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Area forecast discussion 
National Weather Service Wilmington NC 
125 am EDT Thursday Mar 18 2010 


Synopsis... 
low pressure off the South Carolina coast tonight will move east- 
northeast through Thursday night. This will bring rain chances 
near the coast tonight and Thursday. A warming trend is expected 
heading into the weekend...as milder air begins to overspread the 
region ahead of an approaching cold front. The front will bring a 
chance of showers Sunday into Sunday night...with cooler weather 
expected Monday through Wednesday. 


&& 


Near term /until 6 am Thursday morning/... 
as of 930 PM Wednesday...surface low developing east of Savannah 
will begin a slow east-northeast movement overnight. Ahead of 
this low an area of weak isentropic lift has developed which is 
responsible for the modest returns seen on kltx 88d. As expected 
light rain/drizzle have been reported and quantitative precipitation forecast has remained under 
0.10 inch. Could see up to 0.20 inch along the immediate coast as 
this area spreads north-northeast. Farther inland a hundredth or 
two is probably the most that can be expected overnight. Behind 
this precipitation low level winds become more northwesterly bringing in 
drier low level air and causing a slight inversion around 2k feet. 
Though precipitation duration will be limited skies will remain cloudy 
all night. Clouds and northerly winds picking up overnight will 
keep temperatures near climatology. 


&& 


Short term /6 am Thursday morning through Friday night/... 
as of 3 PM Wednesday...upper level low pressure will move across 
eastern Georgia and South Carolina Thursday...reaching the 
Atlantic waters late in the afternoon. The lows eastward progress 
is being held up by a second vorticity maximum rotating through 
the deep eastern U.S. Trough. Until this second vorticity maximum crosses 
Florida and moves offshore Thursday evening...our upper low will 
not be in a hurry to leave. Surface low pressure will develop off 
the SC coast Thursday morning. Models are generally in agreement 
with this...although the westward extent of precipitation is where 
larger uncertainty exists. HPC suggests the GFS/European model (ecmwf) forecasts 
are more believable than the NAM... and following this idea paints 
50-70% probability of precipitation along the immediate coast although with very low quantitative precipitation forecast 
on the order of .01 to .08 inches. Between the coast and I-95 
there are varying orders of "chance" probability of precipitation...with very little 
potential for rain west of I-95. In fact model forecast soundings 
show cloud bases may remain above 7000 feet inland...with the dry 
sub-cloud layer eating up what little rain tries to fall. 


High temperatures have significantly more-than-normal uncertainty 
Thursday. Assuming it remains cloudy and dry inland with light rain 
confined to the coast we should see highs in the middle-upper 60s 
inland...with readings closer to 60 at the coast. Any shift of the 
precipitation shield to the west will lead to cooler temperatures...with 
any eastward shift adding several degrees across the board. 


The upper low will begin to accelerate out to the northeast Thursday 
night...with the column quickly drying before daybreak Friday. 
Friday should be a wonderful late winter day with clear skies and 
highs in the lower 70s in most locations away from the immediate 
coast where an afternoon seabreeze will keep temperatures a few degrees 
cooler. Surface high pressure will move into the eastern Carolinas 
Friday night providing calm winds and excellent radiational cooling 
conditions. Lows should fall into the lower 40s most areas. 


&& 


Long term /Saturday through Wednesday/... 
as of 3 PM Wednesday...dry weather and temperatures more typical of the 
latter part of Spring on Saturday under surface and upper ridging. 
Still not quite jumping on the GFS MOS guidance temperatures but will see a 
lot of middle 70s on Saturday afternoon. 


Sunday will be a continuation of the mild temperatures ahead of the cold 
front...albeit with a bit more cloud cover and some late probability of precipitation. GFS 
is honing in on a Sunday afternoon/evening window for precipitation. Would 
go with higher chances if it was not straddling 00z Monday...but 
mostly low chances for now. 


High pressure will build in Monday through Wednesday. Upper pattern 
looking flatter than previous runs so not sure just how cool we will 
be. Will maintain the going forecast with temperatures just below climatology. 


&& 


Aviation /06z Thursday through Monday/... 
as of 05z...a decreasing area of precipitation from east of lbt to 
near ilm will move northeast and dissipate early this morning. 
Winds will begin to pick up out of the north at 5 to 10 knots as a 
surface low begins to develop offshore. Precipitation should 
re-develop mainly offshore early this morning. The proximity of the 
low to the coastal terminals could allow for MVFR ceilings to 
develop across the coastal terminals after sunrise. Visibilities 
will drop to MVFR at times in showers after sunrise until middle to 
late morning as some of the showers move onshore. The flow/lbt 
terminals should remain VFR this morning through the afternoon. 


By afternoon the surface low will move further offshore with 
precipitation chances ending and improving ceilings. Expect VFR 
ceilings by afternoon with middle to high level ceilings by evening. 
Winds will be from the north around 10 knots. 


Extended outlook...moderate confidence MVFR visibilities Saturday 
morning. Moderate confidence tempo IFR visibilities/isolated 
thunderstorms Sunday evening. 


&& 


Marine... 
near term /through tonight/... 
as of 930 PM Wednesday...gradient will start to tighten up 
overnight as low pressure developing east of Savannah slowly 
moves east-northeast. Wind speeds gradually increase overnight 
and become more northerly as the low approaches. Prolonged north 
to northeast fetch will builds seas high than forecast 10 to 15 
knots would suggest...but seas will remain below headline criteria 
through the end of the period. 


Short term /Thursday through Friday night/... 
as of 3 PM Wednesday...low pressure will develop over the South 
Carolina offshore waters Thursday morning...leading to increasing 
northerly winds and building seas through the day and into 
Thursday evening. Rain will accompany the deteriorating 
conditions...especially over the NC coastal waters. The worst 
conditions are expected to occur Thursday night as the pressure 
gradient reaches its tightest extent. As the low kicks out to the 
northeast after midnight...wind directions will back to a more 
offshore direction...knocking sea heights down rapidly during the 
day Friday. In fact by late Friday morning conditions should be 
rather benign...with winds becoming light and variable under high 
pressure Friday night. 


Seas are already running surprisingly high due to an easterly 10-12 
second swell emanating from strong low pressure across the 
far-northern Atlantic. Increasing winds Thursday into Thursday night 
will build seas as high as 6-8 feet east of Cape Fear...with seas 
perhaps building to 5-6 feet over the S Carolina coastal waters. An 
advisory will be posted for the NC waters...but we will hold off for 
SC since the sea conditions are less certain there. 


Long term /Saturday through Sunday/... 
as of 3 PM Wednesday...light and variable winds will come around to 
south on Saturday as high pressure slips offshore and a decent 
seabreeze sets up. Gradient will tighten on Sunday ahead of the 
cold front. GFS shows 850mb winds cranking up to 40 knots...but 
stability will limit US to about 20 knots at the surface. Open fetch 
will build seas to marginal Small Craft Advisory levels Sunday afternoon and evening. 
Frontal passage Sunday evening will bring winds around to westerly and knock 
down seas. Modest gradient will keep wind speeds around 15 knots through 
Monday. 


&& 


Ilm watches/warnings/advisories... 
SC...none. 
NC...none. 
Marine...Small Craft Advisory from 6 am Thursday to 10 am EDT Friday 
for amz250-252. 


&& 


$$ 


Near term...iii 
short term...tra 
long term...Ras 
aviation...mrr/xxxi 

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