Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 211837 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 235 PM EDT Mon May 21 2018 .SYNOPSIS... A moist and slightly unstable air mass will remain entrenched across central NC through the middle of the week. && .NEAR TERM /THROUGH TONIGHT/... As of 1018 AM Monday... 14Z sfc analysis shows the sfc boundary has shifted south to approx the VA/NC border, or just south. It is along this boundary that we`re seeing a few showers, mainly across the NW Triad region and farther west over the foothills. The latest HRRR shows that with additional daytime heating, convective development along this boundary will become more numerous, particulary this afternoon, and across the northern half of our CWA invof the aforementioned boudnary which is progged to stall across the northern half of the state. Current forecast generally in good shape, so no signif upates this morning. Prev near term disc as of 210 AM Monday...Primary drivers of weather today include a weak backdoor surface front moving in from the north and minor mid level shear axes shifting in from the south. The latest surface map shows a fairly uniform air mass across much of the Carolinas and Southeast, albeit with lingering instability over the eastern CWA, and a weak backdoor front stretches W-E across central VA. This front is expected to drop southward into N NC this morning before washing out and ultimately dissipating as it starts to shift back N tonight. This front is more of a temporary wind shift, with minimal change in low level thicknesses, so impacts should be little more than weak low level mass convergence over the N Piedmont this afternoon. But this resultant forcing in tandem with afternoon heating and subsequent destabilization (tempered a bit by considerable mid and high clouds across the region) should allow scattered afternoon convection to be focused over the N Piedmont and N/C Coastal Plain starting in the mid afternoon, perhaps dropping into the Sandhills, S Coastal Plain and S Piedmont late afternoon into early evening, although this will depend to a large degree on cold pool propagation and a possible inland-moving sea breeze. While moderate CAPE is expected over the N and E CWA, other forcing mechanisms will be absent or very weak, including the aforementioned surface frontal convergence and subtle mid level waves tracking northward through the area between the offshore-centered mid level anticyclone and low pressure over the FL panhandle. Latest model runs are favoring pretty good convective coverage along the frontal zone, and will retain the trend up to likely pops across the N/E with lower coverage in the SW. Rather slow storm motion will raise the risk of locally higher rainfall totals. Storm intensity will be limited by the weak deep layer shear, although forecast soundings suggest elevated D-CAPE with potential for strong wind gusts, and high values of normalized CAPE favor high lightning activity. Expect highs in the low-mid 80s, with some upper 80s SW. Storms should decrease gradually in coverage and strength during the evening and into the overnight hours, although residual surface heat flux and high dewpoints necessitate keeping chance pops in through the night. Lows in the upper 60s to near 70 under mostly cloudy skies. -GIH && .SHORT TERM /Tuesday through Tuesday night/... As of 250 AM Monday... Today`s weak backdoor front should be mostly gone by Tue morning, leaving central NC within a light SW surface flow and light W mid level flow, as deep offshore ridging re-asserts its influence over the Southeast Coast. We will see passage of a mid level shortwave trough to our north Tue/Tue night, in conjunction with a surface trough crossing into the Mid Atlantic region Tue/Tue night NW to SE. Models depict a minimum in PW translating NE through the area during much of the day, and with a general lack in dynamic forcing for ascent and slightly lower forecast MUCAPE on the 3 km NAM with poor deep layer shear, storms may be fewer overall and focused across the far N and NW CWA, just ahead of the surface trough where CAPE will be greater (1500-2000 J/kg vs. the 500-1000 J/kg over much of the remaining CWA). Will have chance pops, highest N and W and lower SE, peaking in the afternoon. Pops should decrease in the evening and overnight, although will hold onto scattered showers over the NW through Tue night with an improving wind field resulting from digging of the mid level shortwave trough over the Northeast. Expect highs again in the low-mid 80s under partly to mostly cloudy skies. Lows Tue night in the upper 60s. -GIH && .LONG TERM /WEDNESDAY THROUGH MONDAY/... As of 235 PM Monday... The subtropical ridge off the Southeast coast will keep central North Carolina in a tropical airmass on Wednesday. A frontal zone dropping southward from the Mid Atlantic will enter northern parts of the region Wednesday night and become stationary close to the NC/SC border on Thursday. The tropical airmass will be supressed to the south of the boundary Thursday and Friday before returning northward into central North Carolina over the holiday weekend. In addition, a tropical low is forecast to move northward across Cuba and the Gulf of Mexico/Florida over the holiday weekend and early next week. Diurnal heating combined with the frontal boundary will generate scattered convection Wednesday afternoon and evening. Scattered c onvection Thursday and Friday should be limited to locations near the NC/SC border and western Piedmont. Convection will overspread the entire region over the 3-day holiday weekend with the return of a tropical airmass and close proximity of the tropical low. && .AVIATION /12Z MONDAY THROUGH FRIDAY/... As of 625 AM Monday... VFR conditions will dominate at central NC terminals through the next 24 hours, although MVFR to IFR conditions are expected through 13z this morning at FAY/RWI, with a lower chance at RDU. Considerable mid level cloudiness based above 8 000 ft AGL is expected today through tonight, with scattered to periodically broken clouds based at 3 500 to 6 000 ft AGL this afternoon. Light winds and high humidity early this morning will support stratocumulus cigs based at 400-1200 ft agl across the eastern sections in the hours around daybreak this morning. Scattered showers are possible near RDU/RWI 15z-18z, then scattered to numerous storms are possible 18z-00z later today at RDU/RWI, with a lower chance near FAY/INT/GSO. Shower/storm chances will decrease quickly after 00z. There is a chance of sub-VFR cigs once again 08z- 12z early Tue morning. Looking beyond 12z Tue, another round of scattered showers/storms with local sub-VFR conditions is expected 18z-00z Tue. This pattern is expected to repeat through the week, with storm chances from mid afternoon through mid evening, and low clouds or fog possible late each night through dawn. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS NEAR TERM...np/Hartfield SHORT TERM...Hartfield LONG TERM...Franklin AVIATION...Hartfield

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