Weather

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fxus61 kakq 211129 
afdakq 


Area forecast discussion 
National Weather Service Wakefield Virginia 
629 am EST Friday Nov 21 2008 


Synopsis... 
low pressure will develop off the North Carolina coast this 
morning...then intensify as it tracks northeast toward the 
Canadian Maritimes this afternoon into tonight. High pressure will 
build in from the west for Saturday into Monday. A cold front 
approaches from the west Monday afternoon...then moves across the 
area Monday night into early Tuesday morning. 


&& 


Near term /through tonight/... 
120+ knots 300 mb jet streak nudging into the Tennessee and lower Ohio valleys 
early this morning has allowed the downstream upper trough to 
become more negatively tilted. This in turn has enhanced the 
850-500 mb deformation/frontogenesis...giving way to strengthened 
qvector convergence and increased uvv's/moisture over S/southeast portions of 
our forecast area downstream of the trough axis. As the trough 
continues to deepen (become more negatively tilted)...expect the 
precipitation shield to become more south-southwest-north-northeast oriented vs. The SW-NE 
orientation currently. 


Low level thicknesses and surface/boundary layer wet bulb temperatures support the 
transition from r or a r/S mix to mainly snow early this morning 
across most locations outside of possibly coastal portions of northestern NC. 
Considerable evap cooling potential will come into play as boundary 
layer dewpoints continue to fall behind the departed cold front... 
therefore even in areas near the coast in northestern NC that see mainly 
rain or a rain/snow mix...expect at least a brief changeover at 
times to snow when the precipitation picks up in intensity a little. 
Accums expected to be on the light side (generally less than a 
half inch)...and mainly on non-paved surfaces given the 
relatively warm (above freezing) pavement temperatures. Have issued 
special weather statements for the morning commute and will 
continue to monitor trends as the deformation zone and asscd 
precipitation pivots through the region. 


&& 


Short term /Saturday through Sunday/... 
expect more scattered snow showers to develop this afternoon across eastern 
portions of the region where 700-500 mb lapse rates are expected to 
steepen between 7-8 c/km. Expect the activity to be most prevalent 
across the lower Maryland Eastern Shore...where the elongated northwest flow will 
allow for airmass preconditioning from the western Great Lakes...while 
downslope drying effects over this part of our forecast area is 
minimized. Meanwhile...anticipate the development of more northwest flow 
Bay effect clouds and enhancement to snow showers/flurries across 
the lower Eastern Shore (accomack and Northampton counties especially) 
as Bay-850 mb Delta-ts climb above 20c. 


I have dropped forecast high temperatures today based on local thickness 
scheme...I.E. Highs near 40 at best over northern areas to low-middle 40s 
elsewhere. Even with these surface temperatures some 8-10f or so 
above freezing...expected mainly snow showers as 1000-850 mb 
thicknesses remain below 1290m during the afternoon with the low level 
cold air advection (freezing level remaining below 1 kft). 


Very chilly night on tap tonight/early Sat morning...with min 
temperatures in the low-middle 20s for most (some upper teens possible over eastern 
Virginia piedmont). 


Sat/sun look quiet as surface high pressure builds over the region from 
the west. Highs in the 40s Sat then slightly warmer upper 40s to lower 
50s sun...with lows near 20 to 34 Sat night...and upper 20s to upper 
30s Sun night. 


&& 


Long term /Sunday night through Thursday/... 
the high shifts offshore Sunday night so expecting an increase in 
overnight lows that night/Monday morning. Freezing temperatures will 
continue inland but should be just above freezing closer to the 
coast. 


On Monday an upper low dives down into the Great Lakes region from the 
north and a front shifts our direction during the middle to late afternoon 
hours. Models now in somewhat better agreement on timing...so have 
bumped up probability of precipitation slightly during the afternoon hours and continue them into 
the overnight hours as the front should push offshore in the early 
morning hours on Tuesday. Some question as to whether we see any snow 
early Tuesday morning with the remaining moisture right behind the 
front. For now...think that temperatures will be just above freezing inland 
(models are coming in slightly warmer than previous runs)...but 
can't rule out a few flakes where it manages to get colder quickly. 


The low pulls up to the north on Tuesday as a high pushes in from 
the Central Plains...so although colder temperatures expected on Wednesday...not 
too much below freezing for mins on Wednesday and Thursday mornings. The high 
remains over the southeast U.S. Through Thursday morning then pushes 
offshore during the daytime hours. 


&& 


Aviation /12z Friday through Tuesday/... 
MVFR conds expected this morng across eastern and southern sections including 
ecg/orf/phf/sby sites...due to -snra/-shsn developing as low pressure 
forms off the NC CST. VFR conds then expected across just about all 
locations from this afternoon into Sat morng...as the low pulls away and hi 
pressure starts to build in from the west. MVFR conds could hang on into Erly 
this eveng across the lower Maryland/Virginia eastern shr. Hi pressure and VFR conds will 
then prevail for Sat into Monday. 


&& 


Marine... 
will continue with Small Craft Advisory headlines starting this morng...then incrsng 
to gale warnings on the coastal waters from later this afternoon into Erly Sat 
morng...as strong northwest winds usher cold/dry air into region. Small Craft Advisory conds 
will last through Sat afternoon for the ches Bay and Currituck sand...with 
the coastal waters ramping down to those conds Sat morng through Sat afternoon. 
Hi pressure and calmer conds all areas Sat night into Erly Monday morng...due 
to hi pressure bldng over the region. Strong SW then west winds expected Monday 
through Tuesday...as a cold front approaches then pushes through the area and 
out to sea. 


&& 


Akq watches/warnings/advisories... 
Maryland...none. 
NC...none. 
Virginia...none. 
Marine...Small Craft Advisory until 6 PM EST Saturday for anz630>632. 
Small Craft Advisory from 10 am this morning to 6 PM EST 
Saturday for anz633. 
Small Craft Advisory from 10 am this morning to 4 PM EST this 
afternoon for anz650-652-654-656-658. 
Gale Warning from 4 PM this afternoon to 5 am EST Saturday for 
anz650-652-654-656-658. 


&& 


$$ 
Synopsis...bkh 
near term...bkh 
short term...bkh 
long term...jym 
aviation...tmg 
marine...tmg 

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