Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Charleston SC
145 PM EDT Sat Mar 28 2015

Synopsis...
cold Continental high pressure will prevail into Sunday. Weak
cold fronts will push into the region late Monday and again
Wednesday...and both fronts should dissipate over or close to the
area. Warmer Atlantic high pressure will then build over the
region through late week. A stronger cold front will arrive late
week or early next weekend.

&&

Near term /through tonight/...
today...an expansive upper level trough will remain anchored over
the eastern United States...while strong high pressure continues
to build toward the East Coast. This pattern will support rather
quiet yet cold conditions across the forecast area today.
Scattered middle level clouds have developed across portions of the
forecast area by early afternoon...however expect sky cover to
remain partly cloudy or less. High temperatures will be the
combined result of gradually weakening cold advection and lower
heights aloft...as well as the warming effects of a downslope flow
component within west/northwest winds and decent insolation.
Thus...temperatures will peak only in the upper 50s north to the
low 60s south...10 to 15 degrees below seasonal normals.

Tonight...cold advection becomes more neutral tonight as the
thermal trough shifts into the Atlantic. However...the axis of
the low and middle level trough...acting almost like a secondary cold
front...will push through the region overnight. This allows the
upstream and cold surface high in the Ohio Valley to build from
the northwest with steadily rising pressures. It also arrives
with another influx of cold Canadian air...enough to allow for
freezing temperatures to penetrate into at least the northwest tier.
After coordination with neighboring offices...have upgraded the
earlier freeze watch to a freeze warning from inland Berkeley
County to Allendale and Hampton...Screven and Jenkins. This is due
to min temperatures of 30-32f for 3-5 hours late tonight and early
Sunday...along with patchy frost. Although a hard freeze is not
expected...these conditions are sufficient to cause damage to
sensitive vegetation and unprotected plants.

Considered a frost advisory from inland Charleston to inland
Jasper in SC and then south/SW through inland Chatham...to
Candler...Tattnall and long counties. However there are two
factors going against anything more than patchy frost. First would
be that the dew point depressions are quite large...some 6-10
degrees. Second...there will be too much mixing in some of these
areas due to the high building in and geostrophic winds holding up
near 10 or 15 knots. Even so...those with frost sensitive plants
might want to take precautions as well...with lows in the middle
30s. It is also conceivable that if the freezing temperatures are able
to penetrate a bit further southeast...a few parts of this
corridor of counties could also require a freeze warning. The
only places that should escape any frost and/or freeze will be the
coastal southeast Georgia zones.

Lows tonight will be very close to records for March 29...coming
within about two degrees at the three climate sites. See climate
section below for more information.

&&

Short term /Sunday through Tuesday/...
Sunday...the surface high will shift over the area. Strong
subsidence will provide a dry and sunny day. Despite full
insolation...after a cold start temperatures will only recover into
the upper 50s across most of southeast SC to the lower 60s across much of
southeast Georgia. These temperatures will average some 10 to 13f below normal
for late March.

Sunday night will start with mainly clear skies/light
winds...supporting excellent radiational cooling conditions.
Then...as the gradient tightens between high pressure shifting south
of the region and a cold front approaching from the northwest...and
as clouds begin to increase...overnight temperatures could hold
steady or even begin to rise. Thus...hourly temperatures indicate
lows within a couple of hours of midnight...perhaps dipping into the
upper 30s at inland locations but in the lower to middle 40s most
locations...followed by slightly higher temperatures around daybreak
Monday. If this scenario is delayed...lower temperatures and even a touch
of frost cannot be ruled out well inland.

Monday...guidance has trended toward an earlier arrival of a cold
front from the northwest...due in part to a fast-moving shortwave
trough aloft which will pass north of the region. Guidance depicts
sufficient forcing for ascent and associated deep layered moisture
to justify chance probability of precipitation advancing from north/northwest to S/southeast as the day
progresses. Also...isolated thunderstorms cannot be ruled out...but
latest guidance suggests that a plume of enhanced low-level
instability will be cut off west of the forecast area...leaving only
weak instability in place over the region. Thus...maintained no
mention of thunderstorms for now...but will continue to assess.
Otherwise...despite cloud cover compressional heating ahead of the
cold front will help temperatures recover into the middle/upper 60s
north and lower to middle 70s south...closer to normal for late March.

