10-11 April 2003 Cut-off Southern Appalachian Snowstorm by Richard Grumm

Transcription

1 10-11 April 2003 Cut-off Southern Appalachian Snowstorm by Richard Grumm 1. Introduction This paper will show the anomalies associated A deep upper level cut-off low moved up the east coast on 10 and 11 April This deep upper level cyclone produced heavy snow in the higher terrain of the southern Appalachian Mountains, primarily in southwest North Carolina. Elevated towns and cities had 3-8 inches of snow with one known report of 11 inches in the mountains of North Carolina. It was a record April snowfall in Ashville, North Carolina and the 2 nd snowiest April day in Ashville. Ashville set a record for snow (4 inches) and liquid equivalent (1.49 inches). The last time that happened in April was 12 April 1988 (see RDUAFD in Appendix-I). North of the low, the strong low-level easterly flow brought moderate to heavy rainfall from the Carolinas to New England. April and May appear to have a preponderance of anomalous upper level lows which produce anomalous weather. A deep upper level low brought a spring snowstorm to the southern Appalachian Mountains in 12 April The MTOTAL with this storm was The most anomalous April storm had an MTOTAL of on 28 April This storm brought unusual cold weather to the southeastern United States with a deep upper level low. The most anomalous spring-time upper level in the region was observed on 28 May Spring time snows in the eastern United States appear to be related to anomalously deep upper level cyclones. For more information, check out the storm rankings page, at the eyewall website. The GFS (MRF) observed MTOTAL for the April 2003 storm was The dominant anomalies were associated with the winds, with an NTOTAL of as shown in Figure 1. Figure 1 Rarity forecasts and observations from the operational GFS (MRF). with the southern Appalachian snowstorm and the rain event of April Method All model and climatic anomaly data were taken off the eyewall website. Data used to produce these images included operational NCEP models and the re-analysis climatic dataset climatology as described by Hart and Grumm (2001). Precipitation data were obtained from the NCEP precipitation website.

2 Figure 2 Multi-sensor Precipitation mosaic for the 24 hour periods ending 1200 UTC a) 11 April and b) 12 April Overview a) Precipitation The observed rainfall from the NCEP re-analysis data over the eastern United States ending at 1200 UTC 11 and 12 April 2003 are shown in Figure 2. The precipitation pattern showed a concentration of precipitation associated with the surface and upper level low on the 10 th of April ending at 1200 UTC on the 11 th. The precipitation over western North Carolina was snow, with thunder snow observed in the mountains (Appendix-II has Ashville hourly observations). Spotter reports and public information statements (Appendix-I) spoke of thunder snow and 4-8 inch snowfall amounts in the mountains of North Carolina. The precipitation on the 11 th ending at 1200 UTC on the 12 th was primarily north of the surface and upper level low with a secondary precipitation area under the upper level low in eastern Virginia and North Carolina. The strong low-level easterly jet brought rainfall back to northeastern Kentucky. The precipitation observed from 12Z 11 April through 12Z 12 April 2003 over Pennsylvania is shown in Figure 3. These reports do not reflect the storm total as the rain began about 3-6 hours before the start of the data in this plot and it is missing about 30% of the total at most locations. The overall shape of the rainfall area shows the influence of the easterly low-level jet over the region bring a band of precipitation to the west, with heavier mountains in eastern Pennsylvania. b) Upper level patterns The 00-hour Eta forecast valid at 1800 UTC 10 April 2003 is shown in Figure 4. These data show the deep and anomalous 500 hpa cyclone with a anomaly over northern Florida. The 850 hpa temperatures, though not overally anomalous, show a small pocket of slightly subfreezing air over the southern Appalachian Mountains. These temperatures suggest marginal conditions for snow at lower elevations. The most anomalous low-level 850 hpa air was over Florida.

