Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
730 PM EDT Sat Mar 28 2015
high pressure will prevail through Sunday. Weak cold fronts will
push into the region 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 next weekend.
Near term /until 6 am Sunday morning/...
quiet and cold conditions will continue tonight. The axis of an
expansive upper trough will linger along the East Coast...while
the center of surface high pressure over the Ohio Valley slips
southeast. The leading edge of the surface high will act as a
secondary dry cold front...introducing a reinforcing round of cold
advection overnight. As a result...expect temperatures to rapidly
fall after sunset and reach the low to middle 30s during the pre dawn
hours. The ongoing freeze warning for the northwest tier counties
looks on track and has needed no change with this forecast
package. Will therefore continue to advertise about 3 to 5 hours
of minimum temperatures at 30 to 32 degrees from inland Berkeley
County to Allendale and Hampton...Screven and Jenkins counties.
The tools utilized to determine frost potential have consistently
returned low confidence in anything more than patchy
frost...likely due to the combined effects of larger dewpoint
depressions and somewhat elevated nocturnal wind speeds within a
tightened pressure gradient. Therefore prefer not to issue frost
advisories for the areas outside of the freeze warning at this
time. It should also be noted that low temperatures will approach
records for March 29...see climate section below for more
Short term /6 am Sunday morning through Tuesday/...
Sunday...after a cold start...temperatures will warm up under
plentiful sunshine to the upper 50s north to lower 60s south by late
afternoon as polar high pressure settles over the area. Those temperatures
are 10 degrees or more below average. Relative humidities will be
unseasonably low as well with values into the teens away from the
coast by afternoon. Fortunately...winds will be light...so fire
weather concerns will not be large.
Sunday night will start with mainly clear skies/light
winds...supporting excellent radiational cooling conditions. I am
concerned that we could get close to freezing near the Santee river
during the late evening hours in the usual cold spots around the
Francis Marion. For now...I did not drop the temperatures below 34...but
this will need to be watched. Frost would also be possible in those
areas. Later at night...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 will begin to slowly rise. Thus...hourly temperatures
indicate lows within a couple of hours of midnight...followed by
slightly higher temperatures around daybreak Monday.
Monday...a weak cold front will move through the area on Monday from
northwest to southeast. Models are a little more aggressive in the
chances of rain with the front...so I have increased probability of precipitation and cloud
cover just a touch. Areas that have temperatures reach 70 or above may get
enough surface instability to generate some isolated
thunderstorms...so I did add a slight chance of thunder during the
afternoon hours...as well as the early evening prior to sunset over
Monday night...the cold front will weaken and push southeast of the
area...ending the threat for rain most places. With the front
slowing down considerably...I cant rule out a lingering shower in
the southern areas...so a slight chance of probability of precipitation remains south of
roughly Interstate 16. If any showers do occur...amounts will be
generally light. Temperatures will be down to the 40s to near 50 by morning
as weak pressure briefly visits the area.
Tuesday...high pressure will work its way into the Atlantic during
the day Tuesday...allowing for a milder southwesterly wind to
develop. Temperatures should get to 70 or above during the
afternoon...although probably 10 degrees cooler during the afternoon
along the coast as the winds back to southerly in SC and south
southeasterly in Georgia. Moisture will also be increasing...so I
have maintained the low end probability of precipitation for south if Interstate 16 that I
inherited from the previous forecast...but that is probably a
touch aggressive...and certainly it does not look anything like a
washout of a day.
Long term /Tuesday night through Saturday/...
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 /00z Sunday through Thursday/...
VFR at kchs and ksav through 00z Monday.
Extended aviation outlook...flight restrictions possible in showers
on Monday. Otherwise...mainly VFR.
overnight...strong surface high pressure will steadily build from
the Ohio Valley and expand across the local waters overnight. The
accompanying cold advection and tightened pressure gradient will
allow northwest winds to veer north and increase. North winds could
surge to 20 knots late across the Charleston County waters and
offshore Georgia zones...however latest thinking is that
conditions will remain below Small Craft Advisory conditions.
Sunday through Thursday...winds will be quite changeable during
the Sunday through Thursday time frame...but except for possibly a
period Monday afternoon ahead of a front...headlines seem
High pressure on Sunday will settle right over the coastal
waters...leading to a weakening in the winds...which should
eventually veer to an onshore direction during the afternoon. Winds
will turn southerly...then southwesterly Sunday night ahead of a
cold front...increasing in speed Monday. There is a chance we will
need small craft advisories Monday afternoon in the Charleston
County waters...along with the outer Georgia waters...but for now I
have kept the sustained winds just below headline criteria at 20
knots or so. Winds will veer to the northwest and come back down in
speed Monday night behind the front...and Tuesday the winds slacken
as the next area of weak high pressure settles over the waters.
Another front is expected to pass through Wednesday...with winds
turning northeasterly behind the front Wednesday night...veering
around to southeasterly by Thursday afternoon. Speeds will be
generally 15 knots or less during the middle of the week.
record lows March 29th...
kchs...32 set in 1969.
Ksav...33 set in 1955.
Kcxm...37 set in 1982.
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.