KGSP issues Area Forecast Discussion (AFD) valid 2019-12-10T15:00:00Z 101 FXUS62 KGSP 101500 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1000 AM EST Tue Dec 10 2019 .SYNOPSIS... Cloudy conditions and occasional rain will persist until a cold front passes east of the region early Wednesday. Rain is forecast to briefly change to a light wintry mix before ending across the North Carolina mountains early Wednesday. Dry but cold high pressure will settle in behind the front for mid-week. Another round of brief wintry weather is possible early Friday as a coastal storm brings another round of moisture, but the bulk of this precipitation should fall as a cold rain. && .NEAR TERM /THROUGH TONIGHT/... As of 950 AM: Low pressure is now the dominant feature seen over the East on latest sfc analysis; the sfc high that helped drive the development of an in-situ wedge over the region has pretty much lost influence over our weather. The sfc low is tracking up into Quebec, and strong southerly flow associated with its warm conveyor belt has shifted out of the CWFA; the flow has veered, to boot. Dense fog has become patchy across the area this morning and should completely dissipate by noon. Therefore, the advisory will expire on time. Have gone with mesoscale guidance for temps the rest of the day. This means generally slightly cooler than previously expected highs even with the eroding wedge due to clouds and precip. Of course, there is high bust potential as any thinning of clouds before widespread precip returns will allow temps to soar. Went with an initial downward trend in PoP this morning given radar and surface obs. However, quickly ramp PoP back up for the afternoon as that part of the forecast still looks on track. No changes to the expected wintry mix late this afternoon into tonight. Precip will continue to spread into the area from the west this morning as low-level convergence advects in ahead of the front. The flow will be oriented for upslope enhancement into the SW-facing ridges of the western mtns. PoPs will expand eastward thru the morning, with the precip shield possibly breaking up a bit due to downsloping as the near-sfc cold advection reaches the foothills, around midday. Said CAA will erode whatever's left of the wedge at that point, leading to lifting ceilings and improving vsbys. Temps will continue to be a very tricky fcst thru today, but almost all guidance (even the NAM and its usual strong preference for wedging) depicts temps getting near 70 in the lower Piedmont, and even the northern foothills rising to near 60 in spots. PoPs overall will peak in the late evening with the best combination of deep frontogenesis and upper divergence/DPVA. Temps will fall enough to support a p-type transition in the highest elevations near the Tenn border around sunset. The frontal zone is wide and characteristic of an anafront; accordingly the cooling is relatively slow aloft. Whereas we might normally expect a quick transition to snow, vertical profiles suggest a period of mixed precip before the onset of snow. The NAM's depicted warm nose is the strongest of the available guidance; the preference for this fcst was a blend of the GFS and EC, which results in mainly sleet for the first few hrs. A few areas of the northern mountains cool fast enough at the sfc to still receive a few hundredths of an inch of FZRA at the beginning of the event this evening. Given the warm ground, this likely will only be on elevated surfaces, and widespread travel issues are not expected due to icing. The anafront also features relatively light post-frontal winds in the upslope zone, which will keep accumulations small. Overall the most favored slopes are expected to see 1 to 1.5 inches, with less than an inch in most populated areas of the border counties. We will advertise the minor travel risk in the HWO and also issue a Special Weather Statement. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY/... As of 240 am Tuesday: NW winds advecting drier air into the forecast area Wed morning will bring a rather abrupt end to clouds and precip chances, with mostly sunny/partly cloudy skies expected by mid-afternoon. Temps are still forecast to be on the cool side, peaking at about 5 degrees below climo in most areas. The remainder of the short term will see a continuation of these dry and cool conditions. And in fact, Thu is forecast to be several degrees cooler than Wed, with increasing clouds expected by the end of the day. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 300 am Tuesday: A dampening short wave trough will eject from the southern Great Plains late Thursday, with additional bouts of height falls expected to follow throughout the weekend. This first wave is expected to be responsible for Gulf Coast cyclogenesis Thu night and Friday. Although timing differences remain among global model guidance, they are converging toward a consensus in developing a classic Miller A cyclone across the Southeast coastal states early in the period, and this is expected to bring the next round of precipitation to our forecast area. It's looking more likely that there will be at least a transient period of wintry weather somewhere in the area late Thu night into Friday, as 1040-ish mb surface high pressure over the northern Mid-Atlantic and southern New England will support a cool/dry (i.e., low wet bulb) thermodynamic environment across the region. Guidance is in quite good agreement that moisture and forcing will overspread the forecast area from the south by daybreak Friday, and wet bulb effects should result in temps cooling to near or below freezing, with any rain falling as the freezing variety where temps do cool to the freezing mark. This will be most likely along/north of the I-40 corridor in the Piedmont and foothills, and across the eastern escarpment of the North Carolina's Blue Ridge. In terms of cold air maintenance/longevity: although the strong nature of the surface high is somewhat disconcerting, it it also quite transient, and is forecast to be somewhere in the neighborhood of Nova Scotia by 12Z Friday. This is not conducive to locking in sub-freezing temps, especially in light of the "self-limiting" nature of freezing rain and the fact that temps are likely to cool to near or just below freezing after the precip has set in. Thus, a fairly quick transition to a cold rain seems likely in the aforementioned areas, but it also seems likely that light ice accums will occur before temps warm back above freezing, mainly on elevated surfaces. It seems prudent to begin talking up this event a bit in hazardous weather outlook and briefing products. Categorical rain chances will otherwise be in place for the entire area through the day Friday, with temps at least 10 degrees below normal in in-situ/hybrid cold air damming regime. This also looks to be a solid rainfall event, with the current forecast calling for 1-2 inches for most locations. Drying conditions are anticipated for Fri night through early Saturday, with seasonably cool conditions expected to round out the medium range. Rain chances do increase to 20-30 percent by late Monday in anticipation of the next frontal system. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... At KCLT and elsewhere: Aviation fcst remains challenging as in-situ CAD wedge remains entrenched over part of the area, but weakly. In advance of an approaching cold front, southwest flow has developed down to the sfc at most sites which had promoted a brief erosion of low cigs. Confidence is fairly low given poor guidance handling of the cigs/fog and the competing effects of downward motion and the damming. However, low cigs have returned and even some low vsby. Expect quite variable conditions into early afternoon. Precip chances ramp up as the front enters the area; the IFR/LIFR level cigs should lift quickly as CAA and veering downslope winds erode the wedge from west to east. Some improvement to MVFR is expected tonight but a strong inversion aloft will inhibit mixing with the CAA, implying a return to VFR will be delayed until almost the end of the period, even at KCLT. Outlook: Drier air will spread in Wednesday, bringing precip chances to an end at all the TAF sites. Dry and cold high pressure should keep conditions settled Wed night and Thursday, but a moist low pressure system is likely to bring back restrictions Thursday night. With cold air in place at the onset of precipitation, FZRA could result in some ice accumulation Thursday night or Friday morning. Confidence Table... 15-21Z 21-03Z 03-09Z 09-12Z KCLT High 80% High 96% Med 75% Med 68% KGSP High 83% High 96% Med 76% High 87% KAVL Med 63% High 80% Med 70% High 88% KHKY Med 65% Med 70% Med 75% Med 78% KGMU High 88% High 96% High 80% High 87% KAND Med 66% High 85% High 88% High 83% 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: www.weather.gov/gsp/aviation && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...JDL NEAR TERM...RWH/08 SHORT TERM...JDL LONG TERM...JDL AVIATION...RWH/08