textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) A prolonged heat wave will maintain hot and humid conditions through the end of the week and into the start of the weekend.

- 2) Typical summertime afternoon and evening thunderstorms this week with increasing rain chances Sunday through Tuesday.

DISCUSSION

KEY MESSAGE 1: A prolonged heat wave will maintain hot and humid conditions through the end of the week and into the start of the weekend.

For Friday, the region will solidly placed along the northern periphery of the subtropical anticyclone aloft centered just to the east of the Bahamas. Low-level thickness and 850 hPa temperatures will change little from Thursday with values holding 1437-1442m and 20-22C respectively. The day will start off rather warm with temperatures likely to warm quickly after daybreak. Soundings and low-level thickness schemes support another day of highs in the upper 90s to near 100 with some locations likely surpassing 100. The record highs at both KCHS and KSAV could be challenged. Guidance does show mid-level moisture drying out a bit in response to a dry slot passing through aloft. This coupled with vertical deep mixing should help mix out dewpoints into the upper 60s inland which will limit heat indices there compared to the past few days. Closer to the coast, some mixing is also likely too occur into the early afternoon before dewpoints surge again with the passage of the sea breeze. Similar to the past few days, dewpoints are expected to pool back into the upper 70s/lower 80s near/behind the sea breeze itself. The inland progression of the boundary does look to propagate inland a bit later than usual with the best pooling of dewpoints likely to occur across the lower South Carolina coast, especially the Charleston County coast. Coastal heat indices are poised to peak in the 108-112 in the coastal corridor with 110-115 along the Charleston County coast. For this reason, a Heat Advisory has been posted for the coastal counties as well as Dorchester and Inland Berkeley with an Extreme Heat Warning for Charleston and Tidal Berkeley where the chances for reaching 113 heat indices for at least hours is the highest. Isolated afternoon convection could provide some relief for a few areas, but a Major to Extreme heat risk persists across all of the Lowcountry and Southeast Georgia. Overnight, lows are only forecast to dip into the upper 70s/lower 80s. These warm overnight conditions could result in prolonged heat stress, especially for more vulnerable communities and those without adequate cooling overnight.

For Saturday, things look a bit cooler overall as the subtropical ridge begins to flatten out yielding slightly cooler temperatures and a bit more in the way of shower/tstm activity. Heat indices are still expected to peak in the 105-110 range across the coastal corridor with 100-105 farther inland. Another Heat Advisory may be needed for these areas depending on how quickly convection sparks, but levels look to fall short of Extreme Heat Warning criteria.

KEY MESSAGE 2: Typical summertime afternoon and evening thunderstorms this week with increasing rain chances Sunday through Tuesday.

Friday and Saturday: This hot and humid pattern will persist across the region. However, forecast soundings indicate slightly less PW and some potential dewpoints to mix a bit lower. The forecast features 20-30% pops for parts of the Lowcountry with 20-50% pops for Saturday, highest over the Charleston Tri- County.

Sunday through Tuesday: The GFS and ECMWF indicates that a H5 trough will ripple over the region and persist during early next week. At the sfc, a cold front is forecast to push south over the region, possibly becoming stationary. In addition, AIFS indicates that a broad sfc low may develop over the forecast area Sunday night, pushing off the coast Monday into Monday night. Forecast soundings indicate the PW will increase to 2 to 2.2" across the region. Synoptic scale forcing along with slight to moderate instability should yield much higher shower and thunderstorm coverage. These storms may potential produce pockets of excessive rainfall given the deep moisture and possible concentration of storm coverage near the coastal low. WPC has highlighted the region with a marginal risk on Sunday and Sunday night. The marginal risk could be expanded into Monday.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

00Z TAFs: Radar indicates isolated convection along the sea breeze this evening, which is generally inland of all of the TAF sites. Given the low probabilities of impacts, we kept all of the TAF sites VFR and dry. Winds should gradually ease later this evening. Another round of convection is possible Friday afternoon, with the highest probabilities for impacts at KCHS. Also, gusty winds are expected with the sea breeze Friday afternoon.

Extended Aviation Outlook: There will be a low end risk of thunderstorm impacts each afternoon and evening through Friday. Showers and thunderstorm coverage and risk of associated restrictions may increase Sunday through Tuesday.

MARINE

Through tonight: It continues to look like the gradient will pinch a bit across the local waters through this evening, producing enhanced south to southwest flow. Wind speeds of 15-20 knots should be common, strongest across the Charleston County waters and along the Charleston County coast (and Charleston Harbor) where sustained speeds could reach 20 knots with gusts pushing 25 knots. A Small Craft Advisory could be needed in these areas through the evening. Winds should then gradually diminish through the late night hours while turning more west- southwest. However, speeds will remain elevated in the 15-20 knot range. Seas are expected to be in the 2-4 ft range, and up to 5 ft in portions of the Charleston County waters.

Friday: Surges in the wake of the sea breeze may result in southwest winds to increase during the afternoon and continue into Friday night with a classic summer nocturnal surge. Wind gusts may peak around 25 kts, possibly requiring Small Craft Advisories. The short duration of the elevated winds should keep wave heights between 3 to 4 ft.

Saturday into Tuesday: The pressure pattern should support south-southwest winds between 10 to 15 kts, with occasional gusts into the low 20 kts. Wave heights are forecast to favor 2 to 4 ft.

Rip Currents: There is a Moderate Risk for rip currents for the Charleston County coast on Friday, and then for all of southeast SC and southeast GA on Saturday. Conditions will feature 2-3 ft swell with wave periods up to 9 seconds. In addition, SSW winds will likely generate a moderate longshore current, possibly supporting rip currents near piers and jetties.

CLIMATE

Record High Temperatures:

July 10: KCHS: 100/1986 KCXM: 99/1986 KSAV: 102/1879

July 11: KCXM: 100/2001

July 12: KCXM: 97/1966

Record High Minimum Temperatures:

July 9: KCHS: 80/1998 KCXM: 85/1998 KSAV: 80/1883

July 10: KCHS: 81/1986 KCXM: 82/1986

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 Friday for GAZ217>219-239- 241. SC...Heat Advisory from 11 AM to 7 PM EDT Friday for SCZ044-045-148- 149-151. Extreme Heat Warning from 11 AM to 7 PM EDT Friday for SCZ150- 152. MARINE...None.