Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
308 PM EDT Tuesday Jul 28 2015
weak stationary front will slowly dissipate the remainder of
today...with high pressure residing just off shore. A cold front
moves through the region Thursday...with high pressure building in
the wake. Another quick moving cold front brushes the middle Atlantic
over the weekend...with high pressure building back in heading
into next week.
Near term /through Wednesday/...
scattered slow moving showers and/or T-storms again today into early
evening but staying mainly south of I-66. Storms are moving much
slower today around 5kt and drier air has worked in farther south
based on latest blended tpw product and 12z iad radiosonde observation so flash
flooding threat significantly lower today than yesterday.
Light flow...mid-level drying and moist soils will be conducive to
patchy dense fog again tonight especially at places that receive
any rain this afternoon.
Flow becomes slightly stronger tomorrow and more srly bringing
dewpoints up with heat indices around 100f. Chance of T-storms
creeps up to about I-70 in north central Maryland.
Short term /Wednesday night through Friday/...
incrsg confidence of convection occurring Thursday as a cold front
approaches from the west. Falling heights on Thursday as upper level
ridge weakens/pushes east ahead of the approaching trough. Near
zonal flow takes shape during the day Thursday with the a more
progressive shortwave rounding the closed low over Canada digging
south into the Great Lakes...providing the extra push to finally
push the cold front through and to the southeast by Friday
morning. 12z model suite coming into better alignment with the
frontal passage time...placing it sometime in the afternoon in the
west...and then offshore by 12z Friday morning.
Ahead of the front Thursday...warm air advection places 850mb temperatures near 20 degrees
celsius. Daytime highs in the low to middle 90s. Moisture advection
also will result in dew points hovering around to right above 70. Heat
indices nearing the century mark...with possible 100-105 range if
temperatures warm slightly more or if dew points increase slightly. Will
continue to watch the trend. The combo of higher temperatures and dew
points should result in a fairly unstable airmass...with SBCAPE
values 1000-2000 j/kg by the afternoon. With 0-6 km shear about 20 kts
and steepening middle level lapse rates also by the afternoon...sufficient
ingredients exist for the development of some stronger storms.
While the surface forcing exists with the frontal boundary...lack of
shortwave energy reaching the middle Atlantic suggests most of the
convection...particularly anything stronger to severe would be right
along the front late afternoon into the evening. Precipitable waters incrsg to around
1.8 inches would also suggest the threat of heavy rainfall.
Front pushes south and east slowly Thursday night...with the precipitation
tapering off as drier air moves in behind the front...dcrsg dew points
back into the 60s...and the 50s for isolated spots. The colder air
doesnt quite make it all the way into the County Warning Area. So while temperatures will
be slightly cooler on Friday...still expecting a warm...yet slightly
Long term /Friday night through Tuesday/...upper level trough
quasi-stationary over region through period. Weak positive vorticity advection affecting
are through weekend. Moderate positive vorticity advection arriving Monday.
Much drier air associated with surface high building across
region Friday will give way to a weak cold front Saturday.
Afternoon heating and a slight increase of moisture may be enough
to trigger an isolated shower or thunderstorm over southern Maryland
during the weekend. Increasing moisture...stronger upper level
support and Lee-side trough lends greater confidence in the
development of isolated showers/thunderstorms Sunday night and
Monday especially along southern and southeast fringes of area.
Aviation /18z Tuesday through Sunday/...only kcho has a chance of
seeing a T-storm this afternoon. All other taf sites should remain
dry through Wednesday morning. Patchy dense fog likely again at kmrb and
kcho with a 1-3 hour period of IFR visibilities. Chance of T-storms increases
Possible MVFR ceilings Thursday morning from on night precipitation.
Expecting conditions to improve to VFR by midday. Outside of
convection...VFR conditions then Thursday aftn-Fri. Incrsg chance of
showers and thunderstorms Thursday afternoon/evening with a cold front moving through. Any
storm could bring conditions down to IFR...with heavy rain...and
Marine...light and variable winds through tonight. Flow becomes
slightly stronger and better defined from the south. Gusts up to
17 knots Lower Bay.
Sub-Small Craft Advisory conditions on all waters Thursday morning. Incrsg southerly
flow ahead of the approaching cold front could bring gusts back to
around 18-20 kts Thursday afternoon and night on the Chesapeake Bay.
District of Columbia...none.