3 Figure 3 Cooperative observation precipitation over Pennsylvania. Data shows an analysis and station plots (inches) for the 24 hour period ending 12Z 12 April The 850 hpa wind anomalies in Figure 4 show extremely anomalous U-winds on the order of SD below normal in North Carolina. These numbers, based on the fine scale Eta were probably considerably lower. The GFS forecasts showed a -6.27SD easterly jet. The re-analysis data will likely provide lower values. What is clear from these data is that the models showed a deep and anomalous upper level low in the south eastern United States with a very anomalous low-level jet. All these conditions coexisted with temperatures cold enough for snow in the higher elevations. The deep 500 hpa anomaly and anomalous 500 hpa temperatures (not shown) produced enough instability for thunder snow in the mountains of North Carolina. Figure 5 shows the progress of the upper level low and wind anomalies by 1200 UTC 11 April The upper level low intensity, though unchanged, was not as anomalous as the previous day to the more northward position of the anomaly. The 850 hpa wind anomalies were right over Pennsylvania with the maximum wind anomalies lining up closely to the orientation of the observed rainfall in Figure 3. The areas of and large easterly wind anomalies were over eastern Pennsylvania. The maximum, over Pennsylvania was -3.82SDs below normal. As shown in Figure 2. The rain on the 11 th was focused to the areas in the strong easterly jet and beneath the track of the 500 hpa cyclone. Showers and thunderstorms were observed beneath the instability associated with the 500 hpa low center. 4. Conclusions An anomalous upper level low and strong easterly low-level jet brought a late season snow storm to the western Appalachian Mountains on Thursday 10 April The strong upper level cyclones in the early to middle spring appear to be common characteristics associated with southern Appalachian snowstorms. This event did have marginally sub-freezing 850 hpa temperatures near the upper level cyclone where the snow was observed. On 12 April 2003, the upper level low moved away from the elevated terrain and snow was a far rarer observation farther north. The rains and heavy rains were primarily confined to the areas impacted by the anomalous low-level jet ahead of the northward moving upper level cyclone. Instability showers and thunderstorms brought additional rain to coastal areas of North Carolina, Virginia, and Maryland. There was clear precipitation pattern over Pennsylvania which could be associated with the anomalous low-level jet. This pattern is a potential forecast tool to refine the areas of moderate to heavy precipitation.

4 Figure 4 Eta 00-hour forecasts valid at 1800 UTC 10 April 2003 showing (left) 500 hpa heights and anomales and 850 hpa temperatures and anomalies. (right) U-winds, anomalies and 850 hpa heights and V-winds and anomalies.

5 Figure 5 As in Figure 4 except from 12Z 11 April Eta. Winds are zoomed into the Northeastern US. Max U anomaly was and V was (southeast of plotted area).

6 Appendix I NWS Raleigh-Durham Statements on the snowstorm April 10 snowstorm PUBLIC INFORMATION STATEMENT NATIONAL WEATHER SERVICE GREENVILLE SPARTANBURG SC SNOWFALL REPORTED ACROSS THE MOUNTAINS AND FOOTHILLS OF WESTERN NORTH CAROLINA...FROM SPOTTERS...COUNTY COMMUNICATION CENTERS...AND POLICE DEPARTMENTS. YANCEY COUNTY...ABOUT 6 INCHES REPORTED EARLIER IN BURNSVILLE. GENERALLY 3 TO 4 INCHES AROUND THE COUNTY...WITH MORE IN THE HIGHER ELEVATIONS. POWER WAS OUT IN MANY AREAS. NOW DRIZZLE AND 35 DEGREES. MITCHELL...4 TO 8 INCHES AROUND THE COUNTY TODAY. NOW DRIZZLE AND 35 DEGREES. MADISON...UP TO 6 INCHES IN THE SPRING CREEK AREA AND ALONG HIGHWAYS 19/23. NO SNOW IN HOT SPRINGS OR MARSHALL. SOME HIGH ELEVATIONS LIKELY HAD MORE. BUNCOMBE...WIDEPSREAD POWER OUTAGES. WEAVERVILLE AREA HAD 8 INCHES. ASHEVILLE ABOUT 4 INCHES AS DID LECEISTER. SWANNANOA HAD 6 TO 7 INCHES. NOW DRIZZLE. HAYWOOD...NOW RAIN AND FLURRY MIX IN WAYNESVILLE. GENERALLY AROUND 3 INCHES IN THE COUNTY WITH MORE AT HIGH ELEVATIONS. JACKSON...2 TO 3 INCHES BALSAM AND OTHER HIGH ELEVATIONS. NOT MUCH IN VALLEYS. NOW DRIZZLE. GRAHAM COUNTY...NO SNOW IN ROBBINSVILLE. UP TO 6 INCHES AT 1 PM IN HIGH ELEVATIONS BETWEEN NANTAHALA AND STECOAH AND STILL SNOWING HEAVILY AT THAT TIME. SWAIN COUNTY...NO SNOW IN BRYSON CITY. HAD NOT HEARD FROM HIGHER ELEVATIONS. NO SNOW IN FONTANA ALONG THE SWAIN/GRAHAM LINE. LIKELY SNOW IN THE HIGH ELEVATIONS ALONG THE STATE LINE BUT NO REPORTS. TRANSYLVANIA COUNTY...2 TO 3 INCHES CEDAR MOUNTAIN. 1 TO 2 INCHES IN BREVARD WHERE IT LARGELY HAS ALREADY MELTED. HENDERSON COUNTY...LIGHT RAIN NOW IN HENDERSONVILLE. ABOUT 2 INCHES IN TOWN. WIDEPSREAD POWER OUTAGES ACROSS THE COUNTY FROM DOWNED TREES AND POWER LINES. AVERY COUNTY...4 INCHES IN NEWLAND WHICH HAS FALLEN OVER THE PAST TWO HOURS. SNOWING HARD WITH SEVERAL MINOR ACCIDENTS. SWAIN...RAIN IN BRYSON CITY. NO SIGNIFICANT PROBLEMS REPORTED. HAYWOOD...2 INCHES IN WAYNESVILLE AND SNOWING VERY HARD. WRECKS ALL OVER THE COUNTY. MORE SNOW AT HIGHER ELEVATIONS BUT NOT SURE HOW MUCH.

