textproduct: Columbia

WHAT HAS CHANGED

Precipitation chances trending downward over the next few days ahead of a cold front this weekend. Updated the 00z aviation.

KEY MESSAGES

- 1. Daily shower and thunderstorm chances continue through the week, with isolated flash flooding possible. Chances on Friday/Saturday are expected to be modulated by the location of a backdoor front.

DISCUSSION

Key Message 1: Daily shower and thunderstorm chances continue through the week, with isolated flash flooding possible. Chances on Friday/Saturday are expected to be modulated by the location of a backdoor front.

The pattern today has shifted slightly, favoring a bit of a reprieve from the robust wet weather we have seen over the last week or so. A broad, closed low is meandering across central California, with narrowing upper ridging extending into Canada. Beneath this, a mid-level trough is moving northward across central Texas and will slowly begin shifting eastward over the next 48 hours. The combination of this & the amplified California low have led to a bit of amplification of the ridging across central Canada, with a downstream trough over the northeastern CONUS digging southward.

This has shifted the orientation of the moisture rich flow across the area. The surge of 2"+ PWs has pushed north and east of the area and we are now seeing more southwesterly or west- southwesterly flow atop the area instead of purely southerly flow that has been pushing wave after wave of showers and thunderstorms through the area recently. The airmass still remains fairly moist and unstable causing a few isolated storms this evening, mainly across the central Midlands and eastern portions of our CWA. However, widespread coverage is not favored and should taper off quickly this evening/overnight. As we get into Thursday, the trough across the northeastern CONUS is forecast to begin shifting further southeastward. A backdoor cold front will arrive in the afternoon and evening with this. Plenty of instability is expected, with RAP soundings showing ~2000 j/kg of MLCAPE. Downbursts with precip loading are the most probably hazards with storms ahead of the front, and I do expect that convective coverage will likely be a bit greater than current runs of CAMs are indicating. Damaging winds with the strongest storms are certainly possible, but flash flooding with training storms is the most likely threat.

Friday through the weekend is more uncertain. Guidance is mixed on where this backdoor front ends up. There is a range of possibilities, from the RRFS-MPAS which pushes the front clear into central GA to the ECMWF which hangs it up in the western Midlands. This will mainly modulate rain chances Friday/Saturday as it is likely that this front begins washing out a bit and lifting northward over the weekend. Given our saturated soils currently, any areas that do see repeat rounds of thunderstorms this weekend will have a flash flood threat associated with them. PWs >2" will promote more efficient rainfall. While showers & storms are expected each day Fri-Mon, the best chances appear to be in the CSRA on Friday and then the central/southern Midlands & CSRA on Saturday, with better chances overspreading the area thereafter. A more robust front looks to push through on Monday night, with guidance suggesting a prolonged period of drying into the middle of next week.

AVIATION /00Z THURSDAY THROUGH MONDAY/

VFR conditions generally expected through the forecast period except possible brief restrictions in evening thunderstorms and predawn fog/stratus.

Overall convection has been less widespread today but a line of storms has formed around 00z just north of Augusta and extending into the Columbia area. More isolated storms may impact OGB as well. Activity should be limited to 03z or so but have included VCSH with a tempo for thunderstorms with some IFR vsby restrictions. Winds generally from the south at 5 knots or less overnight but gusty winds may accompany storms this evening.

Guidance has trended toward no vsby/cig restrictions during the predawn hours but given the extensive low level moisture in place and storms that have impacted terminals today feel something is needed to included a tempo group for vsby and/or cig restrictions during the 08z-12z time frame. Winds pick up tomorrow after 15z from the west at around 6 to 10 knots ahead of an approaching backdoor front. Convection is possible with the front but coverage is uncertain so left out of this forecast.

EXTENDED AVIATION OUTLOOK...Morning fog/stratus possible each day this week. Scattered, diurnally driven convection expected through Thursday until a front moves into the region significantly increasing rain chances into the weekend.

CAE WATCHES/WARNINGS/ADVISORIES

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