017). It will be shown that the storms occurred ahead of this strong cold front in a region of high shear and low CAPE . The issues with high shear and low CAPE (HSLC: Sherburn et al. 2016) severe weather events in the eastern United States is well documented. Frey ET. al. (2016) stratified tornado events across the United States by region and season.


The historic warm episode of February 2017: A record breaking warm episode affected much of the eastern United States from 18 to 25 February 2017. The peak warmth in the eastern United States and Mid-Atlantic region was focused on 23-25 February 2017. On the 24th and 25th of many sites in the east set record high temperatures for the month of February and some sites achieved the all-time high for meteorological winter. Despite the record and the prolonged period of warmth, the pattern over much of the eastern United States was not that extreme. The 500 hPa height and 850 hPa temperature anomalies were ranges 1 to 2 and 2 to 3 above normal. Thus relative to the record warm episode of March 2012 the pattern was not overly remarkable. However, the 2m temperatures and 2m temperature anomalies were more impressive in the CFSR and the GFSBC forecasts. However, both were too cold relative to the observed conditions. The estimation of the GFS and CFSR 2m temperatures suggests they can capture the sense of an extreme event but miss the true impact and magnitude of the event. The often asked question is was this warm episode due to climate change. The short answer is we do not have the data to definitively state that it was. In State College, February 2017 will likely end up being the 5th warmest February on record. It will hold the record for the 6-day highest mean maximum temperature, and it set the all-time February high temperature record and the highest all-time observed winter maximum temperature. These kinds of records were broken at many other stations. However, some of the record warm periods have occurred before with 1930 and 1954 showing up at many sites. Thus there is a strong meteorological component to this event. That said, over time climate change predicts we should have more warm episodes and fewer extreme cold episodes which is supported by the data in Table 1. This implies there could be a climate change component to this event.


Examples of Two February Squall lines Mesoscale features are often difficult to Predict On February 13 and 15 2017 two squall lines cross portions of the Mid-Atlantic region. The first squall line was observed during the evening hours of Sunday 12 February . Radar signatures implied 30 to 40dBZ echoes of 8-10,000 feet, low topped squalls which produced 73 reports of severe weather (Fig. 1). There were reports of lightning though lightning plots implied most of the lightning in Pennsylvania was located at turbine locations. The second squall line occurred in a slightly colder air mass and in central Pennsylvania the squalls contained snow. There were no known reports of severe weather. During the snow squalls visibilities dropped to 1/4SM in snow and blowing snow. Despite temperatures above freezing the rapid snow rates coated the ground . Similar to the 13 February event, there were isolated reports of thunder in the initial squall line though reports came from locations close to windfarms, the data showed most strikes along the line near or over wind turbines (Fig. 2) and larger windfarms. However, western portions of State College had several reports of thunder at least 12 miles from the nearest wind turbine. February snow squalls are not uncommon with strong cold fronts and arctic fronts. However, squall lines containing thunder are more unique and squalls producing severe weather are climatologically relatively rare in Pennsylvania and most of the Mid-Atlantic region. These two events offer a contrast in predictability. It will be shown that the squall line of 13 February was relatively well predicted and the squall line of 15 February was poorly predicted. Another interesting issue is the impact of the wind farms on lightning strikes.


Northeastern United States Snowstorm of 9 February 2017: A strong shortwave produced a stripe of precipitation from the western Plains to East Coast on 8-9 February 2017 (Fig. 1). Most of the precipitation along and north of the track of the surface cyclone (Fig 2) fell as snow (Fig. 3). The highest snowfall totals were observed from northern New Jersey across Long Island and into New England. Though not shown, radar and lightning data showed an intense band of snow and a prolonged period of thunder-snow in the band from southern Maine into Massachusetts. Areas in this band received over 16 inches of snowfall.


Minor Winter Flooding Event in northwestern Pennsylvania 12-13 January 2017 The combination of unseasonably warm air and an intrusion of humid air likely helped produce snow melt in northwestern Pennsylvania and southwestern New York. This likely reduced significant water from the snow pack. This combined with a widespread rainfall event of 1-2 inches (Fig 1) led to localized flooding over the region. Additionally, much of the region affected by the higher rainfall amounts received most of the rainfall in a 6-hour window. Many locations received 1 to 1.5 inches of QPE in a relatively short 6-hour window. The pattern in which the rainfall developed was relatively well forecast (not shown). The NCEP GEFS was able to predict the potential for 1 inch or more QPF but was limited in its ability to produce much more than 1.5 inches of QPF. Due to the frozen ground, snowmelt, and the 2 inches of rainfall, this relatively low end QPF/QPE event did produce minor flooding. The threshold for flooding is typically a bit higher, in the 3 inches and greater range in the warm season. However, in this case the antecedent and current conditions favored a better hydrologic response with relatively low rainfall amounts.


