Area Forecast Discussion
Issued by NWS Columbia, SC

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FXUS62 KCAE 211143

Area Forecast Discussion
National Weather Service Columbia SC
743 AM EDT Mon Aug 21 2017

High pressure centered off the coast will cause increased
moisture in a southeast to south flow. However, the moisture
will remain shallow because of upper ridging and limit
thunderstorm coverage. The thunderstorm chance will increase
Wednesday with developing upper troughing and ahead of an
associated cold front. The front will be in the area Wednesday
night and thursday. It will remain hot ahead of the front
through Wednesday with afternoon heat index values peaking
around 100.


An increase in low-level moisture in an onshore flow plus
heating should help cause at least scattered cumulus clouds
during much of today. There will be areas of more cloudiness and
possibly thunderstorms but believe these will be limited
because of upper ridging. Greater cloudiness and thunderstorm
chances should be in the southeast section closer to deeper
moisture and a weakness in the h5 ridge near the coast. The HRRR
indicated very little thunderstorm coverage in the forecast
area with convection mainly just southeast of the area. We
followed the guidance consensus pop of near 20 percent northwest
to 30 percent southeast. The temperature guidance has been
consistent. Expect highs in the lower and middle 90s with heat
index values peaking around 100. Temperatures will have a brief
drop off because of the solar eclipse, with the temperature
rebound lagging for about an hour after totality ends.

The thunderstorm chance should further diminish tonight with
the loss of heating and continued upper ridging. The
temperature guidance was close. Nocturnal cooling and high low-
level moisture may lead to fog but coverage may remain limited
because of widespread stratocumulus.


Tuesday will be similar to the previous day with increasing
low-level moisture in an onshore flow. Strong heating and
convergence into a weak surface trough will produce scattered
thunderstorms in the afternoon. Upper ridging should continue to
limit coverage and updraft strength. The thunderstorm chance
should diminish Tuesday night with the loss of heating and upper
ridging. The thunderstorm chance should increase Wednesday with
developing upper troughing and ahead of an associated cold
front. The models depict more pronounced pre-frontal troughing
in the forecast area. We leaned toward the higher guidance pops
because of this support. It will remain hot ahead of the front
through Wednesday. The guidance has been consistent with highs
in the lower and middle 90s with maximum heat index values
around 100.


The GFS and ECMWF display the cold front in the forecast area
Wednesday night and Thursday. The guidance consensus supported
pops around 50 percent which was reasonable for now because of
timing uncertainty. The GFS and ECMWF show the front east of
the area Friday through Sunday with a diminished shower and
thunderstorm chance. However, the nearness of the offshore front
adds uncertainty to the weekend forecast. The GFS and ECMWF MOS
plus GFS ensemble mean pops are 20 to 30 percent Friday, and 10
to 20 percent Saturday and Sunday. The MOS has temperatures
near or a little below normal during the period.


Moisture in the lower levels of the atmosphere has contributed
to LIFR fog development at AGS and DNL this morning. Other TAF
sites have visibilities lower than 10, but remain VFR as of
1140Z. Expect improvement with mixing by 14Z. After morning
fog/stratus, expect VFR through the day and into evening. A SE
wind and a gradually moistening atmosphere will provide a slight
chance of afternoon convection, with the better chance towards
the south and mainly late in the afternoon. Confidence remains
too low to include VCSH at any of the terminals given that the
middle atmosphere remains dry. Likely just scattered to broken

Low level moisture will continue to increase and fog/stratus
will be possible again Tuesday morning. However, it could be
hampered by lingering mid-level clouds.

EXTENDED AVIATION OUTLOOK...Scattered mainly afternoon and evening
showers and thunderstorms through Thursday, with increasing
chances each day. Also, low-level moisture could result in early
morning fog and/or stratus.




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