Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 211903 RRA AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 203 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 1050 AM Wednesday... An unprecedented sub-tropical ridge, in terms of record heights and temperatures across much of the eastern U.S., will remain situated about halfway between the coast of the Carolinas and Bermuda today. In fact, to name a couple, 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 to June! 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 should translate to record surface temperatures well into the 70s, to lower 80s, across cntl NC this afternoon. A small chance of showers has also been introduced to the forecast, primarily over the srn Piedmont and Sandhills this afternoon- evening, in closest proximity to an instability axis characterized by 500-1000 J/kg of MLCAPE from cntl SC nwd across the NC Foothills and wrn Piedmont, per near term model guidance, including the HREF. About half of that CAPE will exist above the strong subsidence inversion observed near and just below 700 mb on proximity RAOBs 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. Any resultant convection will consequently be weak, and in the form of shallow showers. 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. && .SHORT TERM /THURSDAY AND THURSDAY NIGHT/... As of 315 AM Wednesday... On Thursday, the Bermuda high will start to sink southwestward towards the coast and at the same time, a second surface high will cross the northern Great Lakes. The frontal zone to our west will get elongated and cut off between the two high pressures. Short wave disturbances coming from the deep south will travel up the Appalachians and then make a hard right turn through the mid- Atlantic states. This will again keep us dry for much of the day but partly to mostly cloudy. Temps again well above normal with low 70s across the north to near 80 degrees in the south. As the day progresses, the northern high will move east towards NY and New England and move into a more favorable position for cold air damming but the disturbances in between the highs leave some question as to how far south the cold wedge will be able to progress overnight Thursday and into Friday. This will leave a tricky low temperature forecast for Thursday night with low 50s in the northeast and upper 50s in the southwest. && .LONG TERM /THURSDAY THROUGH TUESDAY/... As of 330 AM Wednesday... A backdoor cold front will retreat north as a warm front during the day Friday as high pressure off of the SE US coast strengthens and edges a bit further west. This will help to re-intensify southerly flow and warm air advection into the region. In all, only minor influence on temperatures is expected and mainly along and north of the Interstate 85 corridor. These areas may be limited to the mid to upper 60s during the afternoon while the rest of the area should warm nicely, with 70s to near 80 degrees will be possible. Some uncertainty here, as models continue to trend warmer, with some members keeping the boundary north of the VA/NC border altogether. If this scenario plays out, expect temperatures across the northern piedmont counties to be warmer than currently 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. && .AVIATION /18Z WEDNESDAY THROUGH TUESDAY/...
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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 NC 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.
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&& .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...Ellis LONG TERM...JJM AVIATION...MWS CLIMATE...RAH

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