535 
fxus62 krah 082355 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
700 PM EST Thursday Jan 8 2009 


Synopsis... 
an upper level disturbance will move southeast across NC today. High 
pressure will build into the state from the west tonight and Friday. 
Another cold front will approach from the west on Saturday... which 
will increase the chance of showers by then. 


&& 


Near term /through Friday night/... 
as of 1230 PM Thursday... 


Surface analysis this afternoon showed a trough axis over the 
middle-Atlantic and western Carolinas. As of 17z...temperatures across the 
area ranged from the middle/upper 40s in the north/northwest to the lower 50s in 
the S/se. Dewpoints ranged from the upper 20s to the lower 30s. 
Aloft...12z radiosonde observation analysis showed an upper level trough axis over the 
eastern Tennessee/Ohio valleys. WV imagery and RUC model data showed a 20/S 
500 mb vorticity maximum embedded within the trough...located over eastern 
Kentucky at 12z. 


Rest of today: 
large scale lift will continue to increase this afternoon as an 
upper level trough swings east across the mid-Atlantic. The 
vorticity maximum seen in water vapor imagery over the northwest 
North Carolina mountains as of 17z is forecast to move east 
across northern portions of the area this afternoon. Forecast 
soundings show a sufficient depth of saturation (about 10 thousand 
feet of saturation...up through the minus 20c isotherm) during the 
best large scale ascent (18 to 21z) to support the development of 
spotty light precipitation. However...forecast soundings show 
rather breezy westerly flow in the lower and middle levels 
today...which should help keep the lower levels rather 
dry...particularly in locations closer to the foothills/ 
mountains...such as The Triad and western Piedmont. With the 
relatively speedy westerly flow...feel that we have a better 
chance of seeing virga rather than measurable precipitation... 
particularly in the western half of the area where the detrimental 
downslope drying effects of westerly flow will be more pronounced. 
As such...will show the only chance for measurable precipitation 
today in the coastal plain from gsb north to rzz...where a slight 
chance of a shower will be advertised. Elsewhere...will keep out 
measurable probability of precipitation but will still mention a slight chance of a few 
sprinkles. 1000 to 850 mb critical thicknesses appear too warm 
over the area to support any mixed precipitation...with the lowest 
4 to 5 thousand feet above freezing in the east and at least the 
lowest 2 to 3 thousand feet above freezing in the west and 
northwest...with surface temperatures at least in the middle 40s. 
Speaking of temperatures...highs look on track to range from the upper 
40s in the northwest to the middle to upper 50s in the southeast... 
with the thicknesses coming in line pretty close with MOS 
guidance. The only potential problem with the temperature forecast is 
related to middle and upper level cloud cover...which is currently 
quite extensive. If extensive cloudiness persists... particularly 
in the eastern half of the area...current forecast highs may be a 
few degrees too warm. Conversely...if enough sunshine breaks out 
in the west and northwest Piedmont...high temperatures may be a 
few degrees too cool. 


Tonight: 
weak high pressure will begin to build in from the west after sunset 
this evening...becoming centered over the North Carolina mountains 
by 12z Friday morning. Additionally...we will see height rises and 
subsidence aloft as the upper level trough lifts northeast away 
from the area...allowing clear skies tonight. The min temperature 
forecast tonight will ultimately depend on how long a light 
northwest wind persists after sunset. The latest GFS/NAM guidance 
shows the mean sea level pressure gradient beginning to slacken in 
the west and southwest after 06z as the surface high approaches the 
mountains...however...the coastal plain (east and northeast portions 
of the area) may see enough of a pressure gradient to keep a light 
(3 to 5 knot) northwest wind going through sunrise Friday morning. 
Therefore...expect the best radiational cooling potential in the 
western half...and will correspondingly show the lowest temperatures 
(mid to upper 20s) in that area...with upper 20s to lower 30s in the 
east to northeast. 


Friday: 
surface high pressure will slide across the area on Friday...moving 
offshore Cape Fear by sunset Friday evening. Aloft...heights will 
continue to rise in the wake of the departing upper level trough... 
with upper level flow becoming zonal by Friday evening. This pattern 
will result in clear to sunny skies across central North Carolina 
throughout the day Friday. Low level thicknesses start out rather 
cool on Friday... ranging from 1275 to 1290 meters at 12z... 
increasing to 1310 to 1320 meters by the end of the day. These 
thicknesses yield high temperatures ranging from the middle 40s to 
around 50 to 52f...which agrees reasonably well with a MOS 
guidance blend. Warmest readings are expected in the sandhills and 
southwest Piedmont...with the coolest in The Triad and NE coastal 
plain. 


