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Area forecast discussion 
National Weather Service Raleigh NC 
645 am EST Sat Nov 21 2009 


Synopsis... 


High pressure to our north will bring northeast flow to North 
Carolina into Sunday. The next storm system will move into the 
region from the southwest and bring clouds and rain for Sunday and 
Sunday night. 


&& 


Near term /through tonight/... 
as of 315 am... 


Surface high pressure north of North Carolina will keep northeast 
flow over central North Carolina into tonight. Expecting enough 
sunshine through the middle and high clouds to bring temperatures to 
just short of full sun values for northeast flow. Afternoon highs 
upper 50s to lower 60s north... and 62 to 65 south. 


An upper level system dropping into Texas is generating surface 
low pressure in the northwest Gulf of Mexico at this time. Both 
these lows forecast to roughly move east northeast into Sunday. 
The NAM 700 millibar pressure field is perhaps too weak and prefer 
the stronger middle level front of the Canadian and GFS models. The 
GFS and Canadian depict a 700 millibar warm front... barely 
lifting north into North Carolina by sunrise Sunday. Isentropic 
lift is quite weak below this front so any rainfall should be 
late... well under one tenth inch and not stray too far north by 
sunrise. Increased lower cloudiness and more of a breeze than 
Friday night should yield low temperatures in the middle to upper 
40s. 


&& 


Short term /Sunday through Monday night/... 
as of 315 am... 


A 700 millibar warm front near the South Carolina border sunrise 
Sunday will exit to the north after sunset. Given the expected 
northward progress of the front and moderate to weak isentropic 
lift near 850 millibars... at least the southwest three quarters 
should see measurable rain before sunset Sunday. 


As the upper system lifts northeast through the mountains Sunday 
night... low pressure development off the Carolinas occurs and 
becomes the dominate surface low. Good middle level isentropic lift 
in the middle levels should keep rain chances high into Sunday night. 
Most models damp out the parent low pressure trough as it moves 
northeast and keep the surface low weaker than the Canadian as it 
approaches Cape Hatteras. Rainfall perhaps two tenths inch 
northwest to four tenths southeast Sunday night. 


Winds weaken late Sunday night into Monday and heavier 
precipitation may miss US to the east Monday. Precipitation may 
linger longest in the northeast with the exiting surface low but 
rainfall amounts of one tenth inch or less east... and much less 
to the west. Of course if the low tracks closer to the coast... 
these amounts are underdone. Only a slight rain chance in the 
northeast with the exiting low Monday night. 


Overnight lows 45 to 50 Sunday night... and mostly middle 40s Tuesday 
night. Sunday high temperatures near 50 northwest to upper 50s 
southeast. If a cold air mass damming wedge sets up as expected 
Monday...a low overcast with light rain or drizzle possible. The 
current forecast Ten Degree diurnal rise to Monday afternoon would 
need to be dropped to three to six degrees if clouds are able to 
persist into early Monday evening. 


&& 


Long term /Tuesday through Friday/... 
as of 315 am Friday... 


Any light precipitation lingering into Monday night should be gone by 
Tuesday morning...as southwesterly middle level flow Ushers in drier 
air above the remaining cad airmass. The possibility lingering low 
clouds could complicate the temperature forecast on Tuesday as some 
moisture remains below a subsidence inversion beneath developing 
shortwave ridging aloft. Will stick with MOS values with highs 
64-70. 


The forecast gets quite a bit more complicated beyond Tuesday...with 
the handling of a shortwave expected to enter the Pacific northwest 
on Monday and emerge over the north Central Plains by Wednesday. The 
general trend is for the development of a high amplitude longwave 
trough by the end of the week...and while both the GFS and European model (ecmwf) 
resolve such a solution...they currently get the similar answer 
through somewhat different scenarios. The GFS sends the 
aforementioned shortwave across the upper Midwest and the lifts the 
system into Ontario by Thursday...pushing a dry cold front into 
central NC Wednesday night. This is followed by a second shortwave that 
dives south through the Midwest...but with no more to work 
with....points east of the mountains stay dry even as a surface wave 
is induced offshore Thursday morning. The European model (ecmwf) on the other hand 
is much slower with the first shortwave...allowing more moisture to 
be pulled north over a persistent surface ridge along the eastern 
Seaboard. Cyclogenesis along the coast subsequently occurs closer 
to the coast Thursday morning with less eastward progression of the 
surface front...and thus the European model (ecmwf) produces precipitation over the eastern 
third of NC. Given these continued model inconsistencies...feel the 
best course of action is to show below normal temperatures for the end of 
the week...continuing with higher slight chance probability of precipitation in the east 
until some sort of model convergence begins. 




&& 


Aviation /12z Saturday through Thursday/... 
as of 645 am Saturday... 


With models trending slower on the arrival of a storm system 
currently over the western Gulf Coast...VFR conditions will prevail 
through the valid taf forecast. Scattered to broken cirrus and eventually 
cirrostratus will drift across the region today and into 
tonight...all ahead of a storm system set to impact central NC from 
the southwest during the day Sunday. At the surface...high pressure 
to our north will settle over New England...extending a ridge of dry 
air down the East Coast....and providing a light NE wind. 


As the storm system approaches from the southwest Sunday morning 
ceilings will eventually lower to MVFR or IFR levels Sunday through 
Monday....with periods of rain possible at all taf sites. At least 
MVFR ceilings are expected to persist through late Monday...before a 
coastal low moves away from NC and pulls drier air into the area. 
Conditions are expected to remain VFR for the most through the 
balance of the week...although there is the potential for another 
weather system to effect mainly eastern terminals by Thursday. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


Long term...Smith 
aviation...Smith 

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