985 
fxus61 kakq 231133 
afdakq 


Area forecast discussion 
National Weather Service Wakefield Virginia 
633 am EST Monday Nov 23 2009 


Synopsis... 
low pressure tracks northeast along the eastern Seaboard today 
through Tuesday. Weak high pressure builds into the region late 
Tuesday through Wednesday. A weak cold front approaches from the 
west Wednesday night. 


&& 


Near term /through tonight/... 
challenging forecast next 24 hours. All depends on which model one leans 
toward as different models offer up different solutions. The one 
trend that each model does depict is for anthr extended prd of cloudy 
cndtns with little if any drying through Wednesday. 


Through 7 am...current radar loop indctg the widesprd precipitation shield 
moving north iacross southern Virginia is slowly dsptg in coverage as it is having 
a hard time with the dry air at the surface across the northern counties due 
to the high prs nosing S from New England. Expect this first batch 
of precipitation to saturate the lower layers over the next several hours. Rain 
slowly move S-north across the region through 12z. 


Through middle afternoon...low prs lifts NE along Carolina coast with best 
lift and deep moisture prognosticated to continue categorical probability of precipitation all areas. 
Retreating high prs continues in-situ wedge with little (inland) temperature rise 
expeceted. 


Late afternoon...deepest moisture and best lift move NE but lagging short wave nrgy 
keeps precipitation going across the region. NAM keeps moisture locked in while GFS 
moves it NE. Thus...will begin to cut back probability of precipitation (to likely) in the 
grids aftr 18z across southwestern counties first then across the entire southern 
half of the forecast area aftr 21z. Interesting to note that moisture along the 
coast prognosticated to become more showery than stratiformed late this 
afternoon. 


Tmps/qpf...should see at least a 10 degree tmp difference northwest to southeast today. 
Highs remain within a degree or two of 50 across xtreme northwest counties but 
range into the l60s across the southeastern coastal areas. Rainfall totals spited 
to be highest across Erna areas where around one half inch quantitative precipitation forecast expeceted... 
possibly higher if precipitation type becomes more convective. Between a quarter 
to one half inch elsewhere. 


&& 


Short term /Tuesday through Wednesday/... 
tonite...second batch of moisture slow to depart the Northern Neck/Eastern Shore 
areas this evening so will continue probability of precipitation/measurable precipitation there. Tsctns and 
latest soundings indct column remains nearly saturated but little if any trigger 
for precipitation. Thus...expect precipitation to taper off to light rain/drizzle this 
evening and continue through ovrnite hours. Added areas of fog to the grids aftr 
midnite. Lows gnrly in the 40s xcpt near 50 southeastern coastal areas. 


Tuesday/Tuesday night...weak high prs builds over the area but no real trigger 
seen to dry out column. Will continue the cloudy skies as tsctns keep moisture 
at or below 850 mb. Patchy morning drizzle Tuesday with addntl drizzle & fog dvlpng 
late Tuesday night. Highs in the 50s. Lows in the 40s. 


Wednesday...next frontal boundary aprchs from the West. Day starts off cloudy but turns 
mostly cloudy drng afternoon. Will keep slight chance of rain showers in the aftn/eveng... 
as a weak cold front acts on what available moisture remains. Highs u50s-l60s. 


&& 


Long term /Wednesday night through Sunday/... 
by Wednesday night/Thanksgiving day...GFS/ECMWF now both show a fairly weak 
surface pressure pattern across the middle Atlantic...with a developing surface low 
moving up the southeast coast. The low strengthens as it tracks north-northeast and off 
the Virginia convective available potential energy/middle Atlantic coast Thursday ngt/Fri. Carried 20-40% chances for 
showers Thanksgiving day and night (highest probability of precipitation along the coast). 


After that...GFS/ECMWF have trended farther south with the upper 
low...now closing it off in the vicinity of the region Friday/Friday night. 
Still some discrepancies...with a consensus of the GFS/European model (ecmwf) still 
keeping the main shortwave energy over Virginia (rather than the 
carolinas). Given this track...have trended the forecast a bit cloudier 
and added 20-30% chances for precipitation Friday afternoon/evening for central and 
northern zones...but will keep the south dry. Added a bit more in the way 
of cloud cover Sat (ptly sunny) but overall a cool dry pattern should 
prevail west/ westerly low level flow. Highs Fri/Sat...around 50 far north 
to middle 50s south. Dry west/ a slight warming trend Sunday as ridging 
slowly builds back into the East Coast from the SW...highs middle/upper 50s. 


&& 


Aviation /12z Monday through Friday/... 
MVFR to IFR conditions are in place now across the region and 
expected to remain in place for the next 24 hours. The upper level 
trough continues to shear out...but the southwest flow aloft 
continues to push moisture northward. The high pressure system 
over northern New England combined with surface low pressure developing off 
the southeast coast is leading to NE flow at the surface. The low level 
flow is expected to persist for the next few days so the low level 
moisture will linger...but with the moist SW flow...the isentropic 
lift will continue to generate scattered areas of rain through 
about 21 to 00z when the deep moisture finally lift northeastward. 


Later tonight...while the deep moisture and rain end...enough low 
level moisture lingers that fog/drizzle are possible. So have 
maintained the IFR/MVFR conditions tonight as well. 


&& 


Marine... 
the combination of high pressure over northern New England and weak low 
pressure sliding off the southeastern US coast are combining to produce an 
NE flow. The gradient is tight enough to produce small craft 
winds...but it is not anticipated that they will increase in 
strength. The upper level system that is associated with the 
surface low is shearing out as it lifts NE and should be well northern of 
the region by Monday night. With out the dynamic support...the 
surface low is not expected to intensify and should slide NE off the 
New England coast by Tuesday afternoon. This means that the current 
winds of 20 to 25 miles per hour should not strengthen...but they should remain 
in that range through Monday into Tuesday before beginning to weaken 
as the ridge of high pressure moves over the region Tuesday night 
and the wind fields relax. Seas in the 6 to 8 feet range can be 
expected in the coastal zones today and they will take time to relax 
after the winds die down..so it could be into Wednesday before they 
drop below 5 feet. As a result have extended the Small Craft Advisory for an 
additional 12 hours in the 3 northern coast zones...as these are the most 
likely areas to maintain the higher seas in the persistent NE flow. 


The models begin to have some disagreements from Wednesday on as the 
GFS and European model (ecmwf) try to form an area of low pressure and slide it up 
the coast...while the NAM slides a weaker system out to sea. For 
now have just gradually increased the winds...but kept below Small Craft Advisory 
criteria. A strong closed forms over the Great Lakes and Ohio 
Valley on Thanksgiving...which will help develop a strong low 
pressure system off the coast for next weekend. This will produce 
strong off shore flow. Will have to monitor as advisories may be 
required. 


&& 


Equipment... 
NWR transmitter at Windsor NC remains down at this time. Technicians will 
return to the site Monday. A return to service time isn't yet known. 
See pnsakq for more information. 


&& 


Akq watches/warnings/advisories... 
Maryland...none. 
NC...none. 
Virginia...none. 
Marine...Small Craft Advisory until 6 am EST Tuesday for anz630>632. 
Small Craft Advisory until midnight EST tonight for anz633. 
Small Craft Advisory until 6 am EST Wednesday for anz650-652- 
654. 
Small Craft Advisory until 6 PM EST Tuesday for anz656-658. 


&& 


$$ 
Synopsis...mpr 
near term...mpr 
short term...mpr 
long term...lkb 
aviation...ess 
marine...ess 
equipment... 

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