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Wednesday, July 21, 2021

SPC Jul 21, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0300 PM CDT Wed Jul 21 2021 Valid 212000Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE MID-ATLANTIC REGION INTO SOUTHERN NEW ENGLAND... ...SUMMARY... Thunderstorms capable of producing damaging winds and isolated large hail will continue this afternoon from the Delmarva into portions of southern New England. Other isolated strong storms may occur over parts of Arizona and Montana. ...Mid Atlantic into the Northeast... The Slight Risk has been trimmed across northern portions of the Mid Atlantic into southern New England, in the wake of earlier convection and the passage of a pre-frontal trough, though the Marginal Risk has been left in place across these areas due to the potential for an isolated strong storm or two along the primary cold front. Across the remaining portion of the Slight Risk, ongoing storms (including some organized clusters and a couple of supercells) will continue to pose a threat of damaging wind and large hail, before the threat begins to wane this evening. See MCD 1324 for more information regarding the threat in this area. ...Montana... Isolated strong thunderstorms remain possible across parts of western MT late this afternoon and evening, though it still appears that the severe threat will remain relatively localized. See the previous discussion below for more information. ..Dean.. 07/21/2021 .PREV DISCUSSION... /ISSUED 1120 AM CDT Wed Jul 21 2021/ ...Mid Atlantic into Southern New England... Water vapor imagery shows an upper trough moving across the northeast states today, with an associated cold front stretching from southern PA into MA. A moist and unstable air mass is present to the south of the front, where most CAM guidance shows the development of scattered afternoon thunderstorms. Confidence of high storm coverage and greatest severe threat is over NJ and vicinity. However, at least isolated intense storms may occur from central VA into southeastern MA. Damaging winds will be the main threat, but a few rotating cells may develop along with a risk of hail. Activity will move offshore by early evening, ending the severe threat. Refer to WW385 for further details. ...MT... Mostly clear skies are present today over western MT, where surface dewpoints in the 50s and relatively cool temperatures aloft will help to destabilize the air mass. Southwesterly mid-level winds are forecast to strengthen later today as well. Widely scattered afternoon thunderstorms are expected over this region, with the potential of gusty/damaging winds and hail in the stronger cells. Coverage of storms is uncertain. If confidence in higher coverage occurs, an upgrade to SLGT risk may be needed at the 20z update. ...AZ... Easterly mid-level winds will help to move storms that form this afternoon over the mountains of southeastern AZ into the lower deserts this evening. A few of these storms may pose a risk of gusty/damaging winds. Read more LIVE: spc.noaa.gov