textproduct: Columbia

WHAT HAS CHANGED

Confidence increasing in periods of moderate to heavy rain late Wednesday and Thursday. Aviation discussion updated for 12Z TAFs.

KEY MESSAGES

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

- 2. Rain chances pick back up late week and into the weekend

DISCUSSION

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

Summary: The rainfall forecast remains on track for mid-week as an upper level trough digs into the central US, phases with southern stream energy, and shifts eastward. The associated surface cold front will work into the Southeast on Thursday. Models continue to show agreement in overall synoptic pattern despite differences in the timing of the front and other details. This increases confidence in periods of moderate to heavy rainfall but leaves the threat of severe weather uncertain.

Rainfall: Robust moisture transport is expected ahead of this front with PWAT values reaching the NAEFS 99th percentile by Thursday morning and IVT at the climatological maximum. As synoptic scale lift moves over the region late Wednesday into early Thursday, scattered showers and storms should develop. Coverage will likely increase Thursday morning as moisture transport and surface convergence ahead of the front peaks. This activity could bring periods of moderate to heavy rain as forecast soundings Thursday morning depict deep moisture, a long/skinny CAPE profile, and a 30- 40 kt LLJ. With the antecedent dry conditions, the overall flash flooding risk is expected to be on the low side. The front will shift east late Thursday bringing an end to rain from west to east. Gusty winds will also be possible Thursday and a Lake Wind Advisory may be needed.

Severe Threat: The threat of severe weather will be largely dependent on the timing of the front. Some of the timing differences stem from the progression of the upper trough and the interaction with a cut-off low to the west. There is also some uncertainty in the degree of destabilization through the day with widespread rain and cloud cover anticipated. That said, ECMWF EFI values for CAPE- Shear combo still range from 0.6 to 0.8 indicating some potential for deep shear to line up with moderate instability. NCAR ML/AI severe probabilities have remained consistent indicating a potential threat of severe weather, particularly to our east where instability may be higher and to the north in the Mid-Atlantic states where the dynamics will be stronger as the trough lifts.

Key Message 2: Rain chances pick back up late week and into the weekend

Global ensembles tend to favor a somewhat persistent synoptic pattern through the weekend with another trough digging into the central US then shifting east. However a large spread in 500 mb heights indicate much more uncertainty in the forecast compared to the mid- week system. Mean PWAT values from the GEFS and ECMWF ensemble rise back above 1 inch by early next week. Near normal moisture and upper level troughing over the region supports a chance of rain near the end of the forecast period.

AVIATION /12Z MONDAY THROUGH FRIDAY/

VFR conditions will prevail through 12Z Tuesday.

Satellite imagery shows mainly clear skies in place over the area this morning. Winds are generally light/calm at all terminals. While a dry airmass has limited fog formation, there is light patchy fog in some river valleys.

Winds along the back side of Atlantic high pressure will increase from the south-southwest to 5-10 kt during the late morning and afternoon hours today. While they should diminish with the loss of daytime heating this evening, they may not fully decouple. Thin cirrus should begin spreading over the area this afternoon, potentially increasing in thickness/coverage toward this evening and overnight, but will have no impact to aviation interests.

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.