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Southern Plains Severe and heavy rain Event of 6 May 2015: MRMS Rotation Tracks and tornadoes. Severe thunderstorms and strong tornadoes affect the southern plains. Several of the tornadic storms had long-lived rotation tracks. Heavy rain affected the same general regions which had severe weather. The 24 hour rainfall values approched the 100 year return values.


Hail and Severe Event of 20 April 2015: A weak frontal boundary separating shallow cold air to the north east and a surge of warm air produced thunderstorms. On the warm side of the boundary, there was a surge of relatively high CAPE, around 1200 to 1800 JKg-1 and strong low-level shear. This favored the development of large updrafts and potentially rotating storms. Though not shown, there was relatively high low-level helicity in and near the frontal boundary, further increasing the potential for rotating storms. The HRRR, which forecast and analyzed the high CAPE and shear did produce the potential for a quasi-linear area of convection (Fig. 17 & 18) and implied larger discrete cells along the line. Several supercells developed and the four larger and more persistent storms produced large hail. The storm which produced large hail in Centre County also produced a short-lived tornado just to the lee of a significant ridge line. Wind damage with all the storms was quite minimal. Despite the nearly ideal CAPE, shear, and LCL heights only one weak tornado was observed.


Eastern United States anafrontal snow 4-5 March 2015: A nearly textbook example of an anafront brought heavy snow; with areas of 10 to 25 inches of snow to portions of Kentucky and heavy snow in southeastern Pennsylvania and Maryland. The snow fall lacked a strong surface cyclone though it occurred in a region of strong baroclinicity with deep warm air on the south side of the boundary and cold arctic air on the cold side of the boundary. At 250 hPa a strong jet developed. Most of the enhanced snowfall on the cold side of the frontal boundary occurred as the jet entrance region moved along the boundary. The frontal boundary, strong 250 hPa jet with +4 to +5s u-wind anomalies and the potential for significant QPF was well predicted by both NCEP EFSs. The GEFS and SREF predicted the anomalous jet, the area of high QPF and the arctic air under cutting the warm moist air to the south. Relative to other high impact snow events, this event lacked a strong surface cyclone and was not associated with a strong 850 hPa low-level easterly jet. There was no cyclone or “storm” associated with this winter “storm”. This anafront was more about dueling anticyclones than cyclones. The famous Pennsylvania weather forecasting quote about predicting heavy snow “forecast the high forecast the snow” was relevant in this case. There were forecast issues related to snow amounts and the timing of the transition from rain to snow. The existed in both EFS and in the high resolution models. The timing of the transition was critical to the timing of the snow and potential for heavy snowfall. The HRRR was used to show the timing in the Ohio Valley and southeastern Pennsylvania. These forecasts show great promise for rapidly updated high resolution models.


Adriatic snow event of 5 March 2015: Capracotta Italy Extreme Snowfall event. Heavy snow impacted Italy and the Balkans with record snows on both sides of the Adriatic. Brief summary of the conditions. GEFS forecasts, the anomalies with the circulation about the close 500 mb low for this impressive late winter snow event.


Mid-Atlantic Ice Storm 4 March 2015:A significant cold air damming and mixed precipitation event occurred over northern and central Pennsylvania on 3-4 March 2015. This event was somewhat dwarfed by the significant snow event of 4-5 March 2015 which affected a broad region of the United States from the south into the Ohio Valley in Mid-Atlantic region. However, this event did produce significant ice, canceled schools, and disrupted transportation on a more regional and local scale. Additionally, this event was relatively well predicted and showed the value of using ensembles and high resolution models during complex issues related to mixed precipitation and cold air damming. The NCEP SREF and HRRR both performed well during this event. The SREF clearly showed the potential for a modest precipitation event and the potential for warm air overrunning cold air. Thus the SREF produces rain in southwestern Pennsylvania and a high probability of both sleet and freezing rain in central and northern Pennsylvania. At shorter ranges, the HRRR showed the precipitation and cold air damming issues. The HRRR also provided the potential to time and provide relatively precise, within a few hours, information related to the freezing rain event. In this event, the HRRR showed and displayed the potential snow to rain transition rather well. Though not shown here, the HRRR showed a brief period of snow in State College before the snow went to sleet and freezing rain. The 2m temperatures remained below 0C few hours longer than implied by the HRRR forecast. During this event and experience with 6 other events in the November 2013 through March 2015 suggest that as good as the HRRR is at showing trapped cold air in valleys and east of the Appalachian mountains, the model is typically too fast in eroding the cold air. The error is typically on the order of a few hours but new model runs come in aiding in forecasting the potential prolonged persistence of the cold air


