Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
634 am EDT Monday Oct 20 2014
high pressure will shift off the middle Atlantic coast today and
tonight. A cold front will move through the region Tuesday
night...with high pressure building in its wake through late in
Near term /until 6 PM this evening/...
only minor changes were needed for the early morning update...mainly
to account for sky cover trends.
Working off limited model data this morning due to a communications
outage at the National Weather Service telecommunications Gateway.
Thankfully...the weather is poised to remain quiet and very fall-
like across the lowcountry and coastal Empire as Canadian high
pressure maintains its hold.
The center of the anticyclone will eventually shift offshore with
winds turning more onshore as another cold front approaches from
the northwest. Temperatures will modify some from Sunday/S
readings...but are still expected to top out in the 70s today...
ranging from middle 70s north to the upper 70s across the central and
southern portions of the forecast area. The degree of cloud cover
is a bit uncertain given the lack of model data this morning. The
NAM seems a bit bullish on the various forecast soundings showing
skies becoming cloudy later this morning...but given early morning
satellite imagery does show extensive stratocumulus poised to
move onshore as an area of weak isentropic lift pushes inland...
expect at least mostly to partly cloudy skies to occur. The
Prospect for measurable rainfall is low given the meager mean relative humidity
values in place...but a light/brief sprinkle or two moving
onshore can not completely ruled out this afternoon. Probability of precipitation at the
coast will generally be confined to 5 percent or less.
Short term /6 PM this evening through Thursday/...
tonight...a diffuse pre-frontal trough is prognosticated to push off the
coast early Wednesday morning as the main cold front approaches
the North Carolina mountains. Any marine based shower activity
that can develop over the coastal waters will likely remain
offshore given this flow regime and anticipate rain-free
conditions to dominate once again over land areas. Varying degrees
of cloud cover will occur this evening...with a clearing line
prognosticated to slowly work its way from west-east through the night.
Lows will range from the lower 50s inland to the upper 50s/lower
60s at the coast.
Models in good agreement through this period. The main features will
be a deep/closed upper low that will move from the eastern Great
Lakes states Tuesday eastward to just off the New England coast by
Thursday. Dry conditions are expected with temperatures above normal
Tuesday...falling to slightly below normal Wednesday and Thursday
behind the next polar cold front on Tuesday night.
Tuesday...a full latitude trough of low pressure from aloft to
the surface will dominate the entire U.S. East Coast. This will
produce westerly low level flow...which will have a downslope
component that will help warm temperatures to abound 80 to lower 80s
under mostly sunny skies. By Tuesday night...another polar cold
front will move through the area from the north. Again...deep
layer moisture is lacking. Thus...the front is expected to come
through dry. Lows by morning in the upper 40s inland to the lower
50s else where.
Wednesday...high pressure will be centered over the Great Lakes
states with a ridge axis extending southeast into our region...while
a deep low pressure center will move off the Delaware-Maryland-Virginia coast. This
will result in low level northerly flow and weak cold air advection
all day. Expect highs only in the lower 70s under full sun...which
is a few degrees below normal. The center of the surface high will
remain well northwest of the region Wednesday night...which will
keep enough pressure gradient to maintain light north-northwest flow
just above the surface...which may be enough to hold radiational
conditions to less than perfect. Still cool however...with lows
generally in the middle 40s...except lower 50s at the coast under
mostly clear skies.
Thursday...the deep layer low pressure center will be just off the
southern New England coast while a weak area of high pressure at the
surface will cover much of the Ohio River valley and southeast states.
This will result in relatively light afternoon low level northwest
flow with highs again cool in the lower 70s with sunny skies.
Long term /Thursday night through Sunday/...
global model solutions now very different for the middle to later
part of the period. The latest solutions now show the deep/closed
upper low off the New England coast to continue slowly moving
eastward Friday as a deep layer ridge develops over the southwestern
U.S. States by Friday. Between these two features...a short wave upper
trough drops southeastward from the Great Lakes Friday...to off the
middle Atlantic coast by Saturday. Then a deep layer ridge builds eastward
across the south and southeast states Sunday and Monday. This is
drastically different from solutions 24 hours ago...which
featured an upper low over the Gulf states and associated surface
low moving up from the Gulf across Florida then into the southwest
Atlantic. This newer solution is much drier and warmer than
previously expected. Given the extreme uncertainty...made only minor
changes to temperatures through the period...generally holding them
near to slightly above normal. Did introduce slight chance probability of precipitation for
Monday with some moisture return on the east side of the ridge...but
confidence quite low for probability of precipitation given above discussion.
Aviation /12z Monday through Friday/...
Extended aviation outlook...there are no concerns with a greater
than 30 percent confidence level.
today...east to northeast winds will veer southeast this afternoon
in response to high pressure shifting off the middle-Atlantic coast.
Speeds look to remain below 10 knots with seas 2-3 feet.
Tonight...winds are forecast to shift offshore after midnight as a
pre-frontal trough pushes off the coast. Winds will remain
light...generally 10 knots or less with seas 1-3 feet.
Tuesday...no highlights expected as a broad trough of low
pressure remains north and west of the waters with prevailing
westerly flow of 15 knots or less and seas 3 feet or less. Tuesday
night...a cold front will push from north to south. Cold advection
and building high pressure behind the front will increase winds to
15 to 20 knots with some higher gusts and seas 3 to 5 feet. At this
time...conditions seem too marginal to go with a Small Craft
Advisory. By later Wednesday through Friday...winds and seas
expected to remain below any highlights as a broad area of high
pressure remains to the northwest of the region. By Saturday...an
upper level disturbance will move off the North Carolina coast.
This feature will help to increase the northwest winds...but still
not expecting winds/seas to reach Small Craft Advisory levels.