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Area forecast discussion 
National Weather Service Charleston SC
704 am EST Thursday Mar 5 2015

..near record warmth today followed by significantly colder
conditions Friday...

Synopsis...
a cold front will approach from the west and northwest today and
push offshore this evening. Colder high pressure will build over
the area Thursday night and will prevail into Sunday. Next
week...an unsettled pattern will develop between a stationary
front south and east of the region and high pressure centered to
the north.

&&

Near term /until 6 PM this evening/...
mesoscale update...winds have unexpectedly backed slightly along
the lower South Carolina coast...which is helping to drive sea fog
inland from off the Atlantic. Visibilities east of Highway 17 have
dropped to 1/2 nm or less at times. If conditions lower anymore...
a dense fog advisory will be issued. Anticipate conditions will
rapidly improve once the sun gets up and mixing begins. A Special
Weather Statement has been issued to address the fog for the
morning commute.

The next 24 hours are going to be quite dramatic...which is likely
understating what the lowcountry and coastal Empire will
experience as a powerful cold front pushes through the area this
evening.

Morning low clouds moving up from the Gulf of Mexico will mix out
by middle-morning as vertical mixing intensifies ahead of an
approaching cold front. The combination of pre-frontal
compression...periods of insolation and a 1-2c increase in 850 hpa
temperatures over the past 24 hours should result in high
temperatures very near what was experienced Wednesday...if not a
bit warmer if clouds mix out quick enough. Highs in the Lower-Middle
80s look reasonable for most areas away from the immediate coast
where influences from the cooler shelf waters will not be factor.
Strengthening southwesterly winds should suppress or at least
severely delay the inland penetration of the result sea breeze
circulation...especially along the Georgia coast this
afternoon...however coastal locations will remain significantly
cooler than inland areas given the proximity to the chilly shelf
waters.

Most of the day will be dry as a cold front approaches from the
west. Models and current radar data suggest the frontal structure
is anafrontal in nature...so very little in the way of pre-
frontal shower activity will occur with the bulk of the rainfall
occurring along and behind the front itself. Rain will begin to
move into the western zones by middle-afternoon and spread east to
the coast as the cold front moves through the forecast area. Probability of precipitation
will generally be capped at 50-70 percent with the higher
probabilities occurring across interior southeast South Carolina.
Temperatures will fall as the front moves in and rain begins to
fall so have tried to time these thermal falls to reflect the
expected timing of the both the front and onset of rain.
Instability will be sufficient for a few thunderstorms along the front
itself...so a chance for thunderstorms will be maintained for all areas.
There is no risk for severe thunderstorms with this system.

Gusty southwest winds will occur within the warm sector. Gusts
could approach 25-30 miles per hour at times...similar to Wednesday...but
mixing profiles over Lake Moultrie will be poor as warm air flows
over the chilly lake waters. This will yield absolutely stable
conditions over the Open Lake waters with little in the way of
significant wind expected within the thick marine layer. The
degree of gustiness will even be subdued along the Lakeshore as a
robust lake breeze circulation is likely...which will tend to
stabilize the boundary layer.

&&

Short term /6 PM this evening through Sunday/...
tonight...rain chances will steadily end from west-east this
evening as the cold front pushes through. The bulk of the
measurable rains will be offshore 9 PM to midnight with only
isolated to perhaps scattered showers lingering through early
Friday. In the wake of the primary rain band...low ceilings and
periods of drizzle will fill in as colder air pushes south.
Temperatures could be near freezing across upper portions of
Dorchester and Berkeley counties after 4-5 am. Although soundings
show middle-level moisture quickly scouring out by this time...deep
moisture within the boundary layer will support periods of
drizzle. This opens the possibility for a little freezing drizzle
by sunrise Friday...roughly along and north of Saint George-
Ridgeville-Huger-Shulerville line. With only marginally cold
conditions and temperatures holding right at freezing...
significant impacts appear unlikely at this time. However...a very light
Glace on elevated surfaces such as trees...metal objects as well
as some bridges and overpasses can not be completely ruled out.
Ground conditions will be too warm to support ice accumulations
otherwise...which includes non-elevated roadways. Given the
marginal nature of this event...the forecast will call for periods
of drizzle with a chance of freezing drizzle in the aforementioned
areas. The risk for freezing drizzle will surge south after
sunrise as subfreezing wet-bulb temperatures advect south.

Lows will range from around freezing far north to the Lower-Middle
40s south of Interstate 16 in southeast Georgia. One could argue
that a freeze warning is needed for parts of Dorchester and
Berkeley counties now that its beyond 1 March and the growing
season has begun. However...given the transient nature of the
freezing line and considering a much more significant and
widespread freeze will occur Friday night...a freeze warning will
not be issued.

Friday...models have trended drier in the low levels as compared
with recent runs. Thus...even as the coldest air featuring temperatures
around freezing and wet bulb temperatures just below freezing spreads
across inland areas between the edisto river/Interstate 26 and the
Santee river...low level moisture now appears barely sufficient to
support anything more than drizzle. Thus...the probability for any
freezing precipitation has diminished...although a slight
chance/chance of freezing drizzle is still justified across
northern/inland locations through midday Friday...possibly a far
south as a Walterboro-North Charleston-McClellanville line. Even
if a bit of freezing drizzle occurs any trace amounts of glaze
should only occur on elevated surfaces such as trees and street
signs...greatly limiting any impacts.

Farther south/east where deeper moisture will reside and slight
chance/chance probability of precipitation are justified through midday/early afternoon...
temperatures will remain above freezing. In general...guidance has
trended toward a slow but steady drying trend...so probability of precipitation decrease
with time Friday afternoon and fall to near zero Friday night.

