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Area forecast discussion 
National Weather Service Raleigh NC 
941 PM EST Tuesday Dec 2 2008 


Synopsis... 


High pressure will build into the region from the southwest 
tonight bringing cold calm conditions. The high will shift east 
Wednesday allowing for a warming trend. A cold front is then expected 
to sweep through the state late Thursday ushering in another shot 
of colder air for Friday into the weekend. 
&& 


Near term /through Wednesday/... 
as of 945 PM Tuesday... 


1028 mb surface high pressure was centered over Florida this evening. 
The ridge axis extended northward up the Appalachians to near 
Pittsburgh. Satellite and surface observations indicated clear 
skies from NC westward to the Ozarks. With rising heights aloft to 
the west... this means clear skies all night. The surface had 
already decoupled at most locations this evening under the large 
ridge... with calm conditions reported. This will allow for maximum 
radiational cooling. 


The middle evening dew points ranged from 20 to 30 northwest to southeast... with 
temperatures already ranging in the middle to upper 20s rural areas... to middle 
30s urban areas. We look to be well on our way to our projected 
lows of 22-27. There should be some near 20 lows around Roxboro to 
Louisburg... and possibly in rural areas of The Triad region. 


Wednesday: shortwave ridging aloft and associated high pressure at 
the surface will shift offshore... but clear... stable... and dry 
conditions (with a pronounced subsident inversion centered around 
h8) are expected to remain. There may... however... be an increase 
in cirrus from the southwest late in the afternoon and into Wednesday 
evening... as backing flow aloft transports high level moisture into 
our area ahead of an upstream trough aloft. Return flow around the 
low level ridge will result in a moderating trend... with afternoon 
forecast low level thicknesses in the 1315 to 1325 meter range. 
Corresponding high temperatures of 53 to 57 degrees expected. 


&& 


Short term /Wednesday night through Friday/... 
as of 200 PM Tuesday... 


For Wednesday night: the surface ridge will shift east of the 
area... in advance of a cold front approaching from the west that is 
associated with broad troughing digging over the northern U.S. Both 
low and middle level flow increases from the SW overnight... producing 
a strong surface based inversion as 850 mb temperatures warm above +8c by 
Thursday morning. However despite the low level flow out of the 
Gulf... deep moisture is nonexistent as the precursor air mass over 
the Gulf is quite dry through a deep layer. We should see increasing 
high clouds as the cold front... in conjunction with shearing middle 
level shortwave energy... moves in from the west... and the surface 
inversion may be strong enough to lead to areas of advection fog by 
daybreak. Lows 32-36. 


For Thursday through friday: the 12z NAM and GFS agree well on 
a surface frontal passage during the late afternoon through 
evening... and precipitation chances ahead of the front are still looking 
rather small. A potent (but diminishing) low level jet of 30-40 kts 
shifts into the County Warning Area... but the low level winds become westerly ahead 
of the front resulting in downslope drying... and forecast low level 
moisture transport is meager and decreasing as the front nears. The 
middle level shortwave trough holds well to our northwest and shears out... 
the upper jet core remains well to our north... and the column 
remains pretty stable... which all result in limited lift 
mechanisms. Will hold probability of precipitation at no better than 18% Thursday afternoon. 
Thicknesses climb above seasonal normals... but much of this is 
realized in the 925-850 mb layer... rather than at the surface. 
Nevertheless there should be enough heating to allow US to hit highs 
in the middle 50s northwest to middle 60s southeast. Expect clearing skies and a cooling 
and further-stabilizing column Thursday night. With thicknesses 
plunging... have trimmed a couple of degrees off lows... going 
around 28-36. Sprawling polar-source high pressure builds in from 
the west... and just a few high clouds are expected under broad 
cyclonic flow aloft. Highs 46-54 are in line with guidance trends 
and low level thicknesses. -Gih 


&& 


Long term /Friday night through Tuesday/... 
as of 200 PM Tuesday... 


