National Weather Service United States Department of Commerce

Powerful Pacific System Impacting the West; First Significant Snow for Portions of the East

Power Pacific system will continue to bring significant impacts for Pacific Northwest into northern California the remainder of the week. Dangerous coastal affects, heavy rain, flooding, strong winds, and higher elevation mountain snow continues. Meanwhile, a storm across the east is set to bring the first accumulating snow to many higher elevations of the Catskills into the central Appalachians. Read More >

Overview

A strong cold front moved through the eastern mid-Atlantic during the early to mid afternoon hours on April 21. Showers and thunderstorms developed ahead of the front. Some storms became severe, with sporadic reports of storm damage from southern New England southward through much of the coastal mid-Atlantic. The front moved through a couple hours before peak heating, which combined with an air mass that was not overly warm to begin with limited instability. However, steep mid-level lapse rates helped to compensate for this and allowed up to around 1000J/kg of surface based CAPE to develop. Strong frontal forcing and strong shear (deep layer shear of 50 to 60 kt) combined with the sufficient instability to allow convection to develop and intensify. Mainly linear storm modes with some discrete elements were observed. The strongest storms produced hail up to quarter size, aided by cool air aloft thanks to the lapse rates, and scattered instances of downed trees and power lines due to damaging winds. Convection moved offshore by late afternoon, and a much cooler and drier air mass overspread the region.

Image
KDIX radar loop of the April 21, 2021 severe weather at half hour increments from 1530 UTC to 1930 UTC. Note the rapid development between 1530 and 1600 UTC (11:30 AM - 12:00 PM EDT) and how convection consisted of a mix of discrete and linear elements, as opposed to a solid squall line such as on March 28, 2021.

Meteorological Environment/Setup

Synoptic Overview at 12z 21 April 2021 (courtesy of SPC)

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Figure 1: 250-mb analysis Figure 2: 300-mb analysis Figure 3: 500-mb analysis
 
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Figure 4: 700-mb analysis Figure 5: 850-mb analysis Figure 6: 925-mb analysis

Synoptic Overview at 00z 22 April 2021 (courtesy of SPC)

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Figure 7: 250-mb analysis Figure 8: 300-mb analysis Figure 9: 500-mb analysis
 
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Figure 10: 700-mb analysis Figure 11: 850-mb analysis Figure 12: 925-mb analysis
 

Surface Maps (courtesy of WPC)

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Figure 13: 06z surface map Figure 14: 09z surface map Figure 15: 12z surface map
 
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Figure 16: 15z surface map Figure 17: 18z surface map Figure 18: 21z surface map

Observed Soundings
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Figure 19: 12z Sterling, VA sounding Figure 20: 12z Wallops Island, VA sounding Figure 21: 12z Long Island, NY sounding
Environment Environment Environment
Figure 22: 0z Sterling, VA sounding Figure 23: 0z Wallops Island, VA sounding Figure 24: 0z Long Island, NY sounding

Preliminary Storm Reports

prelimreports prelimreports prelimreports
Figure 1: National Preliminary Reports 12z 21 April 2021 to 12z 22 April 2021. Figure 2: Zoomed Preliminary Reports for the same period. Figure 3: April 21 1300z SPC Convective Outlook.

Watches, Warnings, and Advisories Issued

Click here for a county warning area based plot of local storm reports and warnings/advisories issued for the 21 April 2021 event.

Preliminary Local Storm Reports

 

NWUS51 KPHI 220034
LSRPHI

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
834 PM EDT WED APR 21 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1220 PM     HAIL             2 SSW DISTRICT TWP      40.43N 75.66W
04/21/2021  E0.25 INCH       BERKS              PA   TRAINED SPOTTER  

             

1222 PM     HAIL             1 S BOYERTOWN           40.32N 75.64W
04/21/2021  M1.00 INCH       BERKS              PA   TRAINED SPOTTER  

             

1227 PM     HAIL             1 NE EASTLAWN GARDENS   40.76N 75.29W
04/21/2021  M0.25 INCH       NORTHAMPTON        PA   TRAINED SPOTTER  

