340 
fxus62 krah 211000 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
500 am EST Friday Nov 21 2008 


Synopsis... 
a polar cold front will drop to the southeast across the region 
through daybreak. Blustery winds from the northwest behind the front 
will usher on another round of very cold air. The colder than normal 
temperatures will continue through Sunday. 


&& 


Near term /through tonight/... 
as of 230 am Friday... 
update: radar imagery shows that the snow band has been increasing 
in intensity over the last hour... and Forsyth Colorado Fire Department 
reports some accumulation already on cars and grass. Will issue a 
Winter Weather Advisory... with accumulation around an inch... for 
areas along/northwest of a line from Henderson to Durham to Asheboro... 
until 8 am. 


As of 155 am Friday... 


Through today: interesting situation unfolding as radar returns have 
been increasing in a broad band across the northwest Piedmont and 
northern and central foothills in the last few hours. Cloud ceilings 
have been high though... generally above 7000 feet... and with the 
surface dew point depression still 15f or better... this precipitation 
aloft has not been reaching the ground... until very recently. Mount 
Airy has just reported light snow as has Forsyth County... and 
ceilings have started to fall in locations such as Danville and 
Martinsville Virginia. Synoptic scale lift is already increasing: the 
middle level vorticity maximum we've been anticipating has dropped into 
eastern Kentucky with rapidly increasing positive vorticity advection into Virginia/northwest NC... the 130 knots 
upper jet core has settled across WV/northern Virginia... q-vector convergence 
is on the rise this morning especially over northern NC... and strong middle 
level height falls of 140 meters are expected overhead through this 
morning. In addition... 700 mb frontogenesis really ramps up 
particularly over northern and eastern NC... concurrent with strong pressure 
advection on the 1.5 pvu surface. Given this strong lift potential 
and radar trends... have raised probability of precipitation up to 50-80% mainly across the 
western and northern County Warning Area through daybreak... with probability of precipitation spreading into eastern 
NC through middle morning. Will set accumulation as an inch or less 
mainly on grassy locations in the advisory area. The only sticking 
point is the surface wet bulb temperatures which are hovering near 
freezing to the middle 30s across the County Warning Area. But as the polar air begins 
to arrive in earnest with frontal passage this morning... this 
should become less of an inhibiting factor... especially if we get 
strong enough lift. Farther south... including Wadesboro/ Fay/ 
Clinton... the large scale lift should be less this morning... and 
with higher surface wet bulbs... will have precipitation as light snow or 
rain. Highs of 40-45 today... with mostly cloudy skies early (partly 
cloudy south) followed by clearing for the afternoon... looks good 
given that low level thicknesses will start the day around 1280 M. 
Scattered clouds beneath the subsidence inversion aloft should 
develop through the afternoon but otherwise expect a good bit of 
sunshine this afternoon. Winds will then supplant snow as the big 
weather story. Strong cold air advection with sustained winds to 
15-25 miles per hour will knock wind chills down in the upper 20s to middle 30s 
for much of the day. Winds should subside this evening with a 
continued drying column leading to clear skies. Lows 19-23... with 
outlying sheltered areas likely even colder and urban areas possibly 
slightly less cold. Record lows will be approached if not broken. 
-Gih 


&& 


Short term /Saturday through Sunday night/... 
as of 325 am Friday... 


For Saturday/Saturday night: the surface high settles overhead and 
the 850 mb anticyclone drifts overhead during this period. But 
farther aloft... what is now a strong low pushing through the 
interior Pacific northwest dives quickly to the east-southeast... dampening 
into an open wave as it approaches NC. But nevertheless this wave 
remains of respectable strength and should still bring in 
appreciable middle and high cloudiness spreading into NC Saturday 
afternoon from the northwest. NAM/GFS forecast soundings are in good 
agreement on this. Have raised sky cover... starting the increase in 
the late morning. Thicknesses start to rebound as the 850 mb warm 
front heads to our NE and highs should be a degree or two warmer... 
42-46... hardly noticeable though given the frigid start to the day. 
Cloud cover should decrease late Saturday night as the clipper 
shortwave trough exits. Forecast lows in the low-middle 20s still look 
good. 


For Sunday/Sunday night: quiet weather for Sunday with surface high 
pressure over the eastern Carolinas and broad (but weak) ridging in 
the middle levels. Thicknesses rise into the 1290-1300 meter range... 
still well below normal however... and we should see highs still 
about 1-2 categories below normal... 52-57. Our next weather system 
of interest... a polar vortex... drops into the upper Midwest Sunday 
night... and middle level flow into the Appalachians starts to become 
cyclonic and strengthens... as the surface highs shifts offshore. We 
should see increasing high clouds through the night but this should 
have little impact on radiational cooling. Expect lows of 28-32. 
-Gih 


&& 


Long term /Monday through Thursday/... 
as of 455 am Friday... 


For Monday through tuesday: a cold middle level vortex is expected to 
drop southeast across the Great Lakes region during this period. The 
latest GFS and European model (ecmwf) are in good agreement on a faster arrival of 
the next cold front as compared to yesterday's guidance... and have 
hastened the cloud cover increase and raised probability of precipitation Monday afternoon 
to high chances in the northwest County Warning Area... tapering to slight chance in the 
southeast. Chance probability of precipitation will be continued through Monday night as the 
cold front sweeps rapidly east. The front should be well off the 
coast by Tuesday morning... and only a slight chance east of I-95 is 
expected early Tuesday morning. Expect clearing from the west by 
midday Tuesday. Highs 55-62 Monday with thicknesses just 10-15 M 
below normal. Highs Tuesday 52-57... a much less modest cooling than 
we've seen with these recent cold front passages... as the incoming 
surface air mass has its source in the Pacific and rockies rather 
than Canada. Morning lows 37-43. 


For Wednesday through Thanksgiving day: still looks quiet and cool 
in central NC. The middle level vortex sits and spins over the interior 
northeast according to the latest GFS and European model (ecmwf)... although the 
European model (ecmwf) is a bit farther north. Our boundary layer flow will continue 
from the northwest with Lee troughing holding on through the County Warning Area. Will 
continue the dry forecast with considerable sunshine and continued 
below normal temperatures. -Gih 


&& 


Aviation /06z Friday through Tuesday/... 
as of 230 am Friday... 


MVFR to IFR conditions with light snow expected to develop through 
middle morning... with VFR conditions and gusty northwesterly winds 
this afternoon. A vigorous trough and accompanying jet aloft will 
cross our region through midday. Large scale forcing for ascent 
attendant these features... coupled with strong and deep 
frontogenetical forcing on the mesoscale will result in ceilings and 
visiiblities lowering to MVFR/IFR in light snow (perhaps mixed with 
rain inititally) this morning. Skies will rapidly clear from the northwest 
during the early to middle afternoon. Northwest winds at 15 to 20 miles per hour 
will occasionally gust to near 30 miles per hour this afternoon. 


VFR conditions are expected tonight through early Monday. An 
approaching cold front will bring a chance of light rain showers 
late Monday. -Mws 


&& 


Climate... 
record lows for Saturday (november 22): 
rdu....23 (1987) 
gso....17 (2000) 


&& 


Rah watches/warnings/advisories... 
Winter Weather Advisory until 8 am EST this morning for 
ncz007>009-021>025-038-039. 


&& 


$$ 
Synopsis...Hartfield 
near term...Hartfield 
short term...Hartfield 
long term...Hartfield 
aviation...mws 
climate...Hartfield 

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