textproduct: Charleston

WHAT HAS CHANGED

Aviation section updated for 12z TAFs.

KEY MESSAGES

- 1) Locally heavy rainfall possible again today with a risk for a few damaging wind gusts. Rain chances diminish through the week into the weekend.

- 2) Hot and humid conditions will return Tuesday and continue into the weekend.

DISCUSSION

KEY MESSAGE 1: Locally heavy rainfall possible again today with a risk for a few damaging wind gusts. Rain chances diminish through the week into the weekend.

Today: Early morning water vapor and 31/00z upper air analysis show the synoptic pattern aloft is beginning to change across the Southeast U.S. with the pronounced upper ridge centered over the Mid-Missisippi Valley showing signs of building east. Guidance suggests the main mid-level weakness that has been in place for the past few days will begin to steadily weaken today and gradually shift offshore as 500 hPa heights slowly rise. While subsidence with the upper ridge will gradually strengthen this week, modified RAP soundings across Southeast Georgia and the South Carolina Lowcountry show no signs of a capping inversion through the afternoon with both moisture and instability remaining at levels sufficient for scattered showers/tstms again today.

Convection is expected to develop along the mainly the Georgia coast later this morning as weak coastal troughing develops near the beaches. Near term instability progs suggest MLCAPE as high as 1000-1500 J/kg could move across the coastal counties from off the Atlantic prior to daybreak as low-level convergence builds. This could support a round of showers/tstms with the greatest concentration expected to occur across the middle and upper Georgia coastal counties. PWATs around 2.25" are expected in this area with very weak wind fields noted aloft. This could support a risk for minor flooding of low-lying and poor drainage areas, but thankfully much of the Georgia coast outside of the Savannah Metro area and smaller urban centers such as Richmond Hill, Hinesville and Darien can typically handle quite a bit of rainfall before problems occur. The 31/00z REFS supports a potential for localized rainfall amounts in excess of 3"/3-hr in this area.

Later this afternoon, building instability ahead of modest sea breeze circulation should support another round of shower/tstms. By this time, however, drier air associated with the building ridge aloft will begin to arrive with a pronounced dropping of K-Index values across the far interior noted (roughly the Millen- Allendale-Saint George-Moncks Corner corridor). Modest moisture still looks to hold for areas south of this corridor through peak heating, but convection may still not end up being quite as pronounced as would typically be observed during the late summer given the increasing influence of the upper ridge. Wind fields are forecast to remain pretty light aloft (850-200 hPa steering flow of 10 kt or less) so any tstms that develop (especially closer to the coast where PWATs will remain above 2") have the potential to drop some torrential rainfall in a short period. This could yield some flooding issues with the 31/00z REFS supporting a potential for localized rainfall amounts in excess of 4"/3-hr, mainly associated with sea breeze convection across parts of Charleston, Berkeley, Dorchester and Colleton Counties. Increasing dry air aloft could also increase the risk for a few locally damaging wind gusts. Shear will be weak, so tstms will be heavy influences by mesoscale processes such outflow boundary collisions and the sea breeze. Convection will quickly wane this evening.

Rest of the Week: The edge of the ridge axis aloft begins to slide over the region on Tuesday, lowering the convective potential a bit due to the subsidence that it provides into the end of the week. However, weak onshore flow, periods of warm-air advection in the mid-levels, and the daily sea-breeze will lead to isolated to scattered showers and thunderstorms being possible. For Tuesday, there is a bit of a disparity between the global models and the higher resolution convection allowing models (CAMs), where the global models are more aggressive and the CAMs more reserved. So, don't be surprised if the forecast ends up trending drier for Tuesday. For Wednesday into the end of the week, expect much of the same. By Saturday, the ridge weakens and an upper level trough will be developing along the east coast, with shortwaves moving through the flow aloft. This will bring increased rainfall chances Saturday afternoon into Sunday.

KEY MESSAGE 2: Hot and humid conditions will return Tuesday and continue into the weekend.

The ridge of high pressure aloft that was discussed above will also help to keep temperatures above normal next week, with the return of hot and humid conditions expected. On Tuesday, the region will start to slide under the eastern periphery of the ridge axis, bringing widespread temperatures in the upper 80s to lower 90s, with heat index values in the upper 90s to lower 100s. By Wednesday, the ridge axis actually shifts a touch northwards, with the region now on the southern periphery through Saturday. With chances for rainfall remaining on the lower side temperatures will remain above average, with temperatures peaking on Friday and Saturday in the mid 90s. The need for Heat Advisories will continue to be assessed, with the greatest risk occurring for those along the coastal corridor where heat index values of 105-110 deg F may occur. A cold front looks to push through on Sunday, bringing an end to this stretch of hot and humid conditions.

AVIATION /12Z MONDAY THROUGH FRIDAY/

31/12z TAFs: KCHS/KJZI: No major changes from the previous TAF cycle. Risk for some marine-based showers will increase at KJZI by 13z. VCSH was noted for now with most of any activity that forms poised to remain to the south. An area of scattered tstms is expected to form along the sea breeze near KCHS by mid-afternoon. PROB30 for TSRA was maintained 19-21z to account for this. Tstm risk looks to small to include a mention at KJZI as the sea breeze should clear that terminal by 17z. Will need to watch for a little shallow ground fog and/or low stratus forming after 08z tonight, but with a light north drainage wind expected, significant impacts are not expected at this time.

KSAV: No major change from the previous TAF cycle. Risk for showers and possibly a few tstms will increase next few hours as convection fires in the vicinity of a weak coastal trough along the beaches. Risk for tstm impacts look greatest 14-17z so a TEMPO group was highlighted during this time. A few tstms could linger near KSAV through mid-afternoon, so VCTS was noted. The sea breeze should clear the terminal by 20z and should end the risk for tstm impacts. Will need to watch for a little shallow ground fog and/or low stratus forming after 08z tonight, but with a light north drainage wind expected, significant impacts are not expected at this time.

Extended Aviation Outlook: Showers and thunderstorms will bring brief flight restrictions throughout the week, especially during the afternoon hours.

MARINE

Through Tonight: Winds should back to the northeast by daybreak, but then veer to the east to locally southeast by this afternoon as the sea breeze moves inland. Onshore winds will again back to more to the northeast tonight. Speeds look to average 10 kt or less through tonight with seas 2-3 ft.

Tuesday through Saturday: No marine concerns expected. Light and variable prevailing flow will shift onshore each afternoon as the sea breeze circulation develops along the coast, with 2-3 ft seas.

CHS WATCHES/WARNINGS/ADVISORIES

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