             

1230 PM     HAIL             3 WNW UWCHLAN TWP       40.07N 75.68W
04/21/2021  E0.25 INCH       CHESTER            PA   TRAINED SPOTTER  

             

1235 PM     HAIL             TATAMY                  40.74N 75.25W
04/21/2021  M0.25 INCH       NORTHAMPTON        PA   TRAINED SPOTTER  

            PEA SIZE HAIL REPORTED BY A TRAINED SPOTTER 
            IN TATAMY, PA. TIME ESTIMATED BY RADAR. 

1235 PM     HAIL             EMMAUS                  40.54N 75.49W
04/21/2021  M0.25 INCH       LEHIGH             PA   TRAINED SPOTTER  

             

1235 PM     TSTM WND DMG     1 SSW ALLENTOWN         40.58N 75.48W
04/21/2021                   LEHIGH             PA   UTILITY COMPANY  

            TREE DOWN ON WIRES NEAR LEHIGH PKWY. TIME 
            ESTIMATED FROM RADAR. 

1240 PM     HAIL             2 ENE BETHLEHEM TWP     40.67N 75.30W
04/21/2021  M0.50 INCH       NORTHAMPTON        PA   TRAINED SPOTTER  

             

1240 PM     HAIL             1 WSW BETHLEHEM TWP     40.65N 75.36W
04/21/2021  E0.25 INCH       NORTHAMPTON        PA   TRAINED SPOTTER  

             

1245 PM     HAIL             MARTINS CREEK           40.78N 75.18W
04/21/2021  E0.25 INCH       NORTHAMPTON        PA   TRAINED SPOTTER  

             

1245 PM     HAIL             2 WNW FORKS TWP         40.74N 75.21W
04/21/2021  E0.88 INCH       NORTHAMPTON        PA   TRAINED SPOTTER  

             

1248 PM     HAIL             TRUMBAUERSVILLE         40.41N 75.38W
04/21/2021  E0.25 INCH       BUCKS              PA   TRAINED SPOTTER  

             

1250 PM     HAIL             HARLEYSVILLE            40.28N 75.39W
04/21/2021  E0.25 INCH       MONTGOMERY         PA   TRAINED SPOTTER  

             

1250 PM     HAIL             MARTINS CREEK           40.78N 75.18W
04/21/2021  E0.25 INCH       NORTHAMPTON        PA   TRAINED SPOTTER  

             

1250 PM     HAIL             2 WSW WILLIAMS TWP      40.63N 75.24W
04/21/2021  E0.25 INCH       NORTHAMPTON        PA   TRAINED SPOTTER  

             

1250 PM     HAIL             COLLEGEVILLE            40.19N 75.45W
04/21/2021  E0.25 INCH       MONTGOMERY         PA   PUBLIC           

             

1252 PM     HAIL             LANDENBERG              39.78N 75.76W
04/21/2021  E0.25 INCH       CHESTER            PA   TRAINED SPOTTER  

             

1255 PM     HAIL             KENNETT SQUARE          39.85N 75.70W
04/21/2021  E0.25 INCH       CHESTER            PA   TRAINED SPOTTER  

             

1258 PM     HAIL             KINTNERSVILLE           40.56N 75.17W
04/21/2021  E0.50 INCH       BUCKS              PA   TRAINED SPOTTER  

             

0100 PM     TSTM WND GST     WHITE CLAY CREEK        39.73N 75.73W
04/21/2021  M46 MPH          NEW CASTLE         DE   MESONET          

             

0100 PM     HAIL             2 N CENTREVILLE         39.07N 76.07W
04/21/2021  E0.25 INCH       QUEEN ANNES       MD   TRAINED SPOTTER  

             

0100 PM     HAIL             1 W EASTON              40.69N 75.22W
04/21/2021  E0.70 INCH       NORTHAMPTON        PA   PUBLIC           

            DIME SIZE HAIL IN EASTON. TIME ESTIMATED 
            FROM RADAR. 

