Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
937 am EDT sun may 19 2013 


Synopsis... 
a stationary boundary will remain south of the area through 
tonight before lifting north of the area Monday. A weak middle-level 
trough will move through Monday. High pressure builds over the 
western Atlantic midweek...with southerly return flow providing 
warm temperatures to the region. A cold front will approach from 
the Great Lakes late this week. 


&& 


Near term /through tonight/... 
nearly stationary boundary along the coastal Virginia/North Carolina 
border was snaking its way westward through the central Appalachians 
and into the Ohio Valley. Meanwhile...high pressure was located 
over the North Atlantic. 


There hasnt been much change to synoptic features thus far today...and 
really dont anticipate rapid change taftn. Morning lwx radiosonde observation stable and 
satd below h9. While rnk sndg a little less stable...its not exactly 
unstable either. Changed precipitation character to radz for the mrng/midday. 
Radr presentation unimpressive...and have/will scale back probability of precipitation. Do 
think that a few holes will develop in overcast...and that will aide in rain showers 
development lt today. Maybe a thunderstorms and rain too...but that/S looking rather slim. 
Think model cape/Li fields overdone. 


For tonight...models are suggestive that the stationary front will 
start to make its way north. Even if low level flow turns more to 
the south...am not convinced that enough wind in the boundary layer 
will be able to push the front this far to the north. Therefore 
forecast reflects this boundary remaining south of the County Warning Area...keeping 
low cloud cover over the County Warning Area. North of the boundary...lift enhanced 
by a shortwave at 500 mb is likely to bring at least scattered 
showers over the County Warning Area. Stayed close to bias corrected mav/met for 
minima. 


&& 


Short term /Monday through Wednesday/... 
low level ridge will amplify over the western Atlantic during the first half 
of the work week. Broad southerly return flow on the western periphery of the 
high will keep whatever is left of the remnant surface warm front north 
of the area during this time. South of this front...a warm and a 
modestly humid airmass will be in place. Maximum temperatures in the 80s each 
day although clouds may inhibit temperatures from reaching 80f in some 
locations on Monday. 


Both the 00z NAM/GFS forecast a middle-level shortwave trough that is 
currently over the Tennessee Valley to slowly move wd through the middle-Atlantic 
region on Monday. Lift/moisture ahead of the trough in conjunction with 
daytime heating will support diurnal convection over the County Warning Area. Probability of precipitation 
highest toward central Virginia/southern Maryland but still in chance range. 


Middle-level ridge builds overhead Tuesday and Wednesday after the shortwave 
trough exits the area. Coverage of showers and storms will be low 
Tuesday and Wednesday due to an absence of organized lift and a 
strengthening subsidence inversion. Kept forecast dry for most of the 
I-95 corridor and chesepeake Bay as weak steering flow will limit how 
far east convection propagates once initiated over the mts. 


&& 


Long term /Wednesday night through Saturday/... 
shortwave trough and associated cold front will approach from the 
Great Lakes/Midwest regions Wednesday night and Thursday before passing through 
the area on Friday. Chances for showers and storms will accordingly 
increase late in the week and will be highest with frontal passage. Above 
normal temperatures ahead of the cold front will trend much cooler in 
wake of frontal passage Friday into the weekend. 


&& 


Aviation /14z Sunday through Thursday/... 
with moist onshore flow in place...confidence is high in the 
presence of IFR/LIFR flight restrictions through this morning. 
Confidence lowers to moderate regarding the ending time of these 
flight restrictions. Have favored lamp guidance on keeping IFR until 
17-18z. Despite high sun angle...these conditions could linger 
longer given easterly flow. Any improvement that occurs later this 
afternoon is expected to be brief as a stalled frontal boundary is 
forecast to remain to the south of the area tonight. Even if 
easterly flow becomes more south-southeast...a degradation to 
IFR/LIFR conditions is expected overnight per guidance. The only taf 
site with the inclusion of thunder in the grids is cho...rest of the 
sites are expected to be too stable for thunder and only rain showers was 
mentioned in tafs. 


IFR conditions likely to start the day Monday before morning stratus/ 
fog mix out from west to east. Predominately VFR conditions expected Monday 
afternoon although brief restrictions in rain showers/thunderstorms and rain are possible. 
Diurnal fog/low clouds possible at night and early in the morning each 
day in the midweek. Chances for rain showers/thunderstorms and rain will be low during this 
time. 


&& 


Marine... 
easterly flow starts to become southeast later this afternoon and 
evening. Models forecast winds to increase...and southeast winds 
tend to be enhanced over the lower tidal Potomac River and up the 
Bay. Small Craft Advisory in effect this afternoon/evening for these areas. Its 
possible that the Small Craft Advisory could need to be extended into the overnight 
for some of these areas. 


Light southerly flow early in the week will strengthen later in the week. Small Craft Advisory 
conditions possible by Thursday...when a cold front approaches from the 
northwest. 


&& 


Tides/coastal flooding... 
departures have decreased to a half foot or less. If these 
anomalies remain the same...no flooding should occur with the 
upcoming high tide cycle. However...as winds turn more to the 
southeast and then south...departures may increase Monday 
increasing the risk for minor tidal flooding at sensitive 
locations. 


Water levels should tick upward early next week with southerly flow 
persisting and a waxing gibbous moon. This far out...still some 
uncertainty if positive anomalies increase enough to reach minor 
threshold. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory from 6 PM this evening to midnight EDT 
tonight for anz531-532-539-540. 
Small Craft Advisory from 3 PM this afternoon to midnight EDT 
tonight for anz533-534-537-541-543. 


&& 


$$