textproduct: Columbia

WHAT HAS CHANGED

Severe weather threat this afternoon and evening appears limited to areas along and north of the I-26 corridor. Tropical Depression Two forms in the eastern Gulf of Mexico. Aviation discussion updated for 18z TAFs.

KEY MESSAGES

- 1. Conditional severe weather threat this afternoon and evening, mainly north of the I-20 corridor.

- 2. Isolated to scattered convection possible Monday and Tuesday, with greater coverage of storms expected mid to late week.

DISCUSSION

Key Message 1: Conditional severe weather threat this afternoon and evening, mainly north of the I-20 corridor.

Tropical Depression Two has formed in the eastern Gulf of Mexico, and water vapor imagery shows an axis of drier mid level air extending from the western periphery of the TD into the lower CSRA and eastern Midlands. This drier air aloft is having a notable effect on cumulus development this afternoon, with mainly clear skies over eastern portions of the forecast area and rather flat cumulus farther west as of early afternoon. As the day progresses, a short wave dropping across the Ohio Valley will allow a surface trough to sag farther southward into NC by this evening. Increased forcing for ascent could allow for isolated to scattered showers and thunderstorms to develop through the afternoon and evening, but any activity appears to remain generally north of the I-26 corridor based on the latest CAMs guidance.

With strong surface heating and surface dewpoints in the low 70s, SBCAPE over 3000 J/kg is expected to develop. Generous mixing should also yield DCAPE values near 1000-1200 J/kg, so if a updraft can become better established and combat dry entrainment in the mid-levels, a few strong to severe storms will be possible with the risk of damaging downburst winds. Portions of the Midlands and Pee Dee generally along and north of the I-26 corridor remain in SPC's Slight Risk (level 2 out of 5) for severe weather with the severe probabilities dropping significantly farther south and east.

Key Message 2: Isolated to scattered convection possible Monday and Tuesday, with greater coverage of storms expected mid to late week.

Deeper moisture from TD Two appears to remain south of the area on Monday, however weak short wave energy aloft and a surface trough positioned just north of the area could be a trigger for isolated to scattered convection in the afternoon and evening with precipitable water values still up around 2 inches. Coverage should be greatest across northern portions of the forecast area where synoptic forcing is a bit stronger. Moderate to strong instability will still be in place, with an environment not much different than today, so there will be a threat of isolated strong to severe thunderstorms.

The weak surface trough appears to retreat farther northward on Tuesday, with the bulk of any short wave energy associated with an upper level trough remaining north of the area as well. This should limit synoptic forcing across the area, with mainly isolated diurnal convection expected.

Fairly good support from the ensembles and global models in the pattern becoming more active mid to late week as an upper level trough digs across the eastern US, with a series of short waves crossing the area within the northwest flow aloft. Meanwhile at the surface, while there are some timing differences in the models, a cold front drops into the area sometime late Wednesday into Thursday, possibly lingering nearby into the weekend. Deep moisture, with precipitable water values upwards of 2.00-2.25 inches based on the blend, along with synoptic lift will support higher chances for showers and thunderstorms. There is also the potential for strong to severe thunderstorms during this time period.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

VFR conditions expected through the forecast period outside of convection.

Diurnal cumulus developing across the region in daytime heating with some storms developing along a weak trough north of the terminals. Hi-res guidance suggests activity should generally remain north of terminals this afternoon but maybe some storms make it into the CAE/CUB vicinity later so have kept the PROB30 going there, but left mention of storms out of the other TAFs. Breezy south to southwesterly winds 10 to 12 knots with gusts around 20 knots through the afternoon before diminishing after sunset. Some convective debris clouds expected overnight but guidance not showing much stratus/fog threat tonight.

EXTENDED AVIATION OUTLOOK...Potential for restrictions from showers and thunderstorms continues into next week.

CAE WATCHES/WARNINGS/ADVISORIES

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