0100 PM     HAIL             NEWARK                  39.68N 75.76W
04/21/2021  E0.70 INCH       NEW CASTLE         DE   PUBLIC           

             

0103 PM     HAIL             COLLEGEVILLE            40.19N 75.46W
04/21/2021  E0.88 INCH       MONTGOMERY         PA   TRAINED SPOTTER  

             

0105 PM     TSTM WND GST     1 ENE POHATCONG TWP     40.66N 75.12W
04/21/2021  M46 MPH          WARREN             NJ   MESONET          

             

0105 PM     HAIL             PIKE CREEK              39.73N 75.70W
04/21/2021  E0.25 INCH       NEW CASTLE         DE   TRAINED SPOTTER  

             

0105 PM     TSTM WND DMG     UPPER BLACK EDDY        40.57N 75.10W
04/21/2021                   BUCKS              PA   PUBLIC           

            LARGE TREE DOWN. 

0110 PM     TSTM WND DMG     3 N ALEXANDRIA TWP      40.61N 75.02W
04/21/2021                   HUNTERDON          NJ   UTILITY COMPANY  

            TREE DOWN ON WIRES ON RICK RD. TIME 
            ESTIMATED FROM RADAR. 

0115 PM     HAIL             HOCKESSIN               39.79N 75.69W
04/21/2021  E0.50 INCH       NEW CASTLE         DE   TRAINED SPOTTER  

             

0115 PM     HAIL             1 NE BROOKFIELD         39.86N 75.59W
04/21/2021  M0.25 INCH       DELAWARE           PA   TRAINED SPOTTER  

             

0115 PM     HAIL             1 NW TALLEYVILLE        39.82N 75.57W
04/21/2021  E0.25 INCH       NEW CASTLE         DE   TRAINED SPOTTER  

             

0120 PM     HAIL             3 SE CHURCH HILL        39.11N 75.95W
04/21/2021  E1.00 INCH       QUEEN ANNES       MD   TRAINED SPOTTER  

             

0120 PM     TSTM WND DMG     1 WSW CLINTON           40.63N 74.92W
04/21/2021                   HUNTERDON          NJ   DEPT OF HIGHWAYS 

            DOWNED TREE ON I-78 EASTBOUND RAMP TO EXIT 
            15. ONE LANE WAS CLOSED. TIME ESTIMATED FROM 
            RADAR. 

0122 PM     HAIL             SICKLERVILLE            39.72N 74.97W
04/21/2021  E0.25 INCH       CAMDEN             NJ   TRAINED SPOTTER  

             

0123 PM     HAIL             WASHINGTON TWP          39.75N 75.07W
04/21/2021  M0.25 INCH       GLOUCESTER         NJ   TRAINED SPOTTER  

            PEA SIZE HAIL REPORTED IN WASHINGTON 
            TOWNSHIP. 

0125 PM     HAIL             3 NW MILLVILLE          39.43N 75.07W
04/21/2021  E0.25 INCH       CUMBERLAND         NJ   TRAINED SPOTTER  

             

0135 PM     HAIL             SPRINGFIELD             39.93N 75.32W
04/21/2021  E0.25 INCH       DELAWARE           PA   TRAINED SPOTTER  

             

0135 PM     TSTM WND DMG     3 S EASTON              38.74N 76.07W
04/21/2021                   TALBOT             MD   DEPT OF HIGHWAYS 

            US-50 WESTBOUND WAS CLOSED BETWEEN 
            SCHWANINGER RD AND LANDING NECK RD DUE TO 
            DOWN POWER LINES. TIME ESTIMATED FROM RADAR. 

0136 PM     TSTM WND GST     PHILADELPHIA INTERNATIO 39.88N 75.23W
04/21/2021  M52 MPH          PHILADELPHIA       PA   ASOS             

            ASOS STATION KPHL PHILADELPHIA INTERNATIONAL 
            AIRPORT. 

