Area Forecast Discussion
Issued by NWS Charleston, SC

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884
FXUS62 KCHS 221555
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1055 AM EST Sun Jan 22 2017

.SYNOPSIS...
A strong area of low pressure will develop over the Southeast
as it deepens near the southern Appalachians late today and
tonight. This allows for a strong cold front to push through the
area tonight. Weakening low pressure will pass by to the north
on Monday, before high pressure returns through mid week.
Another cold front will cross the area late in the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
***A SIGNIFICANT TORNADO OUTBREAK EXPECTED ACROSS SOUTH GEORGIA
LATE TODAY INTO THIS EVENING***

10 AM surface observations show the warm front stretching
roughly from Colleton County into the southern tip of Charleston
County, then looping just off the coast and parallel to the SC
coast. This feature will lift north of the area into this
afternoon, as a wave of low pressure along the front lifts into
NC. Vigorous cyclogenesis currently developing over AL will
head toward the Appalachians in response to a potent and
abnormally deep closed low near the lower MS valley. The surface
low is forecasted to strengthen/deepen to near 986 mb by this
evening while it pulls a cold front into and through our
forecast area during the early part of tonight.

There will be a decrease in convection late this morning and
early this afternoon, as the first short wave pulls away. This
allows for at least some insolation to develop in the warm
sector behind the northward lifting warm front and in advance of
the eastward moving cold front. This will lead to yet another
springlike day, with possible record high temps if there is
enough heating once convection winds down. In fact, visible
satellite imagery shows breaks in the clouds moving towards our
area, which will help temperatures rise and further destabilize
the atmosphere. A few places south of I-16 in GA could approach
80F, but most locations should be in the mid to upper 70s. The
SC beaches will be the coolest with a S-SW fetch off the cooler
waters.

There remains plenty of dynamics and impressive shear during the
late morning into this evening, but the big question is what
impact or disruption does the morning convection have on
another round of possible severe storms late in the day. The
location and track of both the surface low and that aloft looks
to be displaced a little too far to the W-NW and its movement is
more N-NE rather than E as it draws closer to the CWFA. But the
pressure drops about 10-15 mb from this morning into the
evening. That would certainly be problematic. While the best QG
forcing does stay to our W-NW through the day, and there is
some influx of maritime air into our SC coastal corridor, there
is considerable shear and forcing from both a 50-60 kt low level
jet brushing the coast, mid level dry air and the overall large
scale forcing for ascent with impressive dynamics and upper
divergence. This could allow for another and possibly more
widespread round of severe weather from roughly 4 to 10 pm.
Provided there is enough destabilization that does indeed take
place and the impressive thermodynamics such as 55 TTI, LI`s of
-6 to -8 and SBCAPES of 1500-2000 j/kg are realized. It`s hard
to gauge exactly how this will pan out since this is extremely
unusual for mid winter, but with SHERBS3 values of 1-1.5 and Sig
Tor of 3-4 there could be some stronger tornadoes, with discrete
and long track supercells and bowing segments late in the day
and this evening. SPC has upgraded much of our Georgia counties
to a HIGH risk of Severe Weather, with Moderate risk elsewhere.
An outbreak of tornadoes in Georgia is possible. This is a
unusual situation for anytime of the year, but unheard of for
January!

A noted dry slot will develop behind the cold front as it sweeps
offshore tonight, and rain chances drop off quickly during the
late evening and overnight. However, some late night showers
could move back in across our NW tier as the upper low draws
closer.

There remains a risk for a Wind Advisory for at least the coastal
counties of SC this evening, due to impressive pressure falls as
much as 1-1.5 mb/hr from about 5 pm to 11 pm. That along with a
packed gradient and the passage of a 50-60 kt low level jet
could allow for a few hours of wind gusts near 40 mph. But we`ll
be cautious of having too many warnings/headlines so we don`t
confuse the public.

What is more certain is that winds across Lake Moultrie will
reach at least 20 kt sustained and/or 25 kt gusts this afternoon
through tonight. A Lake Wind Advisory is in effect.

QPF: There remains a risk for at least minor flooding due to
periods of heavy convective rains today, but also a low end risk
for flash flooding. This is most especially in areas that
received heavy rains yesterday. Large scale forcing and the
PWats that are 150-175% of normal will support an additional 1-2"
of rain today into tonight, with locally higher amounts.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Monday: A strong low pressure system centered over the western
Carolinas at daybreak will slowly lift northeast through the day.
Some lingering moisture may result in a few light showers in the
morning, then precipitation chances should decrease later in the day
as drier air works into the area. Slight chance PoPs seem
reasonable. A tight pressure gradient will result in gusty west
winds. Gusts up to 35 mph will be possible during peak heating
hours, especially across the southern zones. Deep downslope flow
will keep highs above normal, mainly in the mid 60s. Lows Monday
night in the low to mid 40s.

