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FXUS62 KGSP 050013 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 813 PM EDT Wed Oct 4 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. Rainfall amounts will be very limited with the frontal passage. Temperatures will remain above normal ahead of the front, becoming below normal this weekend following the frontal passage. Dry conditions will continue early next week.

NEAR TERM /THROUGH THURSDAY/
As of 7:45 PM EDT Wednesday: A broad area of sct to bkn cirrus is currently making its way over the western half of the fcst area, while mostly clear conditions prevail over our eastern zones. I expect the cirrus to continue to sct out as it pushes further east as profiles remain fairly dry, however it could make the low temp fcst more challenging. Also, there's a decent chance for another round of mtn valley fog overnight/Thursday morning, as long as it's not impeded by the increased cloud cover.
With that said, overnight lows should remain a few degrees above normal.

Otherwise, the Rex Blocking pattern will begin to breakdown over- night and into Thursday morning as a digging longwave upper trof propagates across the north-central CONUS during the period with an attendant cold front. The airmass won't change much during the near-term, but the parameters for an airmass change will be coming together. The cold front is shown stretching from the Gulf Coast into the OH/TN Valleys, and all the way to the Hudson Bay by the end of the period Thursday. The moisture return ahead of the front is relatively weak and pre-frontal DPVA isn't potent at all. With that being said, subtle height falls will begin to take place and should help allow temperatures to cool a degree or 2 for Thursday compared to today.

SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/
As of 144 PM EDT Thursday: An intense upper jet diving out of the Canadian Prairies will continue to carve out a potent trough that will dig across the Midwest and Ohio Valley tomorrow night into Friday. Ahead of this feature, a lead shortwave trough will be sliding across the Deep South and into the Southern Appalachians with an associated surface cold front pushing across Middle Tennessee and into the Cumberland Plateau. A ribbon of 1.5" PWATs pooled ahead of the front will encroach on the mountains tomorrow night into Friday where a few isolated to scattered showers will be possible, especially along the Tennessee border. As this initial front moves into the mountains it will be quickly overtaken by a much stronger cold front racing into the area in associated with the previously mentioned potent trough as it takes on a negative tilt and lifts through the Mid-Atlantic.

This much stronger front will quickly sweep across the area Friday night into Saturday morning. The environment east of the mountains will not be conducive for precipitation as moisture return will be negligible ahead of the boundary owing to northerly low-level flow as a coastal low slides north well offshore. Antecedent dry air ahead of the rapidly advancing front will have little time to modify with scant boundary layer moisture present on forecast soundings.
Some moistening of the column may be realized with strong forcing possibly squeezing out a few isolated showers east of the mountains, but this will be an exception and not the ruled with the majority of the area experiencing a dry frontal passage. Any isolated showers should quickly come to an end Saturday morning as PWATs crash to 0.25" in the face of strong dry advection and downsloping.

Temperatures will start off on the mild side tomorrow night with lows running 5-10 degrees above average as cloud cover increases and the boundary layer remains mixed. Warm high temperatures will continue on Friday with temperatures near or just above average.
Cold advection quickly ramps up behind the front Friday night into Saturday with 850mb temperatures crashing to 3-6 C by Saturday evening. Friday night lows will be at or a few degrees below average while Saturday highs will be 10-15 degrees below average.

LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/
As of 229 PM EDT Wednesday: The coldest temperatures so far this fall will be gracing the area Saturday night through Sunday night.
Strong cold advection will usher in a chilly early fall airmass with temperatures running 10-15 degrees below normal. The boundary layer will likely remain mixed Saturday night, but temperatures are still forecast to fall into the low to upper 30s across the mountains and low to mid 40s farther east. Sheltered valleys across the southwest mountains have the greatest chance of seeing frost if winds are able to relax enough. Continued cold advection will keep highs on the cool side Saturday despite plenty of sunshine. Mountain valleys may reach the 60 degree mark with higher elevations remaining in the 50s. Highs outside of the mountains should range in the mid to upper 60s. The coldest night of the period will likely be Sunday night as surface high pressure slides across the Gulf Coast states and into the Southern Appalachians. This will allow for very efficient radiational cooling of the dry airmass with light to calm winds beneath mostly clear skies. Temperatures Monday morning will be in the low to mid 30s across the mountains with upper 30s to low 40s elsewhere. The highest mountain tops and ridgelines will likely see sub-freezing temperatures. Patchy to widespread frost is also expected across the mountains.

The airmass will quickly begin to modify on Monday as low-level flow backs and brings an end to cold advection as a closed upper low wobbles across southeast Canada. 850mb temperatures quickly warm in a quasi-zonal flow regime with further deep mixing of the dry airmass. This will allow for large diurnal temperature swings with chilly temptress Monday morning quickly warming into the 60s to low 70s. This would be a 30-35 degree temperature swing across much of the area, which isn't too uncommon with a dry fall airmass.
Further airmass modification will occur through early next week with temperatures back into the low to upper 70s by Wednesday.

AVIATION /00Z THURSDAY THROUGH MONDAY/
At KCLT and elsewhere: Expect VFR conditions to continue thru the 00z taf period. The only exception will likely be another round of mtn valley fog/low stratus that impacts KAVL Thursday morning between roughly 09 and 13z. I lower visby to 1sm and cigs to 300ft in a TEMPO group, but if the current sct high clouds linger thru the overnight, dewpt depressions may be too great for the restrictions to materialize. In addition, KHKY could also see some brief restrictions, but it is less likely there. For the time being, I included a TEMPO for MVFR visby from 09 to 13z at KHKY. Otherwise, we can expect few to sct VFR clouds thru the period with KCLT expected to see low-end VFR cigs towards the end of the period late Thursday night. Winds will remain light thru the overnight and morning and continue to favor an ELY direction across the lower terrain. They will pick up marginally tomorrow aftn from the NE to ENE. At KAVL, winds will be light to calm overnight and thru the morning.
They will pick up from the SE tomorrow aftn.

Outlook: Persistent sfc high pressure is expected to keep VFR conditions in place at most terminals thru the workweek. The main exception is the potential for mtn valley fog each morning around daybreak. A cold front may bring showers and associated restrictions to the mtns Friday into Saturday.

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

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