Mid-Atlantic Deformation snow 21-22 February 2015:A retreating arctic air mass and 500 hPa trough (Fig. 1a), which produce record cold over the eastern United States (Grumm 2015) and an approaching arctic air mass produced a snow event in the Mid-Atlantic region on 21-22 February (Fig 2). The western edge of the retreating arctic air 850 hPa (Fig. 3) was along the East Coast and western Atlantic at 0000 UTC 21 February 2015. A surge of warm air moved into the Mid-Atlantic region ahead of a second surge of cold air (Figs. 3b-e). At the surface (Fig. 4), a retreating anticyclone was associated with the retreating cold air. A trough developed between the approaching and retreating anticyclones (Fig. 4c-f). A strong surface cyclone or storm was not present during this “winter storm”. Most of the snow fell in the deformation zone between the two anticyclones. It will be shown that forecasts of this event by several modeling systems to include the NCEP GEFS indicated a stronger cyclone than observed. These forecasts systems also produced too much quantitative precipitation too far north relative to the observed location of the more robust observations of snow and QPE (Figs. 2 & 5). Thus human forecast were biased toward more QPF and heavy snow farther north than observed. This paper will document the pattern and anomalies associated with the snow event of 21-22 February 2015. Section 3 section will examine the forecasts produced by the NCEP ensemble forecast systems.


The Big Chill of 18-20 February 2015 A strong arctic front moved into the Mid-Atlantic region and eastern United States on 18 February 2015. The arctic front produced a line of intense snow showers as it moved across portions Pennsylvania. As shown, the 3km NCEP HRRR did relatively well predicting the snow squalls along the arctic front during the afternoon of 18 February 2015. Overall, the NCEP NAEFSBC and SREF forecasts of the arctic frontal passage on 18 February and the deep cold air over the eastern United States on 20 February were quite useful. The in addition to the forecasts of deep cold air over the eastern United States, the SREF showed a large and anomalously strong anticyclone around 1200 UTC 20 February. The large anticyclone, the cold air, and the extensive snow cover, produce nearly ideal radiational cooling conditions over the region. This likely contributed to the 632 record low temperature records tied or broken on 20 February 2015 (Table 2). This arctic outbreak was relatively well predicted by the 3 forecast systems presented. As the NAEFS contains data from the NCEP GEFS and CMC Ensemble forecast system, it can be assumed that these two systems also provided useful guidance. The high resolution HRRR also provides useful information related to details in the potential snow bands associated with strong fronts, aides in post-event analysis, and provides detailed short-range temperature forecasts.


The Big Chill 14-16 February 2015 A series of arctic air masses entered the eastern United States during the month of February 2015. The second in a succession of these air masses was associated with a deep 500 hPa trough (Fig. 1). At 850 hPa a pocket of extremely cold air moved into the region. The 850 hPa temperatures ranged from -20 to -26C with 850 hPa temperature anomalies on the order of -2 to -4s over portions of the eastern United States (Figs 2-a-f). The coldest air “la crème de la crème” of North American cold with 850 hPa temperatures in the -32 to -34C ranged were focused over northern New England (Fig. 2c). The deep trough (Fig. 1) and deep cold air (Fig. 2) were associated likely contributed to cyclogenesis (Fig. 3a). A modest low pressure over the Great Lakes spread snow to the east. As this feature and the strong frontal boundary reached the coast (Fig. 3b) rapid cyclogenesis ensued (Fig. 3c-e). The strong gradient between the deepening cyclone and strong anticyclone to the west produced strong winds. The strong easterly winds north of the surface cyclone (Fig. 4) contributed to the heavy snow eastern New England in areas in close proximity to the -3 to -4850 hPa jet (Figs. 4c-d). Over Pennsylvania, light snow fell ahead of the developing cyclone over the Great Lakes. The total accumulated precipitation is shown in Figure 5. As the cyclone moved eastward and the arctic front swept across the State, it produced a line of convectively enhanced snow which included reports of thunder with the snow. Lightning was also observed along the line. The NCEP HRRR 2100 UTC 14 February analyzed radar (Fig. 6a) and forecasts from 5 previous cycles valid at 2100 UTC 14 February (Fig. 6b-f) show the convective line and simulations of this feature. The HRRR provided extremely useful guidance related to the passage of a very strong front with implied convective snowfall along the leading edge. This paper will document the pattern and anomalies associated with the ECWS of 8-10 February 2015. Section 3 will focus on the pattern and standardized anomalies to put the event into context. As with all high impact weather events, the forecasts and the communications of these forecasts are important. The 4th section will examine the forecasts produced by the NCEP ensemble forecast systems and the HRRR.