7 MACON...MOSTLY RAIN IN THE LOW ELEVATIONS. SNOWING HARD AT HIGHLANDS...BUT SNOW IS JUST COVERING THE GRASS. TRANSYLVANIA...HEAVY SNOW IN BREVARD WITH NUMEROUS ACCIDENTS...SOME MAJOR...ACROSS THE COUNTY. NO CHANCE YET TO GET ACCUMULATIONS. MCDOWELL COUNTY...OLD FORT MOUNTAIN GETTING SLICK. GATEWAY MOUNTAIN AREA ON BAT CAVE ROAD HAS 3 TO 4 INCHES OF SNOW AT AN ELEVATION OF 3800 FEET. MOST LOWER ELEVATIONS ON THE ESCARPMENT ARE JUST SLUSHY...BUT GETTING SLICK. POLK COUNTY...ACCIDENT EVERYWHERE. I-26 FROM EXIT 36 WEST IS AT A DEAD STOP. SNOWING HARD IN THE WESTERN PART OF THE COUNTY BUT NOT SURE OF AMOUNTS. MINOR ACCUMULATIONS OF SNOW WERE REPORTED IN THE LANDRUM AREA OF SPARTANBURG COUNTY...WITH NO PROBLEMS REPORTED BY THE GREENVILLE COUNTY DISPATCH...WATAUGA...BEECH MOUNTAIN 11" 308 PM HEAVY SNOW FLAT TOP MOUNTAIN 14.7" IN BUNCOMBE COUNTY RDU AFD: The 4 inches of snow Thursday was a record snow for the date. The previous record was a trace set in This will go down as the 2nd snowiest April day in Asheville history. The slow movement of the cut off upper low allowed a considerable amount of moisture inflow from the Atlantic to enter the mountains. With the cold air aloft in place we saw heavy snow and at times thunder snow around midday. Snowfall rates exceeded 1-4 inches per hour at times in some spots during the afternoon. As warmer air entrained into the system, snow became mixed with rain this evening. With 1.49 inches of water equivalent, this was a record for the date and the first time since January 2, 1999 that we had a record daily snow and record daily precipitation set. The last time it happened in April was April 12, Models of choice: RUC/Eta flood watch tonight and Thursday. Light to moderate to sometimes heavy rain was developing in the piedmont and getting into the foothills this evening. The activity was moving northnorthwest and should effect the northern tier of the area for tonight. scattered showers here but as daybreak approaches, rain chances will increase as the upper low moves to our south and we get right into the strong lift and dynamics associated. so the rain area should fill in south. Temperatures tonight will be steady in the low 40s. so the potential is there for heavy rain with the strong lift from the upper level system combined with a good, deep feed of moisture from the east off the Atlantic. Helping matters will be a low forming off the carolina coast that should pivot inland during the day. So expect heavy rain, cold and breezy through the early afternoon. winter storm watch for tomorrow afternoon through Friday morning. Some areas especially at 4 and 5,000 feet will start seeing snow by tomorrow morning. as the cold air moves east wrapping around the upper low, those snow levels will lower through the day. Now with heavier falls of precipitation, that'll help drive the colder air aloft down to the surface and dynamically cool the atmosphere. So by afternoon, expect rain changing to a rain/snow mix and then to snow by evening in the Asheville area. This is more likely from the city on north and westward. Precipitation will be heavy at times, especially tomorrow evening. that will be the best time for accumulating snows here in Asheville. Heavy snow all day long in the higher elevations. temperatures will be dropping in the afternoon into the mid 30s as the cold air aloft comes down in. the wind will have a bite to it out of the north and northeast between 10 and 20 mph. so for thursday night, plan on rain and snow showers across the area. Again, mainly all snow from Asheville north. looking at accumulations of snow here in the city of up to 4 inches at this time with locally higher and lower amounts depending on where the heavy snow band sets up. Higher elevations, expecting 6 to 12 to perhaps more than that inches. and remember with warm ground and temperatures that should remain above freezing, snow that does stick will likely do that in heavy snow bursts but once that stops, the snow will likely melt. roads could be treacherous in the higher elevations but only temporarily so here in asheville. temperatures will hold steady or rise through the mid 30s. Friday morning, the chance for rain or snow showers exists, especially north and east as the upper low and surface low should start moving northeast. could end the snow as some light rain here as warm air advects in behind the upper low. Still mostly cloudy all day with temperatures struggling to make 50 and there could be a shower in the afternoon. all this mess moves out and the weekend appears to be nice with partly cloudy skies and highs back into the 60s and 70s. But for Thursday, best time for snow into Asheville looks to be during the afternoon,

