Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 051150
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
650 AM EST Mon Dec 5 2016

.SYNOPSIS...
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/...
At dawn: Overnight rains are coming to an end from the west as
coverage decreases. This is likely the start of our lull through
the afternoon before rain chances ramp up again this evening and
tonight. No significant change to the forecast other than to
update PoP`s based on radar trends.

Today: The main story for the forecast today will be the development
of a cold air damming scenario and the evolution of rain chances.
High pressure over the Ohio Valley will bridge across the central
Appalachians and ridge down the lee side of the spine of the high
terrain. The morning stream of rainfall will gradually come to an
end by early afternoon and we then expect a relative lull in
coverage through much of the afternoon. PoP`s are lowest during the
afternoon, but are still in the chance range to account for some
scattered patches of rain and drizzle. Temperatures will be
extremely difficult to nail down, and the hourly trend has been
constructed using a blend of RAP/HRRR/NSSL-WRF. As such, a very
limited diurnal range is expected for well inland areas where
temperatures will essentially hold steady. This will yield highs
topping out in the mid/upper 50s inland and north, with some low 70s
possible south of I-16 in southeast Georgia. Confidence in
temperatures is quite low and it is entirely plausible that the
current forecast is too warm given the overcast skies and the
developing wedge.

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 show some showalter
indices falling to zero or less so elevated instability will be
present. Have added a mention of slight chance of thunder increasing
from the south 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 SUNDAY/...
A mainly dry cold front will sweep through the area Thursday
night followed by cold and dry high pressure through early next
week.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
IFR ceilings have settled into KCHS while KSAV has been much
slower to come down and currently sits as VFR. Rainfall is
quickly diminishing and this is likely the lull period we will
experience until rain chances ramp up again tonight. At KCHS,
IFR ceilings expected to linger through the morning with lifting
to MVFR this afternoon. At KSAV, the thinking is that IFR
ceilings will work into the site over the next couple of hours
before experiencing a similar lift to MVFR. Both sites are then
forecast to see IFR return this evening as moderate rainfall
overspreads the area. There is a very low end chance of thunder
at KSAV very late tonight, but confidence is too slow to
include in the TAF`s.

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.

&&

.MARINE...
Today: As the weak surface passes by this morning winds will locally
increase across the Charleston County waters into the Small Craft
Advisory range. The current advisory is scheduled to end at 10 am
and that appears reasonable. Behind the departing surface wave winds
will turn more northerly and eventually northeasterly. Wind speeds
should top out at a solid 15 knots this afternoon, but there could
be periods of 15-20 knots. Seas will mainly range 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.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 7 AM to 11 PM EST Tuesday for AMZ374.
     Small Craft Advisory until 10 AM EST this morning for AMZ350.

&&

$$

NEAR TERM...BSH
SHORT TERM...JRL
LONG TERM...JRL
AVIATION...BSH/JRL
MARINE...BSH/JRL


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