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FXUS62 KGSP 031817 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 217 PM EDT Tue Oct 3 2023

SYNOPSIS
Dry high pressure will remain in place across the region through Thursday. A cold front will approach from the northwest on Friday and move east of the area on Saturday. Temperatures will remain above normal ahead of the front, becoming below normal this weekend following the frontal passage.

NEAR TERM /THROUGH WEDNESDAY/
As of 215 PM EDT Tuesday: Fair weather cu will continue to roam the sky as we enter peak heating this afternoon into the evening before dissipating just after sunset. Surface high pressure centered over the southern/central Appalachians will gradually work north in response to a developing Rex Block over the Eastern Seaboard. With an upper anticyclone setting up shop over the Mid-Atlantic/Northeast with the surface high riding underneath. Surface ridging will still nose into the area and continue to provide a northeast to east-northeasterly wind. An upper cyclone will be trapped within the Rex Block south of the anticyclone, just offshore the Southeast Coast. As a result, the mid- and low-level flow will veer more easterly and provide a subtle moisture fetch. In the upper-levels, subsidence will be in place and allow for a nocturnal inversion to form with the easterly fetch trapped underneath. In this case, a deck of stratocu should spread over portions of the area and could allow temperatures to be a degree or two warmer than advertised and derail the extent of the major mountain valley fog, if widespread enough. Overnight lows will be at or slightly above normal due to the stratocu deck.

Conditions will remain dry on Wednesday with the Rex Block fully underway by the end of the period. The CFWA will remain dry with subsidence and high pressure in place. The only monkey-wrench being thrown into the forecast is the alluded stratocu deck and how long it will take to fully dissipate after sunrise, which could alter the temperature forecast a little bit. Otherwise, elevated heights will along with continued high pressure will push highs up to 5 degrees above normal for Wednesday.

SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/
As of 200 pm EDT Tuesday: The latter half of the week continues to look fair and quiet. The rex blocking pattern off the Eastern Seaboard is expected to devolve as an upper trof digs over the nrn Plains and pushes the upper ridge off New England farther east. Our region ends up with a remnant sfc/upper ridge getting pinched between the old upper low off the Southeast Coast and the mid/upper trof amplifying as it moves east across the nrn Plains/upper Midwest thru Thursday night. Temps should remain a category or so above normal, but the end of that is near. The approaching trof from the west is not expected to arrive in time to warrant any precip chances over the mtns before daybreak Friday.

LONG TERM /FRIDAY THROUGH TUESDAY/
As of 137 pm EDT Tuesday: The medium range part of the forecast looks like it will be dominated by a large upper low expected to close off over the Great Lakes by Saturday. Before we get there, we will come under the influence of the associated deepening mid/upper trof on Friday. A lead short wave ahead of the main trof axis will move across the region during the day on Friday, but the initial response looks weak and limited, partially due to relatively weak forcing, but mostly due to a lack of moisture return ahead of the system with the Gulf closed off. Precip chances, such as they are, don't start to rise from the west until late in the day as the actual cold front approaches. The model guidance is in fairly good agreement with the timing of the frontal passage Friday night, but if you are looking for meaningful rain you will probably be disappointed. The moisture just doesn't look like it will be available as it stands right now. Precip probs were kept mostly over the mtns/foothills and in the slight chance to low chance range, and QPF is insulting to those who need rain. The front should usher in the coolest air of the season over the weekend on the strength of a deep NW flow as the deep upper low spins over the lower Great Lakes region. Temps on the order of ten degrees below normal will definitely make it feel like autumn. Wind should stay mixed up enough the first night to not present many low temperature problems, but Sunday night, the second night after the frontal passage, could be problematic. A sfc high will settle over the Southeast Sunday night which will allow the pressure gradient to relax and afford the region a better shot at radiational cooling. Some of the 6k ft peaks will have a good shot at getting low temps below freezing, but more of the mtns will have to deal with the possibility of frost formation in the pre-dawn hours Monday. The previous shift started a mention in the HWO for this, so we will continue that. The air mass will moderate quickly in the early part of next week as high pressure continues, so temps will creep back up toward normal by Tuesday as high pressure stays over the Deep South.

AVIATION /18Z TUESDAY THROUGH SUNDAY/
At KCLT and elsewhere: VFR conditions will prevail at all terminals through the period, with the exception of fog/low stratus potential at KAVL around daybreak Wednesday. Winds are generally east- northeasterly, while KAVL maintains a southeasterly component throughout the afternoon/evening at 4-8 kts before going mostly light and variable overnight. A few cu can be expected throughout the day, but cigs will remain FEW/SCT 040 or higher. Model guidance have picked up on a nocturnal subsidence inversion with moisture trapped underneath, which will lead to an increasing strato cu deck (040-050). Decided to advertise this at most TAF sites overnight with FEW050 as confidence is still low on the exact placement of this cloud deck. Otherwise, another round of mountain valley fog/low stratus is expected overnight through daybreak Wednesday. Have placed a TEMPO for IFR/LIFR fog/low stratus at KAVL around daybreak Wednesday, but this forecast could easily become derailed if the alluded deck of stratocu spreads over the terminal before fog/low stratus can fill in.

Outlook: Persistent high pressure will keep VFR conditions in place across the terminals through the workweek. The main exception is the potential for mountain valley fog around daybreak each morning. A cold front may bring showers and associated restrictions to the mountains late Friday into Saturday.

GSP WATCHES/WARNINGS/ADVISORIES
GA...None.
NC...None.
SC...None.

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