Snow, freezing rain, and shallow arctic Air 8-10 February 2015: NCEP HRRR success story: A strong baroclinic zone in the eastern United States separated arctic air from relatively warm air to the south. Easterly flow on the colder side of the boundary produced a long duration snow event from about 7-10 February. A short-wave moved this baroclinic zone 8-9 February producing snow and areas of heavy snow deep in the cold air and a region of freezing rain as the shallow arctic air drifted southward on 8-9 February. Heavy snow was observed in east-central New York and eastward to Boston. This system had a relatively low predictability horizon (PH) and forecasts from 4-6 February had considerable spread and thus low confidence in where the higher QPF amounts would occur. The GEFS showed large spread in the baroclinic zone in the temperature and mean sea-level pressure fields. Despite the low confidence rather robust forecasts for heavy snow were presented at times from central Pennsylvania to New England. The NCEP GEFS (Figs. 7-9) showed the low confidence at longer ranges. However, the GEFS did correctly focus the higher probability for higher QPF amounts near the region where the higher snow amounts were observed. The NCEP SREF showed warmer 850 hPa temperatures and the potential for the rain snow line to be farther north than the GEFS and EC (not shown). The SREF also showed the potential for mixed precipitation. As the forecast horizon decreased, the spread decreased and the forecasts converged toward the higher QPF and thus snow from central New York into eastern Massachusetts. The SREF and GEFS both indicated considerable uncertainty that may or may not always be utilized by forecasters. Late on 8 and 9 February, as the cyclone began to develop, the precipitation increased. The higher observed QPE amounts (Fig. 5) were observed from 0000 UTC 9-10 February. During this period of time the HRRR showed the surge of shallow arctic air southward along the coast (Fig. 11) and the southward progression of the freezing rain southward with time. The HRRR correctly forecast the shallow arctic air down into northern Maryland by 1200-1400 UTC 9 February into northern Maryland and into western Pennsylvania (Fig. 12) while keeping the relatively warm air anchored in the higher terrain of the Appalachian mountain into central Pennsylvania. These data imply that the high resolution, rapidly updated HRRR (Fig. 6,11, & 12) can simulate shallow cold air masses down the coastal plain and thus provide clues to potential freezing rains. The high resolution of the HRRR and its high resolution terrain provide it with a critical advantage over coarser Regional and Global models. In this case, the HRRR did remarkably well with both the southward advance of the shallow cold air and the persistence of the relatively warmer air in the central Appalachians.


Eastern United States Winter Storm of 1-2 February 2015-Northeast Ground Hog Storm A major Winter Storm brought precipitation from the Great Lakes into the eastern United States primarily from 1-2 February 2015 (Fig. 1). North of the primary cyclone track heavy snow and blizzard conditions were observed. Heavy snow was observed northern Illinois and southern Wisconsin across northern Indian and southern Michigan on 1 February. Chicago’s O’Hare airport received 20.2 inches of snow, the 4th largest snowfall observed at the site. The much of the region of heavy snow was buffeted by strong winds with gusts between 50 and 60 mph. This resulted in snow drifts in the range from 2 to 5 feet (WFO-Chicago). The strong winds were associated with a strong gradient between a strong anticyclone to the north and west and surface cyclone tracing into the Midwest (Fig. 2a-d). As the storm moved into the Mid-Atlantic region, (Fig. 2e-f) a secondary cyclone developed along the coast. The Miller-B type cyclone evolution with the primary cyclone moving into western Pennsylvania and southwestern New York (Fig. 3) allowed warm air to change the snow to rain over most of Pennsylvania, dramatically limiting snow amounts. Based on hourly HRRR 0-hour forecasts, the secondary cyclone developed along the coastal plain in southern New Jersey (Fig. 3d-e). This secondary cyclone became an East Coast Winter storm (ECWS: DeGaetano et al. 2002) producing rain, a wintry, mix and heavy snow from Long Island into New England. The total precipitation for this phase of the event (Fig. 4) indicated that the higher QPE amounts occurred just north of the cyclone center from eastern Pennsylvania across Long Island. Some locally higher QPE was observed over southeastern New England and eastern New York. Most of the precipitation fell as rain in the 32 to 48 mm QPE band along the coast. Most inland areas received snow with heavier snow amounts in eastern Massachusetts. his paper will document the pattern and anomalies associated with the winter storm of 1-2 February 2015. Section 3 will focus on the pattern and standardized anomalies to put the event into context. Section 4 will present some of the uncertainty information provided by the NCEP EFS suite. The human forecasts, which were based on a mix of models and the European Center forecast system were not readily available and are not presented here.


