Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 270830 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 430 AM EDT Thu Apr 27 2017 .SYNOPSIS... A cold front will approach the mountains from the west late this afternoon and evening, then stall out and dissipate over the Mid- Atlantic tonight. A moist southerly return flow on the western periphery of a strengthening Bermuda high will prevail Friday through the weekend. && .NEAR TERM /Today and Tonight/...
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As of 345 AM EDT Thursday... A strong subsidence inversion /cap/ and dry mid-level airmass in place over the region this morning will persist through the majority of the afternoon. A squall line associated with a deamplifying shortwave lifting NE through the TN/OH valley (and an attendant cold front) is expected to track eastward from central TN into the southern Appalachians by early afternoon. Forcing assoc/w the aforementioned wave will weaken at this latitude as it lifts NE through the OH valley this afternoon and the thermodynamic environment becomes increasingly hostile (dry/capped) east of the mountains (in closer proximity to the upper level ridge building along/offshore the SE coast). As such, one would expect the squall line to weaken and gradually dissipate as it progresses east of the mountains late this afternoon. However, mesoanalysis data this morning shows a S/SW LLJ advecting rich low-level (925-850mb) moisture poleward from the GOMEX into the Deep South, directly in advance of the squall line. This plume of low-level moisture will advect northward into the Carolinas immediately in advance of the approaching squall line late this afternoon and evening, beneath steep mid-level lapse rates attendant a modified elevated mixed layer over the region (see 00Z GSO RAOB). With rapid destabilization expected immediately in advance of the approaching squall line, probs are increasing that the MCS will be able to maintain itself as it progresses east of the mountains during the late aft/eve. The bulk of 00Z guidance indicates as much, showing the line progressing through central NC in the 21-04Z time frame. Expect highs in the upper 70s (west) to mid 80s (east). Temperatures may fall 10-15F as outflow attendant the squall line progresses east of the I-77 corridor into the western Piedmont during the late afternoon (21-22Z). With increasing low-level moisture, expect overnight lows in the mid/upper 60s. -Vincent
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&& .SHORT TERM /Friday and Friday Night/...
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As of 400 AM EDT Thursday... Moderate to extreme destabilization (~3000 J/Kg MLCAPE) is expected during the day Friday in assoc/w strong insolation and seasonable low-level moisture (Td lower 60s) beneath steep mid-level lapse rates /a modified elevated mixed layer/. Despite an anomalously favorable thermodynamic environment, weak forcing and a strong ~800 mb cap may preclude convective development over central NC. Highs will range from the mid 80s (NW) to ~90F (SE). As such, a gradient in CINH (highest west / lowest east) will exist across the state. Unless a well-timed shortwave/MCV in SW flow aloft progresses over western NC or upstream convection propagates downstream into western NC (neither appear likely at this time), dry conditions will prevail. In eastern NC where afternoon temps will reach ~90F and CINH erosion will be maximized, isolated convection cannot be entirely ruled out if sufficient convergence is present (e.g. a robust seabreeze penetrates inland). If the cap breaks and deep convection develops (e.g. updrafts survive entrainment) in the SE Coastal Plain 21-00Z, thermodynamic/kinematic profiles would support robust supercells capable of producing destructive hail/microbursts. If the cap holds, dry conditions will prevail through tonight. Expect lows in the mid 60s to lower 70s. -Vincent
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&& .LONG TERM /SATURDAY THROUGH WEDNESDAY/... As of 345 AM EDT Thursday... The Bermuda high`s influence will be strongest over the area on Saturday with low-level thicknesses progged to peak between 1420m(40m above normal) with H8 temps warming to around 18C. Highs Saturday in the upper 80s to lower 90s on Saturday, when we could challenge the record high max of 91 at RDU. Heights are forecast to lower slightly on Sunday, with highs Sunday similar to Friday`s temps. Rain/convective chances through the weekend will be limited by the dominate ridge with best chance across western NC, where convection is most apt to fire where owing to differential terrain heating. Will retain a slight/small chance pops across the western Piedmont. If storms do develop or propagate east into the area on Saturday, strong instability ~3000 J/Kg aided by steep lapse rates aloft associated with a modified EML, coupled with deep layer shear of ~25kts could support some vigorous updrafts capable of producing large hail and damaging thunderstorm winds. Models have slowed down with the timing of the front on Monday, delaying fropa until after 00z Tuesday. This unfavorable nocturnal timing will not bode well for severe potential, with models suggesting only weak instability across central NC Monday afternoon/evening. When coupled with the better trough dynamics lifting off to the north, the potential for severe storms is low at this time. Cooler/near normal temps will follow Tuesday and Wednesday in the wake of the cold frontal passage Monday night. Late next week, medium range models are actually in good agreement in showing the development of a deep trough east of the MS, with unsettled/active weather expected to accompany this dynamic system late Wednesday night through Friday as it progresses east into the region. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... As of 145 AM EDT Thursday... 24 Hour TAF Period: VFR conditions associated with a dry/capped airmass will prevail through mid to late afternoon. Scattered convection (remnants of a squall line expected to cross the mountains this afternoon) will be possible 20-00Z at INT/GSO terminals and 00-04Z at eastern terminals, with the relative best chance at INT/GSO terminals given an increasingly dry/hostile airmass with eastern extent. SW/WSW wind gusts as high as 20G35KT will be possible in assoc/w any convection. Otherwise, calm or light SSW winds early this morning will increase to 10-15 knots a few hours after sunrise, with gusts as high as 20-25 knots this afternoon (after ~15Z), highest at the INT/GSO terminals. Outlook: Though VFR conditions will prevail through the majority of the upcoming weekend, adverse aviation conditions are periodically expected. Examples include IFR/LIFR ceilings expected to develop each morning Sat/Sun/Mon between 08-12Z, lifting/scattering out to VFR by 12-15Z. Additionally, isolated to scattered afternoon/evening (21-03Z) convection will be possible Fri/Sat. Should convection develop and/or propagate into the region Fri/Sat, environmental conditions would strongly support severe weather. Otherwise, expect the best overall potential for convection Monday evening/night in assoc/w a cold front /squall line/ progged to track eastward through the Carolinas. -Vincent && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Vincent NEAR TERM...Vincent SHORT TERM...Vincent LONG TERM...CBL AVIATION...Vincent

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