8 especially late and then during the evening hours. Stay tuned here for updates on possible warnings as they become available.

9 Appendix II Ashville North Carolina Observations METAR KAVL Z 34012KT 2SM +RA BR BKN011 BKN016 OVC021 04/03 A2972 RMK AO2 SLP065 P T SPECI KAVL Z 35010KT 2SM +RA BR BKN006 BKN010 OVC016 03/02 A2972 RMK AO2 P0005 SPECI KAVL Z 36008KT 1SM SN BKN006 OVC012 01/01 A2974 RMK AO2 RAE10SNB10 P0015 SPECI KAVL Z 33007KT 1/2SM +TSSN BKN006 OVC012 00/00 A2973 RMK AO2 TSB40RAE10SNB10 P0028 METAR KAVL Z 34008KT 1/2SM +TSSN BKN006 OVC012 00/00 A2973 RMK AO2 TSB40RAE10SNB10 SLP072 P0031 T SPECI KAVL Z 35006KT 1/4SM +TSSN VV004 01/00 A2972 RMK AO2 P0013 METAR KAVL Z 36006KT 1/4SM +TSSN VV004 01/00 A2972 RMK AO2 SLP066 P0039 T SPECI KAVL Z 35008KT 3/4SM +TSSN BKN007 BKN014 OVC023 01/01 A2970 SPECI KAVL Z 34009KT 3/4SM +TSSN FEW006 BKN013 OVC023 01/01 A2969 RMK AO2 P0011 SPECI KAVL Z 35008KT 5SM -SN BR FEW006 BKN023 OVC028 01/01 A2969 SPECI KAVL Z 35007KT 1SM -SN BR OVC024 01/01 A2969 RMK AO2 TSE30 P0011 METAR KAVL Z 35008KT 2SM UP BR OVC024 01/01 A2969 RMK AO2 TSE30UPB49SNE49 SLP055 P T SPECI KAVL Z 34009KT 3SM -RA BR OVC024 01/01 A2968 RMK AO2 UPE1757RAB00 P0000 SPECI KAVL Z 34010KT 2SM -SN BR OVC004 01/01 A2968 RMK AO2 UPE1757RAB00E07SNB07 P0000 METAR KAVL Z 35009G16KT 2 1/2SM -RA BR BKN009 OVC028 02/01 A2969 RMK AO2 UPE1757RAB00E07B50SNB07E50 SLP057 P0002 T METAR KAVL Z 34011KT 2 1/2SM -RA BR BKN022 OVC029 02/01 A2969 RMK AO2 SLP055 P0005 T