0140 PM     TSTM WND DMG     1 SSW WYNCOTE           40.08N 75.15W
04/21/2021                   MONTGOMERY         PA   UTILITY COMPANY  

            TREE LIMBS DOWN ON WIRES ON ROCK CREEK DR. 
            TIME ESTIMATED FROM RADAR. 

0140 PM     TSTM WND GST     1 SSE SOUTH PHILADELPHI 39.89N 75.20W
04/21/2021  M59 MPH          PHILADELPHIA       PA   MESONET          

            PENNDOT RWIS STATION AT I-95 AND GIRARD 
            AVENUE. 

0140 PM     HAIL             CHESTER                 40.79N 74.69W
04/21/2021  E0.25 INCH       MORRIS             NJ   TRAINED SPOTTER  

             

0140 PM     HAIL             SPARTA                  41.04N 74.63W
04/21/2021  E0.25 INCH       SUSSEX             NJ   TRAINED SPOTTER  

             

0145 PM     TSTM WND DMG     1 SE FLOURTOWN          40.10N 75.20W
04/21/2021                   MONTGOMERY         PA   UTILITY COMPANY  

            TREE DOWN ON WIRES ON TYSON RD. TIME 
            ESTIMATED FROM RADAR. 

0145 PM     HAIL             1 WSW DOVER             39.16N 75.54W
04/21/2021  E0.25 INCH       KENT               DE   PUBLIC           

            SOCIAL MEDIA. 

0148 PM     HAIL             MEDFORD                 39.89N 74.82W
04/21/2021  E0.88 INCH       BURLINGTON         NJ   PUBLIC           

            MOSTLY PEA-SIZED WITH A FEW PENNY TO 
            NICKEL-SIZED STONES. REPORT VIA SOCIAL 
            MEDIA. 

0148 PM     HAIL             1 SE POINT BREEZE       39.92N 75.17W
04/21/2021  E0.25 INCH       PHILADELPHIA       PA   TRAINED SPOTTER  

             

0148 PM     TSTM WND DMG     3 N BRANCHBURG TWP      40.60N 74.69W
04/21/2021                   SOMERSET           NJ   DEPT OF HIGHWAYS 

            UTILITY POLE DOWN ON US-22 EASTBOUND WEST OF 
            READINGTON RD. ALL LANES WERE CLOSED. TIME 
            ESTIMATED FROM RADAR. 

0150 PM     HAIL             2 NW EDGE HILL          39.19N 75.55W
04/21/2021  E0.70 INCH       KENT               DE   PUBLIC           

            DIME SIZE HAIL IN DOVER, DE. TIME ESTIMATED 
            FROM RADAR. 

0155 PM     TSTM WND DMG     3 SSE FAR HILLS         40.65N 74.62W
04/21/2021                   SOMERSET           NJ   PUBLIC           

            A FEW MEDIUM SIZED TREE BRANCHES DOWN. TIME 
            ESTIMATED FROM RADAR. 

0155 PM     HAIL             MORRIS TWP              40.78N 74.46W
04/21/2021  E0.25 INCH       MORRIS             NJ   TRAINED SPOTTER  

             

0156 PM     HAIL             1 ESE WHIPPANY          40.83N 74.40W
04/21/2021  E0.25 INCH       MORRIS             NJ   TRAINED SPOTTER  

            PEA SIZE HAIL REPORTED IN WHIPPANY, NJ. TIME 
            ESTIMATED FROM RADAR. 

0158 PM     TSTM WND DMG     1 N HOLMESBURG          40.06N 75.03W
04/21/2021                   PHILADELPHIA       PA   UTILITY COMPANY  

            POWER OUTAGE DUE TO SEVERE WEATHER NEAR 
            HOLME CIRCLE. TIME ESTIMATED FROM RADAR. 

0200 PM     TSTM WND DMG     FEDERALSBURG            38.70N 75.78W
04/21/2021                   CAROLINE           MD   BROADCAST MEDIA  

            TREE DOWN ON VEHICLE. 

