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Area forecast discussion 
National Weather Service Wakefield Virginia 
1125 am EST Thursday Mar 11 2010 


Synopsis... 
a slow moving low pressure system slides into the middle Mississippi 
River valley today...pushing another warm front across the Middle 
Atlantic States late tonight. The cold front associated with the 
system crosses the area late Friday...with low pressure 
developing along the front on Saturday as it moves to the north. 
Northwest flow returns Sunday into early next week as the low 
slowly moves away from the coast. 


&& 


Near term /through tonight/... 
the radar is mainly dry at this time...but the rain is 
approaching from the south and should arrive late in the day along 
the Virginia/NC boarder and spread northward overnight reaching the Maryland 
portion of the Delaware-Maryland-Virginia after midnight. The big question for this 
afternoon is clouds. The satellite shows lots of clouds across the 
region with some low stratus still hanging on and the high level 
cirrus continuing to flow northward and thicken. But there are 
breaks in the clouds to peaks of sunshine are expected through out 
the day. With the clouds expected to increase...have nudged high 
temperatures down a few degrees to the middle to upper 60s across the 
region. Overall this looks to be the last of the dry warm days 
prior to a wet couple of days Friday into the weekend. 


&& 


Short term /Friday through Saturday night/... 
meandering upper low over the Missouri Valley continues to dominate weather 
over the eastern Continental U.S....with primary challenge in the period 
timing (numerous) precipitation opportunities Friday and Saturday. 


Models are in general agreement with synoptic features as the next 
shortwave pushes into the region. The GFS is a hint faster than 
the NAM...which is more aggressive (and farther north) with 
developing surface low over NC coastal plain. Still...despite these 
differences...both would bring drier conditions and potentially 
even some partial clearing Friday afternoon. There is a little dry air 
behind it and with the area going into the warm sector...expect to 
see some breaks in the clouds and also a mainly dry 
afternoon...although a shower can not be ruled out. Given the 
continued southerly flow...should see mild temperatures on Friday 
so leaned toward the warmer mav numbers. 


By Friday night...the upper low will begin to head east and as it 
does the colder air will finally move east...pushing the cold 
front across the Ohio Valley into the Middle Atlantic States. The 
models are decent agreement on this frontal movement...but where 
the redevelopment of the surface low takes place is very different. 
The NAM continues to be the outlier developing the low off the 
coast. The favored 00z/11mar European model (ecmwf) and GFS both develop the low 
across the Piedmont of the Carolinas and lifting across Virginia into 
southern New Jersey by Sat night. Have followed closer to the track which leads 
to some significant rainfall from late Friday night into Saturday 
when the low and front pass the area. The increasing southeast flow ahead 
of the system should help to further deepen the moisture across 
the region. However...once the low and front pass...the flow turns 
westerly and should help to once again begin to dry out the low 
and middle levels. 


For temperatures Friday night through Saturday...continued to lean 
towards warmer envelope of guidance as the increased moisture should 
hold values up in the upper 40s to lower 50s. Saturday...timing 
could be an issue as the longer the clouds and rain hang around 
the longer it will take to allow for the warming. Bumped highs 
upward a bit in anticipation of some downslope-aided clearing. 
This could also be a mitigating factor for convective chances Sat 
afternoon as the upper low crosses. Will stick with slight chance T wording 
for now during the afternoon Saturday. Highs again 60s to l70s. 


&& 


Long term /Sunday through Wednesday/... 
global operational models and GFS ensembles are rather consistent 
with a large upper low centered over the middle Atlantic and central 
Appalachians Saturday night. At this time the fetch of warm moist 
air associated with the warm conveyor belt is well to the north over 
New England. However...a secondary shortwave trough rounds the base 
of the upper low passing over the local area late Saturday night or 
Sunday morning with a period of rain showers. Systems such as this 
can be difficult to time several days in advance...so the extended 
forecast will continue with a period of chance probability of precipitation north...slight 
chance south for Saturday night and Sunday. The upper low slowly 
drifts south and shears apart late in the weekend and early next 
week before the large scale upper level flow becomes northwest over 
the East Coast by middle week. Conditions should be dry overall after 
Sunday with the exception of Monday afternoon...where one final 
impulse could trigger a few light rain showers. Temperatures through 
the extended period will average near to slightly above seasonal 
normals. 


&& 


Aviation /16z Thursday through Monday/... 
elongated band of warm frontal rain (mainly light) continues to 
lift across the forecast area from S to north early this morning... 
extending mainly north of the Virginia/NC border at sunrise. Ceilings/visibilities 
have deteriorated to at least MVFR over all terminals outside of 
the lower Maryland Eastern Shore (including sby)...with scattered IFR conditions. 
This initial batch of rain lifts north into the lower Eastern Shore later 
in the morning...which will also lead to the development of MVFR/ 
possible IFR conditions over sby by midday. Elsewhere...as rain tapers 
later this am...expect occasional MVFR conditions (mainly cigs) to 
linger through the afternoon. 


A much broader area of rain will then develop from S to north over 
most taf sites once again by early evening (all except sby). The 
result will be a lowering of ceilings/visibilities to IFR tonight (along with 
occasional LIFR conditions). Over sby...an onshore flow and a moistening 
boundary layer will result in developing LIFR conditions as well 
tonight...even prior to the arrival of precipitation. 


MVFR/IFR and periods of precipitation continue at the taf sites Friday and Sat. 
Conds should begin to improve Sat night but a chance of showers will 
continue into sun. 


&& 


Marine... 
winds will remain 10 kts or less today...while gradually backing from 
S to east/southeast ahead as low pressure approaches. The low organizing 
over the Gulf Coast region will move northeast and eventually reach 
a position near the coastal border of north and South Carolina by 
Friday morning. An easterly wind should strengthen during the day 
Friday continuing through Friday night and into Saturday before 
relaxing and becoming southwesterly as the low pressure system lifts 
to the north. Wind speeds appear to be solidly within Small Craft 
Advisory range...in fact...the last 2 NAM runs have indicated an 
increased potential for gales Friday night/Sat morning. Will continue 
to side closer to the GFS/sref solutions...which would favor sca's 
over gale warnings...as feel the NAM may be a bit overdone with the 
strengthening onshore flow given the synoptic setup (i.E. With main 
low still well inland during this time). The upshot...have gone 
ahead and hoisted small craft advisories beginning on Friday...and while 
headlines will likely persist into the weekend (at least for 
lingering high seas)...per local policy have not extended sca's 
beyond Friday night or the 4th period of the forecast. 


&& 


Tides/coastal flooding... 
the latest model extratropical water level guidance is now indicating 
the potential for tidal anomalies on the order of 1 to 1.5 feet above 
MLLW on Saturday. Extended period of onshore flow...with period of 
strong east-southeast winds possible Saturday morning...could bring about some 
minor tidal issues. However at this time...forecast maximum high 
tides remain below maximum astronomical tide or mat levels over most 
sites. One notable exception is Ocean City...where the latest 
projected maximum tide is expected to peak around a half foot above the 
mat...although still around 0.25 feet below minor flooding thresholds. 
Will continue to monitor in the coming days. 


&& 


Akq watches/warnings/advisories... 
Maryland...none. 
NC...none. 
Virginia...none. 
Marine...Small Craft Advisory from 1 PM Friday to 6 am EST Saturday for 
anz630>633-650-652-654. 
Small Craft Advisory from 7 am Friday to 6 am EST Saturday for 
anz656-658. 


&& 


$$ 
Synopsis...mam 
near term...ess 
short term...ess/mam 
long term...ajz 
aviation...bkh 
marine...bkh 
tides/coastal flooding... 

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