textproduct: Columbia

WHAT HAS CHANGED

Aviation discussion updated for 00Z 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.

The forecast for the mid week system generally remains on track as deep moisture with PWAT's near 1.75-1.90" overspread the region by Thursday morning ahead of a cold front. A northern stream trough will start digging southward through the day Tuesday and Wednesday, interacting with a cut-off low currently nearing the California coast. As medium range models reach out toward Wednesday and Thursday, there remains uncertainty in whether this northern trough will phase with some energy from the cut-off low or if this low will remain intact. In the most recent LREF cluster analysis, 70% of the grand ensemble solution falls within two clusters. Each of these favor at least some interaction between the cut-off low and northern stream trough, leading to a slightly slower progressing trough. Overall, this would favor a frontal passage during the late afternoon to evening hours Thursday.

Ahead of this front, robust IVT, warm cloud layer depths between 11,000-12,000 ft, elevated instability, generous upper support, and a 35-45 kt LLJ will bring the potential for periods of heavy rain Thursday morning and into the afternoon. Rain chances could move in as early as late Wednesday, but the primary window comes Thursday. The latest run of deterministic guidance is generally showing between 0.50-1" of QPF from this system with pockets of higher amounts. These amounts are further supported by ensemble guidance with 13z NBM probabilities for greater than 0.50" of QPF reaching 50-70% across the CWA. The Flash Flood risk remains fairly low as 3-hr FFG is around 2.5-3.5" across the FA and 6-hr FFG is over 3.5-4". Some nuisance flooding in urban spots cannot be ruled out in any convection however. A tightening surface gradient and some mixing down of the robust LLJ, should bring gusty conditions where a Lake Wind Advisory could be needed.

For the severe threat, the timing of the front will greatly modulate this risk. Though with less variability across LREF cluster guidance, confidence is slightly greater in a fropa favoring later in the day as messaged above. Strong upper dynamics should overspread the region through the day with deep layer shear exceeding 50-65 kts. The question will remain destabilization and the overlap with deep moisture as strong kinematics look to be in place. Widespread cloud cover and morning showers/weak storms Thursday could limit destabilization and the greatest axis of moisture looks to begin shifting toward the Coast through the afternoon. If instability can build, a couple strong to marginally severe storms could form in this highly shear environment, mainly toward the eastern Midlands, extending into the Pee Dee, but at this time confidence in this is low.

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

Global deterministic guidance and ensembles favor a rather active synoptic pattern through the end of the period, where another trough is expected to near the region by the weekend and into early next week. PWAT's should gradually raise back above 1" behind Thursday's front this weekend, but there remains large spread in the timing of the next trough and any weak shortwaves ahead of it. Overall, more rain chances seem possible in this pattern, especially toward early next week, but confidence in coverage or timing is low.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

VFR conditions expected to prevail through the TAF period.

Passing FEW-SCT cirrus through the period and no CIG restrictions expected. A dry air mass remains in place tonight, though some moisture increase from the southeast is possible toward dawn. This may indicate a low threat for some brief river valley fog at places like AGS/OGB, but otherwise widespread fog is not expected given a 25kt low level jet forecast overnight. Light winds overnight should pick back up from the south by 14z to around 7 to 10 kts. A few gusts around 15 kts may then occur Tuesday afternoon.

EXTENDED AVIATION OUTLOOK...Drier conditions are expected through Tuesday, then a midweek system brings a chance for restrictions once again with showers and thunderstorms starting Wednesday evening.

CAE WATCHES/WARNINGS/ADVISORIES

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