Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 211838 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 235 PM EDT Sun May 21 2017 .SYNOPSIS... Weak high pressure will wedge south into most of central NC as the backdoor cold front temporarily stalls near the South Carolina border. The stalled front will lift back north-northeast as a warm front tonight, in advance of a cold front that will move into the region on Monday. && .NEAR TERM /THROUGH TONIGHT/... As of 1105 AM Sunday... Main adjustment to the near term forecast was to raise afternoon temperatures across sections of the central/northern coastal plain. Otherwise, few, if any, adjustments required to the temperatures or the PoPS. Sfc cold front has stalled near the South Carolina border at mid morning. This feature displayed quite well on the visible imagery. This boundary will serve as a focus for scattered convective development this afternoon into the evening hours. Cool and stable air mass has overspread most of the northern Piedmont. This air mass quite shallow as per 12Z soundings from GSO and MHX. Due to the shallow nature and potential for mixing of drier air aloft into the boundary layer, breaks in the overcast appear likely in the periphery of this airmass, primarily over the coastal plain counties. Thus, have raised afternoon temperatures a few degrees based in this expectation, and observed thinning of the low clouds per latest visible imagery. Highest PoPs (chance) will remain confined to our far southern and western sections as best sfc based instability will reside here (south) and where best low level moisture transport is focused (west). ~WSS A series of upper disturbances and strengthening southerly moist warm air advection within a 25kt LLJ spreading east into the area as a cold front approaches the NC mtns, will give rise to increasing shower chances after sunset and overnight, with a rumble of thunder or two as weak instability develops behind the northward lifting warm front. Lows Sunday night 65 to 70. ~CBL && .SHORT TERM /MONDAY AND MONDAY NIGHT/... As of 415 AM Sunday... Surface cold front over the mtns will inch slowly eastward Monday and Monday night as the amplifying flow aloft becomes increasingly parallel to the low-level boundary. This will set up multiple rounds of convection throughout the day/night, with the lead shower band(s) across the Piedmont Monday morning progressing eastward into the coastal plain during the mid to late morning hours. A second round of convection is expected to re-develop everywhere by the afternoon with daytime heating resulting in moderate instability(1000 to 2000 J/kg) across the area. Modest lapse rates of 5.5-6.0 C/km coupled with bulk shear of only 20 to 25 kts will limit severe threat. Highs ranging from the upper 70s/near 80 west to mid 80s southeast. What once look like a brief lull for Monday night/early Tuesday looks to be setting up for yet another round of showers and thunderstorms with the front hung up over the area as a series of upper disturbances eject NE across through the region, in advance of the digging upstream synoptic scale trough. Instability looks to be marginal, but models suggest increasing deep layer shear Monday night in response to a strengthening mid/upper level jet streak. Additionally, with the front stalled across the area the threat of heavy rain and flooding will begin to increase. Lows Monday night in the 60s. && .LONG TERM /TUESDAY THROUGH SATURDAY/...
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As of 235 PM EDT SUNDAY... Model consensus is a little better depicting a vigorous wave lifting out of the northcentral Gulf and riding deep southwest flow northeast into NC on Tuesday. Timing issues abound, however, as the GFS would like to lift the wave out a little further north and considerably (~9 hours) faster, which maintains a heavier rain axis west of the area, but cuts off the heavier rain abruptly Tuesday afternoon. Per its more consistency over the past couple of days, will maintain our ongoing ECMWF bias in the timing and QPF amounts, with categorical PoPs into Tuesday night and total rainfall amounts Tue-Thu in the 2.25-3 inch range. The bulk of the heavy rain (up to 2 inches) will fall Tuesday and Tuesday night and there will be a threat of flooding, particularly in our more urban areas, as the southwest flow and slow eastward translation of the system will encourage training convection. Strong low level convergence and low level jet of 40-50 Kts will be sufficient to produce strong storms, but severe potential will be diminished due to limited instability with the widespread antecedent cloudiness and weak mid level lapse rates...your basic high shear/low CAPE environment. Highs will range from ~70 northwest to mid and upper 70s in the coastal plain. Convection...particularly stronger convection...will diminish with loss of diurnal influence as the surface wave lifts north of the area early Tuesday night. However, southwest flow maintains relatively deep moisture transport overnight and through the day on Wednesday as a second cold front approaches from the west. As such, will maintain likely PoPs into Wednesday night, and we could see some pretty vigorous storms develop ahead of this second front on Wednesday afternoon as we will get some modest mid level cooling and drying behind the intial wave to steepen mid lapse rates. Will raise PoPs to the 60% range through the day, diminishing to a chance of showers Wednesday night into Thursday as we get dry-slotted by wrap- around from the cutoff low over the northern Ohio Valley. Highs Wednesday and Thursday will range from 75 to 80. Dryness returns at last on Friday and should persiste through the weekend. Initial northwest flow flattens through the weekend, and the cool airmass moderates from the 70s Friday to the low to mid 80s Saturday and Sunday.
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&& .AVIATION /18Z SUNDAY THROUGH FRIDAY/... As of 155 PM Sunday... There is a high likelihood of adverse aviation conditions across central NC through Tuesday as a series of upper level disturbances crosses the moist air mass in place across central NC. Each disturbance will kick-off scattered showers and thunderstorms with associated MVFR/IFR ceilings and visibilities. The first disturbance will cross central NC this evening through Monday morning. Another is expected to affect our region late Monday night into Tuesday. In between the periods of showers and storms, MVFR/low end VFR ceilings will prevail due to the available moisture in the atmosphere. The unsettled weather will persist through Wednesday. An improving weather pattern is anticipated by Thursday with VFR conditions highly probable for Thu-Fri. && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...WSS/CBL NEAR TERM...WSS/CBL SHORT TERM...CBL LONG TERM..mlm AVIATION...WSS is the U.S. government's official web portal to all federal, state and local government web resources and services.