Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC
Issued by NWS Raleigh/Durham, NC
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206 FXUS62 KRAH 181742 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 142 PM EDT Wed Sep 18 2024 .SYNOPSIS... A large upper level low over the western Carolinas will slowly drift east then northeast, tracking along the Mid Atlantic and Northeast coast through Friday. A trailing upper level trough will persist just off the East Coast through the weekend, as an upper level high pressure ridge builds over the Gulf States. This ridge will shift east into the Southeast states early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 930 AM Wednesday... Only minor tweaks to the forecast were needed with the mid-morning update. High temperatures where raised a degree or two across the Sandhills, southern/central Coastal Plain and into the Triangle given the quickly decaying low cloud layer and increasing sunshine. Additionally, water vapor imagery shows the core of the remnant mid- level vort max spinning over central SC is contributing to a streamer of very light showers that may leak up into the southern Piedmont and towards the Triad through this morning, but a weakening/drying trend is expected with northward extent as they become more displaced from the forcing mechanism. Previous forecast discussion as of 400 AM Wednesday... Water vapor imagery shows a decaying mid/upper low (the remnants of the coastal low that affected us with heavy rain on Monday) swirling over the western Carolinas. This low will slowly move east then northeast today and tonight, opening up into a trough as it does so, with the trough axis centered over central NC by early Thursday morning. The surface reflection is very weak at this point, but an occluded front still extends to its NE across northern NC. Anomalous low-level moisture along and north of this front is currently resulting in areas of fog, locally dense, across the northern Piedmont and northern Coastal Plain. Some clearing has occurred across the south, but guidance shows this may result in patchy fog there as well early this morning. Any fog will quickly dissipate after sunrise. As the mid/upper low moves overhead, it will bring mostly cloudy skies and provide weak forcing for ascent. When combined with increased moisture (PW values of 1.5 to 2 inches) and weak convergence along the occluded front, there will likely be some scattered showers around. Forcing looks too weak for anything above chance POPs at this time. LPMM on the HREF indicates some very isolated amounts of 1+ inches will be possible, but it looks too localized for any widespread flooding concerns. The best precipitation coverage will likely follow the front, initially in the north and west before shifting south and east in the evening. Instability of 500-1500 J/kg may also result in isolated storms, with the best chance in the SE where more clearing will take place. However, deep-layer flow will be quite minimal so any storms should be short-lived, and not expecting anything severe. A cool wedge of high pressure building down from the Northeast US will keep temperatures from getting above the mid-70s in the far north, while the SE should get into the lower-80s with more sun. Showers will dissipate on Wednesday night with loss of heating, but there will still be plenty of low-level moisture around. So widespread low stratus will redevelop, with maybe some patchy fog as well. This will keep low temperatures mild, in the mid-to-upper-60s (5-10 degrees above normal). && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 400 AM Wednesday... Mid/upper troughing will extend down the Eastern Seaboard on Thursday. Meanwhile a surface low will linger off the northern Mid- Atlantic coast. This will bring one last day of upper forcing for ascent and above-normal PW values. But with even weaker forcing compared to today, shower coverage should be even less and more focused in the east where the trough axis will be. There could be enough instability for a few storms, but again with such weak flow not expecting anything severe. Temperatures should be a bit cooler than today across the SE as there will be northerly flow around the aforementioned coastal low. Forecast highs are mid-70s to 80 with forecast lows Thursday night in the upper-50s to mid-60s. && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 230 AM Wednesday... This looks to be a largely dry period, with slightly below normal temps favored most days. The mid-upper level low over the Mid Atlantic coast early Fri will continue a drift to the NNE over the coastal Northeast into the weekend, as a trailing mid-upper trough along the East Coast shifts very slowly offshore. This will keep us in a dry NW steering flow through Sun as an anticyclone over SE TX builds slowly eastward into the Mid South and across the Gulf States. At the surface, corresponding low pressure over the far NW Atlantic off the Northeast and New England coast will periodically reload but maintain a slow E drift, as its trailing backdoor fronts extend SW then W through the Carolinas and Mid Atlantic region. Deterministic models vary with backdoor front placement and movement; the GFS and Canadian, for example, show a weak front to our S and W early Fri, before washing it out by Sat and allowing temps to moderate, only to have them knocked back down below normal by a second backdoor front passage Sun night and a cool wedging high nosing down through central NC through early next week. The ECMWF takes this second backdoor front southward through NC earlier (Sat night). Weak low level mass convergence along any backdoor front is possible, and despite generally low PW and a lack of moisture availability from 850 mb up through the mid levels, such convergence could prompt a few shallow showers. But overall, there`s a lack of opportunity for deep moisture return through at least Mon, so will carry nothing more than isolated pops at most, an outcome in line with the latest LREF and other ensemble suites. By Tue, there are indications that the mid level ridge over the Gulf States will shift E into the Southeast, nudged by deep mid-upper troughing over central Canada through the US Plains states, and this onset of WSW flow aloft could support rising chances for convection over the NC mountains into the foothills and perhaps our far NW Piedmont. This timing could shift, however, so will keep Tue pops aob climo for now. Will keep temps near to slightly below normal through Sun, before lowering them a few more degrees for Mon/Tue. -GIH && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... As of 140 PM Wednesday... Isolated to scattered airmass showers and widely scattered storms are possible in the vicinity of all TAF sites through the afternoon hours, lowest confidence at FAY. A northerly wind shift over southeast VA will slowly sag south this afternoon before collapsing through central NC this evening into the overnight hours. Along and behind the wind shift, cigs will quickly fall to IFR and remain there through the early morning hours Thurs. Landing minimums will be most probable at INT/GSO early Thurs morning, but confidence remains low at this time. A slightly higher chance for a shower will also accompany this wind shift as weak instability and minimal inhibition will continue through midnight. Cigs are expected to gradually rise to MVFR by early afternoon. Outlook: An unsettled period will continue through Thursday with areas of overnight and early morning stratus and a risk of some afternoon showers and possibly a storm. Aviation conditions will generally improve for the weekend. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Hartfield NEAR TERM...Swiggett/Danco SHORT TERM...Danco LONG TERM...Hartfield AVIATION...Swiggett/Luchetti