Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 231348
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
948 AM EDT Wed May 23 2018

.SYNOPSIS...
High pressure will remain to the east through Friday while a
trough of low pressure lingers to the west. Developing low
pressure over the Gulf of Mexico will slowly move north toward
the central Gulf Coast this weekend, possibly lingering through
early next week. This pattern will maintain higher than normal
rain chances across southeast South Carolina and southeast
Georgia.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Morning GOES-E water vapor imagery shows the region is pinned
between subtropical high pressure to the east, a fast belt of
the westerlies to the north and a mid-level weakness to the
south and west. The main concern for today is trying to pin down
how much convection will develop. 23/12z roabs sampled a fairly
tropical atmosphere with PWATS in excess of 1.75 inches.
Moisture will continue to surge north through the day with PWATS
progged to reach to near 2 inches by late afternoon. Despite
this increase net moisture, the latest mesoscale guidance has
backed off considerably on the amount of convection that will
develop ahead of the sea breeze this afternoon. It is not
exactly clear why the various CAMs have trended in this
direction, but the trend has been there for several runs now.
There are some signals, especially in the RAP, that dewpoints
along/east of I-95 could mix out in the mid-upper 60s, thus
limiting in overall stability, while farther inland, modified
sounding only yield weak instability at best. Given the latest
model trends and a less than ideal thermodynamic profile, have
opted to lower net pops by 10-30% across the board. Updated pops
range from 50% across interior Southeast Georgia to 10% at the
coast. Although this a net lowering from the previous forecast,
it is not quite as low as most of the guidance would suggest.

With the region pinned between the 3 feature noted above, the
wind profile throughout the tropopshere is fairly light. Any
deep convection that is able to fire is likely to move very
little. High PWATS are in place, so there will be a risk for
locally heavy rainfall and minor flooding of low lying and poor
drainage areas.

Highs will warm into the mid 80s for most away from the beaches.
If convection does fire across the interior earlier than
expected, then highs may only reach the lower 80s in that
region. Will not make that change just yet.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
Tonight: Deeper moisture tends to linger overnight with little
overall change in the pattern. We maintained some 20-30 POPs
into the overnight with somewhat higher chances adjacent to the
CSRA and midlands and perhaps some of inland GA zones. Clouds
may thin later tonight but there could be more stratus once
again with some patchy fog where clouds become thinner. Lows
will be in the upper 60s to around 70 near the coast.

Thursday through Saturday: A cold front near the Santee River
should stall out Thursday morning with Atlantic high pressure
then prevailing through the end of the week. By Saturday low
pressure over the Gulf of Mexico should begin moving north
toward the central Gulf Coast. This pattern will keep a tropical
air mass in place with rain chances staying above normal,
mostly during the afternoon and evening each day. In general we
think the highest chances will be inland near the better
moisture, instability and forcing. Some spots will likely pick
up over an inch through the period. No significant flooding or
severe weather is anticipated at this time. Temperatures will
generally be near to above normal, especially lows.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
Not much pattern change late this week into early next week with
high pressure remaining to the east and low pressure over the Gulf
of Mexico moving north, most likely toward the north-central Gulf
Coast. The low could stall out near the Gulf Coast into early next
week. This pattern will mean the tropical air mass will remain in
place across southeast SC/GA leading to the continuation of higher
than normal rain chances, especially each afternoon/evening.
Temperatures should generally stay near to above normal.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
Isolated to scattered convection is expected with the onset of
the sea breeze by early afternoon. VFR conditions will prevail
with the potential of any thunder too low to mention in either
TAF. Tonight, low clouds are more likely inland with VFR again
the likely outcome. A few showers are possible during the
overnight.

Extended Aviation Outlook: A persistent tropical air mass will lead
to higher than normal rain chances into early next week, mainly each
afternoon. Thunder chances will be low however given the limited
instability. Some restrictions will also be possible from morning
low clouds and/or fog as well.

&&

.MARINE...
Today: Broad Atlantic high pressure will nose into the waters
from the western Atlantic through tonight. The forecast remains
on track with south to southwest flow northern waters and south
to southeast flow GA legs. Speeds mainly less than 15 kt with
seas in the 2-4 ft range, highest beyond 20 NM.

Thursday through Monday: The area will remain between low pressure
to the west and high pressure to the east. A tropical cyclone may
develop in the Gulf of Mexico later this week and track north toward
the north-central Gulf Coast and this should lead to an increasing
pressure gradient locally so we think winds and seas will be on an
upward trend as a result. Advisory conditions will be possible this
weekend into early next week, especially near the Gulf Stream.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$


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