Weather

632 
fxus62 krah 060843 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
340 am EST Tuesday Jan 6 2009 


Synopsis... 
a backdoor cold front will move south into central North Carolina 
this morning... then stall over the southern sections of the state 
today. A storm system developing over the middle Mississippi Valley 
region will move northeast... and pull the front back northward as a 
warm front... into the eastern Piedmont by this evening. A strong 
cold front will push east across the state Wednesday. 


&& 


Near term /through Wednesday/... 
as of 305 am Tuesday... 


Complex weather pattern to affect central NC through this period. 
Main forecast challenges are temperatures and the need for a Flood 
Watch for sections of the Piedmont. 


Surface cold front marking the leading edge of cooler air working 
its way S-SW across the northern coastal plain and far NE Piedmont 
at 07z. Krzz and khnz. Meanwhile south of the front...temperatures remain 
in the mild 50s with lower 60s in the extreme south. Model 
guidance varies on how far south surface front will get before 
stalling. NAM-WRF suggest that front will make it into the 
sandhills and southern coastal plain by 18z while the GFS stalls 
it closer to Highway 64. The truth is likely in the middle but 
having doubts that the front will make it to gsb or Fay as parent 
high driving the front relatively weak (sub1020mb). One thing that 
it does have going for it is it's pocket of cool dry air with 
dewpoints in the teens/single digits. Thus...where the cooler air 
mass does get a foothold and maintains some connect to the parent 
high...the rain falling through this cool stable layer will help 
to lock in/solidify this cad air mass. Needless to say....temp 
forecast will be tricky...especially in vicinity of the front. Will 
likely see temperature variances of 10-15 degrees across some 
counties...depending upon which side of the front they are on. 


Upstairs...a series of disturbances in the fast west-SW flow aloft 
will help to enhance the southerly low level flow...causing widespread 
rain to occur across the region. The GFS shuts off this favorable 
isentropic lift pattern a little sooner (by 18z tue) compared T 
the NAM. Considering that the core of 850mb winds will remain to 
our SW today and water vapor imagery depicting a series of 
perturbations extending to the MS river...favor NAM scenario for 
now. Thus expect widespread this morning with rain gradually 
diminishing from the south this afternoon. 


Tonight...GFS and NAM differ in the erosion of the cad airmass 
over the northern Piedmont. GFS quickly scours out the cool stable 
air from aloft due to increasing southerly jet (60-65kt). NAM-WRF 
depicts a slow erosion from the south and east but suggest 
through its lifted indices field that the cad airmass will 
persist through the night over the northwest Piedmont into the NE 
Piedmont. Forecast experience favors the later scenario. Will 
continue forecast trend of steady temperatures in the cad airmass with 
temperatures slowly rising over the remainder of the County Warning Area overnight. 


Am a bit concern that persistent light-moderate rain may lead to 
some urban flooding problems in the northern Piedmont...especially 
The Triad. Both NAM and GFS depict strong moisture transport in 
the 925-850mb layer off the eastern Gulf...suggesting efficient 
rain production. However best lift associated with strong upper 
level jet will be to our northwest...placing greater emphasis for lift 
for areas west and north of The Triad. For now will hold off on 
watch but will highlight urban flood threat in the severe weather potential statement. 


Finally on Wednesday...strong cod front associated with deepening upper 
level wave will cross central NC Wednesday morning/early afternoon. 
850-500mb lapse rates do hover around 6 degree c/km in the east 
through 18z and both NAM and GFS hint at narrow ribbon of 
instability over the eastern counties. Will introduce a isolated 
thunder int he east with frontal passage. Timing of frontal passage suggest rain 
chances dwindling west-to-east after 16z-18z...ending in the 
coastal plain by 21-22z. Winds behind the front expected to be 
strong with sustained speeds 15-20 miles per hour and gusts 30-35mph. Due to 
saturated top soil...this may lead to some sporadic power outages 
due to downed trees/power poles. 




&& 


Short term /Wednesday night through Friday night/... 
as of 330 PM Monday... 