0200 PM     HAIL             2 WNW NEPTUNE CITY      40.21N 74.06W
04/21/2021  E0.25 INCH       MONMOUTH           NJ   TRAINED SPOTTER  

             

0200 PM     HAIL             1 ENE LAWRENCEVILLE     40.30N 74.71W
04/21/2021  E0.70 INCH       MERCER             NJ   EMERGENCY MNGR   

             

0210 PM     TSTM WND GST     FORTESCUE               39.24N 75.18W
04/21/2021  M58 MPH          CUMBERLAND         NJ   MESONET          

            FORTESCUE NJ WEATHERNET SITE. 

0210 PM     MARINE TSTM WIND FORTESCUE               39.24N 75.18W
04/21/2021  M58 MPH          ANZ430             NJ   MESONET          

            FORTESCUE NJ WEATHERNET SITE. 

0212 PM     HAIL             BRIDGEVILLE             38.74N 75.60W
04/21/2021  M1.00 INCH       SUSSEX             DE   BROADCAST MEDIA  

            TIME ESTIMATED FROM RADAR. 

0212 PM     HAIL             SEAFORD                 38.64N 75.61W
04/21/2021  M1.00 INCH       SUSSEX             DE   BROADCAST MEDIA  

            TIME ESTIMATED FROM RADAR. 

0214 PM     HAIL             BRIDGEVILLE             38.74N 75.61W
04/21/2021  E1.00 INCH       SUSSEX             DE   BROADCAST MEDIA  

             

0214 PM     HAIL             SEAFORD                 38.65N 75.61W
04/21/2021  E1.00 INCH       SUSSEX             DE   BROADCAST MEDIA  

             

0220 PM     HAIL             2 N GEORGETOWN          38.71N 75.39W
04/21/2021  E0.25 INCH       SUSSEX             DE   TRAINED SPOTTER  

             

0220 PM     TSTM WND GST     1 W MILLVILLE MUNICIPAL 39.37N 75.08W
04/21/2021  M54 MPH          CUMBERLAND         NJ   ASOS             

            ASOS STATION KMIV MILLVILLE. 

0222 PM     HAIL             2 NW BERKELEY TWP       39.95N 74.32W
04/21/2021  E1.00 INCH       OCEAN              NJ   PUBLIC           

            AN MPING REPORT OF QUARTER SIZE HAIL. TIME 
            ESTIMATED FROM RADAR. 

0223 PM     HAIL             MILLVILLE               39.40N 75.04W
04/21/2021  E1.00 INCH       CUMBERLAND         NJ   PUBLIC           

             

0224 PM     HAIL             1 WSW MILLVILLE         39.40N 75.05W
04/21/2021  E0.88 INCH       CUMBERLAND         NJ   PUBLIC           

            SOCIAL MEDIA. TIME ESTIMATED FROM RADAR. 

0225 PM     HAIL             2 WSW LAKEHURST         40.00N 74.34W
04/21/2021  E0.25 INCH       OCEAN              NJ   PUBLIC           

            SOCIAL MEDIA. 

0225 PM     HAIL             HELMETTA                40.38N 74.41W
04/21/2021  E0.25 INCH       MIDDLESEX          NJ   TRAINED SPOTTER  

             

0225 PM     TSTM WND GST     MILTON                  38.78N 75.32W
04/21/2021  M44 MPH          SUSSEX             DE   TRAINED SPOTTER  

             

0235 PM     MARINE TSTM WIND 1 WNW LEWES             38.78N 75.16W
04/21/2021  M58 MPH          SUSSEX             DE   MESONET          

             

0235 PM     HAIL             2 NW WALL TWP           40.17N 74.09W
04/21/2021  E0.25 INCH       MONMOUTH           NJ   PUBLIC           

             

0235 PM     TSTM WND GST     1 WNW LEWES             38.78N 75.16W
04/21/2021  M58 MPH          SUSSEX             DE   MESONET          

             

0238 PM     MARINE TSTM WIND INDIAN BEACH            38.68N 75.08W
04/21/2021  M56 MPH          SUSSEX             DE   MESONET          

            MESONET STATION XDEW DEWEY BEACH GUSTED TO 
            56 MPH. 

