textproduct: Columbia

WHAT HAS CHANGED

Confidence in periods of moderate to heavy rainfall Thursday has increased slightly. Aviation updated for 18Z TAFs.

KEY MESSAGES

- 1. Periods of moderate to heavy rainfall are expected during the mid week with the potential severe weather.

DISCUSSION

Key Message 1: Periods of moderate to heavy rainfall are expected during the mid week with the potential severe weather.

There has not been a significant change in the outlook with an incoming system for the mid week. Overall, there is good model consensus in an upper trough swinging out of the northern CONUS toward the region late Wednesday into Thursday. An associated surface cold front will also work toward the region Thursday. Robust moisture transport is expected ahead of this front with PWAT's reaching above the NAEFS 97.5th percentile by Thursday morning. Timing differences remain for the front across deterministic guidance, but spread is a bit less in LREF cluster analysis with each of the clusters now favoring an afternoon/evening frontal passage. Some of the timing differences likely stems from the progression of the upper trough and its interaction with a cut-off low that is currently off the West Coast. As upper support overspreads the region late Wednesday into early Thursday, scattered showers and storms will be possible, likely increasing in coverage Thursday morning. This activity could bring periods of moderate to heavy rain as forecast soundings Thursday morning depict long/skinny elevated CAPE profiles, a 35-40 kt LLJ, and warm cloud depths exceeding 11,000 ft. With the antecedent dry conditions, the overall flash flooding risk is expected to be on the low side at the moment. The front is expected to clear late Thursday, bringing an end to rain chances from west to east. Besides scattered to widespread rain, a tightening surface pressure gradient and the 35-40 kt LLJ could bring breezy conditions Thursday.

In terms of the severe weather risk, as stated in previous discussions, this will be highly dependent on the timing of the front as plenty of shear should be in place (0-6km Bulk Shear near 50-65kts) with solid upper support. AI/ML guidance continues to indicate at least a low end risk and there does seem to be greater consensus on more favorable timing across LREF clusters with this package. The main concern will be pre frontal activity Thursday morning. As stated above, favorable dynamics will be in place for scattered to widespread activity Thursday morning and thus despite the potential for a slower moving front, widespread cloud cover and showers/weaker storms may limit destabilization during the afternoon. The most favorable location for overlapping dynamics, shear, and instability, continues to look just north and east of the FA, perhaps stretching down toward the far eastern CWA, where around 75% of EC Ensemble members show MUCAPE near 1000 J/kg developing Thursday afternoon.

AVIATION /18Z SUNDAY THROUGH THURSDAY/

VFR conditions generally expected through the 24hr forecast period.

Satellite imagery showing mainly sunny skies across the region with some diurnal cumulus across the southern half of the forecast area. Outside of those fair weather VFR clouds winds should becoming prevailing from the west-southwest as the axis of surface high pressure gradually slides east of the region. Winds should subside with sunset although there is a 15-20 knot low level jet which may allow for winds to stay a bit mixed at times, which combined with a dry air mass with dewpoints in the 40s overnight should limit fog potential. Cannot completely rule out some river valley fog at AGS/OGB but confidence not high enough to include in this forecast. Winds should pick up by 14z from the southwest around 5 to 8 knots. There could be a few passing high clouds but no impact to aviation expected.

EXTENDED AVIATION OUTLOOK...Drier conditions are expected through Tuesday, then a midweek system brings a chance for restrictions once again.

CAE WATCHES/WARNINGS/ADVISORIES

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