Monday night and Tuesday...the cold front will weaken...stall and
begin to dissipate over or close to southern counties of the
forecast area. Model solutions differ regarding the southward
progress of the front and the associated pool of enhanced moisture.
Per latest trends...slight chance probability of precipitation are justified Monday night
through Tuesday across southern counties. Otherwise...lows in the
40s/50s Monday night will give way to near-normal temperatures
mainly in the 70s away from the coast Tuesday.

&&

Long term /Tuesday night through Friday/...
through late week...above normal temperatures will prevail... except
perhaps Thursday if showers/thunderstorms become more widespread
than currently forecast. Then...normal to below-normal temperatures
should return next weekend.

Another cold front is expected to settle into the region from the
north Wednesday. Discounting the 28/00 UTC Canadian solution
depicting a progressive front followed by strong high pressure
building from the N/NE...this front should remain weak and will
likely stall and dissipate over or close to the region. Meanwhile...
a shortwave trough aloft will advance through the lower MS River
Valley toward the northern Gulf Coast...and to varying degrees
guidance depicts an expanding plume of moisture/precipitation ahead
of this feature. Give the shortwave ridging aloft over the
region...feel that guidance could be pushing convective precipitation
too quickly toward the east/into the region. Thus...held probability of precipitation below
15 percent with no mention of precipitation Wednesday...but forecast
confidence is low. A better chance for showers and perhaps
thunderstorms should develop Wednesday night through at least
Thursday night as the upper trough passes over the region. A
stronger cold front will arrive later Friday or Saturday...
justifying at least chance probability of precipitation and a mention thunderstorms.

&&

Aviation /18z Saturday through Thursday/...
VFR conditions will prevail through 18z Sunday. Breezy
west/northwest winds this afternoon will diminish by the
evening...eventually becoming north behind a dry cold front after
midnight.

Extended aviation outlook...flight restrictions possible in showers
Monday. Otherwise...mainly VFR.

&&

Marine...
today...waning cold advection and a slight weakening of the
pressure gradient has allowed conditions to improve over the
waters. Small craft advisories are no longer in effect.

Tonight...the improvement in conditions will be short-lived as a
strengthening Continental high builds from the Ohio Valley and
expands across the local waters during the late evening and
overnight. It will also arrive with another round of cold
advection and pressure rises...and a decent north/northeast surge
will occur late. Conditions could actually approach Small Craft
Advisory criteria on the Charleston County Atlantic waters as
sunrise approaches.

Sunday through Thursday...a surge of northeast winds and localized
Small Craft Advisory conditions could persist into Sunday morning...
then a much weaker pressure pattern and slackening winds/subsiding
seas should develop Sunday afternoon into Sunday evening.
Then...ahead of an approaching cold front later Sunday night through
Monday...SW winds will increase and seas will build...perhaps
pushing to Small Craft Advisory levels especially across amz350 and amz374. The cold
front will stall and dissipate by Tuesday...then another weak cold
front will likely stall and dissipate in the area Wednesday.
Thus...a relatively benign pressure pattern primarily governed by
offshore high pressure should hold winds/seas below Small Craft Advisory levels until
late week or early next weekend.

&&

Climate...
record lows March 29th...
kchs...32 set in 1969.
Ksav...33 set in 1955.
Kcxm...37 set in 1982.

&&

Chs watches/warnings/advisories...
Georgia...freeze warning from 4 am to 9 am EDT Sunday for gaz087-088.
SC...freeze warning from 4 am to 9 am EDT Sunday for scz040-042>045.
Marine...none.

&&

$$
Synopsis...
near term...wms/
short term...Spr
long term...Spr
aviation...wms/
marine...wms/
climate...

National Weather Service Glossary of Abbreviations