East Coast Winter Storm of 25-27 January 2015: A major East Coast Winter storm (ECWS: DeGaetano et al. 2002) brought snow and areas of heavy snow to portions of the Mid-Atlantic region and the northeast. The heaviest snow fell over eastern Long Island northward into eastern New England. The storm evolved as a northern stream clipper moved into the Mid-Atlantic region then off the East Coast. This system merged with a southern stream system which led to rapid cyclogenesis southeast of Long Island (Fig. 1). The evolution of the cyclone on 27 January is shown in Figure. 2. During this phase the storm had strong low-level winds and the 850 hPa low-level jet had -5 to -6 u-wind anomalies (Fig. 3). Stuart and Grumm (2006) showed that many high impact ECWS are associated with -3 to -6 850 hPa wind anomalies. The strong u-wind anomalies (Fig. 3) with -6 u-winds with this and with previous storms is often an indication of a meteorologically and climatologically significant event. Not surprisingly, many locations in eastern Connecticut, Rhode Island, Long Island, and Massachusetts had near record snowfall. Worcester, MA set a new record snowfall of 33.5 inches (AP 2015) breaking the old record of 33 inches set on 1 April 1997. Boston had its sixth all-time largest snowfall. A list of locations with over 20 inches of snowfall is summarized in Table 1. This paper will document the pattern and anomalies associated with the ECWS of 25-27 January 2015. Section 3 will focus on the pattern and standardized anomalies to put the event into context. As with all high impact weather events, the forecasts and the communications of these forecasts are important. The 4th section will examine the forecasts produced by the NCEP ensemble forecast systems and what these data implied.


Coastal Cyclone of 23-24 January 2014: An East Coast Winter Storm (ECWS: DeGaetano et al. 2002) brought heavy snow for Pennsylvania along with its neighboring states. The storm evolved from a southern system off the Gulf Coast which then moved up and off the East Coast and eventually combined with a northern Canadian low strengthening the off shore system (Figure 1). The evolution of the low is shown in figure (Figure 2). A large percentage of the historic snowstorms along the East Coast typically involved a surface cyclone, which comes up the East Coast. This particular storm contained an 850 hPa low level jet with -2 to -3σ u-wind anomalies and 2 to 3σ v-wind anomalies (Figure 3 & Figure 4). Forecasts of this storm will be shown from an NCEP ensemble perspective. The extent of the southern surface low into Pennsylvania provided a gradient in snowfall totals and forecasts across the state. The NCEP guidance originally placed the highest precipitation south of Pennsylvania into Maryland and Northern Virginia. However, the highest snowfall totals were in eastern Pennsylvania and New York City area into New England, the areas to the south mainly saw rain. The storm snowfall totals ranged between 1 inch and a maximum of 7-8 inches for the heaviest hit regions (Figure 5). Some of areas in New England with higher snowfall totals were later affected by the 23-24 January 2015 Major East Coast Winter Storm. This paper will document the pattern and anomalies associated with the ECWS of 23-24 January 2015. Section 3 will focus on the pattern and standardized anomalies to put the event into context. As with all high impact weather events, the forecasts and the communications of these forecasts are important. The 4th section will examine the forecasts produced by the NCEP ensemble forecast systems and what these data implied. web 1


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