Area Forecast Discussion
Issued by NWS Charleston, SC
FXUS62 KCHS 051817
Area Forecast Discussion
National Weather Service Charleston SC
117 PM EST Mon Dec 5 2016
A storm system will affect the area through Tuesday evening. A
strong cold front will sweep through Thursday night followed by
cold high pressure through early next week.
.NEAR TERM /THROUGH TONIGHT/...
This Afternoon: Surface high pressure is moving to the less
side of the central Appalachians. The lull in POPs this
afternoon is indicated by radar, especially across the
Charleston Tri-County area. However, further south in our GA
counties the precipitation is building back into the area from
our next system. Made some minor adjustments to the POPs to
account for the current trends, but stayed in the chance range
to keep the forecast somewhat consistent. The temperature
forecast remains difficult with highs across the northern 2/3 of
our forecast area close to the forecast, but the bottom 1/3 are
a more moderated due to the showers that have begun. Again,
trended the temperatures and dew points in the next few hours to
reflect the current observations. The daytime high temperatures
are occurring now in most locations.
Tonight: The cold air damming scenario will become well entrenched
this evening, coinciding with a nice surge in forcing/moisture from
the southwest. A shield of precipitation is expected to rapidly
develop and expand from the southwest in response to a nice forcing
signature. Through the night, the forecast area will fall within the
right entrance region of the 130-150 knot 300 mb jet that becomes
aligned west to east across the Ohio Valley and New England.
Coincident with the divergence aloft, models show a well defined
frontogenetical signature along a sharpening warm front that lifts
northward. PoP`s increase to categorical everywhere and the
combination of forcing and precipitable water values approaching 1.7
inches (which would be near the daily max on record for December
6th), should yield a sizable area of moderate to occasionally heavy
rainfall. Overnight amounts should be in the 1 to 2.5 inch range. As
the warm front lifts north late, temperatures will begin to rise as
the warm sector oozes in. Also of note, models still show some elevated
instability working their way in from the south. We kept a
slight chance of thunder in the forecast, beginning after midnight.
Given the unfavorable timing, no severe weather is anticipated.
.SHORT TERM /TUESDAY THROUGH THURSDAY/...
Tuesday morning the warm front will be lifting north, then
surface low pressure developing off the NC Outer Banks will
bring a trailing cold front through the area Tuesday afternoon.
Strong low-level warm advection Tuesday morning may yield
limited instability but given the strong shortwave approaching
and lingering rains there will likely be widespread cloud cover
in the morning. This will significantly limit the CAPE values
despite impressive low-level wind shear. The lack of expected
instability is the primary reason we are not terribly impressed
with the severe weather potential on Tuesday. However, given the
35-40 kt low-level jet and 60 kt of bulk shear, we cannot rule
out an isolated damaging wind or tornado threat if pockets of
instability can develop. Will mention this marginal threat in
the Hazardous Weather Outlook. Regardless of severe weather
potential, widespread moderate to heavy showers will push
through the area as the highest PWAT airmass shifts in from the
southwest. The cold front is expected to sweep through the area
during the mid afternoon, ending precip rather quickly. Highs
will surge into the mid to upper 70s due to strong southerly
flow and some pre-frontal compression.
Weak high pressure will build in Wednesday through Thursday with
slightly cooler temps and dry weather. Another cold front
approaches late Thursday but things have been trending drier
with this front.
.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
A mainly dry cold front will sweep through the area Thursday
night followed by cold and dry high pressure through early next
.AVIATION /18Z MONDAY THROUGH SATURDAY/...
IFR ceilings continue for KCHS and KSAV. Despite a lull in the
precipitation early this afternoon, surface observations across
the region remain low. We pushed the start of MVFR back to
later this afternoon and then have ceilings around 1,000 ft.
However, would not be surprised if the ceilings stay IFR through
this afternoon given the trends up to now. Confidence in the
"improvement" to MVFR is now low. Both sites are forecasted to
see IFR this evening as moderate rainfall overspreads the area.
There is a low chance of thunder at both sites after midnight,
but confidence was too low to include in the TAFs.
Extended Aviation Outlook: Flight restrictions, mainly from low
ceilings, are expected through Tuesday night as a storm system
affects the area. Additionally, gusty winds are possible at
both terminals in showers and thunderstorms during the day
Tuesday. VFR weather returns Wednesday into late week.
Rest of today: The buoy observations indicate winds have eased a
bit since this morning as the front moves further offshore.
However, we`re continuing to have them peak around 15 kt this
afternoon. Seas will mainly range from 2-4 feet out to 20 nm
and 4-5 feet beyond.
Tonight: The northeast flow regime will prevail for the first part
of the night as widespread precipitation spread across the local
waters. Then, a warm front will begin to lift northward late and
winds will gradually turn more southeasterly and then southerly by
sunrise. Wind speeds will increase during this time and likely reach
the 15-20 knot range. Seas, after falling to the 2-3 foot range
early in the evening, will begin to tick upward and become 2-4 feet
everywhere. With the turn to southerly flow late and the increase in
moisture, there are some indications that fog/low stratus could
become a problem across the waters. Water temperatures are still in
the low to mid 60s, and the overall residence time of the southerly
flow won`t be all that long, so confidence is low in the potential
for sea fog. But, it will bear watching as the time approaches.
Gusty southwest winds and seas rising above 6 ft will yield
Small Craft Advisory conditions for the Charleston nearshore and
GA offshore waters Tuesday into Tuesday evening. Thursday night
through Friday night will then be the next surge after a cold
front sweeps through. Strong offshore flow with cold advection
will likely result in advisory conditions across most if not all
of the waters.
MARINE...Small Craft Advisory from 7 AM to 11 PM EST Tuesday for AMZ374.