959 
fxus62 kilm 212001 
afdilm 


Area forecast discussion 
National Weather Service Wilmington NC 
301 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 warmup 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 am Saturday...a weak upper ridge manages to build on 
Tuesday and this should bring a mild afternoon. Wednesday may also 
bring highs in the low 70s ahead of what will end up being a 
pretty strong cold front on Thursday. Little to no moisture flow 
ahead of the front so its passage should be dry. The ecwmf has 
indicated otherwise for a few runs now but this model has been 
displaying a very wet bias for some time locally in such setups. 
Dry and cool weather for the rest of the period with highs struggling 
to low 60s by day and upper 30s at night. 


&& 


Aviation /20z Saturday through Thursday/... 
middle level stratus associated with weak low pressure wave offshore is 
struggling to advect inland...and coastal terminals have only 
reported scattered ceilings today. Still expect broken 5k foot ceilings at 
ilm/cre/myr through the afternoon and into tonight. Flow/lbt only dealing 
with high level cirrus streaming in ahead of developing Gulf of Mexico 
low pressure. Winds have been generally out of the north/NE at 8 to 14 
knots across the region...and this will continue through the afternoon while 
insolation helps promote mixing. Tonight winds will decrease...but 
likely not decouple...so expect NE winds at 4 to 7 knots through the 
overnight. No fog tonight. 


VFR will be the rule at all terminals through the taf period. Only 
forecast concern will be development of rain tomorrow afternoon. Guidance 
continues to delay onset of precipitation...so have only introduced 
vcsh to flow by 18z tomorrow. All other terminals should remain dry 
through the next 24 hours...with ceilings developing around 4k to 5k 
feet everywhere. Will let future tafs address rain potential as the 
event gets closer. 


Extended outlook...potential IFR due to showers and low ceilings 
Sunday and Monday. Fog possible Tuesday morning. 


&& 


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 Wednesday/... 
as of 3 am Saturday...looks like a fairly quiet pattern for most 
of the period. High pressure remains to the north Tuesday bringing 
a light northerly wind and seas in the 2 to 3 feet range. Changes 
may be underway come Wednesday as the high begins to lift out 
ahead of an approaching cold front. The timing of this front and 
the evolution of surface features associated with it is highly 
uncertain. 


&& 


Ilm watches/warnings/advisories... 
SC...none. 
NC...none. 
Marine...Small Craft Advisory until 9 PM EST this evening for amz256. 


Small Craft Advisory until 7 PM EST Sunday for amz250-252. 


&& 


$$ 


Near term...tra 
short term...iii 
long term...mbb 
aviation...jdw 

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