textproduct: Charleston

WHAT HAS CHANGED

The fog key message was removed. The Aviation discussion was updated for the 04/12z TAFs.

KEY MESSAGES

- 1) Another round of scattered to numerous showers and thunderstorms will impact the region today. Locally heavy rainfall is possible along with a risk for an isolated strong to severe thunderstorm.

- 2) A typical summertime pattern returns from midweek through the weekend, with isolated strong to severe thunderstorms possible each day.

DISCUSSION

KEY MESSAGE 1: Another round of scattered to numerous showers and thunderstorms will impact the region today. Locally heavy rainfall is possible along with a risk for an isolated strong to severe thunderstorm.

Water vapor imagery shows the upper trough to the west has completely cut off from the main belt of the westerlies and has closed off into an upper-level cyclone. The upper low is forecast to meander over northern/central Georgia through tonight with the local area located along its far eastern periphery. A large area of dry air observed on moisture imagery is poised to push in from the southwest later today which will result in a considerable decrease in PWAT values compared to the near record levels observed from the past several days, likely dropping down into the 1.50-1.90" range by this afternoon (lowest PWATs at the coast, highest far interior Southeast Georgia). Despite this, modest instability (MLCAPE 1500-1800 J/kg) is still expected to occur with highs warming into the upper 80s/lower 90s with dewpoints generally in the lower-mid 70s (some upper 70s at the coast in the vicinity of the sea breeze). MLCAPE values should peak in the 1500-2000 J/kg range this afternoon. With the upper low holdings to the west, several around of showers/tstms are poised to impact the region through tonight.

Round 1: The first round is expected to impact parts of the Charleston County coast through daybreak. As low-level convergence builds over the Atlantic near the west wall of the Gulf Stream within a lingering plume of PWATs 2.25-2.45", convection is expected to slowly build. A number of the short- term CAMS suggest convection could consolidate into a fairly large complex, which could brush much of Charleston County, especially along/east of the US-17 corridor, including Downtown Charleston. Given the level of PWATS noted, intense hourly rainfall rates could occur with 04/00z HREF neighborhood probabilities for >1"/3-hr running 60-90% and >3"/3-hr 10-30% (both highest across the middle and upper portions of the Charleston County coast). Depending on how convection evolves, will need to watch for a possible short-term flood threat for this area, especially since will be during the morning commute. Fortunately, the mean 850-250 hPa steering flow remains in the 20-25 kt range out of the southwest which will keep storms fairly progressive.

Round 2: Once convection moves northeast of Cape Romain by mid- morning, dry conditions should prevail. Convection is expected to initiate by early afternoon across interior Southeast Georgia where a region of higher 850 hPa theat-e will be found. There are no discernible impulses forecast to cross the area this afternoon even with the upper low to the west. In fact, the main vorticity channel looks to remain displaced well offshore. This suggests convection will be largely airmass driven with the evolution of showers/tstms likely being driven by mesoscale interactions such as convective outflow collisions with the sea breeze. There are signals the sea breeze could become a large focus for convection along the I-95 corridor from Walterboro south to along the Georgia coast by late afternoon as outflow generated by convection over interior Southeast Georgia intersects it. Pops 40-70% were realigned slightly to favor this scenario, highest along the I-95 corridor from KRBW to near KBQK/KSSI. There will be a risk for locally heavy rainfall, but rainfall rates will be curtailed somewhat given the lower PWATs in place this afternoon. The dry air aloft will elevate the risk for damaging wind gusts, but with such 0-6km weak shear in place, any strong to severe tstms that form will remain pulse-like and closely tied to where updrafts can be locally enhanced by boundary collisions.

Round 3: Similar to the past few nights, nocturnal convection is expected to blossom over the Atlantic early Wednesday morning as convergence shifts back closer to west wall of the Gulf Stream. Some of this activity will likely stream into parts of the lower South Carolina coast into the Charleston Tri-County area as the steering trajectories back from southwest to south.

