Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 090848 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 348 AM EST Mon Dec 9 2019 .SYNOPSIS... The weather will be unsettled, with occasional rain until a cold front passes east of the region Tuesday night and early Wednesday. Rain is forecast to briefly change to snow before ending across the North Carolina mountains early Wednesday. Dry but cold high pressure will settle in behind the front for mid-week, with another storm system expected to bring precipitation late in the week. && .NEAR TERM /THROUGH TONIGHT/... As of 250 AM: Expansive sfc high pressure is centered off the Atlantic coast. Water vapor imagery depicts a shortwave swinging from the Rockies into the northern Plains, and low pressure is developing in the Midwest as a result. Closer to home, a lobe of the offshore high extends inland over the Carolinas, manifesting as a wedge. Dewpoints are mostly in the 20s attm, with temps largely in the lower 40s. The Midwest low is already associated with a broad area of warm advection across the Southeast, and as the warm conveyor belt moves across the area today, precip should result atop the wedge, reinforcing it via evaporative cooling. Model QPF reflects a significant boost to the lift in the upslope areas, which means PoPs only reach the likely range near the south-facing terrain. In fact, as the better midlevel winds exit to the east and north late in the day, the net forcing may diminish enough away from the Escarpment that a lull in precip will occur. However, with the potential for at least sprinkles or the occasional shower to occur, a small PoP has been retained in most areas. The temp fcst will largely hinge on the expectation of evaporative cooling; the NAM as usual reflects this very strongly. However, with the parent high being so far offshore, it is a little questionable how effective the wedge will hold on in areas that see less rainfall. Virtually all guidance, even the NAM, brings sfc winds around to the south today, and this could easily bring temps well above normal if the wedge were not in play. Normally our short-term model consensus product does very well with wedge temps, and it depicts the warmer air making it about as far north as I-85. I did blend in a bit of the NAM`s temps, sharpening the gradient between "fully wedged" temps near the Blue Ridge and north of I-40, and above-normal values in the far southeast. The retreat of the wedge boundary likely will lead to lowering ceilings and possibly areas of fog near the boundary, as the airmass becomes very shallow. The sfc high will move east of Lake Huron by 00z, leading low to midlevel winds to veer, and drier air will advect in above 850 mb. The more westerly winds will be associated with a loss of the WAA and possibly even the onset of CAA at some levels. While mostly cloudy to overcast conditions should continue, PoPs will remain largely at slight-chance away from the better terrain thru the evening. As the cold front nears in the early morning, the westerly flow will strengthen and further increase chances in the SW NC mountains and vicinity. Tonight`s min temps generally will occur in the early evening, as the wedge loses its grip. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... As of 250 am Monday: The leading edge of a rather well-forced/moist cold front will begin to pass through the forecast area near the start of the short term period...moving somewhat slowly across and east of the area by Tue evening. With strong upper jet axis displaced well behind the front, much of the forcing/moisture will be found along and on the cold side of the boundary. Pops therefore increase gradually to categorical across the northwest half of the CWA by the end of Tuesday, reaching a nadir with 80-90 pops across the entire area Tue evening. Event total rainfall amounts will be rather tame...generally .5-1.0 inch in most areas, except across the high terrain of southwest NC, where 2-3 inches are possible. The anafront nature of this scenario will create p-type concerns across mainly the NC mtns Tue night/early Wed, as cold air is expected to overspread the area while moisture and forcing are still in place (at least to some degree). Forecast soundings suggest a narrow transition zone to sleet and/or freezing rain will exist, but the bulk of precip wintry precip would most likely fall as snow. It`s always tricky to nail down accums in these anafront events, and they tend to have a tendency to "disappoint," but current indications are that an inch or two will be possible across the TN border counties, with amounts likely tapering down quickly to little more than a dusting across the interior mountain counties. Surface should be quite warm before the transition to snow occurs, so accums would most likely be confined to grassy surfaces. Winter wx headlines are certainly not warranted at this juncture (and may never be with this event), but we will begin to mention this event in the HWO and briefing materials. The remainder of the short term will see quickly drying conditions Wed morning, with chilly weather expected Wed into Wed night, when temps are forecast to be roughly 5 degrees below normal. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 330 am Monday: A rather progressive upper air pattern will result in unsettled weather during much of the medium range, as a series of short wave troughs are forecast to interact with a baroclinic zone draped across the Southeast. The extended period will begin dry and cool, but pops will gradually ramp up late Thu into Friday, during which time moisture and upglide associated with a developing Gulf Coast surface wave are expected to overspread the Southeast. Owing to the cool and dry conditions, and precip falling Fri morning could fall as light freezing rain, mainly across the mountain valleys near the Blue Ridge and along the I-40 corridor in the Piedmont and foothills. While light accums cannot be ruled out, and pronounced in-situ/hybrid cold air damming will likely develop in response to any falling precip, current guidance indicates that parent surface high pressure will be in an unfavorable location to lock in sub-freezing temps for any significant length of time. Thus, a quick transition to rain is forecast by Fri afternoon. Likely pops will otherwise be advertised by Friday afternoon, continuing into Saturday before slowly tapering off late Sat/Sat night. Dry conditions should return by Day 7. Temps will begin the period below normal, possibly ending up well below normal under Friday`s cold air damming regime, then return to above-normal levels by Day 7. && .AVIATION /09Z MONDAY THROUGH FRIDAY/... At KCLT and elsewhere: Within an strengthening and increasingly moist warm-advection regime, areas of -DZ will occur this morning. Continued development of precip, associated with LLJ into Great Lakes low pressure, warrants addition of -SHRA to the TAFs later this morning, and is expected to promote a cool in-situ wedge. Cigs will lower especially as precip becomes steadier, but likely won`t go IFR until after daybreak. The strong winds aloft imply a LLWS threat for most of the period. Wedge front is expected to creep northward later in the day, likely introducing LIFR if it has not already formed. Weakening flow aloft will reduce precip rates late in the day, but areas of LLWS remain in the fcst. Little to no improvement expected after sunset. Outlook: Widespread restrictions along with occasional rain are expected to persist thru at least early Wednesday. A cold front slips into the area on Tuesday, but drier air doesn`t filter in until Wednesday. Dry high pressure should return during midweek and is forecast to persist through at least Thursday. Confidence Table... 08-14Z 14-20Z 20-02Z 02-06Z KCLT High 85% Med 77% High 80% Med 68% KGSP High 90% Med 75% High 80% Med 76% KAVL Med 68% High 95% Med 75% High 90% KHKY High 100% Med 66% Med 61% Med 78% KGMU High 90% Med 70% High 83% Med 73% KAND High 88% High 90% High 80% Med 75% The percentage reflects the number of guidance members agreeing with the scheduled TAF issuance flight rule category. Complete hourly experimental aviation forecast consistency tables and ensemble forecasts are available at the following link: && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...JDL NEAR TERM...08 SHORT TERM...JDL LONG TERM...JDL AVIATION...08 is the U.S. government's official web portal to all federal, state and local government web resources and services.