0240 PM     HAIL             SEASIDE HEIGHTS         39.94N 74.07W
04/21/2021  M0.25 INCH       OCEAN              NJ   TRAINED SPOTTER  

             

0240 PM     HAIL             DOROTHY                 39.41N 74.81W
04/21/2021  M0.25 INCH       ATLANTIC           NJ   TRAINED SPOTTER  

             

0241 PM     TSTM WND DMG     2 SE LEISURE VILLAGE EA 40.01N 74.13W
04/21/2021                   OCEAN              NJ   PUBLIC           

            GARDEN SHED BLOWN OVER, SECTION OF VINYL 
            FENCE BROUGHT DOWN, AND SHINGLES BLOWN OFF 
            HOUSE. TIME ESTIMATED FROM RADAR. SOCIAL 
            MEDIA. 

0243 PM     HAIL             1 ESE MANAHAWKIN        39.69N 74.24W
04/21/2021  E0.25 INCH       OCEAN              NJ   PUBLIC           

             

0244 PM     HAIL             1 N SEASIDE HEIGHTS     39.96N 74.07W
04/21/2021  E0.25 INCH       OCEAN              NJ   PUBLIC           

             

0245 PM     TSTM WND GST     CAPE MAY                38.94N 74.92W
04/21/2021  M46 MPH          CAPE MAY           NJ   MESONET          

             

0245 PM     MARINE TSTM WIND CAPE MAY                38.93N 74.92W
04/21/2021  M46 MPH          CAPE MAY           NJ   MESONET          

             

0245 PM     HAIL             BARNEGAT                39.75N 74.22W
04/21/2021  E0.25 INCH       OCEAN              NJ   PUBLIC           

             

0250 PM     HAIL             NORTHFIELD              39.37N 74.55W
04/21/2021  E0.25 INCH       ATLANTIC           NJ   PUBLIC           

            SOCIAL MEDIA. 

0250 PM     TSTM WND GST     DEWEY BEACH             38.69N 75.08W
04/21/2021  M48 MPH          SUSSEX             DE   MESONET          

             

0250 PM     MARINE TSTM WIND DEWEY BEACH             38.69N 75.08W
04/21/2021  M48 MPH          SUSSEX             DE   MESONET          

             

0254 PM     TSTM WND GST     ATLANTIC CITY INTERNATI 39.45N 74.57W
04/21/2021  M63 MPH          ATLANTIC           NJ   ASOS             

            ASOS STATION KACY ATLANTIC CITY GUSTED TO 63 
            MPH. 

0255 PM     MARINE TSTM WIND 1 NW SEA ISLE CITY      39.17N 74.71W
04/21/2021  M44 MPH          CAPE MAY           NJ   MESONET          

            LUDLUM BAY. 

0255 PM     TSTM WND GST     1 NW SEA ISLE CITY      39.17N 74.71W
04/21/2021  M44 MPH          CAPE MAY           NJ   MESONET          

            LUDLUM BAY. 

0259 PM     TSTM WND GST     3 NW FENWICK ISLAND     38.48N 75.09W
04/21/2021  M60 MPH          SUSSEX             DE   PUBLIC           

            PERSONAL WEATHER STATION KDESELBY13. 

0300 PM     TSTM WND DMG     1 S GALLOWAY TWP        39.46N 74.51W
04/21/2021                   ATLANTIC           NJ   BROADCAST MEDIA  

            TREES DOWN AT 6TH AVE AND RIDGEWOOD ST. 
            EMERGENCY MANAGEMENT REPORT RELAYED BY 
            BROADCAST MEDIA. TIME ESTIMATED FROM RADAR. 

0300 PM     MARINE TSTM WIND NORTH WILDWOOD          39.00N 74.79W
04/21/2021  M48 MPH          CAPE MAY           NJ   MESONET          

             

0300 PM     MARINE TSTM WIND 3 SSE LANOKA HARBOR     39.82N 74.13W
04/21/2021  M41 MPH          ANZ451             NJ   MESONET          

            KITE ISLAND. 

