220 
fxus61 krnk 211422 
afdrnk 


Area forecast discussion 
National Weather Service Blacksburg Virginia 
1022 am EDT sun Mar 21 2010 


Synopsis... 
a storm system will track northeastward from the lower Mississippi 
Valley into the Ohio Valley through Monday. The associated front 
will move east of the region by Tuesday...with high pressure 
arriving Wednesday. 


&& 


Near term /through tonight/... 
clouds continuing to increase from the south this morning ahead of 
the upper low to the SW...but still fading a bit upon 
encountering dry air aloft off 12z radiosonde observations. As the flow backs more 
southeast this afternoon should see things moisten up just ahead of the 
leading edge of the axis of isentropic lift toward 21z. Local WRF 
also has a few showers in advance of the main deep relative humidity axis making 
it into the far SW zones by middle afternoon so sped up low chance 
probability of precipitation there. Otherwise appears most of the organized coverage will hold 
off until well after 00z when a better fecth off the Atlantic 
takes shape. Degree of clouds and temperatures the main concerns elsewhere 
this afternoon with 12z thickness quite warm out east where still 
some low 70s look possible. However influx of middle deck and 
developing light southeast flow likely to hold readings back a few 
degrees so only raised the east a couple degrees while keeping 
the far SW in the 50s and 60s elsewhere. 


Best shot at heavier rainfall looks to come by midnight across the 
SW and then along/NE of the Blue Ridge 06z-12z ahead of the 
occluded front. New NAM has enough instabitlity to add thunder 
mention most areas into Monday morning followed by a strong dry 
slot by midday so made adjusts. Also wind potential looks a bit 
more enhanced blf/Tazewell vicinity per 850 mb 50 knots southeast jet but will 
hold on headlines until the afternoon package for now. Otherwise upper 
low working into the SW later Monday will likely bring isolated 
convection back into parts of the northwest NC mountains toward middle afternoon. 
This before the cooling cools from the top allowing precipitation to 
change to snow at elevation by evening mainly above 3k feet. 


Previous discussion... 


Forecast still hinging on a blend of the models...with heavier 
leaning in the near term toward the GFS/local WRF. Dense shield of 
cirrus/cs over the Gulf Coast states spreading quickly NE into the southern 
appss early this morning. Seems models are too slow with this and 
will be clouding things up faster. Still could see some breaks in 
the North/East this morning b4 clouds thicken by midday. With the 
thicker cloud cover...decided to cut high temperatures...especially over 
the SW County Warning Area in the mountains/along the Blue Ridge on southeast flow. Temperatures will 
be warmer...close to MOS in southeast WV and the far eastern/northern County Warning Area. 


Trends were for a slower arrival time for precipitation...but the 00z 
models have come in quicker...and with a the southeast flow 
moistening up the low levels by 00z this evening...decided to raise 
probability of precipitation late this afternoon to low likelys in the NC mountains and adjacent 
foothills. However...bulk of showers and isolated storms arrives 
during the evening and overnight. 


Increase probability of precipitation tonight to categorical to high likelys. Pattern with 
strong upper low and good upper divergence...along with a 3-6 hour 
period of strong southeast upslope could spell some heavier downpours along 
the southern Blue Ridge this evening. Add some instability although 
marginal and could see isolated flooding problems. But too early and 
too isolated to warrant a watch. The other concern by early evening 
is the 50 knot jet advertised at 800 mb by the 00z GFS. 
Historically...this favors stronger wind gusts in the mountain Empire of 
SW Virginia in the corridor from blf-jfz south to mkj. However...with 
substantial cloud cover and the fact that this jet quickly veers to 
the south by 06z...think a Wind Advisory is not needed at this time. 
Potential for showers mixing this wind down is there...and could see 
some special weather statements and isolated warnings being issued if the 
strong low level jet does indeed occur as advertised by the GFS. 


The overnight period will be milder with rainfall and clouds...and 
bumped lows up some from previous forecast. 


&& 


Short term /Monday through Tuesday night/... 
a closed upper low will move across the middle-Atlantic region Monday 
into Wednesday. Best Omega and moisture pushes through the forecast 
area early Monday. Used a blend of the European model (ecmwf) and continuity for probability of precipitation. 
Southerly low level winds turn southwest...expect dry slot to form 
from south to north. Kept the mention of isolated thunderstorms as 
cold air advection aloft helps produce modest instability from the 
late morning to early afternoon. Any breaks in the cloud cover may 
allow temperatures to rise to the adjmet values. 


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. A light snow accumulation 
may be possible in the highest elevations. Lowered temperatures for 
Monday night and Tuesday. 


As upper low finally exits off the coast Tuesday night...building 
ridge will shut off upslope precipitation. Trimmed probability of precipitation from south to north 
on Tuesday night. Played lows Tuesday night from the middle 30s to the 
lower 40s. 


&& 


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 /14z Sunday through Thursday/... 
look for an increase in high clouds toward 12z with a middle deck 
arriving this afternoon. However...expect VFR conditions to 
prevail through the afternoon. 


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


Looking at some showers working in from the south this 
evening...overspreading the area overnight. 


Looking at MVFR ceilings and some visibilities with the showers through the 
night. Potential for isolated thunder exists especially from 04 to 12z. 


The strong southeast jet veers to the south overnight. There could be 
some wind shear issues for the lwb-roa-lyh corridor. 


This storm system will keep MVFR conditions in any precipitation 
that accompanies the front through Monday. 


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. 


A return to VFR conditions is expected Wednesday-Thursday in southeast WV. 


&& 


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


&& 


$$ 
Synopsis...reb/wp 
near term...jh/wp 
short term...kk 
long term...ph 
aviation...km/wp 

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