Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
109 am EDT Thursday Apr 2 2015
high pressure will build in from the north tonight behind a weak cold
front. High pressure will shift off the eastern Seaboard Thursday.
A robust fast moving cold front will cross the region Saturday
morning...followed by cooler high pressure through Sunday. The high
will shift into the Atlantic and will push warmer and more humid air
into the region through the first half of next week.
Near term /until 6 am this morning/...
dry conditions will prevail with upper level riding and synoptic
subsidence building through the night. A small area of stratus
and stratocu over southeast Georgia was easing inland and we touched up
some sky cover grids on the midnight update. Otherwise...only
minor adjustments to initiale temperatures in a few spots.
Short term /6 am this morning through Saturday/...
Thursday...an upper level ridge and an area of surface high
pressure will shift well off the coast. Low level flow will become
onshore which will help surface dewpoints increase into the lower
60s across southeast Georgia and upper 50s to near 60 elsewhere by late
in the day. Meanwhile...a shortwave trough aloft will advance into
the region from the west/southwest during the afternoon...which
could aid in the development of isolated showers and thunderstorms
during the afternoon across mainly western zones. Even though weak
instability develops across the southwest by afternoon...
maintained only slight chance/lower end chance probability of precipitation due to lack of
a low level focus. Highs will mainly be from 75-80f...except near
the coast where a vigorous onshore flow will keep temperatures in the
Convection could persist into Thursday evening especially across
western/southwestern counties...so maintained at least slight chance
probability of precipitation in the southwest through much of the evening. As winds veer
toward the south overnight...dewpoints in the 60s will maintain a
longer residence over the cooler waters...which could result in some
sea fog across the nearshore waters. Some of the fog may try to move
a short distance inland. Water temperatures near 60 degrees in many
areas and decent winds of near 10 knots are limiting factors for
dense fog development...unlike back in early to middle March when shelf
water were much cooler. It will be a mild night with lows mainly in
the lower 60s.
Friday...a decent amount of sunshine...850 mb temperatures of 13 to
14 degrees celsius and compressional heating in advance of a cold
front should push high temperatures into the lower to middle 80s. An
isolated shower/thunderstorm cannot be ruled out if/where low level
convergence develops east of I-95...but capped/fairly dry model
soundings suggest that the potential for convection remains too
limited to justify a mention within public forecasts. Also of
note... sea fog/stratus could continue to brush the beaches/Sea
Friday night...ahead of an approaching cold front...temperatures
will only drop into the lower 60s. Cloud cover will be on the
increase especially late as a fast moving storm system and
associated cold front approach the western counties late. Maintained
a slight chance for showers and perhaps a thunderstorms in the north
and west late.
Saturday...the cold front will zip across the region with an
associated chance for showers and perhaps a few thunderstorms along
and ahead of the front. All 12z model guidance and 00z European model (ecmwf)
indicate a very quick cold frontal passage in the morning followed
by a fairly rapid clearing...with mainly sunny skies expected in the
west and north by middle afternoon. Models differ significantly on how
cool the air is behind the front. The 12z GFS and NAM indicate much
cooler air than does the 00z and 12z European model (ecmwf) and especially the 12z
Canadian model. Based on a trend toward more cooler air in the wake
of the front...lowered highs a bit into the lower to middle 70s.
Long term /Saturday night through Wednesday/...
Post frontal passage high pressure will provide cooler and rain-free
conditions Saturday night through Sunday night. Then...high pressure
will push offshore and a return flow featuring warmer temperatures
and increasing moisture will prevail Monday through Wednesday. Odds
favor rain-free conditions through Monday. Then...even though
significant moisture/precipitation should be held west of the region
by the Atlantic ridge through midweek...low probability of precipitation are justified
Aviation /06z Thursday through Monday/...
Ksav...MVFR/IFR ceilings remaining S and SW of the terminal at 05z but
pockets of lower clouds remain possible overnight into middle morning
on Thursday. Until trends dictate we have continued VFR until
near sunrise when MVFR ceilings become more likely into late morning
per latest sref ceiling forecasts and NAM soundings.
Extended aviation outlook...scattered showers and perhaps a
thunderstorm are expected Saturday morning which could result in a
period or two of sub VFR cigs/vsbys.
tonight...onshore flow averaging 15 knots overnight with seas 2 to 4
feet with slightly higher seas beyond 20 nm.
Rest of the forecast period...higher dewpoint air will spread
across the still-chilly shelf waters on Thursday and then likely
persist through Friday night or Saturday morning. Conditions will
become increasingly favorable for sea fog later Thursday or
Thursday night...possibly continuing intermittently or
continuously through Friday night/Saturday morning. Maintained
mention for patchy or areas of fog through Friday...then did not
include it Friday night or Saturday morning due to the possibility
of somewhat lower dewpoint air moving in. Waters temperatures near
60...as opposed to lower to middle 50s during our bigger fog events a
few weeks ago is a limiting factor for dense sea fog.
As surface high pressure shifts offshore...moderate onshore flow is
expected Thursday. Ahead of an approaching cold front S/SW winds
will increase Friday/Friday night...and Small Craft Advisory conditions are possible
especially across amz350 and amz374.
The cold front will cross the waters Saturday...and strong high
pressure will then build into the region enhancing cold advection
and supporting a surge of northeast winds and elevated seas during
the latter half of the weekend into early next week. Small craft
advisories appear likely with this event.
The high pressure will shift offshore next week...and winds will
veer through the east/southeast to S/SW. Winds/seas should remain below Small Craft Advisory
levels through middle week.