Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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692 FXUS62 KRAH 201142 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 642 AM EST Mon Nov 20 2017 .SYNOPSIS... High pressure over the area today will shift offshore late tonight and into the day on Tuesday. A weak low pressure will develop off the Florida coast on Tuesday, and then track quickly northeastward along the Southeast coast Tuesday night. A cold front will move through the area on Wednesday. && .NEAR TERM /THROUGH TONIGHT/... As of 640 AM Wednesday... High pressure and a deep dry air column will dominate today with nary a cloud in the sky until the arrival of some thin cirrus clouds late this afternoon/early evening. It will be a chilly start this morning, with 12z low-level thicknesses averaging around 20 to 23 meters below normal, more typical of late December as oppose to mid/late November. Highs today a good 5 to 7 degrees below normal, ranging from mid 50s north to upper 50s/near 60 south. Aforementioned thin cirrus Monday evening will spread north and possibly thicken tonight within the increasing SWLY flow aloft, in response to a southern stream shortwave trough advancing east across the western and central GOM. Meanwhile, high pressure over the area, will quickly scoot east and offshore. The NAM is quicker with the onset of southerly low-level moisture return late tonight, advecting a stratus cloud deck into southern/southeastern portions of the forecast area during predawn hours. Lows in the lower to mid 30s. && .SHORT TERM /TUESDAY AND TUESDAY NIGHT/... As of 340 AM Wednesday... The aforementioned southern stream shortwave trough over the GOM will progress eastward across the Florida peninsula during the day on Tuesday, and then lift northeast along the SE Coast Tuesday evening/night. The associated weak spin-up of surface low pressure area over the eastern GOM will quickly track NE up the SE coast, remaining offshore. This wave will support yet another quick hitting- light precip event across the area late Tuesday afternoon and Tuesday night, with bulk of measurable precip expected to remain along and east of US 1, where low-level moisture transport will be strongest. Expect QPF amounts to range from a trace across the western Piedmont a tenth of an inch across the Coastal Plain counties. Under a thickening cloud cover, temperatures will range from lower to mid 50s across the NW Piedmont to lower/mid 60s SE. Much warmer overnight lows in the lower 40s NW to near 50 SE. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 400 AM Monday... Medium range guidance is in relatively good agreement with the planetary scale pattern, which is forecast to be a stable one consisting of a persistent mean ridge centered near Baja CA and another strong ridge extending from the w-cntl N. Atlantic nwwd through the Davis Strait and to the North Pole. Between these to ridges, a high-amplitude, longwave trough will become established from near Hudson Bay swwd into the e-cntl GOM mid-late week, and then reload this upcoming wknd. Numerous shortwave perturbations are forecast to round the wrn NOAM ridge and amplify into the base of the ern NOAm trough; and details regarding the timing and amplitude of these shortwave troughs --and their interaction with a persistent frontal zone draped from the ern GOM newd along and offshore the sern US coast-- arise in the model guidance with respect to the timing and wwd influence of at least a couple of episodes of cyclogenesis from the FL peninsula newd along the offshore frontal zone. Otherwise at the surface, a lead moisture-starved nrn stream cold front will collapse sewd, likely in backdoor fashion through the srn middle Atlantic states, late Wed-Wed night, with following cP high pressure poised to expand across most of the ern US through the end of the week. A reinforcing and similarly moisture-starved, nrn stream cold front is then forecast to sweep across NC Sat night-Sun. Sensible weather conditions are consequently forecast to be mainly dry, and mild in the upr 50s to lwr 60s Wed and Sat, ahead of the respective fronts, with cooler 50s, and some upr 40s on Thu, otherwise. At this time, any precipitation chances on the far nwrn edge of the aforementioned coastal cyclones are expected to remain confined to the far sern RAH CWFA, most likely on Thanksgiving Day, and perhaps again with the second low early this wknd. However, the wavelength of the larger scale pattern and associated breadth of the ern US trough would certainly support more amplification than currently indicated over the sern US, so there is room for a wwd jog in the low level frontal zone and associated surface waves, and wrn extent of precipitation into the Carolinas. && .AVIATION /12Z MONDAY THROUGH FRIDAY/... As of 640 AM Monday... VFR parameters across central NC will persist through Tuesday as an area of high pressure will be in control of our weather. Sfc winds tonight through Tuesday will be variable at less than 10 kts. MVFR ceilings are possible Tuesday afternoon through early Wednesday as an area of low pressure skirts northeastward up the Southeast U.S. coast. VFR parameters will return on Wednesday and should remain VFR through the majority of the upcoming week. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL NEAR TERM...CBL SHORT TERM...CBL LONG TERM...MWS AVIATION...WSS/CBL

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