Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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964 FXUS62 KRAH 211035 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 535 AM EST Tue Nov 21 2017 .SYNOPSIS...
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High pressure will drift offshore today. Meanwhile a weak area of low pressure will develop off the Florida coast, and then track quickly northeastward along the Southeast coast tonight. A cold front will move through the area on Wednesday.
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&& .NEAR TERM /THROUGH TONIGHT/... As of 340 AM Tuesday... Surface high over the area this morning will shift offshore, in advance of a northern stream trough and attendant cold front progressing SE into the Ohio and Tn Valley. Meanwhile, the southern stream shortwave trough over the central GOM this morning will lift northeast across the Florida peninsula today and then up along the SE Coast late this evening and tonight. Associated weak spin-up of a low pressure area will move NE up the SE coast overnight, remaining well offshore. Brief surge of low-level moisture transport in advance of this southern stream wave, coupled with strengthening ascent in the jet entrance region, will yield yet another quick hitting and light precip event across central NC this evening and tonight. Bulk of measurable precip is expected to remain along and east of US 1, where low-level moisture flux will be strongest, with QPF amounts ranging from a trace across the western Piedmont to a tenth of an inch across the Coastal Plain counties. Although modulated by a thickening cloud cover, the southerly return flow will support warmer temperatures than yesterday with afternoon highs ranging from mid/upper 50s across the NW Piedmont to lower/mid 60s SE. Much warmer overnight lows in the lower 40s NW to near 50 SE. && .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... As of 350 AM Tuesday... Aloft, a lead shortwave trough now over the cntl GOM will be lifting newd through ern NC and VA Wed morning, while another pair of shortwave troughs now amplifying sewd across the nrn Plains and Intermountain West, respectively, form and cause a positively-tilted trough to edge ewd from the middle Atlantic states swwd to the cntl GOM. At the surface, a cold front now stretching from s-cntl Ontario swwd through the upr Great Lakes, then wwd into across the cntl Plains, will progress sewd and lie invof the cntl Appalachians by 12Z Wed, where it will be slowed and subsequently arc swd in backdoor fashion through the srn middle Atlantic states late Wed and Wed night. After a lingering (slight) chance of rain Wed morning over the nrn Coastal Plain, in association with the lead shortwave trough forecast to lift newd from the GOM, wnwly deep layer flow will favor drying/clearing Wed. It will otherwise remain mild ahead of the aforementioned cold front, with high temperatures exactly where they should be relative to the 30-year avg - upr 50s to around 60 degrees over the nrn Piedmont, and 60s elsewhere. Lows should be in the 30s, coldest north, where relative calm and clear will support good radiational cooling conditions. Increasing mid-high level moisture, downstream of the next shortwave trough lifting newd from the GOM, will meanwhile stream into srn and sern NC and favor lows in the upper 30s to near 40 degrees there. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 400 AM Tuesday... Little change needed to the long term forecast. Medium range guidance remains 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, the latter of which will cause a relatively strong negative NAO pattern to develop. Between these two ridges, and locked in place by the negative NAO, 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 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-- remain 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, cP high pressure will expand across the middle Atlantic states and Carolinas through the end of the week. A reinforcing and moisture-starved, nrn stream cold front is then forecast to sweep across NC Sat night-Sun, with another cP high to follow for Sun-Mon. Sensible weather conditions are consequently forecast to be mainly dry, with generally below avg temperatures except for in a briefly mild, pre-frontal airmass on Sat. As indicated last night, the wavelength of the larger scale pattern and associated breadth of the ern US trough would allow for one or more of the aforementioned shortwave troughs to amplify strongly and perhaps briefly close off from the sern US to the GOM, which would in turn favor a wwd jog in the low level frontal zone and associated surface waves, and wrn extent of precipitation into the Carolinas, most likely on Thu and Sat. Indeed, the 00Z/21st guidance has trended in that direction for Thanksgiving Day; and if these trends continue, current slight chances of precipitation will need to be increased for the srn and cntl Coastal Plain on Thu, with continued dry conditions elsewhere. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... As of 135 AM Tuesday... Sfc high pressure over the area this morning will drift eastward and offshore throughout the day. Low-level dry air associated with this high will support a continuation of VFR conditions through at least 18z, with the resultant SELY return flow leading to gradual moistening and broken/overcast ceilings between 3 to 5kt during the mid to late afternoon. Strengthening low-level moisture advection in advance of a sfc low that will move northeast off the SE Coast overnight, is expected to lead to deteriorating aviation conditions this evening and overnight, especially at the eastern terminals(KRWI and KFAY). Model guidance continues to exhibit higher than average spread with this coastal system, and thus forecaster confidence is lower than normal. Expect ceilings to lower to IFR to MVFR with patches of light rain/drizzle from RDU eastward. It`s possible that KINT and KGSO could also see a period of sub-VFR ceilings during the late afternoon/early evening, however confidence is too low to include in TAFS at this time. A cold front will cross central NC Wednesday. Cold dry air advection will quickly scour out any sub-VFR ceilings Wednesday morning. VFR parameters should prevail across central NC Wednesday afternoon through Saturday. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...CBL NEAR TERM...CBL SHORT TERM...MWS LONG TERM...MWS AVIATION...CBL is the U.S. government's official web portal to all federal, state and local government web resources and services.