The Spring Storm of April 2007 By Richard H. Grumm National Weather Service Office State College PA 16803

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1 The Spring Storm of April 2007 By Richard H. Grumm National Weather Service Office State College PA INTRODUCTION A major spring storm struck the eastern United States from 13 through 16 April The storm produced severe weather from Texas to the Carolina s as it swept eastward. On the northern edge of the storm s precipitation shield, a late season snow storm was observed. Snow fell from Colorado into New York State. Along the coast high winds and coastal flooding were observed as the storm central pressure bottomed out near hpa Ambrose Light, south of KJFK on Long Island at 1000 UTC 16 April. The Super storm of 13 March 1993 had a low pressure of 960 hpa in southern New England and 962 hpa at KJFK (Uccellini et al and Kocin et al 1995) on 14 March In the warm sector and close to the surface cyclone, heavy rainfall was observed. Areas of eastern Pennsylvania received over 3 inches of rainfall. Northern New Jersey was saw rainfall amounts which locally exceed 8 inches. The heavy rains were relatively well forecast 1-2 days in advance and produced some near historic flooding at several locations in New Jersey. From a forecast perspective, this storm offered mixed results. The general track of the storm and the intensity of the storm were relatively well forecast. At least 3-4 days in advance a major cyclone was forecast along the East Coast with an intense storm forecast near Long Island, New York. The winds, shear, and stability implied the severe weather threat quite well. The moisture and model quantitative precipitation forecasts (QPFs) indicated potential for locally heavy rains, with some forecasts showing as much as 3.5 inches of rainfall. However, early forecasts were too cold and implied significantly more snow potential then produced by this storm. The thermal character and thus the precipitation types produced by the Global Forecast System (GFS) and its cloned Global Ensemble Forecast System (GEFS) suggested a significant late season snow event from central Pennsylvania northward into New York State. Consistent forecasts of this snow potential slowly gave way to a warmer solution and rain by 0000 UTC 14 April 2007, a mere 24 hours before the precipitation was forecast to start. Thus, this storm was mixed opportunity. The models and ensemble prediction systems correctly forecast a significant spring storm. However, the snow potential with this storm was grossly over forecast as recently 1 to 3 days prior to the event. It should be noted this storm came on the heals of the large storm of April 2007 which produced heavy snow in the upper Midwest and Great Lakes region. This paper will document the spring storm of April The focus is on the weather it produced as it traversed the country and the impacts the storm had. The focus of the model and ensemble

2 a. b. c. Figure 1 Storm Prediction center reports of severe weather color coded by event type for a) 13 April, b) 14 April and c) 15 April Black triangles represent tornadoes, blue triangles are wind reports and green triangles show hail reports. Daily summaries are provided in Table 1. forecasts will be included in part II. This version will focus on the impacts of the storm with a focus on an overview of the storm and impacts in then eastern United States. I should be noted that most of the heavy snow initially indicated by the models did not materialize over central and western New York. However, as the storm stalled over Long Island snow bands wrapped around the systems west side and produced a fairly large area of snow and heavy snow. The storm also produced strong winds. 2. METHODS Model and ensemble predictions system (EPS) data were retrieved in near realtime from NCEP and archived for redisplay using GrADS. A mix of traditional ensemble displays, as described by Sivillo et al. (1997) and Grumm and Hart (2001) were used. The latter method displayed the ensemble mean relative to the departure of the field in standard deviations (SDs) from normal. The method to compute and develop the climatogoligical data from which the departures were computed is described by Hart and Grumm (2001). In addition to planview images of ensemble displays, time series of spaghetti plots were displayed using plume diagrams. Plumes of precipitation by precipitation type and temperature forecasts are presented. Other ensemble displays included probabilistic guidance as originally described by Sivillo et al (1997). Short-term analysis data were produced using the 13km RUC. The focus of RUC was on the storms location and central pressure at discrete times. Observational data were retrieved from various National Weather Services sources included the Storm Predictions Center (SPC) for severe storm reports, the Heavy Precipitation unit (HPU) at the National Centers for Environmental Predictions (NCEP). SPC Severe weather reports Date Totals Tornado Wind Hail Total

