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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
921 PM EDT Tuesday Jul 28 2015

weak stationary front will dissipate tonight...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/...
an active evening across portions of the mid-Atlantic. Showers
and thunderstorms kept regenerating on outflow boundaries across
the region in a very warm and humid airmass. Heavy rain was
reported with amts of 2-3 inches in the heaviest storms. The
iad radiosonde observation was released late due to thunderstorms over
the office this evening.

Using 00z rnk radiosonde observation and mesoanalysis...instability is present
across the region with weak steering flow. Showers and
thunderstorms have been moving slow today and will continue
tonight. The overall trend is that intensity is decreasing with
the loss of daytime heating. Showers have become more straitform
across the central Shenandoah Valley and central foothills with
moderate rainfall rates. Similar to last evening the next story
is fog.

Light flow and clouds clearing over a very satuated ground will
allow fog...dense at times to form mainly across the central
foothills...Piedmont and into northern Virginia and Washington District of Columbia
suburbs. Heavy rain west of the Blue Ridge will also lead to areas
of fog mainly across the Shenandoah Valley into the eastern
Panhandle of WV. Warm conditions tonight...temperatures in the upper 60s
and low the upper 70s in the metros.

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
drier day.


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

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 /01z Wednesday through Sunday/...
rain showers/thunderstorms and rain diminishing this evening. Thunderstorms in the vicinity still expected at cho
this evening. Fog development tonight will likely impact cho-mrb-
iad with IFR conditions. At this time... kept iad at MVFR but the
recent heavy rain will likely cause them to get to IFR by Wednesday
morning. Sub-VFR conditions possible at dca into Wednesday morning.
Visibilities will improve Wednesday morning. Isolated-scattered rain showers/thunderstorms and rain possible Wednesday
afternoon. At this time...coverage seems limited and thefore kept
out of tafs.

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
gusty winds.


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.


Tides/coastal flooding...
anomalies running around a half foot above normal. Main concern is
Annapolis late tonight. At this time...water levels should remain
just under minor flood. Will need to monitor overnight.
Elsewhere...water levels should stay under flood thresholds.


Lwx watches/warnings/advisories...
District of Columbia...none.


near term...has/lfr
short term...Sears
long term...Sears
tides/coastal flooding...has

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