Area forecast discussion 
National Weather Service Blacksburg Virginia 
743 am EDT Thursday may 23 2013 


Synopsis... 
a strong cold front will move through the region from west to east 
this afternoon and tonight. Meanwhile...a deep upper low will 
swing through the Ohio Valley and middle-Atlantic region through 
Saturday...then shift off the middle-Atlantic coast Sunday. High 
pressure at the surface and aloft will build across the region 
Sunday through the first part of next week. Showers and 
thunderstorms today will give way to much cooler...drier weather 
for the weekend. 


&& 


Near term /through tonight/... 
as of 430 am EDT Thursday... 


Main concern through the near term will be the passage of the cold 
front and associated convection. Radar and surface observation suggest that 
that the actual front is near a day-cvg line. Models in general 
agreement that the front will move into the western part of the 
County Warning Area during the afternoon...reaching the eastern part of the County Warning Area 
before midnight. European model (ecmwf)/GFS both suggest convective activity will 
increase through the morning with potential linear development 
early in the day...perhaps as early as daybreak. Radar suggesting 
at least weak potential for this at the current time. GFS/European model (ecmwf) 
suggest that the best potential for rain showers/thunderstorms and rain activity will be 
across the Piedmont as the day progresses NAM suggests more 
scattered...unorganized activity through much of the day...with 
more organized linear activity late in the day. This theory is not 
totally discredited as the GFS...especially...attempts to move 
frontal activity east too quickly. Considerable instability and 
moisture remain in place...so have advertised categorical 
probability of precipitation...focusing on the highest probability of precipitation in the west during the first 
half of the day...and the east during the later half. Have also 
trended the better threat for thunder from the west through 18z to 
the east after 18z. There remains some threat for severe given 
the degree of instability...convective available potential energy > 1000 across the Piedmont and 
lifted indices during the afternoon to -4 eastern areas as well. 
However...overall coverage of severe should be less than observed 
Wednesday. As of now...Storm Prediction Center has the area in a see text...mainly for wind 
threat...but with colder temperatures aloft spreading into the area with 
an upper low approaching...there should be some hail threat as 
well. 


For tonight into Friday morning...our attention will turn to an 
anomalously deep upper low...a recurring pattern since middle- 
winter...prognosticated to settle across the middle-Atlantic into the 
weekend. A secondary vorticity lobe will sweep through late 
tonight/early Thursday...and bring winterlike upslope clouds/-shra to 
the western slopes of the alleghanys...mainly in eastern WV. 
Increasing downslope should end precipitation across the Piedmont...with 
only small chances for the remainder of the County Warning Area outside the far 
western slopes of SW Virginia/northwest NC...and southeast WV as mentioned above. 
Temperatures will be noticeably cooler by early Friday as we enter a 
period of well below normal temperatures that will last into the 
weekend. 850mb temperatures will drop to near zero c across the northern 
portions of the County Warning Area by the weekend. 


&& 


Short term /Friday through Sunday night/... 
as of 400 am EDT Thursday... 


For Friday morning...will be under a weather pattern found more 
often during the winter months...albeit warmer...with an 8mb 
pressure gradient situated across our area as Canadian high pressure 
builds in from the upper Midwest. Will start the day with residual 
shower activity across the mountains that will linger into early 
afternoon...but the main concern will be strong northwest winds that 
will remain across the region through the day. With cloud cover 
clearing out early...the stronger winds aloft will mix down to the 
surface during the morning...and will not be surprised to see 30+ 
miles per hour gusts common across the mountains and 25 miles per hour gusts across the 
piedmonts. Gusts will be stronger above 3000 feet...and believe we 
will see advisory level gusts across the North Carolina mountains 
into southwest Virginia. The Canadian high pressure will bring 
significantly cooler temperatures...with Friday afternoon highs 
ranging from the upper 50s to the low 60s across the mountains...to 
the low 70s across the piedmonts. Keep in mind that the gusty winds 
may add an additional chill to the air...so keeping a light jacket 
handy may be a good idea. 


