Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 271936 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 336 PM EDT TUE SEP 27 2016 .SYNOPSIS...
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A quasi-stationary frontal zone will meander over the area through late in the work week, as a deep upper level low settles over the Tennessee Valley.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 330 PM Tuesday The near term is a little complicated and uncertain, though likely low impact. The synoptic cold front, which has essentially stalled, stretches from Birmingham through the NC Foothills into central VA. A pseudo warm front/differential heating boundary shows up across northern Piedmont of NC this afternoon, with this morning`s rain having reinforced the upper 60s to mid 80`s temperature gradient across the area. This boundary should mix north a little this afternoon, but fog seems likely to the north across the northern Piedmont and northern coastal plain tonight. Meanwhile, one line of shallow convection is progressing into the coastal plain, while another area of showers/storms has been getting going over upstate SC. Water vapor imagery shows a speed max crossing the mid-Mississippi River valley region and a buckle in the moisture channel over northern AL, both of which may help to maintain the the convection to our west into the Piedmont later this evening. As such, we will maintain 30-40 POPs well into the overnight period. Severe parameters don`t particularly line up well today, with localized 30-35kt effective bulk shear across the northern CWA in the vicinity of the front and better instability to the south. Lows will be in the mid to upper 60s.
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&& .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
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As of 330 PM Tuesday... Depending on the amount of ongoing showers early wednesday morning, Wednesday afternoon could be a rather active. As the stacked low over the Great Lakes digs south toward the TN Valley, increased forcing for accent and the stalled frontal zone over west-central NC will trigger convection, though the timing of greatest coverage is still a little uncertain given that the best height falls and cooling aloft won`t arrive until later Wednesday afternoon/evening, possibly toward the end of peak heating. Deep shear, while strongest to the northwest, will increase to 30-40kt, especially as the low-level flow backs in response to the approaching height falls, which may support supercell activity within a band of convection that appears favored from US-1 east, based on the available CAMs. All of central NC is in a Marginal Risk (per SPC) and this could be upgraded if instability appears to be sufficiently strong. The other concern for Wednesday evening is isolated flash flooding given system parallel southwesterly mean flow and PW near 2 inches. At the moment, Flash Flood Guidance for 3+ hours is well in excess of 2 inches. The GEFS and EC Ensemble means are roughly 1-2 inches for the central and northern Piedmont areas, so urban areas would likely run the greatest risk. However, confidence in the most favored area is not high enough to consider a watch at this time. Even after an evening round of convection "occurs" the ECMWF indicates another round of precip overnight, which isn`t out of the question given cyclonic flow aloft and no airmass change. it`s hard to put much detail into that portion of the forecast until the mesoscale aspects of Wednesday evening play out, but POPs will remain high overnight. Highs Wednesday should be in the mid 70s to lower 80s, with overnight lows in the mid and upper 70s.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/... As of 245 AM Tuesday... An upper level low over the northern Great Lakes will dig southward through the OH valley today/Wed, cut-off over the TN Valley Thu/Fri as an expansive upper level ridge builds over the eastern 2/3 of the CONUS, then lift northward back to the Great Lakes Sat/Sun as the ridge breaks down in response to potent shortwave energy digging southeast from the Pacific NW to the Intermountain West/4-corners region. What remains of the ridge will shift downstream of the OH/TN valleys over the Mid-Atlantic/Carolinas early next week as additional shortwave energy moving ashore the Pacific NW digs SE through the Intermountain West, further amplifying a deep upper level trough over the Rockies. With the above in mind, expect slightly below normal temps Thu/Fri and lingering precipitation in assoc/w the cut-off upper low possible on Thu. A dry mid-upper level airmass wrapping around the southern periphery of the upper low will likely preclude any potential for precipitation on Fri. Dry conditions and near normal temperatures are expected to prevail for the remainder of the long term period, Sat/Sun as the upper low lifts north back to the Great Lakes and Mon/Tue as the aforementioned ridge aloft shifts eastward from the OH/TN valleys into the Mid-Atlantic/Carolinas. -Vincent && .AVIATION /19Z TUESDAY THROUGH SUNDAY/... As of 210 PM Tuesday... Low level moisture in the vicinity of a front draped across western NC and central VA has maintained MVFR ceilings at INT and GSO, with VFR conditions having developed elsewhere. Satellite imagery suggests better potential for clearing at INT than at GSO in the next few hours, before ceilings redevelop overnight and possibly drop back down to IFR or LIFR. Low vsbys will also be possible mainly from KINT to KRWI northward, especially if clouds aloft thin out. Confidence in the nature of possible sub-VFR conditions is much lower at FAY. Otherwise, scattered showers or isolated storms are still possible this afternoon and evening, mainly at INT/GSO/FAY, though models trends have been showing less potential. Scattered showers are possible Wednesday morning, with numerous showers and storms Wednesday afternoon and evening. Looking beyond the 24 TAF Period: The combination of a quasi- stationary front across the Carolinas and a cut-off upper level low settling over the Tennessee Valley will keep unsettled weather in the form of scattered to numerous showers and storms through Thursday. Conditions are expected to improve Friday through the weekend as drier air finally spreads in from the west. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...SMITH NEAR TERM...SMITH SHORT TERM...SMITH LONG TERM...VINCENT AVIATION...SMITH

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