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Area forecast discussion 
National Weather Service Wilmington NC 
635 PM EST Sat Nov 21 2009 


Synopsis... 
increasing moisture from the Gulf of Mexico will enter the Carolinas 
Sunday into Monday with widespread rain expected. A brief warm up is 
expected middle week. A strong cold front Thursday will usher in 
some colder air for next weekend. 


&& 


Near term /until 6 am Sunday morning/... 
as of 3 PM Saturday...multi-layered cloudiness is increasing over 
the Carolinas from the south as an upper trough over Louisiana 
advects Gulf moisture into the area. Aside from some clouds trapped 
under a subsidence inversion around 800 mb...the bulk of this 
moisture is at and above the 500 mb level currently. Models show the 
depth of moisture increasing through the night as surface low 
pressure near the mouth of the Mississippi River moves into southern 
Alabama late. The 12z runs of the NAM and GFS show measurable 
precipitation should hold off until during the day Sunday. This idea 
is supported by recent radar composites showing the front edge of 
the precipitation extending no farther east than Biloxi MS 
currently. 


Low temperatures tonight present an interesting problem. MOS 
guidance suggests we will reach the middle 40s in most areas while the 
raw NAM and GFS models show lows in the 50-55 range. The 12z 
Canadian is in between these two extremes. Insulating influence from 
clouds will be partially offset by weak cool advection below 850 mb. 
Our forecast is for upper 40s inland and around 50 at the coast. 


&& 


Short term /6 am Sunday morning through Monday night/... 
as of 3 PM Saturday...nearly all of the guidance has delayed arrival 
of the precipitation on sun...with light precipitation now spreading over the 
southwest portions of the forecast area Sun morning. Light precipitation 
will continue to spread north during the afternoon hours...covering 
all of the area by late afternoon. Heaviest rain arrives Sun night 
into Monday with increase in isentropic lift. Delayed arrival of 
categorical probability of precipitation to account for slower evolution of the system. 


Low pressure developing off the Georgia/SC coast Sun night will track 
northeast...passing just east of Cape Fear on Monday. Forecast 
soundings and cross sections show plenty of dry air above 850 mb 
spreading over the area Monday morning. Subsidence in the wake of 
exiting low and upper level short wave combined with dry air will 
bring an end to measurable precipitation. Have extended higher probability of precipitation into 
Monday...but think most of the action will be over by late morning. 
Area remains under the influence of The Wedge for the remainder of 
Monday and through Monday night with southwest flow just above the 
surface. Weak overrunning will keep low cloud cover in place with 
some patches of drizzle through the afternoon and overnight. At this 
point it does not look like this batch of precipitation will measure. 


Temperatures will remain below climatology during the day...though coastal 
site may flirt with climatology on Monday depending on how close the low 
gets to the coast. Cloud cover and moisture will help keep overnight 
lows above to well above climatology both night. 


&& 


Long term /Tuesday through Saturday/... 
as of 3 PM Saturday...weak high pressure will be in place across 
coastal Carolinas Tuesday through Wednesday. Pcp water values will be down 
closer to a half inch with plenty of dry air aloft...but moisture 
profiles continue to show a very shallow layer of moisture holding 
on through Wednesday morning. With shallow cool air in place and decent 
subsidence and dry air in place above will hold on to low clouds 
through much of Tuesday. Pattern remains fairly flat in the middle to 
upper levels through middle week until Thursday into Friday when more 
significant trough approaches and moves through. Drier westerly flow 
takes over through middle week. A cold front or pre frontal trough 
moves into the coastal Carolinas late Wednesday into Thursday as coastal 
trough develops off the southeast coast. Looking at middle to upper 
levels..may see a shortwave move through longer wave 500 mb trough 
giving push of drier air initially...but then winds back on Thursday 
tapping into a Little Gulf moisture. This remains mainly off 
shore...but may be just close enough to produce some pcp for Thursday 
afternoon. Therefore will hold onto low end probability of precipitation for Thursday. Then plenty 
of dry air will move in behind strong cold front late Thursday into 
Friday. By Friday high amplitude middle to upper trough swings across the 
East Coast scouring out all the weather and bringing dry and cool 
high pressure for the weekend. Middle to upper level flow will remain 
more westerly through Friday night but by Sat afternoon should get stronger 
and deeper layer cold air streaming into the Carolinas in a deep 
layer northwest flow as 500 mb trough clears the coast. Expect dry but cold 
weather through the weekend. 


