Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1245 AM CDT Sat Jun 27 2020 Valid 271200Z - 281200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE CENTRAL APPALACHIANS EASTWARD TO NEAR THE NEW JERSEY COAST AND VICINITY OF NEW YORK CITY... ...SUMMARY... Severe thunderstorms associated with damaging wind gusts and hail will be possible today across parts of the central Appalachians eastward to New Jersey and far southwestern New England. Marginally severe storms will be possible in parts of the central, southeast and northwestern states this afternoon. ...Central Appalachians To New Jersey and Far Southwest New England... At the start of the period, a linear MCS should be ongoing in parts of the central Appalachians from northeast Pennsylvania into southern New York. Wind damage will be possible along the leading edge of this convective line as it moves eastward across southern New England this morning. In the wake of this feature, a moist airmass will exist across the central Appalachians with surface dewpoints in the mid 60s F. In response, pockets of moderate instability are expected to develop by afternoon across parts of the Upper Ohio Valley eastward to the vicinity of New York City. An outflow boundary from the morning MCS should be present across the area. This boundary will likely be a focus for convective development. Additional convection should initiate off of terrain features in the central Appalachians. By mid to late afternoon, scattered thunderstorms are forecast to develop from eastern Ohio eastward to near the New Jersey coast and Long Island. RAP forecast soundings along this corridor at 21Z show unidirectional wind profiles from the west-southwest with speed shear in the mid-levels and steep low-level lapse rates. This environment should be enough for damaging wind gusts with the more organized multicell line segments. For this outlook, have left the slight risk area similar to the previous outlook. This is mainly due to uncertainties with the forecast which include the location of the outflow boundary and resulting distribution of instability. ...Central Plains/Mid Mississippi Valley/Southern Great Lakes/Ohio Valley... An upper-level ridge will move slowly eastward from the central Plains into the lower Missouri Valley today. Further west, a shortwave trough will move into the central High Plains. Beneath these two features, a moist airmass should exist with surface dewpoints generally ranging from the lower 60s F in the west-central Kansas to near 70 in the lower Missouri Valley. As surface heating takes place during the day, moderate to strong instability is expected to develop across much of central Plains eastward into the Lower Missouri Valley. Although convection should remain isolated during the day, a few thunderstorms could form along pre-existing boundaries in areas that destabilize the most. These storms may have a marginal wind damage threat. This threat is expected to continue into early evening as a 40 to 50 kt low-level jet strengthens across the region. Further west into eastern Colorado, convection is expected to develop in the higher terrain of central Colorado. These storms will move eastward into the central High Plains during the afternoon just ahead of a subtle shortwave trough. Although instability in eastern Colorado should be relatively weak, lapse rates will be very steep. This along with deep-layer shear of 30 to 40 kt should be enough for semi-organized multicells with hail and a few marginally severe wind gusts. ...Southeast... A very moist airmass will be in place across the Gulf Coast States today with surface dewpoints mostly in the upper 60s and lower 70s F. In response to surface heating, moderate to strong instability is expected to develop across much of the moist sector by afternoon from far east Texas eastward into Alabama. Some models suggest MLCAPE will peak near or above 3000 J/kg across parts of the Southeast. This magnitude of instability combined with steep low-level lapse approaching or exceeding 8.0 C/km should be enough for an isolated wind damage threat with the stronger pulse storms or multicells. ...Northern Rockies/Northern High Plains... An upper-level trough will dig southeastward into the Pacific Northwest today. West to southwest mid-level flow will be in place ahead of the trough across much of the northern Rockies and northern High Plains. As the upper-level trough approaches the northern Rockies this afternoon, scattered thunderstorms are expected to develop in the higher terrain of western and central Montana. These storms will move eastward into the lower elevations across central and eastern Montana late this afternoon into early evening. Although instability should be relatively weak, steep lapse rates and moderate deep-layer shear may be enough for a marginal severe threat. ..Broyles/Bentley.. 06/27/2020Read more
from SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day1otlk_1200.html
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