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Area forecast discussion 
National Weather Service Wakefield Virginia 
1001 PM EDT sun Mar 21 2010 


Synopsis... 
low pressure over Tennessee will track northeast through the Ohio 
Valley overnight and Monday. The low will eventually move off the 
coast of New England by Tuesday afternoon. High pressure will 
build across the area Wednesday and Thursday...with a cold front 
approaching from the west on Friday. 


&& 


Near term /until 6 am Monday morning/... 
latest weather analysis showing strong negative tilt upper low centered over 
MS...with surface low tracking into eastern Tennessee. Still expect some 
precipitation to move in from the west-southwest after midnight...and bringing 
likely probability of precipitation into western zones towards Erly morning (but will keep quantitative precipitation forecast amts 
low/gnly less than .10-.20"). Temperatures remain mild overnight west/ southerly 
flow and clouds...ranging through the 50s. Have added a schc thunderstorms to 
far southern zones after 3am...as h850 warm air advection slowly tracks in from the 
SW. 


&& 


Short term /6 am Monday morning through Wednesday/... 
main batch of steady precipitation should be moving into the County Warning Area from the 
SW by daybreak. With warm air advection and low level SW jet both NAM and GFS 
depict negative showalter indices (instability aloft) over much of 
Virginia/NC during the morning hours. This suggests a decent chance for elevated 
thunderstorms during the morning hours and have added this to the forecast. Will 
carry 80% probability of precipitation on Monday...but am concerned about a split precipitation 
pattern...with fairly minimal quantitative precipitation forecast values over much of central Virginia. 
Based on latest data not looking for any severe threat given time of 
day...but some gusty winds are possible given the prognosticated instability and 
shear. 


Best lift & instability (along with the frontal bndry) lifts NE into Northern 
Neck & Eastern Shore areas drng Monday afternoon. Data suggests dry punch of 
air from the SW dmnshs precipitation chances aftr 18-20z or so across southwestern counties. 
Will keep skies M cloudy for now...but would not be surprised to see 
some prtl sunshine mainly S of i64 late Monday afternoon. Highs in the 
M-u60s...near 70 southwestern counties given any sun. Only carrying 20% probability of precipitation 
Monday night/mainly north as County Warning Area becomes entrenched betwn the surface low 
well to our north and the upper low still off to the west. Lows mainly in 
the 40s. 


Tuesday/Tuesday night...consensus forecast has the upper low centered across S central 
Virginia at 12z Tuesday...tracking east-northeast off the Delaware-Maryland-Virginia between 18z Tuesday and 00z 
Wednesday. With cold air aloft and instability west/ daytime heating...have 
retained at least 20% probability of precipitation on Tuesday...despite a dry low level flow. Highs 
Tuesday tricky depending on amount of cloud cover...gnly going middle/upper 
50s NE to the lower l60s west of ches Bay where more sun is expected by 
later afternoon. Becmg mclr Tuesday night...lows mainly 40-45. 


Wednesday...next fast moving northern stream shortwave moves well to our 
north on Wednesday...in low levels a downsloping westerly flow will 
prevail allowing mostly sunny skies... deep mixing and warmer 
temperatures. 12z mex guidance highs in the upper 60s north to lower 70s 
central and S look reasonable and have stayed close. 


&& 


Long term /Wednesday night through Sunday/... 
latest GFS/European model (ecmwf) continue to show backdoor surface front dropping S of 
the forecast region Wednesday night. Strengthening west-east upper level deformation 
zone will allow for a quick increase in some middle/upper level cloudiness 
over the middle Atlantic region on Thursday...still well out ahead of the upper 
trough/surface low pressure system. For now feel there would be 
sufficient breaks/transparency for at least a partly sunny forecast. As 
for temperatures...cooler along the coasts with the ensuing onshore flow 
following the backdoor front (highs upper 50s/lower 60s)...65 to 70 
farther inland. 


Thursday night/Fri...medium-range models continue to show best rain 
chances during this portion of the extended forecast...as another cold front 
drops S of the forecast area...then stalls somewhere along the Carolinas 
while serving as an anafront as low pressure tracks across the Tennessee Valley 
and southern middle Atlantic region. Given the time frame (still 4-5 days out)... 
for now have kept probability of precipitation capped at or below 50%. 


Temperatures cooler Friday-sun as the next system tracks through and with the 
low level cold air advection behind it for the weekend (esp on sat). Sat looks to be the 
coolest day...as low level thicknesses support highs in the 50s 
area-wide. Lows over the weekend are currently prognosticated in the 
middle/upper 30s over most inland locales. 


&& 


Aviation /02z Monday through Friday/... 
S to SW winds will continue gusty at times this afternoon. Satellite and 17z 
metars indicated a cumulus field had developed over NC and spread 
into southeast Virginia. Radar had rain over parts of NC and sprinkles were 
added to the taf beginning at 05z at ric and ecg and spreading NE. 
Conditions will deteriorate later in the morning as rain becomes 
continuous. IFR conds are forecast at ric...orf and phf. NAM 
indicates 40-50 kts of wind from the S/southeast at h92 on Monday morning 
across eastern portions of the area. Winds will be gusty at times 
which may result in crosswind issues at phf and orf. Isolated 
thunderstorms are also possible Monday morning mainly around orf and ecg but it is 
premature to include in the taf at this point. 


The SW edge of the rain will pull NE Monday afternoon. Rain may end at 
ric just prior to 18z but left out of the taf for now. Surface 
winds turn to the SW and VFR conditions follow within a few hours. 
This is consistent with both mav and met guidance at all of the 
taf sites. VFR conds will continue Monday night. 


An upper level low moves over the region Tuesday. Conditions will generally 
remain VFR but there will be chances for late am/PM showers (espcly 
sby). VFR/dry conds are expected Wednesday and Thursday. The next weather system 
will bring a chance of rain Friday. 


&& 


Marine... 
raised flags (sca's) for Monday over the Bay/coastal waters as S/southeast 
flow increases ahead of an approaching low pressure system. Look 
for building seas over the coastal waters on Monday...before seas 
relax a bit over the southern waters Monday night. Winds/waves are 
expected to increase again (to at or above Small Craft Advisory criteria) on Tuesday over the 
coastal waters as well as Currituck Sound and the lower ches Bay...as 
the low moves along the New England coast...deepens...and gives 
way to increasing gradient (wrly) flow and low level cold air advection. Therefore 
despite the lull that is anticipated Monday night...decided to keep 
the Small Craft Advisory headlines going through the day on Tuesday across all waters. 


&& 


Akq watches/warnings/advisories... 
Maryland...none. 
NC...none. 
Virginia...none. 
Marine...Small Craft Advisory from 7 am Monday to 7 PM EDT Tuesday for 
anz630>633-654-656-658. 
Small Craft Advisory from 10 am Monday to 7 PM EDT Tuesday for 
anz650-652. 


&& 


$$ 
Synopsis...lkb 
near term...lkb/tmg 
short term...lkb 
long term...bkh 
aviation...jef 
marine...bkh/jef 

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