textproduct: Columbia

WHAT HAS CHANGED

Aviation discussion updated.

KEY MESSAGES

- 1. Heat Advisory in effect from Noon to 8PM along and south of I-20 with heat indices up to 108F possible.

- 2. Severe thunderstorms and damaging winds are possible today. Drier conditions then move in for the late week.

DISCUSSION

- 1. Heat Advisory in effect from Noon to 8PM along and south of I-20 with heat indices up to 108F possible.

Oppressive heat and humidity are forecast to develop again across the area today, especially across the coastal plain. A deepening trough across the eastern CONUS will result in a broad surface low developing, along with a cold front rapidly approaching the area from the north. Southwesterly winds are forecast to increase today, helping to push warm and moisture rich air into the region. While dewpoints are forecast to remain in the 70s and yield very humid conditions, there is some uncertainty in just how hot we will get today. It seems like we'll be able to get into the low-mid 90s but the latest hi-res guidance suggests that a decaying MCS from the OH Valley will be pushing towards the region by midday. This could throw a major wrench into the temp forecast for today. However, even still, confidence is fairly high that areas along and south of I20 should see max heat index values up to 108F.

- 2. Severe thunderstorms and damaging winds are possible today. Drier conditions then move in for the late week.

There is considerable uncertainty regarding the forecast for today in many aspects, but especially as it pertains to the forecast for strong to severe storms this afternoon/evening. The synoptic pattern has not changed too much since this time yesterday, though the specifics of the environment ahead of this setup have changed slightly. A shortwave in the Great Lakes is still forecast to dive into the Appalachians and help deepen the trough that is in place, bringing a cold front through the region by Wednesday as well. However, there is substantial uncertainty as to what this shortwave actually does as it moves southeastward near our area. There is an area of convection currently organizing across Michigan. 00z CAMs - across the board! - were in love with this cluster of convection and universally forecast it to dive southward and be near the area by midday. In this scenario, the weakening cluster of storms would almost certainly make it difficult for the eastern portions of the area to see a ton of destabilization during the afternoon hours. Some of these solutions even favor scattered thunderstorms then developing across the western Carolinas and Georgia, a scenario that seemed unlikely as recently as yesterday.

However, there is some reason to be skeptical of this idea. For starters, the CAMs were varying degrees of horrible at actually initializing convection earlier this evening and none of them really managed the evolution of said convection well in the first 5ish hours of the 00z runs. The HRRR has been slowing the arrival time of this cluster (and weakening it) over the last several runs. CAM guidance has not done well with this synoptic regime this week, routinely struggling even upstream in the Great Lakes where convection has been well forced. It does seem reasonable to expect that this convection will exist in some form, likely forced by the aforementioned strong shortwave. So this leaves us with the need to assess what the environment awaiting the forcing will be like. This is uncertain as well, as there is a good bit of spread amongst REFS/HREF members over the degree of destabilization and mixing that occurs this afternoon. Looking at satellite derived PWs, it does look like there may be some slightly drier air pushing southeastward within the overall flow, which may aid in allowing us to mix slightly more than would be expected if it were a bit more moist (though not mixing as much as the HRRR suggests). But even if dewpoints bump down into the 68F-70F range, SBCAPE should still be around 2000+ j/kg, especially across the northern Midlands nearest the approaching front. Wind profiles actually look quite robust for July as the trough settles into place, with 30-40 knots of effective shear per RAP and REFS mean soundings. This the combination of forcing, instability, and shear (especially in the northern half of the area) is enough to justify the Slight Risk from SPC that is there. But the exacts of this setup? Who knows. Forecaster confidence is low in this setup, especially since the background synoptic environment favors convection to our north and northeast. But given we have actual shear overlapping with summertime moisture and instability, damaging wind gusts with strong multi-cell clusters is the most likely severe hazard.

The front is forecast to slide through the area on Wednesday. Drier, albeit not cooler, air is forecast to arrive behind it, providing quite the relief from the oppressive humidity that we have been experiencing the last several weeks. PWs are forecast to fall to around 1", which should result in dewpoints falling into the 60s across the area. It won't be anything too crazy, but it will be a nice break from oppressive humidity. As the well-advertised trough continues to retrograde westward, an increase in moisture is expected over the weekend. Still expecting rain chances to increase Sat-Mon as a result.

AVIATION /10Z TUESDAY THROUGH SATURDAY/

Brief morning fog/stratus possible.

Light and variable or calm winds early this morning may allow for brief restrictions. However convective debris cloudiness over the area will hinder a widespread fog threat. We continued tempo groups for vsby restrictions around sunrise.

Winds pick up from the southwest by 15Z today and increase to around 10 knots by 19z with some gusts up to 20 knots through the afternoon ahead of an approaching front. Convection is expected to develop along the front late today but remain to our north through 00Z. Non-convective gusts should subside after 00Z. As thunderstorms continue south through the evening we expect activity to diminish but storms could still near CAE/CUB between 00Z and 06Z. If they remain in tact through the evening there could be strong winds at CAE and CUB but confidence is low.

EXTENDED AVIATION OUTLOOK...Rain chances increase mid-week with restrictions possible as a front pushes into the Southeast. Drier air moves in for the second half of the week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Heat Advisory from noon today to 8 PM EDT this evening for SCZ016-022-027>031-035-037-038-041-135>137. GA...Heat Advisory from noon today to 8 PM EDT this evening for GAZ063>065-077.