Eastern United States Warm Episode of 11-12 January 2017: A large ridge of the southern United States (Fig.1) with +1 to +2500 hPa height anomalies brought period of warm weather to much of the eastern United States from 11 to 13 January 2017. During this period of time about 809 maximum temperature records were set or tied and 540 maximum low temperature records were set or tied (Table 1). The warmest day over much of the Mid-Atlantic region was 12 January 2017 when temperatures peaked in the 60s over much of Pennsylvania and Maryland. The warmest day over the Mid-Atlantic region was the 12th of January ahead of the cold front. There was a surge of high precipitable air ahead of the front (Fig. 3c) and the 850 hPa temperatures were well above 0C and were +1 to +2 above normal over much of the eastern United States with the highest standardized anomalies over the northeastern United States.


The California Extreme Precipitation Event of 8-10 January 2017: A strong Pacific jet and a surge of high precipitable water (PW: Fig. 1) brought extremely heavy precipitation to portions of California on 8-9 January 2017 (Fig. 2). Rainfall amounts in excess of 200 mm were observed and there were reports of extreme snowfall to include. NBC news reported “epic snowfall” in the Sierras. The Los Angeles Times reported flooding, record snow, extremely high winds at higher elevations, extreme snowfall, and blizzard conditions in the Sierras on 8-9 January 2017. Squaw Valley reported winds of 99 MPH with gusts as high as 159 MPH . The strong Pacific system was associated with a near classic pattern for significant precipitation events in California which included a surge of high PW (Fig. 1) also known as an atmospheric river (AR: Neiman et al . 2008), a deep trough off the West Coast, a strong Ridge over the north Pacific and a ridge to the east (Fig. 3). This results in a strong 250 hPa jet (Fig. 4) moving into then over the affected region (Junker et al. 2008). It will be shown that the general pattern was well forecast and thus the NCEP GEFS was able to forecast the general regions of heavy rain in California. This probably was an easy forecast as the mountains act as fixed forcing for the impinging moisture and energy from the Pacific. This paper will provide an overview of the event and examine NCEP GEFS forecasts of the QPF. This was a multi-day event and the focus was on the heavy precipitation from 0000 UTC 8 to 0000 UTC 10 January. There was a significant event prior to this and another surge of precipitation after this time.


East Coast Winter Storm of 7-8 January 2017:A winter storm produced snow and areas of heavy snow from North Carolina to southeastern New England on 7-8 January 2017 (Fig. 1). The snow overnight Friday into Saturday fell mainly from North Carolina to Virginia (Fig. 1: upper). Much of eastern North Carolina had ice pellets and freezing rain which limited the snow totals. Coastal Virginia and the Delmarva had heavy snow during the morning and early afternoon of 7 January. Later in the day the snow shifted to Long Island (Table 1) and southern New England (Table 2). Some of the higher snowfall amounts, in the 12 to 19 inch range were observed in southeastern Massachusetts and Rhode Island. The broad area of 12 to 16 inches of snow in southern Massachusetts was where the higher snow amounts were observed (Fig. 1: lower). The snow was the result of a relatively deep 500 hPa low (Fig. 2a), a surge of cold arctic air into the eastern United States (Fig. 2b), a surge of deep moisture along the East Coast (Fig. 2c), an elongated low off the east coast. The snow fell in the region of strong northwesterly flow between the cyclone and the strong anticyclone to the west (Fig. 2d). Note that during the peak of the event in the southeast the 850 hPa temperatures were in the -8 to -12C range as far south as northern Georgia (Fig. 2b). This paper will examine the East Coast Winter Storm (ECWS) of 7-8 January 2017. The focus is on the pattern and forecasts of the event. The storm occurred over a weekend which may have limited the overall impact of the storm. Additionally, with the exception of Boston, MA the major cities of the Mid-Atlantic and Northeast did not receive significant snowfall with this event. web 1


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