Temperatures will hold around normal or above with sunniest and warmest day 
expected on Wednesday. By late Thursday into Friday cold and dry high pressure 
will build in behind strong cold frontal passage. 850 temperatures drop 
below 0c by Friday evening and with plenty of dry air in place...expect 
temperatures down in the 30s for overnight lows both Friday night and Sat 
night. Day time highs Friday and Sat will be below normal...below 60 
inland to just near 60 along the coast. 


&& 


Aviation /00z Sunday through Thursday/... 
surface high pressure over the terminals is providing a dry 
northeast flow in the low levels. This is expected to continue 
through sunrise resulting in VFR conditions. Moisture near an 
inversion around 5k could result in at least scattered to tempo 
ceilings at that level overnight. Otherwise overnight...scattered 
middle level clouds and broken high level clouds are expected. 


After sunrise VFR is expected through 18z at the terminals with middle 
level ceilings 8-12k developing as virga overspreads the terminals 
from southwest to northeast. Eventually the rain will develop 
ceilings 4-5k as moisture pools at the inversion level. After 18z 
should see a better chance of MVFR ceilings developing as the lower 
levels continue to moisten up. Although there is a slight chance of 
IFR ceilings by late afternoon at Flo/cre/myr...widespread IFR 
ceilings should hold off until after this taf valid period. 


Extended outlook...widespread IFR Monday. IFR due to fog possible 
Tuesday and Wednesday mornings. 


&& 


Marine... 
near term /through tonight/... 
as of 3 PM Saturday...a stationary front nearly 500 miles off the 
Carolina coast developed a weak low pressure center early this 
morning. This low is still out there although its influence on our 
weather is becoming smaller. Attention is now shifting to the Gulf 
Coast where a new low is developing over coastal Louisiana. This low 
will move east to around Mobile Alabama by daybreak Sunday. The pressure 
gradient between this low and high pressure over New England and the 
eastern Great Lakes will keep a healthy NE wind blowing through the 
night into Sunday. Wind speeds should average 15-20 knots with some 25 
knots gusts possible...especially in The Frying Pan Shoals vicinity. 


Output from our local Swan wave model was almost perfect on 
initialization and will be used for the forecast tonight. A Small 
Craft Advisory will remain posted for the NC waters for 6 feet seas 
out beyond 15 miles from shore. We will also keep an advisory going 
through the evening south of Murrells Inlet until the 6 feet seas die 
away there. Any significant rainfall should hold off until sometime 
during the day Sunday. 


Short term /Sunday through Monday night/... 
as of 3 PM Saturday...northeast flow continues through the period 
with surface low prognosticated to pass east of the waters Sun night into 
Monday. Pinched gradient on sun will keep winds around 20 knots with seas 
4 to 6 feet into Sun night. Strongest winds and highest seas will be 
across NC waters...so will continue Small Craft Advisory for zones 250 and 252. Will 
likely need a scec for SC zones when the time comes. Gradient 
relaxes when surface low passes the waters early Monday. High pressure 
continues to ridge down the East Coast as the low moves off to the 
northeast on Monday...keeping low level flow out of the north. Lighter 
winds...10 to 15 knots Monday and Monday night...and a slight offshore 
component will allow seas to fall pretty quickly...to 2 to 4 feet by 
Monday evening. 


Long term /Tuesday through Thursday/... 
as of 3 PM Saturday...weak high pressure over waters through middle 
week with benign winds and seas expected. Overall expect light off 
shore flow 10 kts or less through early Thursday with seas less than 3 
feet. By Thursday afternoon gradient may increase as cold front pushes into 
the 
Carolinas from the west and coastal trough develops to the south. 
Expect winds and seas to increase through late Thursday into Friday as 
cold front approaches and moves through. May see seas up near 4 to 5 
feet in outer waters in stronger westerly flow Thursday night into Friday 
morning. 


&& 


Ilm watches/warnings/advisories... 
SC...none. 
NC...none. 
Marine...Small Craft Advisory until 6 am EST Monday for amz250-252-256. 


Small Craft Advisory from midnight tonight to 6 am EST Monday 
for amz254. 


&& 


$$ 


Near term...tra 
short term...iii 
long term...rgz 
aviation...mrr 

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