3 Figure 2 NAM 00-hour forecasts of MSLP showing the cyclone evolution in 12-hour increments from a) 0000 and b) 1200 UTC 13 April and c) 0000 and d) 1200 UTC 14 April Isobars are every 4 hpa. 3. OVERVIEW a. Severe Weather Severe weather was first observed on the 12 th of April in Texas panhandle were there were 35 reported events (Table 1). As the system strengthened and began to move eastward, the severe weather moved eastward across Texas on the 13 th (Fig. 1a), across the Gulf Coast on the 14 th (Fig 1b) and to the East Coast (Fig. 1c). The storm produced 20 tornadoes and 321 severe weather reports as it moved eastward (Table 1). The warm sector of this storm, ahead of the advancing cold front, experienced a significant severe weather event. Relative to the super storm of 1993, this storm was a prolific producer of severe weather producing 20 tornadoes. Of these tornadoes, 11 were reported on the 14 th in the Gulf States, comparable to the 11 tornadoes in Florida observed with the superstorm of March The squall line that moved across Florida with the 1993 event produced 11 tornadoes and 47 deaths (Kocin et al 1993). b. Storm track The storm track was developed based on NAM 00-hour forecasts and RUC13 analysis. For brevity only NAM data is shown in Figures 2 & 3 but the composite tracks were derived from NAM and RUC13 data. Figure 2 shows the MSLP from the 00- hour NAM in 12-hourly increments from 13/0000 UTC through 14/1200 UTC. The earlier cyclone which created flight hazards and delayed air traffic along the

4 Figure 3 As in Figure 2 except zoomed over the eastern United States showing NAM 00-hour forecasts of MSLP showing the cyclone evolution in 12-hour increments from a) 0000 and b) 1200 UTC 15 April and c) 0000 and d) 1200 UTC 16 April Isobars are every 4 hpa. East Coast on the 12 th can still be seen over southern New England at 13/0000 UTC (Fig. 2a). The developing storm was located over New Mexico. The storm tracked into the Mississippi Valley by 14/1200 UTC (Fig. 2d). By 15/0000 UTC the storm had an elongated area of low pressure extending from Kentucky southward into Alabama (Fig. 3a). Interestingly, there was no anchoring cold anticyclone, often associated with major winter storms, over southeastern Canada or northern New England. As the storm moved eastward, a second cyclone developed to the south of the more northern center in Figure 3a and a 996 hpa closed contour was shown over the Carolina s by 15/1200 (Fig. 3b) and a 972 hpa contours was displayed over western Long Island by 16/1200 UTC (Fig. 3d). This was about 2 hours after the 967 hpa reading off the coast of southwestern Long Island at 16/1000 UTC. The RUC13 showed a closed 974 hpa contour over western Long Island from 16/0000 UTC through 16/1200 UTC and these data were used to produce the tracks shown in Figure 4. c. Precipitation Liquid equivalent rainfall is shown for the period of 12/1200 UTC through 15/1200 UTC is shown in Figure 5. These data show the early rain north of the cyclone center over Colorado and Kansas which moved eastward into the Midwest and up the East Coast by the

