textproduct: Columbia

WHAT HAS CHANGED

Threat for any conditional severe diminishing this evening with sunset. Confidence remains high in a potential severe weather outbreak on Monday. Aviation discussion updated for 00z TAFs.

KEY MESSAGES

- 1. A significant severe weather outbreak is possible on Monday, with destructive wind gusts (75+ mph) and strong tornadoes (EF2+) possible as a line of storms pushes through the area. A Moderate Risk (4/5) of severe weather exists for part of the area, with the rest of the area in an Enhanced Risk (3/5).

- 2: Subfreezing temperatures possible multiple nights this week. There is potential for a hard freeze Tuesday night into early Wednesday.

DISCUSSION

KEY MESSAGE 1: A severe weather outbreak is possible on Monday, with potential for destructive wind gusts (75+ mph) and strong tornadoes (EF2+) as a line of storms pushes through the area. A Moderate Risk (4/5) of severe weather exists for part of the area, with the rest of the area in an Enhanced Risk (3/5).

There is not too much change in the overall thinking regarding potential for a severe weather outbreak on Monday. SPC continues to highlight the area in a moderate risk across the northern Midlands and Pee Dee area to near the Columbia Metro area with and enhanced risk of severe weather elsewhere across the forecast area. A dynamic and deepening 500mb trough will be moving into the Great Lakes and MS Valley region by 12z Monday with additional shortwave energy and a jet streak further amplifying the base of the trough which will induce a negative tilt to the trough during the afternoon hours enhancing the already strong upper forcing. A strong cold front and associated QLCS line of convection is expected to be pushing into the southern Appalachians to Gulf coast by 12z with a very favorable downstream environment conducive for severe weather.

Hi-res soundings at 15z across the forecast area show an environment featuring MLCAPE values approaching 1000 J/kg, 0-6km bulk shear over 40 kts with PWATs near 1.5 inches, strong low level lapse rates associated with the height falls and a veering hodograph with 0-3km SRH values around 200 m2s2 and STP values around 1. Hi-res models depict a broken QLCS line moving into the forecast area late morning and crossing through the afternoon hours with continued destabilization expected ahead of the line favoring possible intensification and the highest impacts occurring over the Pee Dee and eastern Midlands. The environment favors embedded supercells within the QLCS line and isolated discrete supercells developing ahead of the line cannot be ruled out where if they materialize would provide a threat of strong tornadoes. The overall timing of the line crossing the area appears to be around 10-11am to 4-5pm.

Outside of the convective winds the gradient winds will be strong due to a very strong low level jet with 850mb winds 50-60 knots with deep mixing expected which could result in non- convective gusts in the 40-50 mph range ahead of the front and within the strong cold advection behind the front. A wind advisory was considered and after collaboration with neighbors the decision to hold off was made to allow later shifts to re- evaluate the need for one but it is very possible one will be needed.

KEY MESSAGE 2: Subfreezing temperatures possible multiple nights this week. There is potential for a hard freeze Tuesday night into early Wednesday.

Not much change is seen in the outlook behind Monday's strong front as much cooler and drier air is expected to build in with robust CAA driving temperatures from near freezing to sub- freezing. 850 mb temperatures from 18z Monday to 12z Tuesday should drop nearly 15 to 20 degrees and rapid cooling is expected at the surface. This should aid in bringing temperatures between 29-34F Monday night but with the complication of surface winds remaining elevated overnight with the tight pressure gradient from the front slowly loosening into Tuesday. The most recent NBM is fairly similar to its previous run, though the experimental NBM 5.0 is 2-3 degrees warmer than the deterministic NBM. Recent LREF probabilities for lows less than 32F are around 40-60%, highest north of I-20 and the NBM remains more aggressive with a large area of probabilities greater than 60% along and north of I-20. The greater uncertainty exists mainly in the Eastern Midlands into the Pee Dee. With the fairly high probabilities for temperatures near to below freezing, the Freeze Watch remains in effect Monday Night into Tuesday morning for counties along and north of I-20, though Burke County GA and Barnwell County SC are included in this watch as well. Trends will continue to be monitored to determine if a potential expansion is needed.

Surface high pressure builds overhead on Tuesday, likely bringing the coldest night of the week Tuesday night into Wednesday morning. Near ideal radiational cooling conditions should yield temperatures into the upper 20s for much of the FA with increasing potential for a hard freeze, especially across the northern Midlands and into the Pee Dee region. Another night with freeze products will likely be needed with probabilities for temperatures less than 32F over 60-80% for the FA. Moisture is expected to gradually increase Wednesday as high pressure departs and thus temperatures warm a bit. Overnight lows could near freezing again, but confidence in this is not high at the moment, though Frost products at minimum could be needed.

AVIATION /00Z MONDAY THROUGH FRIDAY/

VFR conditions to start off the period, the ceiling restrictions once again become possible tonight through much of Monday.

Very dynamic cold front will be approaching the forecast area through the overnight hours, then will push through during the morning and afternoon hours on Monday. Period starts off with vfr conditions, but with increasing moisture through the night, guidance is bringing ceilings back down into mvfr between 03z-06z at all sites. Have mostly followed that trend for ceilings, with mvfr or lower then expected for the remainder of the period as the front moves in. On top of ceiling restrictions, additional aviation threats through the period will be centered on winds. First will be the main synoptic scale surface winds, as they increase ahead of the approaching front overnight and early Monday morning. Sustained winds will increase to above 10 knots by midnight, then continue to increase through the night. By Monday morning winds will begin to top 20 knots just ahead of the front out of the south to southwest, then rapidly turn west/northwesterly behind the front, with sustained winds between 20-25 knots expected. In addition, wind gusts will be quite strong as a rather intense low-level jet moves through the region and those winds mix down to the surface. Wind gusts outside of any storms will top 30 knots on Monday. Even strong winds will be possible along the squall line as it moves through each taf location, with initial estimates gusts over 40 knots along the line. In addition, the squall line should bring ifr or lower visibilities as it moves through. Exact timing of this front will be adjusted the closer it gets on Monday, but for now have included a 4 hour tempo group at each taf location, with timing centered around the period of highest confidence at this time. All in all, a rather hazardous flying day on tap for the aviation community Monday.

EXTENDED AVIATION OUTLOOK...Strong winds possible into Monday night. A return to VFR conditions and lighter winds is anticipated for Tuesday onward.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Freeze Watch from late Monday night through Tuesday morning for SCZ018-020>022-025>028-030-035-115-116. GA...Freeze Watch from late Monday night through Tuesday morning for GAZ040-063>065-077.