textproduct: Charleston

WHAT HAS CHANGED

All sections have been updated.

KEY MESSAGES

- 1) Rounds of showers and thunderstorms are expected to impact southeast Georgia and southeast South Carolina through Tuesday. Locally heavy rainfall will be the primary concern, and isolated flash flooding will be possible. Also, a few strong to marginally severe storms could occur with damaging wind gusts.

- 2) Near-normal temperatures are expected to return end of this week and warming into the weekend.

- 3) Minor coastal flooding is likely for portions of the southeast South Carolina coast with the evening high tide cycles through Wednesday.

DISCUSSION

KEY MESSAGE 1: Rounds of showers and thunderstorms are expected to impact southeast Georgia and southeast South Carolina through Tuesday. Locally heavy rainfall will be the primary concern, and isolated flash flooding will be possible. Also, a few strong to marginally severe storms could occur with damaging wind gusts.

Through tonight: Aloft, the forecast area will sit within the influence of a mid-level trough and just to the east of a developing closed low (albeit weak). At the surface, a weak boundary will sit west to east across the area, perhaps more across southeast SC. This setup should provide a focus for shower and thunderstorm development this afternoon within an environment of precipitable water values solidly in the 2.1-2.25 inch range. Hi-res model solutions show a good bit of variance in possible solutions, but the general consensus is for development to first take place across southeast GA and perhaps closer to the coast. Then with time through the evening and into the overnight, convection could steadily shift more inland as it is driven by subtle cyclonic flow around a weak area of low pressure that develops to the west. Rainfall amounts will likely be highly spatially variable due to the convective nature of development. But, there is support in the HREF for pockets of 1-3 inches to be common along with isolated pockets of 3-5 inches possible. The HREF 3-hour rainfall probabilities suggest a widespread 70% or greater chance of 1 inch or more just about everywhere, and 30-50% probabilities of 3 inches or more favored for inland areas away from the coastal corridor. The exact placement of the heaviest areas of rain are difficult to pin point, as mesoscale boundary interactions will have a lot to do with that. The entire forecast area remains in WPC's Slight Risk in the Day 1 Excessive Rainfall Outlook. This makes sense given the residual wet conditions from the last few days, and the potential for intense rainfall this afternoon and into tonight. Current thinking is that as convection shifts inland with time, we should see most of the rain potential end for the forecast area through the overnight hours. Also of note, though the environment doesn't look particularly support of severe thunderstorms, we could still see a few strong to marginally severe storms. Instability should be a bit weaker and we certainly don't have much mid-level dry air available. But, we could see sufficient updraft enhancement near boundary interactions to produce a wet microburst threat.

Tuesday: The aforementioned boundary should sink a bit to the south which would also shift the focus and threat for heavy rain a bit south as well. However, the environment will remain very moist and thunderstorm activity is expected just about everywhere. It is hard to put much confidence in narrowing down the location of the greatest threat for locally heavy rain. So, the entire forecast area remains within WPC's Marginal Risk in the Day 2 Excessive Rainfall Outlook. The HREF produces similar 3-hour probabilities as today, with the entire area in the 70% or greater area for 1 inch or more and support for 30-50% chances of 3 inches or greater (perhaps focused a bit more on southeast GA). So we will have to watch for locally heavy rain as well as another risk of an isolated strong to marginally severe storm.

KEY MESSAGE 2: Near-normal temperatures are expected to return end of this week and warming into the weekend.

A nice reprieve from the heat of the last week with broad troughing over the eastern US cuts off into a upper low through the remainder of the week. At the surface a subtle "cool" front will be stalled just south of our area come Tuesday into Wednesday. While this really wont usher in significantly cooler temperatures, the combination of the weak upper low to our west and light surface convergence along the boundary zone will likely provide increased cloud and storm coverage through Wednesday keeping temperatures more seasonable. By the weekend, the upper low will have drifted westward towards the Mississippi Valley allowing for upper ridging to try and nose into the deep south and the weak boundary to mix out. While this should dry us out, it will also result in a gradual warmup into the weekend with the return of heat indices in the triple digits once again.

KEY MESSAGE 3: Minor coastal flooding is likely for portions of the southeast South Carolina coast with the evening high tide cycles through Wednesday.

Astronomical influences including the lunar perigee (7/13) and the new moon cycle (7/14) will combine to produce elevated tide levels through at least the middle of the week. The evening high tide cycle will be the most elevated each day. Though winds won't be particularly strong or supportive of large tidal departures, minor coastal flooding will be likely especially for Charleston and coastal Colleton counties. Coastal Flood Advisories could be needed each evening.

Upcoming evening high tide (~8pm): The astronomical high tide this evening is 6.57 ft MLLW meaning we need a departure of 0.43 ft to reach 7 ft MLLW and the minor coastal flooding threshold at Charleston. At the preceding low tide, the departure is between 0.5-0.6 ft and our local tide forecast tool suggests a high tide around 7.2-7.3 ft MLLW. Therefore, a Coastal Flood Advisory has been issued for Charleston and coastal Colleton counties for the 6-10pm time period.

AVIATION /18Z MONDAY THROUGH SATURDAY/

MVFR ceilings at area terminals have slowly improved the VFR this afternoon and should continue into the mid to late afternoon. The chances for showers/thunderstorms increase during the afternoon with TEMPO groups for thunderstorms and 4SM vsbys included at all terminals between 20-24Z today. Precipitation appears to diminish quickly and move inland by 24Z at all terminals. Lower cigs arrive overnight tonight behind a weak front stalling across the area. Continued MVFR to VFR cigs through 06Z with IFR cigs arriving at all terminals beginning at CHS/JZI around 09Z and at SAV around 10Z before returning to MVFR around 15Z.

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

MARINE

Today through Wednesday: A weak cold front will continue to slip through the region from north to south today and into Tuesday, yielding variable winds. This front is expected to increase the shower and thunderstorm activity over the waters, with isolated to scattered thunderstorms becoming strong to severe, primarily during the late afternoon and evening hours. The front is expected to remain just off to our south of Wednesday, though chances for thunderstorms will continue. Outside of thunderstorms, marine conditions are expected to remain well below Small Craft Advisory criteria.

Wednesday - Sunday: West-southwesterly surface flow is expected to develop Wednesday and remain into the weekend, with continued sub Small Craft Advisory criteria. Winds may briefly increase to near Small Craft Advisory criteria by Saturday into Sunday as the surface gradient tightens.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...Coastal Flood Advisory from 6 PM to 10 PM EDT this evening for SCZ149-150. MARINE...None.