5 L L L L L L L Figure 4 Estimated storm track of the cyclone from 0000 UTC 13 April to 1200 UTC 16 April Lowest pressure was 972 hpa in the NAM and 974 hpa in the RUC th. Amounts over 64mm were confined snow. An incredibly large region to southwestern Kansas and the experienced heavy rainfall for the 24 Appalachians during this period. The hours ending at 16/1200 UTC and the lines of convection can be seen with rainfall continued during the day on the more southwest to northeast bands over 16 th over most of the affected region. Texas and Arkansas which was observed Over 5.5 inches of rain was observed in on the 12 th and 13 th ; an area of rain over Central Park in New York City setting a Mississippi, Alabama, and Georgia and new record for the date which was 1.8 more convective rains over the inches set back on 15 April Total Carolinas. rainfall eventually topped 7.5 inches in The rain after 15/1200 UTC was more intense over the eastern United States and is shown in Figure 6. These data show heavy rain over New Jersey and southern New York State. An area of 6 to 8 inches of rainfall is shown in southeastern New York State with a broad area of over 3 inches of rainfall covering nearly all of New Jersey, extreme eastern Pennsylvania and eastern New York State. An area of 3 inches of rainfall also covered a large area of southeastern New England, though some of this precipitation was New York City. The snowfall is shown in Figure 7. These data show the general area of snow from Kansas into the Midwest. There was a gap in the snowfall the storm moved into the Midwest with a snowfall minimum over Illinois. During the 15 th the snowfall shifted eastward. Warm air on the front side of the storm limited snowfall early in the event. But as the storm moved up the coast, cold air was pulled into the system increasing the snowfall over portions of the central Appalachians from West Virginia into

6 Figure 5 Climate prediction center precipitation analysis (mm) for 1200 UTC 12 through 15 April Values in mm. New York State. Snowfall was highly elevation dependent. In Pennsylvania Mahoney City received 8 inches of snowfall overnight on the 15 th into the 16 th while lower elevation towns in the same region saw 1 to 2 inches of wet snow. The elevation dependence was observed in New York State too. The heavy wet snow in northeast Pennsylvania and New York affected mainly the higher elevations and interior regions of the States. Though light snow reached as far as Philadelphia. The main impact of this storm outside of the interior sections where the wrap-around snow bands brought heavy snow will be the rains along the coast from New Jersey into southern New York State. The heavy rainfall produced flooding along the Delaware basin and lesser streams and tributaries in New Jersey and easternmost Pennsylvania. Major flooding was observed on Neshaminy Creek, in Pennsylvania and the Saddle, Ramapo, Raritan, and Millstone Rivers in New Jersey. Flooding was also observed in rivers in Maryland, Virginia, and West Virginia. d. Winds As with most deep storms, this system produced strong winds.coastal regions of New York, New Jersey, and Connecticut has winds of 40 to 48 kts. The strongest winds were reported at West Hampton Beach on eastern Long Island.

7 Figures 8 & 9 show the 850 hpa winds over the region at 15/1200 UTC and 16/1200 UTC. These data show the strong easterly winds on the front side of the storm and strong southerly winds in the warm air. The strong easterly winds reached over 60 KTS and were over - 5SDs below normal over southern New England by 16/1200 UTC. Some areas had wind anomalies in excess of -6SDs below normal. The stable cool air at lowlevels may have precluded mixing such gusts to the surface. The strong southerly winds impacted the precipitation totals over New Jersey and southern New York. e. Precipitation issues Early forecasts of this storm implied that widespread snowfall was a potential threat from this storm. Ultimately, the storm did produce a significant spring storm, but not of the magnitude implied by earlier forecasts. Additionally, the snowfall was heavily elevation dependent, especially over Pennsylvania. The 850 hpa temperatures shown in Figure 8 & 9 show that the 0C contour was over northern Pennsylvania at 15/1200 UTC. This was about 200 km north of forecasts (not shown) from the GEFS and GFS initialized at and prior to 13/1200 UTC. The storm was warmer than forecast. By 16/1200 UTC the cold air had wrapped around the cyclone, with warm air intruding over most of eastern New England. The intrusion of cold air late Sunday into early Monday produced snowfall over central New York and central Pennsylvania. Locally Figure 6 Multi-sensor liquid equivalent precipitation (inches) over the eastern United States for the 24-hour period ending at 1200 UTC 16 April 2007.

