Area Forecast Discussion
Issued by NWS Columbia, SC

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000 FXUS62 KCAE 251026 AFDCAE Area Forecast Discussion National Weather Service Columbia SC 626 AM EDT Fri May 25 2018 .SYNOPSIS... The region will remain moist and unstable the next few days between a ridge offshore and weak low pressure to the west. Scattered to numerous showers and thunderstorms, mainly during the afternoon and evening, are expected. Periods of heavy rain are possible Sunday and Monday as a plume of tropical moisture is directed into the area. && .NEAR TERM /THROUGH TONIGHT/... Upper trough moving into the Ohio/Miss Valleys will pick up upper disturbance, currently just SW of our FA, and shift it north along the Savannah R basin and into the S Apps. Weak surface boundary, currently stretched across the NE FA, expected to shift north some as well. Upper and surface high to remain centered to our SE. Moist atmosphere to remain in place, with current and progged PW values 1.7 to 1.9 in. With a moist atmosphere and the above features in place, some convective potential exists at any time this morning, especially CSRA near the upper disturbance. Otherwise, will expect scattered to numerous afternoon/evening convective coverage developing. Weak to moderate instability progged. Isolated severe possible, but main threat will again be locally heavy rain/flooding due to the high moisture levels with any slow moving, merging or training cells. Model blend on most elements looks ok. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... The lingering weak upper trough over the southeastern states will continue to shift north of the forecast area on Saturday being picked up by an upper trough moving east across the Ohio Valley into the Mid-Atlantic region. There will not be much upper forcing available on Saturday to aid in convective development but sufficient moisture will remain in place to interact with surface heating and result in scattered showers and thunderstorms. Forecast instability will be moderate with LI values around -5C but wind shear is marginal with generally unidirectional south to southwesterly winds at 20 knots or less. Cannot rule out the possibility of a severe storm or localized flooding threat. Will keep pops in the chance range. Saturday night should provide a brief break in the wet pattern we have been stuck in as mid level drier air moves over the region and precipitable water values drop to around 1.2 inches. We continue to monitor the possible development of low pressure in the Gulf of Mexico and NHC has given a 90 percent chance of tropical cyclone formation. Whether or not the system becomes tropical or not, a surge of tropical moisture is expected to move into the forecast area Sunday afternoon with precipitable water values rising well over 2 inches. Strong moisture transport combined with atmospheric destabilization should yield widespread showers and thunderstorms along with a heavy rain threat which will carry into the overnight hours Sunday night. Possible diffluent flow at 500mb will also provide support for widespread rainfall. Temperatures during this period will be near to above normal for highs, with Saturday being the warmer day in the mid to upper 80s, and above normal for lows in the upper 60s to around 70 degrees. && .LONG TERM /MONDAY THROUGH THURSDAY/... Active weather is expected through the extended forecast period. Much of this part of the forecast will be dependent on what happens with the low pressure system in the Gulf of Mexico and where it eventually tracks and how strong it may become. The GFS, which had been an eastward outlier compared to the NAM/ECWMF/CMC, has come around to a more westerly solution with the low pressure system but is still on the right side of the other guidance. Nevertheless, there is high confidence in an abundantly moist airmass with precipitable water values around or above 2 inches throughout the period with persistent southerly flow off the Gulf of Mexico. Expect above normal chances of rainfall throughout the entire forecast period as the surface and upper low remain to our west through Wednesday, and then cross the forecast area on Thursday. WPC rainfall forecast has 2-4 inches of rain across the forecast area during this period and depending on how much rain occurs over the weekend, a flood watch may need to be considered early next week. Temperatures during this period will be near normal for highs in the mid to upper 80s and above normal for lows in the upper 60s to lower 70s. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/...
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Upper vort just W/SW of DNL/AGS will move north today. Latest radar and high resolution models indicate shower activity will continue to affect DNL/AGS this morning, and the other TAF sites later today, with thunder potential increasing with daytime heating. Expect variable restrictions this morning in low clouds and rain/possible fog, then general diurnal improvement, outside convective areas, during the day, with scattered to numerous afternoon/evening SH/TSRA. Convective coverage expected to Begin decreasing and shifting north late today/early tonight, with deteriorating CIGs expected again late tonight into early Sat morning. EXTENDED AVIATION OUTLOOK...Late night and early morning IFR to MVFR ceilings and visibilities with fog and stratus, along with mainly diurnal showers and thunderstorms through early next week as a very moist air mass remains over the region.
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&& .CAE WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.