textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Confidence is increasing regarding widespread rain for Thursday.

Rain chances are decreasing for next week.

KEY MESSAGES

1. Rain chances will return to the forecast area by tonight with a developing frontal zone. Rain is most likely Thursday morning, slowly tapering off through Friday night from northwest to southeast. The chance of flooding currently looks low. 2. Warm and dry weather returns for the weekend with the next chance for rain toward mid next week.

DISCUSSION

Key message 1: Rain chances will return to the forecast area by tonight with a developing frontal zone. Rain is most likely Thursday morning, slowly tapering off through Friday night from northwest to southeast. The chance of flooding currently looks low.

As low pressure system moves north of the Great Lakes, a cold front will progress southeast thru the Corn Belt and Ohio Valley today. Broad cyclonic flow over the eastern CONUS will be reinforced by the system's upper low. Gradient will tighten between that system and the weak high pressure now centered over the Bahamas; 850mb winds look to reach 35-50 kt within warm-advective SW flow, primarily above the PBL. Winds will be maintained across our forecast area through the early morning hours, and low-end gusts will be possible throughout the area. However, mixing depth will be limited and only very high elevations are expected to see gusts approach Wind Advisory criteria. During the daytime hours Wednesday, mixing will allow brisk gusts of 25-30 mph at times across lower elevations as well. Between the winds and abundant cirrus associated with the cyclonic upper jet, mins this morning will be well above normal. Despite continued high altitude cloud cover during the day, today's max temps will trend 7-10 degrees warmer in the Piedmont owing to the low-level WAA. Winds weaken late in the afternoon as gradient relaxes.

The cold front will sag far enough south to erode the midlevel inversion by this afternoon over the mountains, but it looks to stall with moistening return flow across the TN Valley and southern Appalachians tonight into early Thu, basically leaving a stationary front over the area. Some mechanical lift into the W and SW facing ridges warrants onset of small PoPs late this afternoon in our far west; frontogenetic response leads to a band of QPF showing up in most models across TN and NC overnight into early Thu which results in a swath of 50-80% PoPs extending across the CWA. Temps will be on the order of 10 above normal Wed night, but only a few above normal Thu. All precip will be rain through this event as temps remain above normal, though trending gradually cooler thru Fri night.

A weak shortwave embedded in the longer trough is shown to swing across the lower OH and TN Valleys sometime Thursday, with timing varying among models. Most models do in turn develop a sfc wave along the front, though the timing/track of that wave is dependent on the shortwave, and how quickly and when PoPs taper off Thursday thus varies among models. Most likely time for precip looks to be Thursday morning from the range of present guidance, and thus PoP is over 80% over almost all the area at that time; PWATs look to peak at 1" to 1.2" around then, with the richer moisture shunted to our south. Chances diminish generally from NW to SE, Thu afternoon onward. There is also another, better defined shortwave shown to swing into the Deep South Thu night which looks likely to develop another frontal wave to our southwest. Though the resulting precip could pass by to our south, via this second feature, the 24/12z GDPS develops a band of heavy precip in our CWA Thursday night, and the 25/00z NAM brings a separate round of precip Friday night following a lull during most of Friday. The main implications of these differences is to slow the decline of PoPs from the NBM; likely PoPs (60-70%) linger SE of I-85 into early Friday morning, with small 20-30% chances lasting thru Friday in that area, not completely dropping out of the forecast until early Saturday. Per ensemble QPF probs suggesting low chance of exceeding 2" total QPF even in upslope areas, this rainfall looks most likely to be beneficial in light of the ongoing drought.

Key message 2: Warm and dry weather returns for the weekend with the next chance for rain toward mid next week.

Once showers clear out on Friday, the remainder of the weekend and into the beginning of next week remain quiet. High pressure expands over the region with the axis shifting toward the northeast by the start of the week. Meanwhile, height rises through the weekend bump up temperatures into the upper 60s and a run for the 70s east of the mountains for Sunday. Guidance has the high pressure shifting to the NE by Monday, bringing in a wedge and slightly cooler temps. Some of the guidance tries to increase rain chances toward the very end of the forecast period, but not a whole lot to bet on. Confidence remains very low for the next shot of rain as PoPs at this time are capped at a slight chance (15-30%). Aside from a rather quiet period, there should be enough moisture to keep RH values well above any critical levels, keeping fire concerns low at this time.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: VFR conditions are expected to continue through the daylight hours on Wednesday. If there will be a problem, it would be the wind, which is expected to remain SW but with frequent gusts as stronger SW flow aloft traverses the region. The wind should start to let up late in the day. Meanwhile, the next batch of precip will be taking shape to the west. The latest guidance shows that low clouds would finally start moving in from the west with the precip after 00Z over the mtns, and then after 03Z east of the mtns, perhaps not reaching KCLT until operations ramp down around midnight. The uncertainty of this arrival will be handled at all terminals with an initial PROB30 group in the evening, then more of a prevailing restriction for the rain and associated low clouds afterward. For this issuance, we will stick to MVFR, but indications are that conditions will eventually deteriorate to IFR in the early morning hours Thursday.

Outlook: A cold front tracks across the terminals on Thursday, bringing showers and associated restrictions through at least Thursday night. Activity may linger through early Friday before dry conditions return late Friday into the weekend.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.