Apr 21, 2021 2000 UTC Day 1 Convective Outlook
Updated: Wed Apr 21 19:20:47 UTC 2021 (20210421 2000Z Day 1 shapefile | 20210421 2000Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20210421 2000 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 11,225 6,479,061 Springfield, MA...Bridgeport, CT...New Haven, CT...Hartford, CT...Stamford, CT...
MARGINAL 28,890 22,036,370 New York, NY...Virginia Beach, VA...Newark, NJ...Norfolk, VA...Jersey City, NJ...
Probabilistic Tornado Graphic
20210421 2000 UTC Day 1 Tornado Probabilities Graphic
Probability of a tornado within 25 miles of a point.
Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.
Day 1 Tornado Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Probabilistic Damaging Wind Graphic
20210421 2000 UTC Day 1 Damaging Wind Probabilities Graphic
Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point.
Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Day 1 Wind Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
15 % 11,270 6,500,081 Springfield, MA...Bridgeport, CT...New Haven, CT...Hartford, CT...Stamford, CT...
5 % 29,092 22,312,441 New York, NY...Virginia Beach, VA...Newark, NJ...Norfolk, VA...Jersey City, NJ...
Probabilistic Large Hail Graphic
20210421 2000 UTC Day 1 Large Hail Probabilities Graphic
Probability of hail 1" or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of hail 2" or larger within 25 miles of a point.
Day 1 Hail Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
5 % 13,084 4,195,105 Virginia Beach, VA...Norfolk, VA...Chesapeake, VA...Newport News, VA...Hampton, VA...
   SPC AC 211920

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0220 PM CDT Wed Apr 21 2021

   Valid 212000Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN NEW
   ENGLAND TO WESTERN LONG ISLAND...

   ...SUMMARY...
   Locally damaging winds remain possible across the Mid Atlantic Coast
   region and into southern New England through around 6 PM EDT.

   ...Discussion...
   Ongoing evolution of convection across the SLGT and MRGL risk areas
   remains consistent with meteorological reasoning described in prior
   outlooks.  As storms continue to develop and spread northeastward,
   potential for locally damaging winds continues.  

   The outlook is being trimmed on the western and southern sections,
   in tandem with convective advance.  A small northeastward expansion
   of the SLGT risk area will also be included -- across southeastern
   Vermont and parts of southern New Hampshire, ahead of a
   northeastward-moving band of storms evolving over the Hudson Valley
   area.

   Risk will diminish into this evening, due to a combination of storms
   moving offshore of the Mid Atlantic region, and
   diurnal/boundary-layer cooling.

   ..Goss.. 04/21/2021

   .PREV DISCUSSION... /ISSUED 1133 AM CDT Wed Apr 21 2021/

   ...Mid-Atlantic States to New England...
   Surface temperatures continue to steadily warm at midday with
   moderate insolation in the wake of showers moving into New England.
   A 1001 mb surface low over northeast Pennsylvania/northern New
   Jersey at 16Z will continue to steadily deepen as it quickly
   transitions northeastward into southern/coastal New England through
   early evening. The longest residence time of the warm sector over
   inland areas this afternoon will be across the coastal North
   Carolinas and the immediate coast of New Jersey/New York into
   southern New England.

   Influenced by the amplifying upstream wave, strengthening of low to
   mid-level southwesterlies should yield upscale evolution to
   quasi-linear segments along coastal portions of the Mid-Atlantic
   into southern New England. More broken cells should be maintained
   farther south where a few transient supercells are possible. A
   well-mixed boundary layer with an inverted-v thermodynamic profile
   and pressure rise/fall couplet may support strong to isolated severe
   gusts with scattered damaging winds as the primary hazard. Farther
   south, marginally severe hail is possible where storm mode can
   remain semi-discrete before convection quickly spreads off the coast
   by late afternoon.

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   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z