textproduct: Columbia

WHAT HAS CHANGED

The Heat Advisory for Lancaster and Chesterfield Counties today was allowed to expire. Another Heat Advisory for the whole CWA remains in effect from 11am to 8pm Saturday. Aviation discussion updated for 00z TAF issuance.

KEY MESSAGES

- 1. Heat Index values up to 110 Saturday elevating the risk of heat related illnesses.

- 2. Severe storms possible Saturday afternoon or evening.

- 3. Heat Advisory for Lancaster and Chesterfield Counties has expired.

DISCUSSION

KEY MESSAGE 1. Heat Index values up to 110 Saturday elevating the risk of heat related illnesses.

The center of the upper ridge shifts west on Saturday. Weak downslope winds and increasing moisture ahead of a cold front will push afternoon temperatures into the upper 90s to near 100 degrees and Heat Index values to near 110. There will also be a greater vulnerability to heat related illness due the uptick in outdoor activities this Saturday. A Heat Advisory has been issues for the entire forecast area.

There is some potential for thunderstorms, especially in the northern FA late Saturday afternoon and evening. But we believe that the later timing of the storms will still allow Heat Index values to exceed advisory thresholds through the bulk of the afternoon.

KEY MESSAGE 2. Severe storms possible Saturday afternoon or evening.

As ridging shifts west, we will see shortwave energy dig into the Mid Atlantic States on Saturday afternoon and evening. A cold front well to our north will be the focus for convective initiation. As storms develop and move south and east during the day they will encounter very steep lapse rates which would support damaging winds in some of the stronger storms. The overall limiting factors for severe weather into SC will be the limited moisture available and the fact that the stronger dynamics should remain to our north. With PWAT values only 1.6 to 1.8 inches we may only see scattered thunderstorms that do not develop into an MCS. However, even if the stronger storms do remain to our north, an outflow boundary could still push into the northern FA in the evening. This could spark at least scattered convection through the evening and overnight as it progresses southward through the forecast area.

With a surface boundary over the region on Sunday we can expect at least a chance of showers and thunderstorms as an upper level shortwave crosses into the forecast area. With a deeper trough and higher atmospheric moisture we would expect scattered to numerous thunderstorms Sunday afternoon and evening. Cloud cover could ultimately hinder potential instability and lowers the threat of severe weather. That said a few storms may still become severe, especially if cloud cover starts breaking earlier than expected.

KEY MESSAGE 3. Heat Advisory for Lancaster and Chesterfield Counties has expired.

Heat indices are gradually falling below 100F in the Advisory area and thus the Advisory has been allowed to expire. Overnight lows tonight should stick near the mid 70s before another very hot day is expected Saturday as discussed in Key Message #1.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Mainly VFR conditions through the TAF period, with some fog and stratus possible near daybreak.

Similar conditions are expected through the overnight period tonight with mostly VFR conditions and developing restrictions early Saturday morning with lingering low level moisture trapped below some drier air aloft. Guidance generally favors restrictions affecting the southern terminals so have kept conditions VFR at CAE/CUB and brought in restrictions elsewhere, once again in a tempo group with overall limited confidence in coverage. Restrictions will likely dissipate shortly after sunrise and are not expected to linger as long as this past morning with moisture a bit more shallow. A front approaches this evening which will lead to isolated storms developing, although coverage near terminals will likely be limited so have excluded any mention of tsra from the TAFs with this issuance.

EXTENDED AVIATION OUTLOOK...Chances for restrictions and thunderstorms increase Sunday. Diurnal storm chances continue early next week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Heat Advisory from 11 AM to 8 PM EDT Saturday for SCZ016-018- 020>022-025>031-035-037-038-041-115-116-135>137. GA...Heat Advisory from 11 AM to 8 PM EDT Saturday for GAZ040- 063>065-077.