311 
fxus62 krah 210732 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
230 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 50-70% 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 areas. 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. Scattered 
clouds beneath the subsidence inversion aloft should linger through 
the afternoon... and winds will then supplant snow as the big 
weather story this afternoon. 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 245 PM Thursday... 


Friday and Friday night: 
expect rapid clearing by the early afternoon hours as the potent 
shortwave moves east of central NC and strong synoptic subsidence 
takes over. Throughout the day...1040 mb high pressure to the west 
will advance closer to the area...and the mslp gradient tightens 
considerably. Based on previous experience...this is a pretty good 
setup for a breezy day...strong synoptic subsidence behind an upper 
level wave...strong pressure rises throughout the day (10mb/12hr)... 
and forecast soundings showing 25-35 knots flow in a 6kft mixed layer. The 
GFS forecast soundings are stronger with the low level flow...showing up 
to 30-35 kts in the mixed layer...while the NAM is weaker...showing 
about 25 kts int he mixed layer. Have decided to go with a 
compromise between the two...showing sustained northwest winds around 15-20 
miles per hour...with gusts up to 30 miles per hour. A few isolated gusts could hit up to 35 
miles per hour...particularly if the GFS soundings prove more accurate. High 
temperatures should be pretty chilly on Friday...with thicknesses falling 
throughout the day. Temperatures will ultimately depend on the exact timing 
of the strongest cold advection...and at this time will advertise 
highs ranging from 40-45f across the area...lowest in the northwest 
Piedmont. If you were to perfect forecast the thicknesses off the 
GFS/NAM...it is possible some locations may not make it out of the 
upper 30s. For Friday night...with clear skies and winds becoming 
light after midnight...expect very chilly readings in the upper 
teens to lower 20s (19-23f) across the entire area. 


Saturday and Saturday night: 
synoptic subsidence begins to wind down by Sat morning as the upper 
trough to our east moves further offshore. Both the GFS/NAM show 
another shortwave upstream of the area digging southeast toward the 
middle-Atlantic and Carolinas by Sat evening/night. At the surface... 
strong high pressure will build directly over the area during the 
day Saturday. Although the next shortwave should bring a good slug 
of middle/upper level moisture with it Sat eve/night...the associated 
lift will not be strong enough to overcome the very dry/stable 
airmass associated/west strong high pressure at the surface. As such...expect 
mostly sunny skies through late morning to early afternoon... 
becoming mostly cloudy later in the afternoon through roughly 
midnight Sat night. High temperatures on Sat look pretty chilly. With 
morning thicknesses around 1270m...morning temperatures in the upper teens 
to lower 20s...afternoon thicknesses no higher than 1295m...and 
increasing middle/upper level cloud cover ahead of the approaching 
shortwave Sat afternoon...have decided to drop high temperatures several 
degrees below MOS guidance...and will go with a range of 41-45f. Low 
temperatures Sat night will be somewhat tricky due to the presence of 
considerable middle/upper level cloud cover for at least the first half 
of the night. Both the GFS/NAM show cloud cover decreasing after 
06z...and this should allow for at least a few hours of optimal 
radiational cooling. Will forecast lows a few degrees higher than Sat 
due to the extensive cloud cover expected through at least 
06z...ranging from 22-25f. 


Sunday: 
surface high pressure should still be directly overhead on Sunday 
morning...moving slowly to the east during the day. Aloft...we will 
see subsidence behind the departing shortwave...with increasing 
heights aloft. The net result of this will be sunny skies. Low level 
thicknesses are expected to rise up to 1315m or so during the day 
Sunday...and with full sun...expect highs to rebound into the lower 
50s across the area. Will forecast a range from 50-55f. -Vincent 


&& 


Long term /Monday through Thursday/... 
as of 245 PM Thursday... 


Timing of early work week front has accelerated in the computer 
models and rain could begin before noon Monday in the northwest 
Piedmont. Front will clear our eastern parts before sunset Monday. 
All indications are this will be a rain event for central North 
Carolina. However the freezing level will be dropping during the day 
and the cold air will be chasing the moisture. A short period of 
nonliquid very light precipitation not impossible but chances too 
small to mention in forecast products. 


Other than being a little breezy ahead and behind the front... 
generally fair weather expected other than the above mentioned 
precipitation event. Afternoon highs mostly in the fifties. Lows 
mostly in the thirties. 


&& 


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 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...Vincent 
long term...rlh 
aviation...mws 
climate...gih 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)