448 
fxus61 krnk 211422 
afdrnk 


Area forecast discussion 
National Weather Service Blacksburg Virginia 
922 am EST Sat Nov 21 2009 


Synopsis... 
high pressure will remain in the area today into early tonight. Low 
pressure developing across the Gulf Coast states will impact our 
weather Sunday while high pressure wedges down from the northeast. 


&& 


Near term /through tonight/... 
satellite loop showed high clouds spreading east across the region. 
Thickness values support forecast highs for this afternoon from the 
upper 40s in the northwest to the lower 60s in the southeast. The 
extent/coverage of cloudiness may effect temperatures. The northern 
stream shortwave moving across the Ohio Valley today. For tonight 
the northern stream shortwave slides slowly into the middle Atlantic region 
with shortwave ridging occurring over our area....while the trough 
over Texas slides into the Gulf Coast states. Question for tonight 
is how long does it take to saturate the low levels of the 
atmosphere ? Trend of models have been slower with the arrival of 
precipitation. Will review 12z runs before making adjustments in 
probability of precipitation. Decided make a few minor adjustments in clouds and 
temperatures. 


Previous discussion... 


The 00z models have continued to trend drier this run with the 
system coming out of the Gulf coastal region. The 00z GFS at this time...is 
doing a better job handling the cs pouring over the region ahead of 
the northern stream trough across the Midwest. These high clouds have 
limited fog development...but still should see some thinning across 
the river valleys so kept some patchy fog in this morning. 


For today...the main forecast challenge will be extent of cloudiness 
and effect on temperatures. During the late fall months the GFS seems to do 
a better job with high clouds in terms of its relative humidity depiction in the 
upper levels...and the amount of opaqueness is sometimes 
underestimated. For now...with the northern stream shortwave moving 
across the Ohio Valley today think the high clouds will get shunted 
east...with some lower clouds arriving in the northwestern County Warning Area...although 
not impressed with that at this time. With this in mind...have run with a 
middle of the Road approach for sky cover with 40-60 percent 
coverage. Highs as such will remain close to the previous forecast 
of 50s in the mountains to lower 60s out east...close to a MOS blend. 


Tonight...the northern stream shortwave slides slowly into the Middle 
Atlantic States with shortwave ridging occurring over our 
County Warning Area....while the trough over Texas slides into the Gulf Coast states. 
Have backed off on probability of precipitation thinking that the rain shield stays south of 
our County Warning Area until after 12z Sunday. However...kept the chance in there 
for potential of northern stream shortwave sliding further east. The east-southeast 
flow late tonight may help saturate the low levels over the NC mountains 
all in all...pretty dry tonight...with clouds decreasing in the 
north but increasing after midnight over the south. Lows tonight 
will remain close to MOS...dropped a couple degrees from previous 
forecast due to slower arrival of clouds and some evap cooling 
potential with virga in the south. 


&& 


Short term /Sunday through Tuesday night/... 
models continue trending slower...drier and keeping heavier rain 
southeast of the County Warning Area on Sunday. Because of this was forced to reduce and 
slow probability of precipitation...however did not go as low as guidance since models 
may probably again change drastically. Although low pressure moves 
up the coast on Monday...an upper level disturbance entering from 
the west in its wake...may allow another rain shield to develop 
later Monday. Tuesday should provide am opportunity to dry out 
with temperatures close to 60f again. 


&& 


Long term /Wednesday through Friday/... 
European model (ecmwf) indicates upper level ridging approaching the area Tuesday 
afternoon into early Wednesday morning...while the surface wedge 
hangs on through the end of the week. It also suggests a surface and 
upper low pressure system developing over the Midwest and pushing 
toward the area by middle to late week. Have at least chance to slight 
chance of probability of precipitation across the County Warning Area through the extended 
period. With thicknesses 1000-850mb and 850-700mb thicknesses 
lowering by middle week...there could be some light snow mixed with 
rain in the higher elevations of southeast West Virginia. In these 
same areas mainly along the ridges...surface low temperatures could 
also reach freezing or a few degrees below. 


GFS has been having a hard time with run to run consistency in the 
extended. It has been faster and more progressive with pushing the 
systems through the forecast area. Due to this have leaned more 
toward the European model (ecmwf) as it seems to be a bit more consistent in each run 
the past few days. 


&& 


Aviation /14z Saturday through Wednesday/... 
high pressure will remain over the region through this evening. 
Fog has been sparse thanks to thick high level clouds overhead. As 
such should only see a small period this morning of MVFR fog at 
lwb. Otherwsie expect VFR conditions through the period with 
cirrus/cs ceilings. 


Low pressure is going to slide northeast from the Gulf Coast 
states into the southeast coastal area by Tuesday. Look for 
lowering ceilings during the day Sunday with rainfall expected at the 
taf sites by midday. Models seem to be slowing the rain start 
times down...so with that in mind we may not see sub VFR 
conditions until later Sunday. 


Tuesday will feature improving conditions as the surface low tracks 
northeast up the eastern Seaboard with high pressure building over 
the middle Atlantic for middle week. 


&& 


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


&& 


$$ 
Synopsis...PM/wp 
near term...kk/wp 
short term...km/wert 
long term...reb 
aviation...ams/PM/wp