8 heavy snow as observed, mainly at higher elevations in Pennsylvania as shown in the 2-day accumulated snowfall for the 48 hour period ending at 1200 UTC 17 April Figure 10 shows the NAM moisture flux and moisture flux anomalies at 15/1800 UTC. These data show high values of moisture flux and moisture flux anomalies on the order of 5SDs above normal. For several hours, the high values and anomalies in the moisture flux were directed over New Jersey into southern New England. The high values of the precipitable water (PW: not shown) and the strong and anomalous southerly low-level winds (see Figs. 8 &9) contributed to the high moisture flux over the region where the heaviest rains were observed. 4. CONCLUSIONS A major spring storm affected the United States from 12 to 16 April The storm brought severe weather from Texas to the Carolina s as it moved eastward. Along the northern edge of the storm, snow was observed with some areas of heavy snow from Kansas into interior portions of the northeastern United States. But the real impact of this storm was heavy rainfall and high winds in the eastern United States, mainly along the coastal regions from New Jersey into southern New England. This area saw widespread heavy rainfall with amounts of 3-6 inches, strong winds with a deep surface cyclone which produced coastal flooding. Long Island, New York was hit hard by the coastal flooding. The heavy rainfall in New Jersey and southern New York produced flooding along the Delaware River and many smaller streams. This was an impressive cool season rainfall event. The combined effects of the weather produced along its path as it traversed the United States will likely put this storm in the record books for a number of reasons. Along its northern edge, it produced a late season snowfall from Kansas to New England. In the warm air, it produced a significant severe weather event which spanned 3 days affecting the areas from Texas to North Carolina. Tragically, it also produced a deadly tornado. The more memorable part of this storm will be in the heavily populated corridor from Washington to Boston. The heavy rains and flooding in New Jersey and New York along with the massive areas affected by rainfall amounts in excess of 5 inches will long be remembered. The deep low pressure, around 967 hpa off of Long Island, New York will also put this storm into a memorable category. A companion paper will examine the forecasts of this storm from the NCEP models and EPS s. Suffice to say the deep low, strong winds, coastal flooding, and severe weather aspects of this storm were relatively well forecast. The big weakness was in the details with the precipitation type which at longer ranges appeared to be poorly forecast. 5. ACKNOWLEDGEMENTS Data sources used included the Mid- Atlantic River Forecast (MARFC) data on rainfall amounts and flood stages. Public information statements from various NWS offices were used to extract snowfall and wind gust data. The Brookhaven NWS office produced and

9 extensive list of wind reports. The Storm Prediction Center, Climate Predictions Center, and other NWS sites provided valuable data with which to analyze this storm. 6. REFERENCES Uccellini,L.W., PJ. Kocin, R S Schneider, PM. Stokols, and RA. Dorr, 1995: Forecasting the March 1993 Superstorm., BAM,76, Kocin, PJ, P. Shumacher, R. Morales, and L.W. Uccellini, 1995: Overview of the March 1993 Superstorm, BAMS,76, Figure 10. NAM forecasts initialized at 1800 UTC showing moisture flux and moisture flux anomalies at 925 and 850 HPa.

10 Figure 7 Snow analysis provided by NOAA showing 24 hour snowfall for the event ending at 1200 UTC on 15 and 16 April

11 Figure 8 NAM 00-hour forecasts initialized at 1200 UTC 15 April 2007 showing (left) a) 850 hpa temperatures (C ) and b) heights (m) and (right) 850 hpa winds (KTS) and a) U-wind anomalies and b) V-wind anomalies. The heights are contoured every 30 m, isotherms every 2C. Anomalies for all fields are in standard deviations from the 30-year means. 8.

12 Figure 9 As in Figure 8 except NAM 00-hour forecasts valid at 1200 UTC 16 April Figure 10. The 2-day accumulated snowfall over Pennsylvania from COOP data.

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