Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
657 PM EDT Monday Oct 20 2014
a trough of low pressure will cross the area and will move off the
coast overnight. A cold front will move through the region Tuesday
night...with high pressure building in its wake through late in
Near term /until 6 am Tuesday morning/...
after a tranquil evening...an amplifying upper trough will push a
a surface trough through the region overnight. Associated forcing
for ascent was already producing showers upstream over the
southern Appalachians...and this regime will shift toward the
coast overnight. To varying degrees...latest guidance depicts
showers developing especially over coastal counties and adjacent
coastal waters as forcing for ascent interacts with an airmass
featuring dewpoints in the 50s/60s. Maintained ongoing slight
chance/chance probability of precipitation from late evening/around midnight through early
Tuesday morning mainly east of I-95...but cannot rule out a few
showers farther inland. Will also continue to assess the
potential for greater coverage and even isolated thunderstorms
overnight as depicted by the more aggressive/more unstable 18z NAM.
Meanwhile...clouds...southerly flow and elevated dewpoints will
likely hold temperatures up until late tonight when offshore winds
will usher drier air into the region and will push cooler temperatures in
the 50s into the region and toward the coast.
Short term /6 am Tuesday morning through Thursday/...
Tuesday...light rain/showers will continue to lift northeast and
further offshore from the area early as moisture associated with a
parent low pressure system slowly shifts off the northeast coast.
West/northwest flow along the southern periphery of the low will
help produce fairly warm temperatures ahead of a dry cold front approaching
the area late. Latest 1000-850mb thickness values support overall
high temperatures near 80s degrees and given a downsloping wind component
along with mostly sunny skies...have maintained high temperatures in the
lower 80s for most locations.
Tuesday night into Wednesday...a dry cold front will shift through
the area from the north/northwest Tuesday night...with little to no
impact over the southeast in regards to precipitation. However...cooler
conditions are anticipated behind the front as a north/northwest
flow develops along the southeast side of high pressure centered
over the Midwest. Tuesday night temperatures should remain in the low/middle
50s for most areas...but temperatures could dip into the upper 40s inland
where frontal passage occurs earliest. High temperatures will be some 5-10 degrees
cooler than the previous day on Wednesday as the north/northwest flow
persists between high pressure centered over the Midwest and a broad
low pressure system just off the northeast coast. In general...high
temperatures should range in the low/middle 70s...coolest over southeast South
Carolina. Wednesday night lows should also be the coolest of the
week...with perhaps a small contribution from radiational cooling.
In general...lows should dip into the middle 40s inland to low 50s near
Thursday...very little change is anticipated in regards to the dry
weather pattern...but surface winds should gradually back to northwest
over the course of the day. This supports a gradual modification of
temperatures...with afternoon highs 1-2 degrees warmer than Wednesday...but
still in the low/middle 70s over most locations.
Long term /Thursday night through Monday/...
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 /00z Tuesday through Saturday/...
VFR through Tuesday. Low probability for showers/brief flight
restrictions 04z-06z until 10-12z...but limited mention to vcsh
within 00z tafs.
Extended aviation outlook...VFR conditions are expected at both chs
and sav terminals through this weekend.
tonight...ahead of an approaching surface trough/on the periphery
of high pressure retreating offshore/away from the coast...the
remnant sea breeze circulation will give way to S/SW winds as high
as 15 knots. Then...winds will veer toward the west/northwest at similar speeds
as the trough passes late. Seas will hold around 2-3 feet...highest
across the outer Georgia waters. Isolated to scattered showers are
forecast after 9 or 10 PM...and isolated thunderstorms cannot be
ruled out especially beyond 20 nm overnight.
Tuesday and Wednesday...conditions are expected to be well below
Small Craft Advisory levels Tuesday as a broad trough of low
pressure begins to slowly shift off the northeast coast. In
general...westerly flow will remain at or below 15 kts with seas no
higher than 3 feet. A dry cold front will then approach the area and
shift over the coastal waters from north to south Tuesday
night...resulting in a fair amount of cold air advection over the
waters while high pressure builds from the northwest. As a
result...winds/seas will be enhanced Post frontal passage late Tuesday into
Wednesday...but conditions should remain just below Small Craft
Advisory levels. In general...west winds will become north between
15-20 kts behind the front while seas build to 3-4 feet...highest in
offshore Georgia waters.
Wednesday night through Saturday...winds and seas are 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.