411 
fxus61 krnk 210112 
afdrnk 


Area forecast discussion 
National Weather Service Blacksburg Virginia 
912 PM EDT Sat Mar 20 2010 


Synopsis... 
high pressure will push off the southeast coast tonight. 
Meanwhile...a storm system will track eastward into the lower 
Mississippi Valley. This system moves slowly northeast across the 
Tennessee Valley Sunday and past our region by Tuesday afternoon. 


&& 


Near term /through Sunday/... 
seeing some debris middle/high clouds spreading in from the SW 
tonight but expect most to thin later given very dry air aloft 
per 00z radiosonde observations and latest Canadian model sky forecast. However 
possibly enough clouds west early on combined with more warm 
advection aloft to offset temperature falls somewhat despite low 
dewpoints. Thus will keep trend of mostly clear east to pc far 
west...with lows ranging from 35-40 valleys to the middle/upper 40s 
elsewhere...except low 50s on the higher ridges. 


High pressure will shift east over the Atlantic while a low pressure 
system approaches from the southwest. Dry weather will continue 
through the day Sunday as the models have slowed down the 
progression of the low. Main change was to trim back probability of precipitation 
until the late afternoon and may even need to be trimmed back a 
little more. 


Maximum temperatures are expected to range in the 60s west and lower 
70s east. 


&& 


Short term /Sunday night through Tuesday night/... 
the short term portion of the forecast will be dominated by a closed 
upper low moving across the middle-Atlantic region Monday and Tuesday. 


Short range models are in fair agreement with the evolution of this 
closed low...with the slightly slower NAM preferred based on model 
trends of slowing the system down with time. Sunday night the 
forecast area becomes under an increasingly divergent upper level 
flow...and surface winds turn east-southeast tapping Atlantic moisture. Considering 
how dry the atmosphere will be to start...it will take some time to 
advect enough moisture for precipitation to develop. Best chances for 
precipitation will be in the southwest. 


Best Omega pushes through the forecast area early Monday... 
and as low level winds turn southwest expect dry slot to form from 
south to north...and breaks the clouds could allow temperatures to 
warm to much warm NAM MOS values. As a result...bumped up high temperatures 
a few degrees. Also...introduced chance for thunder forecast area 
wide as cold air advection aloft helps produce modest instability 
from the late morning to early afternoon. 


Cold pool aloft resides overhead on Tuesday...and thus temperatures should 
be a few degrees colder. Lingering low level moisture combined with 
upslope winds behind surface front that passes Monday night should 
result in precipitation along western slopes. Forecast profiles suggest 
some snow may mix with the rain across the higher elevations late 
Monday night/early Tuesday. 


As upper low finally exits off the coast Tuesday night...building 
ridge will shut off upslope precipitation. This will allow winds that will 
be quite gusty at times Monday and again Tuesday to subside. 


&& 


Long term /Wednesday through Saturday/... 
the weather pattern in the long term portion of the forecast will be 
characterized by a split flow regime...with two systems impacting 
the area. The first system will be exiting the forecast area early 
in the long term period...and the second southern stream system 
arriving late the period. In between these two systems...the medium 
range models disagree about how far south the northern branch of the 
jet stream drops. 


The 06z/20 GFS brings a shot of colder air into the forecast area 
Wednesday night...while the 00z/20 European model (ecmwf) keeps an upper level ridge 
in place Wednesday into early Friday. The global wind oscillation 
(gwo) is in a transitory phase moving toward a phase that is 
typically good for a mean Southeast Ridge in the east. Also...the 
global atmospheric angular momentum anomaly (glaam) does not show 
any increase which would favor the arrival of colder air in the east. 


Therefore...leaned toward the European model (ecmwf) for this forecast with fairly 
mild conditions in the middle of the week. 


Following the European model (ecmwf)...dry weather is expected Wednesday and Thursday 
before weak shortwave energy and an increasing upper divergent flow 
ahead of a southern stream system bring a small chance for rain 
late Thursday night. Considering the trends of the medium range 
models have been slower and deeper with this system...feel the 
better chance for precipitation will be on Friday...with slightly cooler 
temperatures on Saturday. 


&& 


Aviation /01z Sunday through Thursday/... 
clouds will be on the increase overnight at all taf sites as a 
southerly flow will bring in the moisture. However...expect VFR 
conditions to prevail through the taf period. 


Southeast winds may gust to 25kts at blf Sunday afternoon...as a strong 
closed system approaches from the Gulf Coast states. 


A storm system will approach the region on Sunday afternoon into 
Monday with MVFR conditions likely in any precipitation that 
accompanies the front. 


Low ceilings...sub VFR on the backside of this system is expected into 
Tuesday...especially in blf/lwb on northwest flow...along with potential 
for rain or snow showers. 


&& 


Fire weather... 
very low relative humidity values this evening will gradually 
recover overnight with readings rising to between 60 and 80 
percent by morning. Should see moisture return on Sunday as winds 
turn southeast...with afternoon humidity levels in the 35 to 
45 percent range. 


&& 


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


&& 


$$ 
Synopsis...reb 
near term...reb 
short term...ph 
long term...ph 
aviation...km/reb 
fire weather...