Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 230212 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1012 PM EDT Sat Sep 22 2018 .SYNOPSIS... A backdoor cold front will sag slowly southward through the area late tonight through Sunday. Cool high pressure will build in behind the front and will linger across central North Carolina through early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 1012 PM Saturday... Good chance of thunderstorms with heavy rainfall across the Northern Piedmont near the Virginia border... Clusters of thunderstorms have been affecting only the far northern Piedmont from Roxboro east to Louisburg and Warrenton. The thunderstorms have been even more numerous over southside Virginia in the vicinity of Martinsville and Danville, where flash flooding has been reported and Warnings are currently in effect. The cold front has been pushing south through Virginia, aided by the thunderstorm outflows coming south out ahead of the front. Since the thunderstorms have been generating on the north side of the boundaries, the storms have been slow to reach the northside of the Triad or Triangle areas. With the outflows getting closer and the front also nearing from the north, the chance of showers and thunderstorms will be confinded to the areas mainly along and north of Winston-Salem to Greensboro to Raleigh and Rocky Mount through midnight, then the chance gradually coming south as the clusters weaken with time. Until then, the Flash flood guidance continues rather low - with around 2.5 or so as a general threshold in 3 hours or less across the northern Piedmont. We will have to monitor these areas closely if merging and/or training of the thunderstorms develop. The wind shift with the front will arrive later tonight. The CAA will be slow to arrive, with areas mainly along and north of I-40 expecting to see 60s later tonight. Expect cloudiness to spread south as well and this will keep temperatures up overnight to the south of the front. Lows near 70 around and south of Fayetteville and Troy. && .SHORT TERM /SUNDAY AND SUNDAY NIGHT/... As of 350 PM Saturday... A cold air damming event will begin Sun. The parent surface high to our north will settle over NH/VT as it helps push the frontal zone further south into the CWA Sun and Sun night. Overrunning flow will abate somewhat during the morning, but considerable low level moisture will persist over the Piedmont. Despite the lack of instability or DPVA, minor waves of upper divergence will pass by W and N sections of NC during the day, and this combined with renewed moist isentropic upglide late will prompt the need for a chance of light rain or drizzle across the N and W CWA, within the CAD wedge. As is usually the case during such events, there will be a tight gradient of temps and clouds, and as a result the forecast confidence in both elements close to the frontal zone is low. Expect highs to range from the upper 60s NW to the low-mid 80s SE. Lows should be mostly in the low to mid 60s with skies becoming cloudy areawide overnight as the front settles along S and E edges of the forecast area. -GIH && .LONG TERM /MONDAY THROUGH SATURDAY/... As of 345 PM Saturday... Monday morning a surface cold front will be stalled across our central zones with a CAD event firmly in place. During the day Monday more clouds are forecast with highs only around 70 degrees towards the Triad and highs in the mid 80s near the coastal plain. One noticeable difference in forecast soundings for Monday is the presence of dry air around 750 MB with the overrunning signal not near as bullish as it was yesterday. To be more precise, the upglide signal is there (as seen on 300 K and 295 K surfaces), but the moisture is now lacking (as mentioned above). Condensation pressure deficits also reflect this change as noted above in the GFS forecast soundings. Tuesday morning, the mid level ridge will anchor off the coast of North Carolina with a low level disturbance pulling northeast around the ridge. This disturbance will help to finally break the CAD Tuesday afternoon. Late Tuesday into Wednesday the upper level trough axis will be centered over the central plains with North Carolina in between the trough and ridge axis. Multiple rounds of PV will move overhead across VA during the day with PWATs forecast to be around 1.80" (90th percentile for this time of year is 1.60"). Due to the more than sufficient lift and moisture, the chance of precipitation may have to be raised in subsequent forecast packages. Late Thursday an upper level disturbance over Ontario will rotate through the main upper level trough axis with a surface cold front pushing into the western zones. The chance of showers and thunderstorms looks pretty good with this front as a trailing jet streak will provide lift in the form of a RRQ. The upper level disturbance will then pull northeast with a secondary shortwave diving southeast into Manitoba. This shortwave will provide a reinforcing cold front late Friday into Saturday morning. It should be noted though that confidence is low on frontal timing in the extended as an upper level low is forecast to break off over the Pacific as the longwave trough axis begins to modulate in amplitude. Model trends have been for a less amplitude longwave trough and a stronger ridge across the southern United States. This trend would favor a warmer forecast with cold fronts potentially not making it as far south. && .AVIATION /00Z SUNDAY THROUGH FRIDAY/... As of 738 PM Saturday... 24 hour TAF period: Scattered showers/storms have developed along a southward-moving backdoor cold front. As the effective front dips south of the border and convection outflow progresses even farther south, IFR cigs will develop and spread southward through tonight at INT/GSO/RDU/RWI, with a trend to MVFR cigs after 05z at FAY. Cigs will very slowly rise from IFR to MVFR at INT/GSO/RDU during the late morning and early afternoon Sunday, with mostly VFR cigs at RWI/FAY. Scattered showers/storms are possible near INT/GSO/RDU/RWI 23z-06z, followed by dissipation overnight. Redevelopment of a few showers/storms is possible at all sites Sunday aft/eve. Surface winds will be mostly from the E or NE, gusting periodically at INT/GSO/RDU Sunday morning. MVFR cigs are expected at all sites except FAY (VFR) through 21z. -KC/GIH Looking ahead: Conditions areawide should drop to IFR Sun evening and persist through daybreak Mon as the backdoor front settles to our S, with patchy drizzle. Slow improvement to VFR is expected at all but INT/GSO Mon, where no better than MVFR is expected through the day. A return to IFR is expected Mon night areawide. Tue/Wed will see sub-VFR conditions each night/morning with improvement to VFR each afternoon, along with scattered showers and storms. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Badgett NEAR TERM...Badgett/Hartfield SHORT TERM...Hartfield LONG TERM...Haines AVIATION...KC/Hartfield

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