Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 161346 AFDRAH AREA FORECAST DISCUSSION National Weather Service Raleigh NC 945 AM EDT Tue Jul 16 2019 .SYNOPSIS... A hot, sub-tropical ridge aloft will remain over the southeastern US through Wed. A trough, including the significantly weakened remnants of Barry, will drift across the central Appalachians and southern middle Atlantic states Wed night through Thu night. The sub-tropical ridge will then redevelop northward across the central Appalachians and Virgnias through the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 945 AM Tuesday... Only minor adjustments to temps, cloud cover, and pop timing with this iteration. High thin clouds over the area have thinned further this morning and should allow for plenty of insolation today. CINH persists but is quickly dwindling, and with elevated MLCAPE already exceeding 1000 J/kg over the E half, we should start developing surface-rooted convective towers prior to noon, particularly in the E where both PW and dewpoints are a bit higher. With no obvious focus for convective initiation, we`ll need to closely watch for subtle boundaries and perhaps old outflows that may only reveal themselves with uniform mixing over the next couple of hours. Given this, it`s tough to determine exactly where the best storm chances will be, but have maintained a bit better chance E than W considering the greater moisture depth there. Still expect temps to peak in the mid-upper 90s, as observed 850 mb temps are well above normal, in the 90th percentile, at GSO (~19C). Peak heat indices will be firmly in the 102-105F range in the E, a bit lower W, so no changes at this time to the heat advisory. -GIH Previous discussion from 345 AM: Another Heat Advisory has been issued for the NC Coastal Plain and ern Sandhills for this afternoon. Note that in addition to the Eastern Region Directive for Heat Advisory criteria of 105 degrees for two consecutive hours or more, WFO RAH`s policy is also for three or more consecutive days of 102 to 105 degrees. Today/Tuesday will mark the sixth day of (at least) that criteria in the Advisory area. The forecast will be generally a persistence one, with perhaps a slight increase in coverage of diurnal, pulse convection through this evening. A subtropical ridge --a strong one at 594 dm and 324 dm at 500 mb and 700 mb, respectively-- will remain situated over the sern US through tonight. A mid level shear axis that was draped across SC and swrn NC during the past 36 hours left behind a relative maximum in moisture around 700 mb; and this moist axis will drift around the nrn periphery of the foregoing subtropical ridge and across cntl NC and VA through this evening. At the surface, a lee trough will sharpen over the Piedmont with diurnal heating, while a another weak convergence axis/wind shift may linger over nern NC/sern VA, in the vicinity of the weak frontal zone that settled into that region early Monday, and since weakened. A continued slight uptick in low level moisture/surface dewpoints will occur today, owing to a more-sly/onshore component to the low lvl flow across the Carolinas. With generally persistence high temperatures, that uptick in low level moisture will favor higher heat index values (than previous days) throughout cntl NC, with near 100 degree readings creeping wwd across all but the nrn and nw Piedmont, and with another day of 102 to 107 degrees across the Advisory area. That uptick in low lvl moisture, and the ewd drift of the aforementioned mid level moist axis, will favor the development of isolated to locally scattered pulse convection this afternoon, some of which may linger over mainly the ne Piedmont and nrn Coastal Plain through midnight. The presence of high DCAPE values, to as much as 1200-1400 J/kg, may support a couple of episodes of severe wind gusts. Persistence lows in the 70s, coolest where earlier convection occurs. && .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... As of 410 AM Tuesday... A mid and upr low over MO/AR this morning, and which includes the remnants of Barry, will devolve into an open wave, positively-tilted trough through early tonight, then drift ewd across the nrn middle Atlantic and cntl Appalachians Wed through early Thu. Weak mid level height falls of 10 to perhaps 20 meters, and the right entrance region of a related weak swly upr jet streak, will consequently overspread cntl NC late Wed afternoon and Wed night. At the surface, the Appalachian-lee trough will again sharpen over the Piedmont, with upr 60s to around 70 degrees surface dewpoints to the west of that trough, and lwr to middle 70s ones to the east. Low lvl thickness values are forecast to increase by about 5 meters over those of Mon and forecast for Tue; and this should translate to a degree or two hotter temperatures. When combined with slightly higher surface dewpoints than those forecast for Tue, heat index values will likely near 105 degrees over all but the nrn and nwrn NC Piedmont, with 105-109 degree values common in the Coastal Plain and ern Sandhills. Consequently, the Heat Advisory area will likely be expanded wwd to include at least another tier of counties in cntl NC for Wed. Scattered showers and storms will again loosely focus in the lee/Piedmont trough, and the sea breeze, with subsequent development/outflow propagation throughout cntl NC through Wed evening. Slightly greater coverage may occur over the nwrn and nrn Piedmont after 6 PM, then continue through early Wed night, owing to the best overlap there of the incoming weak forcing for ascent accompanying the shortwave trough aloft with maximum diurnal heating. The influence of the shortwave trough and jet streak will also yield effective bulk shear values in the 20-30 kt range, which may provide for multi-cell storm and cold pool organization, and strong to marginally severe wind gusts. Generally persistence lows in the 70s. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 100 AM Tuesday... Strong mid and upper level ridging will follow the weak, remnant trough that was Barry late in the week through the weekend. Hot temperatures will prevail Thursday through Sunday, with some cooling with increased thunderstorm and associated cloudiness by Sunday and Monday. As the weak mid/upper trough (Barry remnants) pulls out to the NE and off the northern mid-Atlantic coast/New England Thursday, the heights again rise aloft Thu-Sat leading to a large upper ridge over the region again. This will leave only a weak surface trough, strong surface heating, and daily intense instability to focus isolated to widely scattered PM thunderstorms. Daily highs should reach the mid to upper 90s, with lows in the 70s. Heat indices of 105-108 will be likely into the weekend. The chances of much needed showers and thunderstorms are expected to increase by Sunday into early next week as a pattern change to less ridging and more troughing over the region occurs. A cold front or two should stall over the region during this time frame acting as a focus for convection. && .AVIATION /11Z TUESDAY THROUGH SATURDAY/... As of 655 AM Tuesday... Mainly VFR conditions are expected through the TAF period. However, widely scattered showers and storms are expected to develop with diurnal heating this afternoon, but given a lack of focusing mechanisms, such development is apt to be random and with a probability of occurrence at any given point only around 20 or so percent. Outlook: Increasing moisture ahead of the trough aloft and related remnants of Barry will provide a better chance of mainly diurnal (afternoon-evening) convection Wed-Fri, and also patches of early morning stratus. && .RAH WATCHES/WARNINGS/ADVISORIES... Heat Advisory from noon today to 7 PM EDT this evening for NCZ011- 027-028-042-043-078-088-089. && $$ SYNOPSIS...MWS NEAR TERM...Hartfield/MWS SHORT TERM...MWS LONG TERM...Badgett AVIATION...MWS

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