Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 181525 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1125 AM EDT Fri Aug 18 2017 .SYNOPSIS... A weak surface trough will hold over eastern North Carolina today, while a second stronger trough sits over western North Carolina. An upper level disturbance and accompanying weak surface cold front will cross the region from west to east late this afternoon through tonight. The front will stall out over the coastal Carolinas on Saturday, as weak high pressure builds over the western and central Carolinas through the weekend. && .NEAR TERM /Today through tonight/... As of 1125 AM Friday... 1130 AM update... current forecast in good shape as skies morning stratus and fog has eroded and skies have largely cleared allowing lots of insolation. Temperatures have responded accordingly and now range in the mid to upper 80s in most locations, except across the far northeast Piedmont and Coastal Plain where clouds persisted the longest and the temperature recovery was delayed. Heat index values exceed 100 in many locations already and the heat advisory looks good. Cumulus is begining to develop in the across the northern and northwestern Piedmont. Still expect a few showers and storms to develop in this area and the Coastal Plain this afternoon before the main area of convection associated with the approaching front and upper trough approaches later this afternoon and evening. -Blaes Previous forecast... Latest model output and observed trends support increased odds of organized storms later today through this evening, particularly over the northern Piedmont. But confidence in convection coverage through the first half of the day is less certain. The current surface map shows a subtle boundary sitting along eastern NC just inland from the coast, and a second more distinct trough along the eastern edge of the Foothills. An area of drier air noted on water vapor imagery is currently moving across central NC, mainly along and south of highway 64, limiting clouds and any potential for elevated convection early this morning. But this will soon give way to increasing moisture and dynamic forcing for ascent as a shortwave trough extending from PA through E KY/E TN moves in from the west this afternoon. Models depict fairly early destabilization today given the rapid mixing with heating and very warm 925 mb temps. MUCAPE is expected to rise to 1500-2000 J/kg in the east by midday, increasing further to 2000-3000 J/kg late. Deep layer bulk shear, as was the case yesterday, remains fairly low at 15-20 kts, but is projected to rise slightly late today across the N and NW as the trough approaches. Areas of stratus and patchy fog this morning should mix out and disperse by late morning, and the heating along with high dewpoints, especially in the east, should prompt convection by midday. But given the dry air preceding the trough, the coverage may be limited and focused near the eastern trough where weak low level mass convergence will reside, and in far NE sections where moisture will be a little better. Following the latest HRRR runs, which depict this scenario fairly well, will have small chance pops over the far NE (VA border) and along the eastern Coastal Plain through early afternoon. Areas well east of I-95 will likely see a bit better coverage given the mid level lapse rates well above 6 C/km. Later today, with the arrival of the better moisture, steep lapse rates through a deep layer (7-7.5 C/km in the low levels and 6-6.5 C/km in mid levels), and dynamic forcing for ascent with both mid level DPVA and upper divergence, expect scattered to numerous showers and strong storms, with the greatest coverage across the northern Piedmont and northern Coastal Plain very late afternoon through evening. A few severe storms are possible, mainly north of Highway 64, if the deep layer shear can surpass 20-25 kts. Damaging winds would be the greatest threat, with a secondary hail risk. With strong heating and mixing in most places today, low level thicknesses 15-20 m above normal, 925 mb temps perhaps exceeding 27C, and dewpoints holding in the mid 70s if not higher, heat index values will likely approach or exceed 105F in many areas outside of the Triad and VA border counties. Will have a heat advisory in place for the Triangle region and points south and east today from noon to 6 pm. Expect highs of 91-96. The incoming front should jump into the Piedmont trough and progress east and southeast overnight, with storms pushing into the SE CWA and decreasing in coverage late. Expect lows from 71 NW to 76 SE. May see some light fog in the SE late. -GIH && .SHORT TERM /Saturday through Saturday night/... As of 345 AM Friday... Much quieter day with the surface frontal zone and mid level shear axis pushing to our SE and weak surface high pressure building in from the west. A drier column with warm and subsiding mid levels will result in mostly sunny skies NW and partly sunny SE, where enough low level moisture will remain to foster flat convective cumulus, topped with lingering mid level moisture streaming over eastern NC. Will retain low chance pops only in the SE sections, with dry weather elsewhere. Thicknesses dip slightly but remain near or just above normal, so expect highs of 89-94. Fair Sat night with no pops. Lows 69-73. -GIH && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 310 AM Friday... Sunday will begin with a frontal zone along the NC coastline and continental high pressure moving into the area from the northwest. This should give us very low precipitation chances for Sunday and keep flow very stagnant over the area with very little forcing aloft or at the surface. A similar pattern in place for Monday but the offshore surface high becomes a bit more dominant, meaning southerly return flow increases and potentially some more clouds and shower activity but lagging continental high to the north should keep us relatively dry and hopefully keep cloud cover suitable enough for eclipse viewing. The Piedmont Trough should be more active on Tuesday and coupled with a few shortwaves moving in from the southwest, will result in a slight increase in precip chances on Tuesday afternoon but still only lower end of the chance range. On Wednesday a stronger surface low in the northern stream will bring a cold front to central NC and with it, our best chances for rain for the week. This front will linger over the eastern part of the state through Thursday, keeping precip chances higher in the east. Expect temperatures in the low 90s through the period until Thursday when the front will drop temps back into the mid 80s. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... As of 630 AM Friday... VFR conditions are expected for much of today through tonight at most central NC terminals, with a few exceptions. At RWI, periods of IFR conditions, primarily IFR cigs with MVFR to IFR vsbys, are expected early this morning, through around 13z-14z, after which time these low cigs will quickly mix and disperse to VFR. Scattered storms are possible near FAY/RWI early today, starting as early as 15z with the potential lasting through the afternoon, although they will remain scattered and not a prevailing condition. More numerous showers and storms are expected starting after 19z at INT/GSO, spreading eastward to RDU then to RWI/FAY during the 21z-05z time frame. MVFR to IFR conditions with very heavy rain and strong gusty and erratic winds are possible with any of these showers and storms later today into early tonight. Any lingering showers will be diminishing soon after 06z as a front passes to our east and weak high pressure builds in from the west. Looking beyond 12z Sat, MVFR vsbys in fog are possible early Sat morning at RDU/RWI/FAY and early Sun morning at RWI/FAY. But otherwise, VFR conditions and dry weather are expected Sat through much of Mon with only a small chance of afternoon showers near FAY. The chance of late-day storms and early-morning fog returns late Mon into Tue, as the air mass over NC becomes increasingly hot, moist, and unstable. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... Heat Advisory from noon today to 6 PM EDT this evening for NCZ025>028-040>043-073>078-083>086-088-089. && $$ SYNOPSIS...Hartfield NEAR TERM...Blaes/Hartfield SHORT TERM...Hartfield LONG TERM...Ellis AVIATION...Hartfield is the U.S. government's official web portal to all federal, state and local government web resources and services.