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Area forecast discussion 
National Weather Service Wakefield Virginia 
147 PM EDT Sat Mar 20 2010 


Synopsis... 
high pressure becomes centered off the middle Atlantic coast through 
Sunday. Low pressure tracks across the northern Middle Atlantic 
States Sunday night and Monday. The trailing cold front crosses 
the area Monday afternoon. 


&& 


Near term /through tonight/... 
13z msas surface analysis is beginning to show a Lee-side trough 
developing east of the Blue Ridge as temperatures climb quickly 
this morning foreshadowing a very mild afternoon. Surface dewpoints 
increased slightly overnight...but 12z soundings from 
lwx...rnk...gso...wal...and mhx continue to show very dry air aloft 
suggesting dewpoints should fall again as the mixed layer depth 
grows through the afternoon...so minimum relative humidity values 
will again be in the 15 to 25 percent range. The surface wind 
will shift to the southwest today with the presence of the Lee- 
side trough. The thermal difference between the land and water 
will lead toward cooler and more moist conditions near and 
along the coast. The current forecast is in good shape with no 
updates necessary this morning. 


Previous near term discussion... 
a very mild start to Spring expeceted today (132 pm) as surface high 
moves off the middle atlntc coast. Some high level cloudiness expeceted from 
time to time out ahead of next frontal systm to the northwest. Full sun with 
850 mb temperatures around 8c results high temperatures in the M-u70s west of Bay. 70-75 
over eastern counties with local c-breezes keeping temperatures within a few degrees 
of 70 at the beaches. 


Almost the same scenario as ystrdy with Lee trough east of mts helping 
to dry out colunm. Low rh's will again be a factor until green up. 
Went closer to the lower mixed layer deep temperatures in the grids which are 
between 5-10 degrees lower than model guidance. This puts relative humidity values between 
15-25% most areas away from the water. Will highlight this in fwf. 


&& 


Short term /Sunday through Monday/... 
M clear to pt cloudy tonite as surface high remains off the coast and 
frontal boundary slides S across PA. Lows in the 40s as wind shifts into 
a srly drctn. Tsctns show high/middle level cloudiness increasing sun out ahead 
of next systm apprchg from the SW. Anthr mild / dry day expeceted with 
850 mb temperatures sprtg readings in the l-m70s...65-70 at the beaches. Relative humidity 
values remain on the low side (30-40%). 


Upper low apprchs from the SW Sun night & Monday. Models forecast a 6-8 hour 
prd of decent lift Monday am into Erly afternoon in assctn with a cdfrnt 
aloft that will swing across the region out ahead of the apprchg 
cyclone. Expeceted quantitative precipitation forecast between 1/2 to 3/4 inches may make for a rthr wet 
Monday morning commute spclly west of ches Bay. The best lift exits NE of 
forecast area Monday afternoon. Drier & cooler air aloft moves in behind the front. 
Surface temperatures remain mild...so scattered showers/tstrms are possible in the 
wake of the front. The extent and coverage of any activity Monday 
afternoon and evening will be determined by the exact track of the upl level 
low. Lows Sun night 50-55. Highs Monday in the m60s but could reach 70 
across southern tier counties given any late afternoon sun. 


&& 


Long term /Monday night through Friday/... 
rain chances linger Monday night and on Tuesday as low pressure slowly 
traverses the middle Atlantic region from west to east. System at this point is 
nearly vertically stacked...though both the GFS/European model (ecmwf) are similar in 
showing the middle level low passing across the Carolinas to near the 
Virginia/NC border. Over the forecast area...any dry-slotting we see on Monday 
will be filled in a bit Monday night and Tuesday...as given the southern 
track of the upper low...our chances for wrap-around showers are 
improved somewhat than if the upper vorticity were to scoot by north of the 
region. The upshot is maintaining at least minimal probability of precipitation in all areas 
Monday night and Tuesday...highest over the northern areas (where the activity 
is expected to be more scattered vs. Isolated). 


Otherwise...cooler on Tuesday with the wrap-around moisture and 
predominately northerly flow in place. Highs in the low-middle 50s over 
the lower Eastern Shore and far eastern Virginia to the upper 50s/lower 60s elsewhere. 


Wednesday and Thursday still look tranquil weather-wise with deep-layer ridging 
moving across the middle and southeast Atlantic Seaboard. The late March sun along 
with the downslope flow and gradually developing warm air advection in the llvls 
will help temperatures warm into the lower 60s over the lower Eastern Shore to 
65-70 elsewhere. Thursday looks to be the warmest day as 850 mb temperatures are 
projected to climb to near +8c per the latest European model (ecmwf) (with +10c temperatures 
knocking on the door to our sw). This should help boost surface temperatures 
well into the 70s over the interior portions of central and S-central Virginia 
(i.E. At least middle 70s). 


12z GFS indicated a rather large departure for the Friday-Sat period 
compared to it's 06z predecessor. However...12z European model (ecmwf) has trended 
much the same way as the GFS...sticking with the more consistent 
European model (ecmwf)...though a bit slower than the GFS with the initial cold frontal passage 
Thursday night/early Friday. It's looking more like a cooler/wetter pattern 
taking shape Friday-Friday night as a secondary low forms along the front 
over the southeast region...then gradually pushes off the Carolina coast. 
For now...given the time frame (7 days out)...for now have decided 
to add a chance of rain to the forecast with temperatures slightly cooler 
than guidance. 


&& 


Aviation /17z Saturday through Wednesday/... 
clear skies covered the taf sites at 17z. Scattered clouds above 12k feet will 
begin to show up Sun morning. Most of the cloud development will occur 
beyond 18z. Winds are expected to back toward southeast by late Sat afternoon 
as a sea breeze develops. 


Conds deteriorate Sun night as the next weather system approaches. Periods 
of MVFR/IFR and wdspr precipitation can be expcted Monday. There will also be a 
chance for embedded thunderstorms Monday. Some improvement but a continued chance of 
showers can be expected Monday night and Tuesday. This will be followed by 
VFR/dry conds Wednesday and Thursday. 


&& 


Marine... 
srly flow will continue this morning with winds 5-10 kts over the 
Bay/ctrk sand and 10-15 kts over the coastal waters before decreasing 
to 10 kts or less late am/early PM. Winds increase to 10-15 kts 
along the southern coastal waters tonight. 


Next chance for sca's will be on Monday as S/southeast flow increases ahead of 
the next low pressure system. This low will move over the middle Atlantic 
region into Tuesday and eventually pushing offshore on Wednesday. Could see a 
brief time of low end gales Tuesday night into Wednesday before high pressure 
start to build back into the region. 


&& 


Fire weather... 
upper level ridge prevails over the middle atlntc region through Sunday. A very 
dry airmass is situated underneath the ridge & afternoon min relative humidity values 
will fall to between 15-20 % range inland...with higher values along 
the immediate coast. A light northwest wind will shift to SW this afternoon. An 
upper level low prs systm apprchs from the SW sun. This will bring 
a moistening trend by sun with rain showers expected by Monday. 


&& 


Akq watches/warnings/advisories... 
Maryland...none. 
NC...none. 
Virginia...none. 
Marine...none. 


&& 


$$ 
Synopsis...mpr 
near term...mpr/ajz 
short term...mpr 
long term...bkh 
aviation...lsa 
marine...ccw 
fire weather... 

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