Aside from any precipitation type concerns...Friday will provide
temperatures some 35-40f colder than those of Thursday...only
recovering into the upper 30s to middle 40s.

Friday night...confidence is high that sub-freezing temperatures
will occur across most South Carolina locations and across southeast
Georgia locations west of I-95...and a hard freeze is possible well
inland. Per collaboration with weather forecast office jax...will defer decisions
regarding freeze watches/warnings to subsequent forecast packages.

Despite sunshine...temperatures will remain below normal
Saturday...ranging from the middle/upper 50s north of I-16 to the lower
60s south.

Saturday night...tranquil conditions will prevail...and low temperatures
will range from the Lower/Middle 30s well inland to the middle 40s along
the Georgia coast.

Sunday...mid/high level clouds will increase/thicken ahead of a wave
of low pressure approaching from the west. However...no rain is
expected during the daylight hours. On the positive side...
temperatures should recover into the 60s Sunday...closer to normal
for early March.

Lake winds...favorable mixing profiles will develop this evening
and persist through the night as cold air intensifies. Expect
north winds of 20 knots with frequent gusts to 25 knots to prevail
through the night...so a lake Wind Advisory will be issued. Gusty
conditions will persist through Friday morning before diminishing.
Waves on the Open Lake waters will build to 1-2 feet.

&&

Long term /Sunday night through Wednesday/...
between a stationary front off the southeast coast and across the Florida
Peninsula and high pressure centered north of the region...a
baroclinic zone will waver over the region and will contribute to
unsettled conditions next week. While a mostly cloudy regime
punctuated by periods of precipitation and finally devoid of any
cold air appears likely...confidence regarding complex forecast
details remains low. For instance...we maintain chance probability of precipitation Monday
and Tuesday...but some guidance suggests that likely/categorical
probability of precipitation will eventually be needed for this period. Also...the potential
for thunderstorms could increase...but the uncertain details
regarding individual waves of low pressure and an associated
uncertain potential for an influx of any significant instability
does not justify any mention of thunder.

&&

Aviation /12z Thursday through Monday/...
band of low clouds will affect kchs through about 13z. Otherwise
VFR at the sites through late afternoon. Conditions will crash
rapidly to sub-IFR by 22-00z as a cold front pushes through.
Light to moderate rains will affect the terminals with the front
with low clouds/drizzle to filling in behind the main rain band
has it pushes offshore. LIFR ceilings are most likely at kchs. Gusty
winds will affect the terminals for the next 24 hours...both in
the warm sector and in the Post-frontal cold sector.

Extended aviation outlook...VFR likely Friday PM through this
weekend. Flight restrictions possible in rain early next week.

&&

Marine...
expanded the marine dense fog advisory to include the Charleston
Harbor. Harbor control reported zero visibilities in The Harbor.

Today...nasty sea fog will persist over the nearshore waters
through early afternoon as warm/moist air flows over the chilly
shelf waters. The most widespread/dense sea fog will occur over
the South Carolina nearshore waters where southwest winds will
provide longer parcel residence times over the cold shelf waters.
Will extend the marine dense fog advisory through 2 PM...although
it is possible the Georgia nearshore waters can be removed prior
to that given the less than ideal trajectory structure that far
south. Otherwise...south to southwest winds of 15 knots or less will
persist with seas 2-4 feet nearshore waters and 3-5 feet over the
Charleston County and Georgia offshore waters.

Tonight...conditions will deteriorate rapidly this evening as a
cold front pushes off the coast and strong cold air advection
ensues. The combination of strong Post-frontal cold air advection
and strong isallobaric pressures rises will support gales for
much of the night over the Georgia offshore waters and the
Charleston County waters where frequent gusts to 35-40 knots gales
will occur. The highest winds will occur over the Georgia offshore
waters...roughly beyond 40 nm...where contraflow along the
western wall of the Gulf Stream will be at its strongest. The gale
watch will be upgraded to a warning for these waters beginning at
6 PM and continuing through the night. For the other nearshore
zones...including the Charleston Harbor...winds will remain just
below gale force...so small craft advisories will be maintained.
Seas will build to 4-6 feet nearshore waters and 6-8 feet offshore
waters.

Friday through Monday night...gales will transition to small
craft advisories Friday...and elevated north/NE winds and associated
seas will persist into Saturday. A more tranquil regime is
expected as high pressure shifts over the waters Saturday night
through Sunday. Then...forecast confidence decreases next week as
winds/seas will depend on a highly uncertain storm track. The
latest forecast holds winds/seas well below Small Craft Advisory levels...but an
offshore track followed by deeper areas of low pressure will
translate to stronger winds and higher seas.

&&

Climate...
record highs for 5 March...
kchs... 85/1997
kcxm... 84/1923
ksav... 86/1955

Record low maximums for 6 March...
kchs... 43/1960
kcxm... 43/1960
ksav... 42/1901

&&

Chs watches/warnings/advisories...
Georgia...none.
SC...lake Wind Advisory from 4 PM this afternoon to 2 PM EST Friday
for scz045.
Marine...dense fog advisory until 2 PM EST this afternoon for amz350-
352-354.
Small Craft Advisory from 6 PM this evening to 4 PM EST Friday
for amz352-354.
Gale Warning from 6 PM this evening to 11 am EST Friday for
amz350.
Gale Warning from 6 PM this evening to 2 PM EST Friday for
amz374.
Dense fog advisory until 10 am EST this morning for amz330.
Small Craft Advisory from 6 PM this evening to 11 am EST
Friday for amz330.

&&

$$

St/Spr

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