For Wednesday night/thursday: behind the departing occluded/cold 
front... a secondary polar front drops southeast through NC/VA. Forecast 
soundings from the GFS show a somewhat steep mixed layer up to 
around 800 mb through Thursday with around 40-50 kts in the upper 
half of this layer. Given the intense downward momentum Transfer 
expected with westerly flow and strong cold air advection... these 
winds could easily mix down to the surface. With an upper jet core 
of 150 kts diving over SC... an intrusion of stratospheric air is 
also possible which could augment winds near the surface. Will go 
with sustained winds near 20 miles per hour with gusts 30-35. The GFS has 
trended drier than its earlier runs... including within the critical 
Crystal growth layer aloft... and it takes the vorticity maxima 
diving into the trough base to the north and south of NC. The European model (ecmwf) 
does take sheared vorticity through our area and spits out trace 
amounts of precipitation over Thursday... and given the vertical thermal 
profile... it would fall as light snow showers. Do not have enough 
confidence in the availability of moisture to reintroduce a mention 
of a flurry or snow shower... but later shifts will watch for any 
trend toward moistening and/or a stronger vorticity maximum 
dropping overhead. Morning lows around 34-42. Low level thicknesses 
crash Wednesday night through Thursday into the 1275-1280 range. 
Will have temperatures reaching their highs in the early afternoon 
then dropping through the afternoon as the colder air rushes in. 
This shakes out to highs of 46-54. -Gih 


&& 


Long term /Saturday through Monday/... 
as of 340 am Tuesday... 


A trend toward below normal temperatures appears likely as we head 
Sunday into Monday. Models generally agree in a cold frontal passage 
around Saturday night and early Sunday. It appears from the latest 
models runs that the moisture may be more limited with this front 
than in previous model runs. The European model (ecmwf) continued to be the most 
robust with the moisture... but even that model had cut back on the 
moisture since the previous forecast. The European model (ecmwf) was more suppressed 
with a potential southern wave development Sunday into Sunday night 
than in the previous runs... and this will have to be watched. The 
GFS was also more suppressed southward toward the Gulf. We will go 
with a slight chance of rain Saturday afternoon into Sunday morning 
with the passage of the front... then no pop Sunday afternoon into 
Monday for now... leaning toward the more suppressed models with the 
potential southern wave. 


The surface high expected to build in from the west-southwest behind the front 
Sunday afternoon/night is of Yukon origin but should have had the 
opportunity to modify somewhat once it arrives. It will still be 
mild to warm ahead of the front Saturday... with highs 55-63 and 
lows Saturday night to 35-42. Expect much cooler temperatures Sunday 
into Monday with highs in the 40s (lower 50s se)... and lows in the 
20s. 
&& 


Aviation /08z Tuesday through Saturday/... 
as of 100 am Tuesday... 


This will be a problematic forecast for aviation purposes through 
Wednesday as we will have low ceilings/visibilities and rain to deal with. In 
addition... there may be low level wind shear just north of the warm 
front Tuesday night. 


Through daybreak... expect a lowering of ceilings and visibilities as the large 
shield of rain moves into the area from the southwest in the next 
few hours. All areas should see IFR/LIFR ceilings/visibilities between 
09z-12z... lasting through the morning. The low level front should 
stall somewhere around kfay... and this will keep the terminals to 
the north and west socked in much of the day... both ceilings and visibilities. 
There is a chance that kfay will break out into MVFR or higher ceilings 
and unlimited visibilities later today... as the warm front pushes just 
north of of them. Most models suggest that the warm front will push 
into the krwi area between 00z/03z... allowing a wind shift to the 
south-southwest and a rise in visibilities and ceilings. Krdu should be the battle zone and 
may not have a warm frontal passage (per the 00z/06 nam). However... 
the other models show the warm front pushing just northwest of krdu between 
around or before 06z/07. 


The front will gradually lift north through the area as a warm front 
late today and tonight. The front will likely meet resistance with 
the developing hybrid cad event over the northern counties into the 
kint/kgso/krdu areas. A strong low level jet (40-50 knots at 2 kft) is 
forecast to develop overnight...which may lead to low level wind 
shear beginning somewhere between 00-03z Wednesday...and lasting 
through at least 12z Wednesday morning. 


Wednesday through thursday: 
a strong cold frontal passage is expected during the day Wednesday...and 
adverse aviation conditions in the form of widespread showers and 
gusty west southwesterly winds are expected throughout the 
day...with the front exiting east of the area by around sunset Wednesday 
evening. Although isolated thunderstorms cannot be completely ruled 
out in association with the cold frontal passage...instability will 
be very marginal...and the probability that convection will develop 
is quite low. VFR conditions with gusty westerly winds are expected 
on Thursday behind the front. A lighter wind regime and VFR conds 
expected on Friday with a surface high in our vicinity. -Vincent/mws/Badgett 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...Badgett 
near term...wss 
short term...Hartfield 
long term...Badgett 
aviation...Badgett 

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