Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1247 AM CDT Fri Jul 24 2020 Valid 241200Z - 251200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE NORTHERN PLAINS... ...SUMMARY... Severe thunderstorms capable of damaging winds and large hail are expected this afternoon and evening over the Dakotas and Nebraska, and possibly overnight into western Minnesota. Sporadic strong wind gusts are also possible over parts of the Mid Atlantic during the afternoon. ...Synopsis... Southwest winds aloft are forecast to strengthen from the Rockies into the northern Plains, in between an upper high over OK and an upper trough dropping southeast into MT late. At the surface, a front will move into the central Dakotas during the afternoon, extending southwestward into western NE and northeast CO by 00Z. Persistent southerly low-level winds across the Plains will maintain substantial boundary-layer moisture ahead of this front, contributing to strong instability during the afternoon. To the east, heights will gradually rise over the Northeast due to the departing trough, with only minuscule diurnal pressure falls expected over the Mid Atlantic where moisture and instability will linger, supporting isolated storms. ...Northern Plains... A subtle disturbance is forecast to move north/northeast across CO and WY toward the northern High Plains during the afternoon, with 30-40 kt midlevel wind speeds. Meanwhile, a surface trough will deepen over the western Dakotas and NE, with strengthening convergence. Forecast soundings reveal capping will be eroded fully by 21Z, at which time scattered storms are likely to form. Initial development is likely around the Black Hills, then southward across western NE and northeast CO. Temperatures aloft will be rather warm given the upper ridge over the Plains, but height falls will eventually occur over the northern Plains as convection flourishes. Cellular convection is expected to merge into a line or MCS, primarily with damaging winds. A QLCS tornado cannot be ruled out as the low-level jet increases after 00Z. ...Texas Coast... Tropical storm Hanna is forecast to be just off the Middle to Lower TX Coast by the end of the Day 1 period (Saturday morning). Wind fields will increase with the approach of this system, but associated convection/rain bands that are coincident with the stronger SRH are expected remain offshore. See the Day 2 outlook for further information regarding severe probabilities into Saturday. ...TN Valley into the Mid Atlantic... Pockets of moderate instability may develop this afternoon, aided by slightly cool temperatures related to a subtle upper trough. Storms will have little focus for development except for terrain effects and any remnant outflow boundaries from early day precipitation. Weak winds aloft will favor pulse convection capable of locally strong wind gusts. ...Southern MT... Strong heating will lead to perhaps 500 J/kg MUCAPE over central/southern MT, as 500 mb temperatures remain around -10 C. This may favor isolated thunderstorms forming over the mountains. However, subsidence will be an issue as winds aloft veer in response to the northern Plains trough, limiting storm coverage/strength. Height falls do occur overnight as the next upper trough approaches, but instability will be minimal. As such, severe probabilities now appear to be less than 5%. ..Jewell/Cook.. 07/24/2020Read more
from SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day1otlk_1200.html
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