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: Another round of thunderstorms is expected across portions of the area this afternoon and evening. The environment is quite similar to the last few days as well. Modifying the 12z KCHS RAOB indicates steep low-level lapse rates, moderate surface-based instability, and sufficient mid- level dry air to yield DCAPE values well in excess of 1,000 J/kg. Flow through the column is westerly again, including near the surface, which will keep the sea breeze less progressive and pinned along the coast. Dewpoints across southeast GA are expected to mix out into the mid to upper 60s (along and south of I- 16), which should keep thunderstorm chances much lower there. The focus will therefore be along the Savanna River and to the northeast across our southeast SC counties. Regarding initiation, it appears the early to mid afternoon with a few storms starting to develop perhaps along the sea breeze. There could be a strong to marginally severe storm with these as they develop, but it looks like the main severe threat will come later in the afternoon and into the evening. Upstream clusters of storms are expected to develop across central GA and the SC Midlands and track east-southeastward toward the area. This is when the most significant severe threat is anticipated, primarily in the form of damaging wind gusts. There is also potential for locally heavy rainfall, and even some flash flood risk, in areas where storms move across the same areas aligned within the westerly steering flow. The severe threat could extend beyond sunset, putting the main time period of concern roughly 4-10pm.

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 /18Z SATURDAY THROUGH THURSDAY/

The 18z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV. The main forecast challenge is once again thunderstorms. The best chance for storms this afternoon and evening will once again be at KCHS and KJZI. Showers and storms are already starting to develop back to the west and the potential for impacts will increase through the afternoon and into the evening. TEMPO groups have been added at KCHS and KJZI to account for gusty winds and reduced visibilities within heavy thunderstorm rains. For now the timing is roughly 21- 01z, but this could need to be refined with amendments as storms evolve. Thunderstorm coverage should dissipate by late evening, but depending on how outflow boundaries evolve there could be some risk of storms into the early morning hours.

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 Temperatures:

July 11: KCHS: 102/1986 KCXM: 100/2001 KSAV: 103/1980

Record High Minimum Temperatures:

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

July 12: KCHS: 79/2020

CHS WATCHES/WARNINGS/ADVISORIES

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