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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 230757

Area Forecast Discussion
National Weather Service Charleston SC
357 AM EDT Tue Apr 23 2019

High pressure will prevail through Thursday. Low pressure and a
cold front will bring unsettled weather on Friday followed by a
return of high pressure for the weekend.


Deep layered high pressure will holds its influence on the
region today. Westerly flow atop the southern Appalachians and
Blue Ridge will induce a broad lee-side trough extending from 
the South Carolina Upstate into the North Carolina and Virginia 
Foothills by afternoon. This will yield a prevailing southwest 
flow through sunset, except east of the I-95 corridor where 
winds will back to the south to locally southeast with the 
passage of the resultant sea breeze circulation. H8 temperatures
will rise another 3-5C by the diurnal maximum and with only 
thin cirrus passing through aloft, highs should easily warm into
the mid-upper 80s along/west of the I-95 corridor; trending 
cooler to the coast. Resultant and cool Atlantic influences will
keep the beaches much cooler with most only topping out in the 
lower 70s.


Tonight: A weak pressure gradient will persist through the night
as the lee-side trough remains intact. A few knots of wind
coupled with thickening cirrus midnight through daybreak
Wednesday will support a warmer night with lows ranging from the
upper 50s/near 60 inland with mid-upper 60s at the beaches.

Wednesday: The mid-levels will consist of semi-zonal flow. At the 
surface, high pressure offshore will move away from our area. 
Meanwhile, a cold front will approach from the distant north while a 
storm system develops over the Southern Plains. Aside from some 
increasing high clouds, neither of these features will reach our 
area, allowing the high to bring dry conditions. Southwest surface 
winds, 850 mb temperatures, and low level thickness values all 
support our highest temperatures so far this year. Highs should make 
a run for the 90 degree mark in many locations. This is a few 
degrees below the record highs for this date. Temperatures will be 
cooler and winds will be higher at the beaches due to the afternoon 
sea breeze. Overnight lows will remain mild, generally not falling 
below 60 degrees.

Thursday: Mid-level ridging is forecasted to strengthen over the 
Southeast during the day. Though, the ridge will get pushed offshore 
in the evening and overnight as a trough approaches the Southeast 
towards daybreak Friday. At the surface, high pressure will be far 
offshore during the day and moving away. Meanwhile, a storm system 
and its cold front continues to approach from the west, making its 
way into the Southeast by daybreak. The outer periphery of the high 
will continue to give our area dry weather during the day and into 
most of the night. Continued southwest winds will allow daytime 
temperatures to make another run for the 90 degree mark in many 
locations, again just a few degrees short of the records. Though, 
it's possible highs may be limited if high clouds increase faster 
than expected. Overnight lows will be very mild, well into the 60s. 
Showers from the storm system should start reaching our inland 
locations late, possibly bringing measurable rainfall.

Friday: The mid-levels will consist of a trough and its associated 
vorticity moving over the Southeast during the day. At the surface, 
a storm system and its cold front will slowly cross through the 
Southeast during the day. Moisture advection ahead of the front 
appears okay with PWATs generally exceeding 1.25". Though, the 
plume could be better defined. But the models are in good 
agreement showing showers across our area during the day and the
forecast reflects this. Rainfall amounts are somewhat 
uncertain, depending on what model and how the system evolves. 
But measurable rainfall is expected. There is some instability, 
so we are forecasting thunderstorms, but nothing severe. Highs 
will be "cooler" barely making it to 80 degrees.


A storm system with its associated cold front is expected to move 
offshore Friday night, bringing showers to an end. High pressure 
returns for the weekend. Some models hint at sea breeze convection 
developing early next week.


VFR. Shallow ground fog is possible at both KCHS and KSAV, but
no impacts are expected.

Extended Aviation Outlook: VFR through Thursday. Low pressure and a 
cold front could produce periodic flight restrictions on Friday. VFR 
returns Saturday.


Through Tonight: Light south to southwest winds will back to 
south and southeast this afternoon as a sea breeze develops 
along the coast and propagates inland. Local sea breeze 
enhancements are likely near the coast and Charleston Harbor 
where winds could approach 15 kt at times. Otherwise, winds 5 kt
this morning will increase to 5-10 kt this afternoon. South to 
southwest will surge to a solid 10-15 kt tonight as high 
pressure off the Florida East Coast shifts to the southeast. 
Seas of 1-2 ft nearshore, 2-3 ft offshore today will build to 
2-3 ft nearshore, 3-4 ft offshore overnight.

Wednesday through Saturday: High pressure will move offshore 
through Thursday, with the main driver in the winds each of 
those two afternoons being the sea breeze. It's possible wind 
gusts may briefly reach 25 kt in Charleston Harbor both days. By
Friday a storm system and its cold front moving through the 
waters may bring increased winds and seas. But at this time no 
Small Craft Advisories are expected. High pressure returns for 
the weekend.