Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
407 PM EDT sun Mar 29 2015
high pressure will move south of the area tonight. 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 from the northwest next weekend.
Near term /until 6 am Monday morning/...
the center of surface high pressure will steadily track southeast
off the coast overnight...maintaining rather quiet yet cold
conditions across the area. Details regarding temperature trends
and frost potential will be strongly dependent on the exact
movement of the high pressure center...lending some uncertainty
to the forecast.
Considering significant dry air in place...as seen in dewpoint
values in the teens and 20s this afternoon...expect temperatures
to fall rapidly after sunset. Model guidance is in good agreement
indicating that temperatures will likely reach minimums right
around midnight...then begin a slow warming trend during the pre
dawn hours...as southerly flow increases and the pressure gradient
tightens. There is a small window of opportunity when temperatures
fall to around 37 degrees across portions of inland
Berkeley...Dorchester...and Colleton counties in South Carolina
that patchy frost could develop. However...latest thinking is that
the introduction of a low level jet to the west and the brief
duration of temperatures in the ideal range will prevent
scattered to widespread coverage. Trends will need to be closely
monitored during the evening time frame...as more rapid cooling or
a more persistently weak gradient could support better frost
coverage and necessitate the issuance of a frost advisory.
Low temperatures will range from the upper 30s inland Charleston
tri County region to the low 40s elsewhere...with minimums
reached around midnight before clouds and winds increase ahead of
a cold front to the west.
Short term /6 am Monday morning through Wednesday/...
Monday...thinking on general scenario for Monday remains little
changed. Weak band of convergence associated with a cold front will
move through the region from northwest to southeast through the day.
Upper support during the morning will come from the right entrance
region of a weak jet moving through the middle Atlantic. This should
create enough upper level divergence to allow for modest upward
motion...supporting a line of showers. With dew points no higher
than the 40s...it will be tough to squeeze much quantitative precipitation forecast out during the
morning as the line moves quickly from the upstate into the northern
half of our County Warning Area by early afternoon. The upper support diminishes
during the afternoon as the boundary moves through the remainder of
the County Warning Area...but temperatures in the south will noticeably warmer as
there will be several more hours of low level warm/moist
advection...as well as more of a chance for some broken sunshine
early in the day. Therefore...while the probability of precipitation actually decrease a bit
later in the day...some surface based instability may develop...so I
have kept the mention of thunder for the southern half of the area.
High temperatures will range from the middle 60s north to middle 70s south...with
the coolest temperatures due to earlier cloud cover and rain more than
Monday night...the lingering low chances of rain in the south should
come to an end by midnight with clouds diminishing. Air mass behind
the front can be traced back to the Pacific rather than polar
regions...so lows will be seasonable with middle 40s inland to near 50
at the coast.
Tuesday...as another jet passes through the northern Gulf and
Florida...some higher clouds will move in from the west over
Georgia. The NAM hits that there could some some isolated
showers...or maybe even a thunderstorm as temperatures warm well into the
70s south of Interstate 16...but my current belief is that this will
hold off to the west and the day will be dry.
Tuesday night...another weak front will pass through the area late
Tuesday night. This front will have paltry moisture to work
with...especially over the northern third of the County Warning Area...and another
uninspiring jet maximum...so I have gone a little below MOS guidance and
capped the probability of precipitation at 20 percent. With more clouds around...overnight
mins should be milder...holding in the 50s throughout the County Warning Area.
Wednesday...the front should stall to the south and west of the County Warning Area
as high pressure moves through the Middle Atlantic States. Calling the
high a weak wedge might be giving it too much credit...but it will
act to allow the winds to turn onshore for Wednesday...keeping high
temperatures down into the 60s near the coast. Inland...the low levels will
be warming with 925 mb temperatures running 16 to 18 c by afternoon in the
Georgia counties. Normal mixing down brings temperatures into the middle to
possibly upper 70s. With the uncertainty this far out in the exact
final resting place of the front...I have left in a slight chance of
showers south of Interstate 16.
Long term /Wednesday night through Sunday/...
through late week...above normal temperatures will prevail
then...normal to below-normal temperatures should return next
As an upper shortwave trough ejects east/northeast across the
region...at least chance probability of precipitation for showers and thunderstorms are
justified especially Thursday. Between the departing shortwave and
an approaching cold front...Friday could turn out to be a very warm
day with highs in the 80s and probability of precipitation below 15 percent with no mention
of precipitation. A stronger cold front will arrive Friday night or
Saturday. Timing remains uncertain...but chance probability of precipitation and a mention
thunderstorms are centered on Saturday. Drier and cooler conditions
are then expected Sunday.
Aviation /20z Sunday through Friday/...
VFR at kchs and ksav through 18z Monday.
Extended aviation outlook...mainly VFR.
tonight...winds are expected to steadily veer from northeast late
this afternoon to southwest during the pre dawn hours...as surface
high pressure steadily slips directly over the marine zones and
progresses farther offshore. A decent nocturnal low level jet
after midnight will support south/southwest flow increasing to 10
to 15 knots across a majority of the marine zones outside of the
Monday through Friday...winds will increase out of the southwest
ahead of the front Monday. At this time...it looks like we will have
a few hours just below Small Craft Advisory winds in the nearshore Charleston County
and the offshore Georgia waters...roughly between 10 am and 6
PM...so this will need to be watched closely by future shifts. Any
increases in winds would cause the need of an Small Craft Advisory.
Winds will come down fairly quickly Monday night as front moves away
and a weak surface ridge moves in behind...causing the pressure
gradient to collapse.
Another weak front will move through the waters late Tuesday
night...but currently I think winds will be 15 knots or less with
On Wednesday...weak high pressure will build over the middle
Atlantic...causing marine winds to veer to the east...then
eventually southeast by Thursday and south or southwest on Friday as
the surface high moves into the western Atlantic. Again winds should
be 15 knots or less.