0305 PM     MARINE TSTM WIND 1 W OCEAN CITY          39.28N 74.60W
04/21/2021  M45 MPH          CAPE MAY           NJ   MESONET          

            POINT POINT. 

0310 PM     TSTM WND GST     1 SW BEACH HAVEN        39.55N 74.26W
04/21/2021  M61 MPH          OCEAN              NJ   PUBLIC           

            PERSONAL WEATHER STATION KNJLONGB89. 

0312 PM     MARINE TSTM WIND 5 SE MYSTIC ISLAND      39.52N 74.32W
04/21/2021  M62 MPH          OCEAN              NJ   MESONET          

            RUTGERS/ TUCKERTON MESONET GUST TO 62 MPH. 

0315 PM     HAIL             LITTLE EGG HARBOR TWP   39.58N 74.36W
04/21/2021  M0.25 INCH       OCEAN              NJ   TRAINED SPOTTER  

             

0316 PM     TSTM WND GST     2 SW BEACH HAVEN        39.54N 74.26W
04/21/2021  M62 MPH          OCEAN              NJ   MESONET          

            CWOP SITE EW4321. 


&&

$$

Radar Interrogation

Notable Radar Imagery (GR2Analyst)

Environment Environment
Figure 1: KDOX radar reflectivity factor for the 0.5 degree elevation angle valid 1726 UTC 21 April 2021. A broken line of severe thunderstorms is visible over eastern Maryland and entering into Delaware. Sporadic reports of penny to quarter size hail were received from these cells. Steep lapse rates (a rapid decrease in temperature with height) caused many storms on April 21 to contain at least small hail, with a few containing severe hail of one inch or greater in diameter, or the size of a quarter. Figure 2: KDOX radar reflectivity cross section taken along the line of storm cells seen in Figure 1. Multiple distinct, tilted updrafts are seen. Storm tops reached only 20,000 to 25,000 feet, quite shallow for severe convection and especially for convection producing severe hail. But this is not uncommon in early season severe weather, and the cool temperatures aloft helped compensate for the relatively shallow convection.
Environment Environment
Figure 3: KDIX radar reflectivity factor for the 0.5 degree elevation angle valid 1824 UTC 21 April 2021. A semi-discrete cell is seen passing directly over Millville, New Jersey. A wind gust of 54 mph was measured at the Millville Municipal Airport as this storm passed, and quarter size hail was reported in and around Millville. This storm had previously crossed Delaware Bay and produced a 58 mph wind gust in Fortescue, New Jersey. Figure 4: KDIX radar reflectivity cross section for the storm shown in Figure 3. This time, the cross section is taken across (from west to east) as opposed to along (from southwest to northeast) the storm(s), but it shows a similar picture. A strong, tilted updraft is seen, with the tilt evidence of strong wind shear. Reflectivity values of 60dBz (demarcated by the pink pixels) extend up to 20,000 feet in elevation, and this was likely the most robust storm of the day.
Environment Environment
Figure 5: KDOX radar reflectivity factor (left) and base velocity (right) for the 0.5 degree elevation angle valid 1828 UTC 21 April 2021. In addition to hail, a number of instances of damaging winds were observed on April 21. Storms in southern Delaware intensified as they approached the coast. Base velocity values of nearly 60 kt (69 mph) are seen at less than 1000 feet above ground level. Multiple instances of tree and utility damage and several measured gusts of 55 to 65 mph were recorded in southern and southeastern Delaware. Figure 6: Same as Figure 3 valid at 1911 UTC 21 April 2021. Like the storms in southern Delaware, storms over southern New Jersey also intensified near the coast. This line of cells began to take on a bowing appearance as it moved offshore, a signature which is often accompanied by strong to severe wind. Multiple wind gusts over 60 mph were measured in Atlantic and southeastern Ocean Counties, with tree and utility damage also reported.