Friday night: 
a shortwave trough is expected to drop southeast through the upper 
Midwest toward the Tennessee/Ohio valleys Friday night...with an associated 
surface low deepening over the Tennessee/Ohio Valley. At the same 
time...surface high pressure off the Carolina coast will move 
further east into the Atlantic. This will set up a pattern of 
southerly flow and strengthening warm advection Friday night. 
Forecast soundings do not show any saturation...however...and the 
warm advection appears to manifest itself primarily in the form of 
warming low and middle level temperatures throughout the night. Expect 
mostly clear skies for a good portion of the night...with the 
possibility of increasing clouds from the northwest by sunrise Sat 
morning as the low pressure system over the Tennessee/Ohio valleys moves ever 
closer to the middle Atlantic and Carolinas. Expect low temperatures Sat 
morning to be several degrees milder than Friday morning given the 
strengthening warm advection and a light southerly wind at the 
surface...and will forecast lows in the lower to middle 30s across the 
area. -Vincent 


&& 


Short term /Saturday through Sunday/... 
as of 305 PM... 


Model disagreement in timing of the Saturday cold front with GFS 
Canadian and NOGAPS faster with the front well offshore by sunrise 
Sunday... and NAM and European model (ecmwf) still west of the North Carolina sounds. 
The slower solution is followed due to the consistency of the 
European model (ecmwf)... its slower timing... HPC preference of this model. 


Thus will have southwest winds across central North Carolina until 
midnight... with winds shifting northwest as the front move east 
toward dawn Sunday. This will produce rising dewpoints and steady to 
slightly rising temperatures before temperatures begin to fall as 
winds shift. Rainfall amounts of at least one third inch seem 
possible. Rain exits the east near or before sunrise. 


Temperatures will be mild ahead of the front with Saturday high 
temperatures of 54 to 61. With surface high pressure dropping to 
Texas before lifting northeast to North Carolina... temperatures 
will not drop greatly Sunday... with highs 45 to 50. Saturday night 
lows highly dependent on the timing of the front. The guess for now 
is a large temperature gradient...middle 30s northwest to lower 50s 
southeast. 


&& 


Long term /Sunday night through Thursday/... 
as of 305 PM... 


Chaos continues from the weekend regarding model continuity. The 12z 
GFS model run raised the 1000 to 850 millibar thickness about a 
hundred meters from the 06z run...a magnitude of change I have not 
seem before. The 12z European model (ecmwf) continues steady and will be followed in 
general. However am not yet completely confident in the sharp 500 
millibar trough Tuesday evening over Tennessee...which generates a 
significant coastal low off the Carolinas. There is enough model 
agreement however to warrant rain chances Tuesday... highest in the 
southeast. Otherwise the forecast is dry. 


In general clear to partly cloudy with perhaps more cloudiness 
southeast Tuesday. Afternoon high 45 to 55 Monday and Tuesday before 
falling to 40 to 45 Wednesday before recovering to 41 to 47 
Thursday. Low temperatures not far from 30 Monday and Tuesday 
morning before dropping to low to middle 20s Wednesday and Thursday 
morning. && 


Aviation /00z Friday through Tuesday/... 
as of 700 PM Thursday... 


Lingering middle and high clouds will quickly exit the coastal plain 
region 
as upper vorticity moves offshore. Clear skies are then expected for the 
remainder of the night as high pressure builds toward the area 
from the west and subsidence aloft overspreads the area in the wake 
of the departing upper level trough. Light/variable or light 
westerly winds are expected on Friday. 


VFR conditions are expected to continue Friday afternoon into Friday 
night. A return to MVFR/IFR conditions is expected by Saturday 
afternoon or evening as a cold front approaches the area from the 
west. Expect breezy conditions ahead of the front on Saturday... 
with 15 knots sustained south to southwest winds with gusts up to 25 
knots look very reasonable given the flow forecast in the mixed 
layer... depending on how much cloud cover we have and how much we 
can mix. MVFR/IFR conditions in shower activity are most likely 
between 21z Sat and 12z Sun morning...before a return to VFR 
conditions behind the cold front by or shortly after sunrise Sunday. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...Vincent 
near term...Vincent 
short term... 
long term... 
aviation...bl/Vincent 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)