Tuesday into Wednesday: Weak upper ridging will take place Tuesday
while surface high pressure begins to build into the area. On
Wednesday, the surface high will drift east over Florida and then
into the Atlantic ahead of an approaching cold front. Should be a
quiet and pleasant couple of days. Temperatures will continue to be
above normal, with highs warming from the mid 60s Tuesday to low 70s
on Wednesday.

Lake Winds: Winds will become gusty around and over Lake Moultrie on
Monday. Winds are expected to gust to 25 knots thus a Lake Wind
Advisory is in effect.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
A cold front will approach Wednesday night and then cross the area
Thursday. Moisture looks to be fairly limited so current PoP scheme
only features 20-30% chances. High pressure will return in its wake
and persist through much of the weekend. Temperatures above normal
Thursday will finally moderate closer to normal behind the cold
front.

&&

.AVIATION /16Z SUNDAY THROUGH THURSDAY/...
Convective rains associated with a nearby warm front are slowly moving
offshore. Improvements to MVFR or maybe even VFR are expected
into this afternoon as the risk for convection diminishes. But
this will be short-lived as additional convection is expected to
arrive in the form of a squall line or as discrete storms late
in the day and this evening. Additional flight restrictions are
expected. TEMPO groups will be added/amended as radar trends
develop.

It also will become windy through this evening. S-SW winds will
reach near 30 kt at times due to stronger winds mixing into the
lower levels, combined with a tightening gradient in response
to the upstream low.

Extended Aviation Outlook: Periods of flight restrictions are
likely into Monday. VFR will return thereafter. Gusty winds
will impact the terminals Monday.

&&

.MARINE...
Today: Hazardous weather conditions will develop across the
coastal waters in response to a developing area of low pressure
across GA and AL. While winds will be tempered a little by warm
advection and has been altered over the SC waters by the morning
convection, given 30-40 kt or geostrophic winds at 1000 mb, we
have SCA`s in effect for all waters throughout the day,
including Charleston Harbor. S-SW winds will reach as high as
20-30 kt, and seas will build to 4-7 ft within 20 nm and 6-9 ft
across the outer GA waters.

Tonight: The SCA`s will transition into a Gale Warning for
AMZ374, and a Gale Watch for AMZ350, but continuing elsewhere.
This occurs with large isallobaric pressure falls followed by
strong height falls aloft. It is possible that the other waters
could also require a Gale Warning at a later time.

Severe Weather: There looks to be two rounds of severe weather
possible; first this morning and the second this evening.
Damaging winds and possible waterspouts are the most common
hazards, although lightning strikes and reduced visibility in
heavy rains will also occur.

Monday through Friday: Hazardous marine conditions will be ongoing
at the start of the period as a strong low pressure system remains
nearby. At least Small Craft Advisories will be in effect for all
waters including the Charleston Harbor. There will be a chance for
gale force gusts, especially in the outer Georgia waters. Will
continue to assess trends. The low will lift northeast away from the
area on Tuesday and winds/seas will begin to subside. High pressure
will build in its wake and prevail through mid week with no marine
issues expected. A cold front will cross the waters Thursday with
another round of small craft advisories possible.

&&

.CLIMATE...
Record High Minimum Temperatures for Sunday January 22:
KCHS: 59 set in 1972
KCXM: 66 set in 1937
KSAV: 64 set in 1937

Record High Maximum Temperatures for Sunday January 22:
KCHS: 80 set in 1999
KCXM: 76 last set in 1937
KSAV: 80 last set in 1937

Record Rainfall for Sunday January 22:
KCHS: 1.77 inches set in 1999
KCXM: 1.67 inches set in 1973
KSAV: 1.33 inches set in 1966

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...Lake Wind Advisory until 7 PM EST Monday for SCZ045.
MARINE...Small Craft Advisory until 7 AM EST Tuesday for AMZ352-354.
     Gale Watch from 7 PM EST this evening through late tonight for
     AMZ350.
     Small Craft Advisory until 7 PM EST this evening for AMZ350.
     Small Craft Advisory until 4 PM EST this afternoon for AMZ374.
     Gale Warning from 4 PM this afternoon to 1 AM EST Monday for
     AMZ374.
     Small Craft Advisory until 1 AM EST Tuesday for AMZ330.

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...ECT
AVIATION...
MARINE...
CLIMATE...



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