For Friday night/saturday: high pressure at the surface spans the Tennessee 
Valley and middle-Atlantic region Friday night... but high clouds should 
be on the increase as the axis of the longwave trough approaches 
from the west. By midday Saturday both the GFS and European model (ecmwf) show at 
least a 130 knots jet diving in from the northwest... the left exit region of 
which passes over central NC as a clipper cold front approaches from 
the northwest. This possible jet-induced forcing for ascent along with 
incoming middle level positive vorticity advection should be enough to 
produce significant middle and high clouds for Saturday afternoon into 
early evening. Forecast soundings do show a rather deep moist layer 
extending from around 750 mb up through 400 mb particularly across 
the north... and the vertical wet bulb profile in this area is all 
at or below freezing. But these soundings also indicate a high cloud 
base (over 7000 ft) so even with departing high pressure at the 
surface... the subcloud layer is likely to be too dry for any 
precipitation given the indicated forcing mechanisms. But... if the 
models trend even more moist and/or develop stronger lift in later 
runs... a few flurries may need to be added over the northern 
forecast area on Saturday. Expect lows of 26-32 and highs of 
45-53... in line with projected thicknesses that are 15-20 M below 
normal and factoring in the clouds and reduced insolation. 


For Saturday night/sunday: the longwave trough axis shifts offshore 
Saturday night as the surface clipper front drops through... and 
decreasing cloud cover is expected. As surface pressures lower over 
the interior northeast states while Arctic high pressure drops into 
the middle Mississippi Valley... the resultant mslp gradient should 
keep winds up a bit Saturday night... and with good mixing on 
Sunday... it could be rather blustery. With temperatures likely to top out 
only in the upper 40s to lower 50s Sunday... wind chills may 
struggle to get out of the 30s in some spots. 


For Sunday night through tuesday: middle level troughing deepens off 
the New England coast through Monday as the chilly surface high 
builds into and over the midatlantic region. Thicknesses remain well 
below normal and temperatures should follow suit... with lows in the 
middle-upper 20s and highs again in the upper 40s to lower 50s. Then 
for late Monday Onward... the models really start to diverge with 
the timing and amplitude of middle level trough that digs over central 
then eastern noam. The 12z/02 GFS keeps a closed vortex sitting off 
the Southern California coast and is less amplified and faster with the northern 
portion of the trough... bringing it through the Great Lakes into 
the western slopes of the Appalachians by late Tuesday with 
expansive precipitation spreading over NC. In contrast the 00z/02 European model (ecmwf) and 
the 06z/02 GFS kept the trough full-latitude and phased and thus was 
much more amplified and slower. The non-phased solution seems much 
more likely climatologically speaking... but with this much 
difference among the operational models and with the gefs showing a 
large spread in solutions around its mean trough position... will go 
for a slow increase in clouds Monday through Tuesday... and a chance 
of rain starting Tuesday... mainly west. Will hold onto highs in the 
upper 40s to lower 50s Monday... rising to the low-middle 50s for 
Tuesday. The surface ridge will be exiting to the NE but if it 
deposits residual dry and cool surface air over the Piedmont as the 
rain arrives... an in situ damming event could result and temperatures 
could be much cooler in the western County Warning Area on Tuesday. -Gih 


&& 


Aviation /03z Wednesday through Sunday/... 
as of 630 PM Tuesday... 


VFR conditions expected for the taf period. A surface high will settle 
over the region tonight...allowing winds to go calm in most 
locations. The surface high shifts offshore during the day Wednesday... 
with winds turning light out of the south by late morning/early 
afternoon. Ridging aloft will keep skies mainly clear through the 
period. 


Looking ahead... a nocturnal low level jet... characterized by 
southwesterly winds of 30 to 40 kts at 2000 feet... is forecast to 
develop over central NC late Wednesday night. If the surface winds 
decouple and go (near) calm as expected... then low end low level wind shear could 
be a concern in the 08 to 12z time frame early Thursday morning. A 
cold front will move through central NC Thursday afternoon or 
evening... and result in generally VFR... 3 to 7 thousand feet ceilings 
and a few spotty showers. VFR conditions are expected to prevail 
through Sunday... though forecast uncertainty increases by this 
weekend... when storminess over the northeastern US may threaten. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...Badgett 
near term...Badgett/mws 
short term...Hartfield 
long term...Hartfield 
aviation...jfb 

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