Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 160803

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
403 AM EDT Sun Jul 16 2017

Weak high pressure over the midwest will control the weather
today. An upper level trough will affect the weather Monday and
Tuesday. After another period of tranquil weather during the
middle of the week, another cold front may approach the region
by the end of the week.


Surface front is located across the Carolinas this morning,
while high pressure is located over the Ohio Valley. The
shortwave trough which will have some effect on our weather
during the early part of the week is diving across Ontario.
Satellite indicates there may be a few patches of fog in the
western valleys, but it`s more pronounced west of the
Appalachians where dew point depressions are smaller.

Overall tranquil conditions are expected today as the weak high
influences the weather: light winds, scattered cumulus, and
near seasonable temperatures. Some instability will develop over
the Appalachians as steep lapse rate plume spreads eastward,
but soundings indicate a stout low level cap. Have limited
slight chance POPs to the highest elevations where terrain
circulations will at least have a chance to be a trigger.

Return flow will commence tonight and start advecting in higher
dew points. This could lead to more of a low cloud or fog
threat, though specifics are not clear at this time. While steep
lapse rates will still be found aloft, am not seeing much of a
trigger to overcome stable low levels, so have tapered POPs back
some. Several degrees will be added to the lows...which will be
in the mid 60s to lower 70s.


The upper level trough will be closed off north of Lake Ontario
by Monday morning, with lower heights having expanding down the
Appalachians. There are still a range of solutions as to how
convection will unfold Monday, though the consensus is that the
best chance will be in the mountains, where terrain circulations
will help serve as a trigger since other low level features
will be lacking. Better mid level forcing will be north of the
region, though can`t rule out some small perturbations in the
trough. Meanwhile the front to our south will be washing out,
though some models hint that some remnant troughing or even weak
surface lows could try to work north Monday or Tuesday. These
features could provide a focus for convection as well. By
Tuesday, the closed low will be filling and lifting to the east
as broader troughing approaches from the west. With gradual
height rises, would expect less thunderstorm coverage for
Tuesday, with the trough axis moving to our east overnight and
ending the precipitation threat.

Monday will have the better chance of having a few strong
storms when lapse rates will be steepest aloft, resulting in
around 2000 J/kg of MUCAPE. The problem will be a stable layer
between 5-10kft AGL, though if it can be broken, pulse storms
could briefly produce hail and strong winds in an environment
with around 20 kt of deep layer shear.

Temperatures will remain near normal Monday, but rise a few
degrees Tuesday as 850 mb heights begin to rise. Humidity will
also return with dew points into the lower 70s.


High pressure over the western Atlantic will overall dominate
the weather Wednesday, and it should linger into Thursday. This
will provide hot, increasingly humid weather, but overall, it
should be dry, with only isolated thunderstorms possible.

Some uncertainty regarding the end of the week exists with
regard to a shortwave crossing the region, which looks likely to
send a cold front from Canada southward into our area. Timing
and push southward are both uncertain, but a greater chance of
showers and thunderstorms exists by Friday, with an increased
chance of cooler air moving in by Saturday.


Weak high pressure will provide VFR conditions with light winds
today. Return flow commences tonight and rising dew points
could result in low clouds or fog. Model solutions are split at
this time so will be conservative with 12Z TAFs.

Upper level disturbance will approach Monday but thunderstorm
coverage still doesn`t look especially widespread, with MRB,
CHO, and IAD perhaps having slightly higher chances. Any
stronger storms could contain gusty winds and hail though. After
a lull overnight, some scattered storms may redevelop on

VFR overall in the long term, but risk of showers and
thunderstorms increases Friday.


High pressure will result in light winds today, becoming
southerly by tonight. With southerly flow established Monday and
Tuesday, will have to monitor for periods of channeling, though
the overall wind flow in the atmosphere is forecast to be
light, so am not seeing much potential for a SCA. Isolated to
scattered thunderstorms will be possible Monday and Tuesday as
well, primarily during the afternoon and evening.

Sub SCA overall in the long term, though risk of thunderstorms
will increase Friday.


Southerly flow developing across the region as high pressure
shifts offshore will start to build anomalies today, but since
anomalies have fallen substantially since yesterday, the threat
of any flooding today appears reduced. By Monday afternoon,
minor flooding at high tide is possible at our most vulnerable
sites (most likely Straits Point). This would likely continue
into Tuesday.




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