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Nowcast

• 

• Now as of 1:56 PM EDT on May 21, 2012

  Light to moderate rain showers and drizzle will move northwest across central Maryland...eastern parts of the West Virginia Panhandle...northern Virginia...and locations north and west of the District of Columbia through 400 PM. Rainfall amounts will average one tenth of an inch. Temperatures will be in the middle to upper 60s.

Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Monday
• 
• High: 75 °
• Low: 63 °
• T-Storms

• Tuesday
• 
• High: 79 °
• Low: 59 °
• T-Storms

• Wednesday
• 
• High: 81 °
• Low: 61 °
• Chance of T-Storms

• Thursday
• 
• High: 82 °
• Low: 57 °
• Chance of T-Storms

• Friday
• 
• High: 82 °
• Low: 64 °
• Chance of T-Storms

Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
1205 PM EDT Monday may 21 2012 


Synopsis... 
low pressure will near the middle-Atlantic today and tonight. A low 
pressure trough will become stationary across the middle Atlantic 
region during the middle part of the week. Weak high pressure will 
return for the end of the week into the weekend. 


&& 


Near term /through tonight/... 
an easterly flow around low pressure over southern Virginia has 
allowed for plenty of clouds along with rain and drizzle this 
morning. Most of the rain has fallen across the northern and 
central County Warning Area where isentropic lift has been a bit deeper. 


Low pressure will remain over southern Virginia this 
afternoon...but it will gradually weaken. Deep moisture will 
continue to advect into our region during this time. Weakening 
dynamics suggest that precipitation would diminish. However...deep 
moisture will allow for some instability this afternoon which will 
lead to more showers along with isolated thunderstorms. The best 
chance for showers and T-storms will be across locations where the 
marine layer mixes out. There are two areas where that is most 
likely. The first is across the Potomac Highlands into the central 
Shenandoah Valley and the second is near and east of the I-95 
corridor where a coastal front may assist in mixing out the marine 
layer this afternoon. Any sunshine that does occurr will lead to 
moderate instability this afternoon...which could cause 
thunderstorms to contain locally heavy rain. Severe T-storms are 
not expected due to weak shear profiles. 


Surface trough enters Potomac Highlands tonight and pushes toward Blue Ridge 
overnight. Best probability of precipitation/quantitative precipitation forecast focused along/ahead of this feature...with low 
likely/high chance shower probs exhibited from shen valley to I-95 corridor 
overnight. Rmnt thunder from any afternoon cvctn could linger past 
sunset...but should diurnal diminish afternoon midnight. Minima today will be 
quite mild for late may...50s west of Blue Ridge with l60s east. 


&& 


Short term /Tuesday through Wednesday/... 
unsettled weather conditions will continue Tuesday into Wednesday as 
a low pressure trough becomes stationary across the County warning forecast area. The trough 
will serve to focus precipitation through the period. 


Increased probability of precipitation during this period. Some instability during the 
afternoon...so left mention of thunderstorms in the forecast into 
the evening hours. Upper level steering will be light...and surface 
flow will be from the southeast. This should allow for the highest precipitation 
amounts along the eastern slopes of the Blue Ridge mountains. At 
this time...rainfall amounts will average between one half and one 
inch. Locally higher amounts along eastern facing higher terrain. 
Still below 2sd for precipitable waters . 


Took blend for temperatures. 


&& 


Long term /Wednesday night through Sunday/... 
low pressure trough should dissipate by early Thursday. High 
pressure sets up across the deep south...with southerly flow across 
the middle Atlantic. Some differences between the models for 
Thursday...NAM is drier than the GFS. No real focus for the 
precipitation...so saw no reason to change probability of precipitation at this time. Weak high 
pressure returns for later Thursday into Saturday. 


A cold front should begin to approach the Mason Dixon line by the 
end of the weekend. 


&& 


Aviation /16z Monday through Friday/... 
marine layer will result in IFR ceilings and MVFR/IFR visibilities this 
morning. Ceilings may improve to MVFR levels this afternoon. 
However...there is a concern that IFR conditions could persist 
well into the afternoon hours...especially across kmrb...kiad and 
kcho. 


Rain and drizzle may taper off late this morning and early 
afternoon...but showers and isolated thunderstorms are expected to 
develop this afternoon. IFR ceilings and visibilities are expected again for 
tonight along with some showers. 


Sub VFR conditions likely Tuesday into Wednesday as a low pressure 
trough sets up across the middle Atlantic. Thunderstorms possible 
during the afternoon and evening hours. 


Weak high pressure returns for the end of the week into the weekend. 


&& 


Marine... 
low pressure will weaken over southern Virginia this afternoon. An 
east-southeast flow will gust close to Small Craft Advisory criteria during this 
time. Have capped gusts around 15 knots for now due to the 
weakening pressure gradient. Wind speeds are expected to remain 
below Small Craft Advisory criteria tonight. 


Showers and isolated T-storms are expected to develop this 
afternoon. 


Southeast/S flow remains rather light across the County warning forecast area during the period. A 
low pressure trough will set up just west of the waters for Tuesday 
into much of Wednesday. The trough should dissipate by 
Thursday...with weak high pressure returning for Friday into the 
weekend. 


&& 


Tides/coastal flooding... 
positive tidal anomalies this morning range from 0.50-1.00 feet. 
Ely flow will persist today...causing anomalies to increase. 
However...aftn/eve high tide is astronomical lower of semidiurnal 
cycle so no issues expected even with increasing anomalies. Late tonight/early 
Tuesday...the higher astronomical high tide is expected and minor 
inundation is possible. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...none. 


&& 


$$ 
Synopsis...sbk/nwl 
near term...sbk/bjl 
short term...nwl 
long term...nwl 
aviation...sbk/bjl/nwl 
marine...sbk/bjl/nwl 
tides/coastal flooding...bjl