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Area forecast discussion 
National Weather Service Wakefield Virginia 
409 PM EST Thursday Dec 4 2008 


Synopsis... 
a cold front will move across the area this afternoon and 
evening...then push off the middle Atlantic coast after midnight. 
High pressure builds into the region from the west Friday and 
Friday night...while a weak low pressure system forms off the middle 
Atlantic coast. Another cold front approaches from the west on 
Saturday...then moves off the coast Saturday night. High pressure 
rebuilds over the area from the west Sunday and Monday. Low 
pressure will track across the lower Mississippi and Tennessee 
valleys on Tuesday...as a warm front extending east of the low 
approaches from the middle Atlantic region from the south. 


&& 


Near term /until 6 am Friday morning/... 
rather meager/narrow region of low-middle frontogenesis and 
associated moisture (1000-500 mb rh's above 70%) will quickly 
push across the forecast area this evening. Narrow...fast moving favorable 
dynamical forcing...along with short-lived moist S/southeast low level 
trajectories ahead of the front...will make for a brief period of 
light showers or sprinkles at best. Continued with the 20-30% 
probabilities for anything measurable (i.E. At or above 0.01"). 


&& 


Short term /6 am Friday morning through Sunday night/... 
cooler on Friday behind the front...as local low level thickness scheme 
supports a mav/met blend forecast of highs from the lower 40s north to near 
50 S. 


Friday night could get a little interesting over southeastern areas of the 
forecast area...including the Tidewater Virginia region and especially northestern 
NC. GFS/NAM/sref and even the European model (ecmwf) all show a weak surface-850 mb low 
developing in the low level baroclinic zone...in the Gulf Stream along 
the stalled front ahead of the deepening middle/upper level low across 
the Midwest region. For now...it appears frontogenetic moisture 
shield northwest of the low track will continue to necessitate low probability of precipitation 
(20-30%) along the southeastern Virginia/northestern NC coastal areas Sat night... 
while at the same time low level (100-850 mb) thicknesses and surface wet 
bulb temperatures support the possibility of a r/S mix. At this point the 
depth of forcing/moisture appears to weak for anything other than 
a light event...and considering the light intensity expected (lack 
of evap cooling at low levels)...it's certainly not a guarantee 
that a mix or even a changeover to snow will develop. For 
now...this is something to keep an eye on. 


Another potential weather concern in the short term will come Sat 
night...ahead of the deepening 700-500 mb trough. Compared to the 
NAM/sref/ECMWF...the GFS remains the most bullish in terms of 
low-middle level fgen lift support and deepening moisture. Much will depend 
on how quickly (and how far s) the middle level trough deepens...or how 
soon the trough becomes negatively tilted before moving off the 
middle Atlantic coast. For now have maintained the low chance probability of precipitation in the 
forecasts...but certainly given the lowering low level thicknesses (aob 
1300m between 1000-850 mb for all but coastal locations)...along with 
lowering surface temperatures/wet bulbs and freezing levels while this 
forcing would be occurring...have left the mention of snow or r-S 
in the forecast Sat night into early sun. This will be another feature 
that will bear some watching over the next 24-48 hours. 


Long term /Monday through Thursday/... 
hi pressure will build over the region and just off the CST Sun night and 
Monday. The hi will move off the northern middle Atlantic CST Monday night into Tuesday 
morng...while a frontal boundary pushes through the Midwest into the lower MS 
vlly with low pressure developing on the southern end of the front. Some light 
precipitation in the warm advection area well ahead of the front/low pressure area 
may break out late Monday night or Erly Tuesday morng over our western cnties. The 
precipitation could be in the form of light snow rain showers across the northwest cnties or a 
mix of light snow and rain rain showers across the SW cnties. It will be brief 
however...as east-southeast winds in advance of the low to our SW...will quickly 
warm up temperatures in the morng for just chance of rain all areas throughout 
the day. The chance for rain will continue Tuesday night through Erly Wednesday eveng 
as low pressure tracks NE from the Gulf CST states up across the region. 
Colder air filters in behind the exiting low pressure area and cold front 
late Wednesday night into Thursday morng...resulting in a slight chance of snow rain showers 
or flurries. Hi pressure/drier air will then build in during Thursday. 


Maximum temperatures will be in the lower to middle 40s Monday...in the middle 40s to upper 
50s Tuesday and Wednesday...and in the upper 30s to upper 40s Thursday. Min temperatures will 
be in the upper teens to upper 20s Monday morng...in the upper 20s to upper 30s 
Tuesday morng...in the 40s to lower 50s Wednesday morng...and in the upper 20s to 
near 40 Thursday morng. 


&& 


Aviation /20z Thursday through Tuesday/... 
Erly evening hours...satellite shows cloud shield moving into the area as 
front approaches County Warning Area...as ceilings lower from west to east expect MVFR 
ceilings all sites overnight and into Friday as front stalls over southeast 
Virginia/NE NC.. have tempo -shra at ric but looking unlikely as front 
nears. Winds gusting to near 20 knots this afternoon and should diminish 
to 10 knots or less around sunset. 


High pressure and VFR conditions return late Friday through early Sat. The 
next frontal system moves across the region late Sat through early sun 
bringing increasing cloud cover and a chance of precipitation. High 
pressure then builds back in late sun through Monday. 


&& 


Marine... 
will maintain Small Craft Advisory for the ches Bay and coastal waters from Fenwick isl to 
the NC/Virginia border until 6 PM this eveng due to SW winds 15 to 20 knots 
with gusts to 25 knots in advance of cold front. Hi pressure will build in from 
the west later tonight and Friday...then move across the area Friday night into the 
first part of Sat. Winds will become northwest or north overnight...but will not go 
any higher than 15 knots. North or northwest winds will start out 15 to 20 knots on Friday 
then diminish during the afternoon (except may southern two coastal zns) as hi 
pressure builds closer to the area. May have to issue Small Craft Advisory for seas for NC 
CST Friday and Friday night. Othrwise...don't expect Small Craft Advisory conds Sat eveng/night 
with next frontal boundary that swings across the region. Small Craft Advisory conds likely 
though sun into Sun night...as strong west or northwest winds prevail behind that 
system. 


&& 


Akq watches/warnings/advisories... 
Maryland...none. 
NC...none. 
Virginia...none. 
Marine...Small Craft Advisory until 6 PM EST this afternoon for 
anz630>632-650-652-654-656. 


&& 


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

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