Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
121 am EDT Monday Mar 30 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 this morning/...
the center of surface high pressure has translated off the coast
this evening. An increasing low level pressure gradient and increasing
SW flow atop the stable surface layer should result in temperature
readings that steady out between 3 and 5 am...perhaps rise nearing
Low temperatures will range from the lower and middle 30s in the
central corridor and closer to the core of the high...upper 30s
and lower 40s most elsewhere. Minimums will be reached prior to 3
am before some clouds and low level wind fields increase ahead of
a cold front to the west. We expanded mentions of patchy frost to
cover much of the region inland from US-17.
Short term /6 am this 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 /06z Monday through Friday/...
VFR at both kchs and ksav through 06z Tuesday. However...with an
approaching cold front there could be a shower or two at either
site during the early-middle afternoon...with a remote possibility
of thunderstorms and rain near ksav to occur. Otherwise...a west/SW flow will be
gusty during the late morning into the afternoon...during the
time when the axis of the strongest low level jet moves through in
sync with peak heating.
Extended aviation outlook...mainly VFR.
overnight...winds are expected to steadily veer to southwest
during the pre-dawn hours...as surface high pressure steadily
progresses farther offshore. A decent nocturnal low level jet
will support south/southwest flow of 10-15 knots across a majority
of the marine zones outside of the Charleston Harbor. Seas 2 to 3
feet on average overnight.
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.