textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Revised expectations for rain and possibly isolated thunderstorms in the area through this morning.

More specific information about possible dense fog this morning and Friday night.

KEY MESSAGES

1. Light rain may return to northeast Georgia, the Upstate, and the southern Piedmont of North Carolina between now and morning, but with rates low enough that flooding is not expected. A few rumbles of thunder may be heard. 2. At least patchy dense fog through daybreak Friday, causing very low visibility on some area roadways. Cannot rule out redevelopment of fog Friday night. 3. Dry and warm through the weekend with cooler and wetter conditions to start next week.

DISCUSSION

Key message 1: Light rain may return to northeast Georgia, the Upstate, and the southern Piedmont of North Carolina between now and morning, but with rates low enough that flooding is not expected. A few rumbles of thunder may be heard.

Synoptic cold front will progress through the lower MS Valley toward the Gulf Coast this morning, with aid of shortwave embedded in longer, low amplitude trough over the central/eastern CONUS. The core of that shortwave is over Arkansas as of 11 PM Thu, set to take a more easterly track toward the Carolinas, maintaining a weak sfc wave along the frontal zone. Hence the front is effectively stalled over GA/SC/NC, with warm advection now occurring north of it, producing widespread showers north of I-20 to our southwest. Active t-storms on the south precip near the boundary are tracking SE into the more unstable air, but steering flow should take some of the stratiform rain east across the Savannah Valley in the wee hours of Friday, warranting continued PoPs for our southern CWA. Isolated lightning strikes in the stratiform shield may cause occasional rumbles of thunder, and/or elevated embedded storms may develop above the inversion. Backing 925-850mb flow in the early to mid morning ahead of the shortwave may also lead to new development of precip north of the front. Rainfall rates generally look to be 0.05"-0.10"/hr, but any embedded convective cells could produce totals of 0.5" to 1" in an hour; seeing the responses which occurred in some cases Thursday, if this were to occur over a poorly drained area or small urban stream bankfull conditions could be reached, but that looks unlikely. Precip chances diminish to below 15% as the shortwave passes Friday morning to midday (west to east).

Weak high pressure looks to strengthen over the area briefly behind the departing front and low during the day Friday. A shallow cold pool is present over the NC Piedmont from the weak in-situ CAD which developed Thursday. Rapid drying is unlikely behind the frontal wave owing to weak winds aloft, and most guidance indicates low cloud cover will be trapped under the CAD inversion through the afternoon. Hence have gone below NBM for max temps Friday afternoon, with highs in the mid 50s across the NC Piedmont, ranging up to the lower 60s in the Savannah River valley and mountain valleys of SW NC.

Key message 2: At least patchy dense fog through daybreak Friday, causing very low visibility on some area roadways. Cannot rule out redevelopment of fog Friday night.

Plentiful surface moisture this morning. Low stratus and areas of fog are already present across most of the lower elevations. Dense fog appears already to be present at some airports located near rivers, and obs in other areas are bouncing between a half mile and 2-3 miles. While similar conditions are likely to continue into early morning, not overly confident at this time the dense fog will become widespread enough to warrant a Dense Fog Advisory, since there are a couple of limiting factors. One, high altitude cloud cover generally will increase north of the warm front and ahead of the approaching shortwave, limiting radiational cooling from the top of the fog layer which is associated with prolonged dense fog. Two, onset of rain could promote just enough mixing to allow vsby to improve for a time. For now, we have taken the approach of a Special Weather Statement for patchy dense fog across the NC Piedmont, but could still issue an Advisory for part of the area in the early morning, particularly if/where fog lasts into the hours of the morning commute. Enough warming/mixing is anticipated during the day that fog should mostly dissipate by 10 AM to noon.

Drying behind the front will be slow, and dense fog could redevelop Friday night, as current forecast min temps are below the crossover temp due to the moist afternoon conditions, and a shallow layer of weak easterly flow should be present. However, low stratus could form instead.

Key message 3: Dry and warm through the weekend with cooler and wetter conditions to start next week.

Weak mid/upper troughiness locked in over the Southeast will begin to shift offshore over the weekend as the flow aloft turns quasi-zonal with warm prevailing westerlies and an uptick in thicknesses. Expect temperatures to rise into the mid and upper 60s Saturday and well into the 70s on Sunday once the trough axis fully shifts east of the CWFA. Changes begin to take place Sunday night into Monday as a strong continental surface high (~1040mb) shifts from the Northern Plains into Great Lakes region on Sunday, and sets up shop over the northeastern CONUS by Sunday night and Monday. A southern stream jet stretching from the Rockies through the Southern Plains and Southeast will send a series of shortwaves starting with the first wave late Sunday into Monday and bring the next chance of precipitation as a cold front slips across the region during this time frame. Following the fropa Sunday night, the synoptic pattern supports Cold Air Damming, with variations amongst deterministic guidance on how strong the wedge will actually be. Other shortwaves will reside within the mean flow of the southern stream jet and bring a chance for additional precipitation through mid-week next week. QPF amounts are on the lighter end, but thermal profiles could support the potential for a wintry mix, mainly freezing rain across the typical cold spots during CAD in the northern mountains and adjacent foothills, as well as the highest elevations. Ensemble guidance, most notably the LREF has probabilities of about 20-40% for at least 0.01" of freezing rain in the aforementioned locations. Will have to continue to look at trends leading up to the early part of next week, especially as we get closer in time and start to receive more high resolution guidance, but confidence continues to remain low on the extent of any wintry p-types this far out. Temperatures will be too warm elsewhere for wintry weather, but a drastic drop in temperatures will be evident on Monday compared to Sunday as afternoon highs may end up 20-25 degrees cooler compared to Sunday thanks to Cold Air Damming.

Guidance keeps CAD in place through Tuesday before scattering out the wedge as the surface high shifts into the northwestern Atlantic. These are sometimes tricky as far as the timing of CAD erosion. Current forecast shows temperatures rebounding by next Wednesday with continued low-end PoPs and much warmer temperatures Wednesday through D7.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: A lot of fog and low stratus around the Piedmont and Foothills TAF sites overnight, but with high clouds and some showers moving into the Upstate, could see quite varying conditions at times. Will handle the showers with VCSH at the Upstate sites thru 12z. Overall, expect LIFR to VLIFR at those sites thru about 14z, then some improvement to IFR, and finally MVFR in the mid to late aftn hours. At KAVL, mainly MVFR conditions are expected overnight thru late morning, then VFR. With moist conditions expected again tonight into Saturday morning, LIFR cigs and possibly some fog will form again across most of the area. Winds will be light thru the period, mainly out of the NE across the Piedmont and SE at KAVL.

Outlook: Fog and low stratus should clear out and VFR conditions should return by midday Saturday. Another front may bring showers and associated restrictions Monday into Tuesday.

GSP WATCHES/WARNINGS/ADVISORIES

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