textproduct: Charleston

WHAT HAS CHANGED

The Aviation Section has been updated for the 12Z TAF issuance.

KEY MESSAGES

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

- 2) Hot and humid conditions will continue today, with slightly cooler temperatures Monday and Tuesday.

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

DISCUSSION

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

Today and Tonight: Aloft, a ridge extending across the western Atlantic and Gulf will slowly weaken as a trough progresses across the Mid-Atlantic States. The trough will help nudge a weak cold front toward northern zones this evening, then slowly southward with time across Southeast South Carolina overnight, before reaching Southeast Georgia by around daybreak Monday. Ahead of the front, a very warm and moist airmass will set the stage for afternoon showers and thunderstorms locally heading into peak heating, with activity possibly occurring along a pinned sea breeze initially prior to across interior South Carolina as the front inches closer to the region late afternoon into early evening. Moderate instability, weak shear, strong low-lvl lapse rates and notable DCAPE could support strong to severe thunderstorms locally with damaging winds being the primary hazard with strongest activity. The latest SPC Day 1 Convective Outlook continues to advertise a Slight Risk for severe weather across much of Southeast South Carolina and for areas along/north of I-16 across Southeast Georgia. A Marginal Risk is in place for remaining areas south of I-16 across Southeast Georgia. Similar to the previous day, westerly flow could push precip activity back toward coastal locations later in the day. Convection could also persist a bit longer during evening hours given the arrival of the front overnight.

Although storm motions should be slightly faster than the previous day, steering flow still remains on the weaker side, and with deep moisture residing locally (PWATs 2.0-2.25 inches), the potential for isolated flooding remains locally, especially if multicellular thunderstorm activity congeals/clusters along boundary collisions (sea breeze and outflows) during the day and/or occurs across/near the slowly southward moving/stalling front during the overnight period. Precip activity could also pose an additional risk for flooding along coastal locations should it occur during the evening high tide cycle (see Key Message #3). The latest WPC Day 1 Excessive Rainfall Outlook highlights the Tri-County Area in a Slight Risk and remaining areas of Southeast South Carolina and Southeast Georgia in a Marginal Risk for excessive rainfall/flash flooding potential.

Monday and Tuesday: A weak cold front stalling across the region early week should remain the primary focus for shower and thunderstorm activity locally early into possibly mid week. A few thunderstorms could become strong to marginally severe each afternoon with damaging winds being the primary concern and the potential for precip activity could linger into night periods given the vicinity of the front/stalled boundary. Weak low pressure could develop along this feature as well, and if so would enhance moisture pooling/convergence locally and increase heavy rainfall potential. The latest SPC Day 2 Convective Outlook advertises a Marginal Risk for severe weather across the entire area Monday while WPC highlights the entire area in Marginal Risk for excessive rainfall Monday and Tuesday.

KEY MESSAGE 2: Hot and humid conditions will continue today, with slightly cooler temperatures Monday and Tuesday.

Before storms start forming in the afternoon, the dangerous heat wave continues with afternoon highs in the mid to upper 90s. Dewpoints will be highest along the coast and up into the tri-county region in the upper 70s, resulting in heat index values peaking near 110 degrees. Thus, the previously issued Heat Advisory looks to be in good shape. Inland areas will see a touch less moisture with dewpoints in the lower 70s, keeping heat index in the lower to mid 100s.

Monday - Tuesday: Slightly cooler air is brought down into the region as a weak cold front pushes through Monday morning, with the front remaining across/south of the region into Tuesday. In addition to the front, the increased cloud coverage and rainfall will bring high temperatures down into the upper 80s to lower 90s both Monday and Tuesday. Heat index values may still eclipse 100 for areas along the coast, with inland areas peaking in the 90s.

H500 heights begin to rise again on Wednesday as the ridge strengthens, with temperatures rising back up into the lower to mid 90s. Temperatures look to further climb into the mid 90s Thursday and Friday.

KEY MESSAGE 3: Minor coastal flooding is possible during the evening high tide cycles today 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 this evening. The greatest threat is expected Monday and Tuesday evening 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 /12Z SUNDAY THROUGH THURSDAY/

VFR conditions should prevail at CHS/JZI/SAV terminals through 16Z Sunday. Shower/thunderstorm chances increase during afternoon hours, with CHS/JZI likely to see the greatest impacts in regards to gusty winds and reduced vsbys. However, similar impacts are possible at SAV as the afternoon progresses. TEMPO groups for thunderstorms, gusty winds (15-25 kt) and reduced vsbys (4SM) have been included at the CHS/JZI terminals between 20-24Z Sunday, followed up by PROB30 groups for thunderstorms and gusty winds/reduced vsbys between 00- 03Z Monday. At SAV, PROB30 groups between 21-03Z have been included for thunderstorms and similar gusty winds/vsbys as CHS/JZI. VFR conditions should then return at all terminals by around 04Z Monday, then persist through 12Z Monday. However, there could be some lower cig concerns at CHS/JZI as a weak front enters the region late in the TAF period. Confidence in timing/duration is too low to include in the latest TAF issuance.

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

MARINE

Today through Thursday: Ahead of an approaching cold front, winds will remain out of the west this morning, becoming breezy out of the southwest as the sea breeze pushes ashore this afternoon. Similar to the past couple days, afternoon thunderstorms are possible and will likely bring a risk of strong winds along with them. 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 through Tuesday as the front sags south, 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.

Rip Currents: A moderate risk for rip currents will remain from the South Santee River down to Savannah today, due to 1-2 ft swells with wave periods up to 9 seconds along with increasing tidal influences from the upcoming lunar perigee on Monday. In addition, SSW winds will likely generate a moderate longshore current, possibly supporting rip currents near piers and jetties.

CLIMATE

Record High Minimum Temperatures:

July 12: KCHS: 79/2020

CHS WATCHES/WARNINGS/ADVISORIES

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