Scientific Forecaster Discussion
fxus62 kchs 190220
Area forecast discussion
National Weather Service Charleston SC
920 PM EST Sat Nov 18 2017
a cold front will push offshore early Sunday, then stall south
and east of the region. High pressure will build over the area
and prevail through early next week. A series of low pressure
systems could then track northeast along the stationary front,
producing unsettled conditions later Tuesday through mid to
late week. High pressure will expand over the region late week
or next weekend.
Near term /until 6 am Sunday morning/...
the only change made for the late evening update was to increase
pops to 40%. The rest of the forecast is on track.
A pronounced cold front is moving across Tennessee into deep
south early this evening and is delineated by a sharp line of
showers/tstms. The cold front and accompanying line of
convection will move steadily east tonight, reaching far
interior areas around 3 am and coastal areas by 6 am. Do expect
at least some weakening of the line as it moves across southeast
South Carolina and southeast Georgia during the instability
minimum, but upper forcing associated with the right entrance
region of the upper jet will remain robust. Rap showalter values
go slightly negative just ahead of the front suggesting the
presence of some weak elevated instability which may be just
enough to support a risk for a few tstms despite no surface
based instability. Plan to introduce a slight chance of tstms
for the early morning hours. High resolution models are similar
in showing a distinct line moving west-east across the forecast
area, so 40% pops may be a bit low, especially inland will keep
pops capped at 40% for this update, these may need to be
increased with subsequent updates.
Lake winds: winds will increase on Lake Moultrie tonight ahead
of a cold front, then lull late as the front passes. At this
time it appears as though winds will mainly stay below advisory
levels /25 kt/ in the warm advection regime given the lake water
temperatures in the 50s leading to less than favorable vertical
mixing profiles. The strongest gusts will likely occur around
the Lake Shores.
Short term /6 am Sunday morning through Tuesday/...
clouds and a few showers could linger early sunday; otherwise, the
shortwave trough and associated surface cold front will push quickly
offshore Sunday morning, followed by sunshine and drier conditions.
Downslope flow and insolation will combine to offset cold air
advection, pushing high temps into the mid/upper 60s most locations.
The cold front will eventually stall south/east of the region, and
this stationary front could play a role in our weather beginning
Dry high pressure will expand over the area Sunday night, yielding
clear skies and light winds. Strong radiational cooling will result,
allowing temps to drop into the mid 30s inland and upper 30s to
lower 40s closer to the coast. Accordingly, we introduced a mention
of frost into the late Sunday night/early Monday forecast, and we
may need to consider a frost advisory for Sunday night for some
portion of inland zones where the winds would be more likely to go
calm. A few of the chilliest inland locales could even bottom out
briefly around 32f early Monday, but the probabilities for freezing
temps remain too low to justify a freeze watch.
Cool high pressure will persist Monday, limiting high temps to the
upper 50s north to the mid 60s far south.
Monday night into tuesday: east of an amplifying longwave upper
trough, forcing for ascent, moisture advection and a coastal trough
will combine to produce increasing/thickening clouds, and showers
should eventually develop. Latest guidance suggests that sufficient
dry air will persist to maintain rain-free conditions at least
through Monday night. Then, the chance for showers should increase
Tuesday. Coverage of showers remains somewhat uncertain due to a
persistent south/southeast to north/northwest moisture gradient. The
latest forecast advertises Tuesday afternoon pops ranging from
likely far south/coastal waters to slight chance well inland.
Long term /Tuesday night through Saturday/...
the period looks potentially unsettled though there are substantial
differences in the global models. A series of shortwaves/jet maxima
will dive into the longwave trough anchored over the eastern U.S,
pushing the trough deeper into the Gulf of Mexico. Meanwhile, a
surface stationary front will persist south/east of the forecast
area. This scenario could support serial surface cyclogenesis over
the Gulf, with individual lows tracking northeast along the
stationary front and just off the southeast coast through mid to
late week. Given ongoing uncertainty regarding the
evolution/positions of key features, maintained ongoing/
conservative pops in the 20-30% range into late week.
Aviation /02z Sunday through Thursday/...
* wind shift associated with cold front
* low-end risk for low level wind shear
A cold front is on tap to cross the kchs and ksav terminals
early Sunday, roughly in the 09-11z timeframe. A broken line of
showers will accompany the front, but some weakening is
possible prior to the front reaching the terminals. Will carry
vcsh 09-12z with low-end IFR cigs for now. There is a low-risk
for tstms given the presence of some elevated instability, but
the risk is too low to justify a mention. Pre-frontal low-level
jetting will intensify this evening, but there looks to be
enough surface winds to keep conditions below low-level wind
shear thresholds. Winds will shift to the northwest behind the
front with steady clearing expected.
Extended aviation outlook: periods of flight restrictions are
possible later Tuesday or Tuesday night through Wednesday.
tonight: high pressure will be giving way to an approaching
cold front. Elevated south/southwest winds will occur ahead of
the front, although will be limited somewhat given the warm
advection pattern leading to less favorable vertical mixing
profiles. Advisories will go into effect this evening across the
Charleston County nearshore waters which will be first to see
25 kt wind gusts and 6 foot seas, then around midnight for the
rest of the waters, except Charleston Harbor which should hold
off until a bit later.
Sunday through thursday: in the wake of cold fropa, offshore
winds will diminish, and Small Craft Advisory conditions should
end by midday Sunday across nearshore waters and around mid-
afternoon beyond 20 nm.
Between high pressure building from the northwest and an offshore
stationary front, expect an extended period of northeast winds next
week. As a series of surface low pressure systems develop and track
along the stationary front/off the southeast coast, the pressure
gradient is expected to increase mid to late week. Small Craft
Advisory winds and/or seas could develop over portions of the waters
Wednesday night through Friday.
Marine...Small Craft Advisory from 2 am to 7 am EST Sunday for amz330.
Small Craft Advisory until noon EST Sunday for amz352-354.
Small Craft Advisory until noon EST Sunday for amz350.