KEY MESSAGE 2: A typical summertime pattern returns from midweek through the weekend, with isolated strong to severe thunderstorms possible each day.

As the inland stationary front dissipates Wednesday while the upper low lifts northeast, Atlantic high pressure will build westward, placing the forecast area along its western periphery through the weekend. This pattern will support slightly less coverage than what has been seen over the last few days, with a daily round of scattered convection. The sea breeze and any remnant outflow boundaries will serve as the primary focus for afternoon and early evening thunderstorm development.

While instability will be somewhat limited Wednesday, gradually warmer temperatures and persistent moisture will contribute to a slight increase in instability each day noted by SBCAPE values rising from 1500 J/kg to 2500-3000 J/kg by Friday. Forecast soundings indicate DCAPE values of 800-1000 J/kg during each afternoon and evening period, which could support a few storms capable of producing damaging wind gusts. Deep moisture, with PWAT values near 2 inches each day, will make for efficient rainfall rates with some storms capable of dropping a quick inch or two of rain. Additionally, slow storm motions averaging 10 kt or so, combined with training or repeated rounds of convection, could increase the potential for isolated instances of minor flooding, especially in low-lying and poor drainage areas.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

04/12z TAFs: KCHS/KJZI: Thunderstorms will remain near both terminals through 14z before exiting to the northeast. KJZI still has the best chances for direct impacts and a TEMPO group for this was maintained and extended slightly. Later today, the greatest coverage of showers/tstms this afternoon look to remain west of the terminals. Overnight, yet another surge of showers/tstms moving onshore from off the Atlantic is poised to impact both terminals. There remains some timing and coverage differences in the various guidance, so introduced a PROB30 for TSRA from 08-12z to trend for now.

KSAV: An unexpected surge of low stratus moved north of the Altamaha River a few hours ago and will impact KSAV through mid- morning. An amendment to include IFR cigs was issued just after 10z and these conditions were maintained for the 12z TAF package. Cigs may briefly drop below alternate minimums through 14-15z, but as daytime heating builds, suspect prevailing cigs will remain at or above the critical 600 ft level. Risk for showers/tstms will increase this afternoon with data supporting a cluster of storms nearing the terminal by late afternoon along the sea breeze. For now, a TEMPO group for TSRA was included 21-24z with conditions limited to MVFR. The need for a prevailing group with possibly lower conditions will reassessed with later forecast cycles.

Extended Aviation Outlook: Periodic flight restrictions are expected in showers and thunderstorms this week.

MARINE

Through Tonight: Convection early this morning and again late tonight will pose a risk for mariners. Gusty winds, locally heavy rainfall with vsbys 1 NM or less and cloud-to- water lightning are all possible. Otherwise, a southerly wind regime will linger into tonight. Winds outside of convection look to remain 15 kt or less, although a few gusts to 20 kt could occur this afternoon near the beaches and Charleston Harbor and overnight across all waters. Seas will average 2-4 ft.

Wednesday through Saturday: High pressure in the Atlantic will be the dominant synoptic feature through the weekend. This will generate a typical summertime wind field with increased winds along the land/sea interface each afternoon associated with the sea breeze. Marine conditions are expected to remain below Small Craft Advisory criteria through the weekend.

Rip Currents: There is a moderate risk of rip currents for the Charleston County beaches today due to lingering 2 foot swells at 7-8 seconds, combined with elevated onshore flow. There is a low risk for all remaining beaches. Low risk for all beaches for Wednesday.

Waterspouts: Forecast sounding output and elevated non-supercell tornado parameters values over the nearshore and offshore waters look to support a risk for waterspouts this morning. Satellite and radar already suggest a few cloud lines are trying to develop over the Atlantic which could become the focus for waterspouts through late morning. A Marine Weather Statement was issued to alert mariners to this possibility.

CHS WATCHES/WARNINGS/ADVISORIES

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