Winds will remain somewhat gusty Saturday...although not as strong 
as those on Friday...as a low pressure system deepens off the New 
England coast. Will start the morning off with rather cool 
temperatures...ranging from the middle/upper 30s across the mountains 
to the middle 40s further east. Believe winds will remain strong enough 
to prevent frost from developing in the cooler spots in the 
mountains...however those with tender vegetation should keep an eye 
on the forecast just the same...and be prepared to protect these 
plants if necessary. At any rate...temperatures will rebound into 
the low/middle 60s west...to the low 70s east by afternoon. 


High pressure remains in control of the weather pattern on Sunday... 
although temperatures will begin to moderate as low pressure over 
New England begins to push northeast into Canada. Weather is 
expected to remain dry once again...but will be keeping an eye on a 
warm front stretching from the lower Great Lakes to the central 
Appalachians that will lift northward during the day. This warm 
front will play a role in our weather as we get into the next 
workweek. 


&& 


Long term /Monday through Wednesday/... 
as of 1245 PM EDT Wednesday 


Expect cool trough to linger into Sunday before lifting east. 
Meanwhile a strong trough digs into the Pacific northwest by Monday 
with increasing heights arriving over our area next week. The 
weather pattern will be mainly dry Sat night-Monday...but as we can 
see with a westerly flow aloft the chance that convection that fires 
along a warm front over the Ohio/middle Mississippi Valley could shift 
east and affect our mountains. 


Will lean toward an European model (ecmwf)/wpc blend solution. This will keep the 
chance probability of precipitation in the area by Monday afternoon through Tuesday. The 
warm front lifts north toward the Great Lakes by midweek with 
heights continuing to build. At the surface...high pressure will be 
situated across the middle Atlantic coast then shift off the coast 
midweek. Dewpoints will be increasing again by next week with temperatures 
warming back to normal or just above. 


&& 


Aviation /12z Thursday through Monday/... 
as of 740 am EDT Thursday... 


Frontal system responsible for convection last two days will 
finally move through the forecast area late in the taf valid 
period...generally from 20z west to 04z east. The air mass remains 
quite moist and unstable for morning hours...thus scattered popup 
rain showers noted through much of the County Warning Area at 12z. This should 
continue...then increase in aerial coverage and intensity as the 
day progresses and the frontal system approaches. Expect the bulk 
of rain showers/thunderstorms and rain activity to occur within the 14z-00z time frame from 
west to east. Lingering IFR ceilings Dan/lyh/roa should lift to MVFR 
or better by mid-morning. Mostly MVFR ceilings will accompany 
rain showers/thunderstorms and rain activity today. Otherwise...it should be VFR after middle- 
morning at most sites. Scattered -shra should persist behind the 
stronger convection until the front actually arrives during the 
afternoon west to evening east. Pattern shifts to more of a 
winter pattern after this evening...with typical upslope 
clouds/-shra lwb- blf with MVFR ceilings...as a deep upper low sinks 
down across the middle-Atlantic region for the weekend. Further east... 
toward roa/lyh/Dan...ceilings should improve to VFR with precipitation ending 
by late afternoon/evening. Visibilities mostly VFR...except MVFR in areas 
of br/-shra and/or thunderstorms and rain. Winds south-southwest-SW 5-8kts until frontal passage 
this afternoon...then west-southwest-west 7-10kts...with low end gusts blf and 
roa especially. 


Medium confidence in ceilings and visibilities through taf valid period. 
Medium confidence in thunderstorms and rain timing and coverage. 
High confidence in wind speed/direction through taf valid period. 


Extended aviation conditions...strong high pressure will bring 
much drier air by Friday with VFR likely to prevail through the 
weekend into early next week. Some upslope clouds linger into Friday 
across southeast WV. Increasing potential for northwest flow driven thunderstorm 
activity early next week...most conditions overall VFR for the 
most part. 


&& 


Equipment... 
the National oceanic and atmospheric administration weather radio at Roanoke Virginia is off the air this 
evening due to a problem at the transmitter site. Restoration time 
is unknown at this point. 


&& 


Rnk watches/warnings/advisories... 
Virginia...none. 
NC...none. 
WV...none. 


&& 


$$ 
Synopsis...rab 
near term...rab 
short term...nf 
long term...wp 
aviation...rab 
equipment...jh