Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 212103 RRA AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 400 PM EST Wed Feb 21 2018 .SYNOPSIS... Anomalously strong sub-tropical high pressure off the East Coast will remain through tonight, then gradually weaken and drift southward through the end of the week. Meanwhile, a backdoor cold front will settle slowly south and into northern NC by early Fri. && .NEAR TERM /THROUGH TONIGHT/... As of 330 PM Wednesday... An unprecedented sub-tropical ridge, in terms of record heights and temperatures across much of the eastern U.S., will remain similarly strong and situated about halfway between the coast of the Carolinas and Bermuda for the next 12-24 hours. In fact, the observed 500 mb heights of 591 and 595 dm at GSO and MHX at 12Z Feb 21, respectively, are the highest on record on any day, any year, from roughly October 1 to June 1! Moreover, the strength of the ridge would be near record heights even during the peak of the warm season in July and August - truly remarkable. Not surprisingly, the 850 mb temperature at both NC upr air sites is also a daily maximum; and this has translated to record temperatures at all three cntl NC climate sites this afternoon (77 at GSO, 77 at RDU and 80 at FAY). In the past few hours, showers have indeed percolated over SC and mainly wrn NC; and a few of these may drift into the srn/wrn Piedmont and wrn Sandhills this afternoon-evening, in closest proximity to an instability axis characterized by around 500 J/kg of MLCAPE from cntl SC nwd across wrn NC. About half of that CAPE will exist above a strong subsidence inversion observed near and just below 700 mb on proximity RAOBs from this morning, however, per modified bufr forecast soundings across the region. And as such, (EL) equilibrium levels and level of maximum ascent (LMA) heights will likely exist between 9 and 12 k ft, respectively, with parcels consequently failing to breach the cap and realize the second LFC and associated instability above. Convection will consequently remain weak and devoid of lightning. Given little change in synoptic pattern, with the sub-tropical ridge and associated "Bermuda" surface high holding firm, a repeat of late night low overcast and areas of fog is anticipated, with unseasonably mild and muggy lows in the lwr to mid 60s. Like this past morning, some fog may become dense, particularly east of US Highway 1, as indicated by both HRRR and 15Z SREF guidance. && .SHORT TERM /THURSDAY AND THURSDAY NIGHT/... As of 400 PM Wednesday... A persistence forecast will be hard to beat for Thu, given only a slow weakening and swd retreat of the anomalous sub-tropical ridge off the sern U.S. coast. That is, low overcast and fog will gradually lift and morph into a mainly broken stratocumulus field through midday-early afternoon. The clouds will remain shallow and capped and unsupportive of precipitation, however, owing to a continued strong (and lowering per bufr forecast soundings) subsidence inversion related to the sub-tropical ridge. High temperatures should consequently again climb to record or nearly so values, in the middle 70s to around 80 degrees. A cold front, analyzed at 21Z from ern PA and NY swwd through cntl TN and the lwr MS Valley ,will have moved to near the Mason Dixon line by Thu morning; and this boundary is forecast to make slow swd progress across VA throughout the day. The NAM and GFS are very similar in their depiction of mass fields, but the NAM becomes most aggressive in driving the front into cntl NC Thu night. The NAM is preferred given superior representation of terrain and vertical model resolution in the low levels, which should best capture the associated shallow, post-frontal airmass. As such, the front is forecast to drift to near or just north of US Highway 64 by Fri morning, with continued mild upr 50s to lwr 60s and redevelopment of fog and low stratus ahead of it, and 50s and a small chance of post- frontal rain/showers behind it. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... As of 230 PM Wednesday... At this time, current thinking is the cool wedge will settle as far south as US-64 Friday morning and gradually move northward Friday afternoon and evening. With that in mind, temperatures will remain in the 60s to the north of the boundary. In fact, readings near the NC/VA border may not get far above 60F. To the south of the wedge, maximum readings will climb to around 80F as high pressure off of the SE US coast strengthens and edges a bit further west. With such a tight temperature gradient on Friday, confidence is rather low. If the boundary does not move south into central NC on Friday, then temperatures in the northern Piedmont will be much warmer than forecast. A more unsettled pattern returns by the weekend, as a surface low is expected to strengthen across the central Great Plains by Saturday and eject northeast through the Great Lakes Region through Sunday. This will push an associated cold front through central North Carolina Sunday and Sunday night before stalling it along the coast Monday and Tuesday. This will reintroduce showers and possibly a few thunderstorms as early as Saturday afternoon in the form of pre- frontal showers, with the best forcing arriving Sunday afternoon and evening as the front progresses through. Another wave will intensify along the front Monday into early Tuesday before a cooler area of high pressure takes over, keeping conditions a bit more seasonable. Wednesday starts dry but rain showers may move into western sections by late in the day with warm air advection ahead of the next system. && .AVIATION /18Z WEDNESDAY THROUGH TUESDAY/... As of 150 PM Wednesday... Amidst the influence of an unprecedented sub-tropical ridge aloft off the sern US coast, and an accompanying "Bermuda" surface high, unseasonably mild and humid conditions will persist over much of our region during the next several days. Given little change in the overall weather pattern through Thu, a persistence forecast will be followed, with scattered to broken high MVFR-low VFR cumulus cloud bases this afternoon expected to be replaced by an expanding area of LIFR overcast and LIFR-MVFR visibility restrictions in fog late tonight-early Thu. Like today, the early day low stratus and fog will lift through MVFR range while morphing into a cumuliform cloud layer through 17-18Z, before lifting and/or scattering further to VFR during the afternoon. lastly, the relative warmth and humidity may support the development of a few showers over the wrn Carolinas this afternoon; and a couple may may skirt the GSO and INT vicinity this evening, though the probability of occurrence at either site is too low to include in the TAFs. Outlook: A persistence forecast of late night-morning stratus and fog, lifting and scattering to high MVFR-low VFR each afternoon, will generally continue through the weekend. However, a backdoor cold front will settle into at least nrn NC on Fri, roughly along or just north of US Highway 64; and this boundary will result in a more prolonged period of IFR-MVFR ceilings along and north of it through most of the day Fri. Otherwise, the next chance of rain will accompany a frontal zone that will settle into and possibly stall over the Carolinas late Sun into early next week. && .CLIMATE... RDU Records: Date | High Year | High Min Year ------------------------------------------ 02/21 76 2011 55 1939 02/22 75 1897 60 1897 02/23 79 1980 57 1962 02/24 81 1982 58 1985 GSO Records: Date | High Year | High Min Year ------------------------------------------ 02/21 74 2011 50 1954 02/22 74 1925 57 1980 02/23 74 2017 52 1922 02/24 79 1982 55 1985 FAY Records: Date | High Year | High Min Year ------------------------------------------- 02/21 80 1991 61 1953 02/22 77 2003 56 1989 02/23 80 1922 55 1922 02/24 83 1930 60 1975 && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...MWS LONG TERM...JJM/Franklin AVIATION...MWS CLIMATE...RAH is the U.S. government's official web portal to all federal, state and local government web resources and services.