Scientific Forecaster Discussion
fxus62 kchs 231752
Area forecast discussion
National Weather Service Charleston SC
1252 PM EST Thu Nov 23 2017
a series of low pressure systems will move up the coast today
through Friday. A cold front will shift through the area
Saturday night, followed by dry high pressure through the
middle of next week.
Near term /through tonight/...
no updates with this package.
Previous discussion: radar imagery and surface observations
indicate the main band of rain has gobbled it's way a little
further inland than what was originally expected. We upped the
pops throughout the entire area to account for this trend. In
some areas the pops may seem underdone. But it didn't seem
reasonable to jump from slight chance probability of precipitation to likely pops when
quantitative precipitation forecast amounts aren't significantly higher. Also, lowered the
temperatures as the rainfall and northeast flow are keeping them
down and should make it hard for them to reach the original
highs. Expect temperatures to be well below normal today.
Surface low pressure developing near The Big Bend of Florida
this morning is moving northward. The axis of a mid level trough
is forecast to slide east through the day, reaching the SC/Georgia
coast late this evening. As the trough pushes east, low pressure
is expected to track northeast off the southeast coast. It
appears that a compact rain shield will exist northwest of the
low center as high pressure remains centered over the Delmarva.
Given the passage of the deeper forcing and instability, the
greatest rainfall rates will remain off shore today. It appears
that rainfall totals from today through tonight will peak around
a quarter of an inch across the coastal counties, very far
inland will struggle to see measurable rainfall.
Tonight: low pressure will slowly track northeast, leaving at least
partly cloudy conditions across the County Warning Area through the night. Steady NE
winds should strengthen slightly late tonight as the pressure
gradient increases in the wake of the low. However, the GFS and NAM
indicate very weak 850 mb cold air advection late tonight, generally cooling around a
degree during the pre-dawn hours. A few weak showers will likely
linger near the coast for most of the night, but conditions should
become dry toward dawn Friday. Using a blend of MOS, low
temperatures should range from the low 40s inland to around 50
across the beaches.
Short term /Friday through Sunday/...
the latest model suite is in better agreement on a less
amplified upper shortwave moving through on Friday which
translates to a slightly weaker and farther east coastal low
track. We held onto slight chance showers along the immediate
coast Friday with greater coverage over the waters, though even
this might be a tad overdone if the 00z GFS verifies. Cold
advection will yield highs in the low to mid 60s.
Dry weather is anticipated Saturday through Sunday. A broad
upper trough pattern will persist, with weak surface high
pressure moving east of the area on Saturday, allowing a
southwest flow to bring warmer temperatures in the upper 60s. A
potent northern stream shortwave will drop through the area
Saturday night, pushing a dry cold front past the area.
Expansive dry high pressure will spread over the southeast
United States on Sunday, dropping high temps a few degrees.
Long term /Sunday night through Thursday/...
dry high pressure will prevail Sunday night through Wednesday. A
cold front may approach late Wednesday or on Thursday,
potentially bringing scattered showers to the area. Sunday and
Monday will be slightly cooler due to the influence of cold high
pressure. Temps warm up during mid week as the high shifts east
and southerly flow develops.
Aviation /18z Thursday through Tuesday/...
radar shows bands of light rain moving over our area, continuing
into this evening. Surface observations show ceilings have
lowered to MVFR. But there is a band of IFR ceilings closer to
the coastline, moving northward. We have tempo IFR in both tafs
to account for this. But the timing may need to be amended based
on observational trends. Conditions should improve in the
evening as the center of the surface low passes off our coast
and pulls the rain away. Tonight, VFR conditions with steady NE
winds is expected at kchs and ksav.
Extended aviation outlook: MVFR ceilings could continue into
Friday at both terminals, then VFR returns.
surface low pressure developing near The Big Bend of Florida is
interacting with high pressure centered over the Delmarva
region. The high is expected to move very little through
tonight while the low is forecasted to track NE off the Georgia and
SC coast today into tonight. This pattern will support gusty NE
winds across the marine zones today and tonight. Wave guidance
indicates that wave heights will increase with 6 foot seas
spreading into the nearshore zones this afternoon. Winds are
forecast to increase this evening and tonight, with gusts around
25 kts common. A Small Craft Advisory is in effect outside the
Marginal small craft conditions will extend into Friday over the
nearshore waters and into Friday night for the offshore Georgia
waters, mainly due to some residual 6 ft seas. Then, sub-
advisory conditions will prevail through at least the middle of
next week as high pressure takes hold.
Marine...Small Craft Advisory until noon EST Friday for amz350-352-354.
Small Craft Advisory until 3 am EST Saturday for amz374.