textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Showers and thunderstorms through Tuesday could produce locally heavy rainfall and isolated flooding. A few storms could also become strong to severe.

- 2) Hot and humid conditions will continue into Sunday.

- 3) Minor coastal flooding is possible during the evening high tide cycles Sunday through Wednesday.

DISCUSSION

KEY MESSAGE 1: Showers and thunderstorms through Tuesday could produce locally heavy rainfall and isolated flooding. A few storms could also become strong to severe.

Through tonight: Thunderstorms have already come and gone across the area as we head into the evening. Outflow will continue to work across southeast GA, but dewpoints are low enough to keep the potential for storm development low. Some residual light rain will persist across portions of the Charleston Tri-County for the next couple of hours. Otherwise, we anticipate dry conditions through the overnight.

A weak cold front settling into the region Sunday evening into Monday will serve as the primary focus for showers and thunderstorms Sunday into Tuesday. Deep moisture, with precipitable water values between 2 and 2.5 inches, and weak steering flow will support efficient rainfall rates and slow storm motions, increasing the potential for localized flooding. While weak shear should limit storm organization, sufficient instability with SBCAPE values on the order of 1500-2000 J/kg could support a few strong to marginally severe thunderstorms.

Guidance also suggests the potential for a weak surface low to develop along the nearly stationary front early next week. Should this happen, enhanced moisture pooling would further increase the heavy rainfall potential. WPC continues to highlight the area with a Marginal Risk (Level 1 of 4) for excessive rainfall each day Sunday through Tuesday. Minor flooding is possible across low-lying and poor drainage areas, and metro locations such as Downtown Savannah and Downtown Charleston.

KEY MESSAGE 2: Hot and humid conditions will continue into Sunday.

Dangerous heat will continue Sunday before showers and thunderstorms develop. Afternoon highs in the mid to upper 90s combined with dewpoints in the upper 70s near the coast will support peak heat index values around 110-112 degrees. Another Heat Advisory has been issued for the coastal counties and Berkeley County on Sunday.

Dew points across inland locations should mix into the lower 70s, keeping peak heat index values around 102-106F. While still hot in areas away from the coast, heat index values will likely remain below our Heat Advisory criteria of 108F.

KEY MESSAGE 3: Minor coastal flooding is possible during the evening high tide cycles Sunday through Wednesday.

Increasing astronomical tides associated with Monday's lunar perigee and Tuesday's New Moon will bring a risk of minor coastal flooding beginning Sunday evening. The greatest threat is expected Monday through Tuesday as astronomical tides continue to increase, mainly along the coastal Charleston and coastal Colleton Counties. Additionally, heavy rainfall occurring near or during the time of the evening high tide cycles could exacerbate flooding.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

The 00z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV. Thunderstorms have already ended around KCHS and KJZI, and we do not anticipate any development near KSAV this evening. The overnight should be dry and VFR. Attention then turns to what will likely be another round of thunderstorms Sunday afternoon. Confidence in exact timing and placement isn't particularly high so we have just added in a PROB30 at all 3 sites starting around 20z.

Extended Aviation Outlook: Shower and thunderstorm coverage, along with risks of associated restrictions, will increase Sunday through Tuesday.

MARINE

Through tonight: Southwest flow will become established across the local waters this afternoon and strengthen into the evening. Wind speeds should mostly peak in the 15-20 knot range, but a few gusts up to around 25 knots will be possible. The strongest winds are expected along the land/sea interface with the sea breeze. It is possible that conditions could become marginally supportive of a Small Craft Advisory for the Charleston County waters, but the probability is considered low at this time. Seas should average 2-4 feet. Also, another round of thunderstorms is expected to develop inland this afternoon and evening. These storms could eventually reach the coast and the coastal waters, producing frequent lightning and strong wind gusts through the late evening hours.

Sunday through Thursday: A weak cold front will gradually settle into the South Carolina waters Sunday evening into Monday, causing winds to weaken and become more variable. Scattered showers and thunderstorms will increase across the coastal waters Sunday through Tuesday, with a few storms capable of producing strong to severe wind gusts, frequent lightning, and locally higher seas. Outside of convection, winds and seas are expected to remain below Small Craft Advisory criteria.

CLIMATE

Record High Minimum Temperatures:

July 11: KCHS: 80/2016 KCXM: 82/2016

July 12: KCHS: 79/2020

CHS WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from 11 AM to 7 PM EDT Sunday for GAZ217>219-239- 241. SC...Heat Advisory from 11 AM to 7 PM EDT Sunday for SCZ045-148>152. MARINE...None.