textproduct: Charleston

WHAT HAS CHANGED

All Key Messages and sections were updated.

KEY MESSAGES

- 1) Isolated to scattered showers and thunderstorms today with increasing rain chances at the coast early Saturday morning.

- 2) The typical summertime pattern will continue this weekend into next week. The threat for isolated strong to severe thunderstorms and localized heavy rainfall will exist each day.

DISCUSSION

KEY MESSAGE 1: Isolated to scattered showers and thunderstorms today with increasing rain chances at the coast early Saturday morning.

A deep onshore flow regime will hold through tonight with the region positioned along the western fringes of the Bermuda- Azores high. Some nocturnal shower/tstm activity is expected to develop offshore through daybreak, which may make a run for the beaches, particularly along the lower South Carolina coast. GOES-E Total Precipitable Water products are highlighting a large area of drier air characterized by PWATS <2" lurking just offshore. The latest RAP shows PWATs falling through day as this drier airmass pushes onshore, but surface dewpoints still look to remain near climatological normals for early August (lower- mid 70s) aided by the flow off the Atlantic. Still, net moisture values will be considerably lower compared to the past few days which will tend to limit convection somewhat, especially with a fairly progressive sea breeze expected to move inland through the afternoon. Pops today were limited to 20-40%, highest interior Southeast South Carolina, and is consistent with the 07/01z NBM. Modified soundings do show elevated DCAPES which could yield a risk strong or locally damaging wind gusts with any tstms that form. Highs will warm into the lower 90s away from the beaches with heat indices peaking 100-105.

Deep moisture will begin to surge from the south tonight as the upper ridges weakens and retrogrades farther offshore and a broad mid-level weakness to the west nudges east. This will generally result in an uptick in marine-based noctural convection overnight into early Saturday, but there are mixed signals on exactly how much coverage will occur as daybreak approaches. Moisture convergence does not look overly strong given the fairly weak wind fields noted both aloft and at the surface, so suspect some of the more reliable CAMs are likely too aggressive in the amount of convection that will form over the Atlantic late. Given the uncertainties, opted to maintained output from the NBM, which highlights pops as high as 40-60% at the coast late. Adjustments may be needed in later forecast cycle updates as additional data and model output are received.

KEY MESSAGE 2: The typical summertime pattern will continue this weekend into next week. The threat for isolated strong to severe thunderstorms and localized heavy rainfall will exist each day.

As the pattern with deep high pressure over the western Atlantic and Eastern Seaboard continues, daily scattered convection chances will remain possible. High-res convection allowing models continue to suggest that Saturday will start off with morning showers and thunderstorms as a shortwave moves onshore, with additional development possible on any remnant outflow boundaries throughout the afternoon hours. PWATs near 2 inches are expected throughout the day, with forecast sounding profiles showing long and skinny CAPE profiles, both indicating locally heavy rainfall will be possible. REFS and HREF would both suggest scattered 50-70% chances for an inch in a 6 hour period, with scattered 30-50% chances for 2 inches in a 6 hour period. Difficult to narrow down areas most prone to the heavy rainfall threat given the scattered nature of the convection, with WPC indicating a level 1 out of 4 (marginal) risk of Flash Flooding across the region. Given 1000-2000 J/kg of SBCAPE an isolated risk for severe weather can't be entirely ruled out, with the rather weak 10 knots of 0-6km bulk shear suggesting storms will remain pulse-like. Given the high water loading potential and ~500 J/kg of DCAPE, strong wind gusts would be the primary risk.

For Sunday into next week, showers and thunderstorms will be primarily driven by the expected afternoon sea-breezes and outflow boundaries. Given similar environmental conditions each day, locally heavy rainfall and an isolated strong-severe thunderstorm will be possible Sunday and into next week.

As for temperatures, Saturday is looking to be a touch cooler given the earlier start to showers and thunderstorms, with highs in the mid 80s to lower 90s. Sunday into next week will likely see high temperatures in the lower to mid 90s, likely getting closer to the mid 90s by Tuesday. Will have to see how the dewpoint temperature forecast trends, as areas along the coast currently have heat index values peaking into the upper 100s on Monday and Tuesday, which would necessitate Heat Advisories being posted.

AVIATION /10Z FRIDAY THROUGH TUESDAY/

07/12z TAFs: KCHS/KJZI/KSAV: There are no major concerns for today. Isolated showers will meander near the terminals through mid-morning. VCSH was highlighted to account for this. This afternoon, the risk for tstms look to remain west of the terminals with the sea breeze passing through a bit earlier than normal. The risk for showers/tstms will increase along the coast by daybreak Saturday as a surge of tropical moisture pushes onshore. The risk through 12z is highest at KSAV. VCSH was noted after 09z to trend for now.

Extended Aviation Outlook: Periodic flight restrictions are expected in showers and thunderstorms through early next week. Locally heavy rainfall may produce brief IFR or lower visibility restrictions.

MARINE

Through Tonight: Southeast winds 15 kt or less will persist into tonight with the region located along the western flanks of Atlantic high pressure. Seas 2-3 ft today will build to 3-5 ft tonight.

Saturday through Tuesday: Atlantic high pressure will be the dominant synoptic feature through the weekend. Light to moderate winds prevail with gusty winds along the land/sea interface each afternoon associated with the sea breeze. Be alert for the potential for nocturnal jetting to set up Sunday into early next week, keeping breezy conditions in place along the beaches overnight. This feature is typically poorly resolved by models, but the combination of the weak surface trough shifting inland with persistent strong high pressure offshore and very warm water makes for a favorable setup.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...None.