Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 171845

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
245 PM EDT Mon Jul 17 2017

An upper level trough will affect the weather today and
Tuesday. After a period of hot but mostly dry weather during
the middle of the week, another cold front may approach the
region by the end of the week.


Upper trough continues to progress into the region while frontal
boundary stalls and washes out across the Ohio Valley and up
into western PA/NY. Relatively steep mid level lapse rates still
exist with 700-500 mb lapse rates around 7 C/km per latest SPC
mesoanalysis. This combined with surface temperatures near 90F
and dew points near 70F has led to the development of 2000+ J/KG
of MLCAPE and the earlier indication of capping inversion has
weakened. While shear remains quite low (less than 20 knots
0-6km), this amount of instability is certainly enough to
provide for an isolated pulse severe threat with both hail and
downburst potential, especially northwest of I-95. In addition,
with little steering flow, thunderstorms will be slow moving,
so heavy rain will need to monitored as well.

Convective activity should wane with the loss of daytime
heating this evening, leading to a mostly dry night overnight.
With areas of rain today, light boundary layer winds, and high
dew points, there is the potential for patchy fog as well. Lows
tonight from the mid 60s to mid 70s.


Upper trough will remain overhead on Tuesday, and another round
of scattered afternoon/evening showers/thunderstorms is
anticipated. Could once again be an isolated severe and heavy
rain risk with atmospheric conditions similar, although mid
level lapse rates are a bit weaker. High temperatures in the
upper 80s to around 90F.

The upper trough will continue to slowly weaken and gradually
pull away Tuesday night into Wednesday. So a relatively calm
night expected again Tuesday night after any evening convection
ends, with the potential for patchy fog. By Wednesday, there
could still be an isolated shower/storm but coverage should be
less than today and Tuesday. Temperatures rise a bit into
Wednesday with highs from 90-95F. This combined with dew points
in the upper 60s to around 70F will give heat indices 95-100F
locations east of the Blue Ridge and 90-95F west. Warm and muggy
conditions Wednesday night, with lows 68-78F.


The main story for the end of the week will be the heat. High
temperatures Thursday and Friday will be in the mid to upper 90s
with dew points in the lower 70s, heat advisories will be possible.
Temperatures over the weekend may be more variable due to storms and
a wavering front, but still above normal with high humidity.

In terms of synoptic patterns, a mid/upper level trough will be just
east of the Mid-Atlantic coast on Thursday. Forecast instability is
meager Thursday afternoon, so the best chance of an isolated shower
or storm will be over the mountains.

The subtropical ridge then will be able to exert influence on the
weather through the remainder of the period, though it will largely
remain centered to our south and west. As such, a cold front will be
able to work down from the north on Friday, but fail to make it
completely through the forecast area. With flow a little north of
west aloft and the front in the vicinity, thunderstorm chances will
increase Friday, but especially Saturday into Sunday, with MCS
activity (or remnants) possible.

Model spread increases later Sunday into Monday with how to handle
next piece of energy moving across Canada, but with potential for
the front to still be in the area, forecast will have a chance of


Primarily VFR expected through the period. The main weather
concern today and Tuesday will be scattered showers/thunderstorms
during the afternoons which may bring brief gusty winds and
heavy rain resulting in reductions to ceiling/visibility.
Otherwise, patchy early morning fog is possible both Tuesday and
Wednesday morning, especially where rain falls during the
afternoon and evening.

It looks like any isolated thunderstorms will remain away from the
terminals (in the mountains) on Thursday, though there could be more
scattered coverage across the area on Friday. Otherwise VFR
conditions are likely.


Generally sub-SCA southerly winds expected across the waters
through Wednesday night. Main weather concern will be for
thunderstorms, which could pose a risk both this afternoon and
Tuesday afternoon.

Light westerly winds on Thursday with high pressure in control. A
cold front will approach from the north on Friday, but will likely
not push completely through the area. While there are no clear
opportunities for SCA conditions, scattered thunderstorms could
affect the waters on Friday.


South flow persists across the Chesapeake Bay region through
midweek. Water levels at the preferred high tide (late
afternoon/evening) will approach minor coastal flooding thresholds
at particularly sensitive sites like Straits Point (St. George
Island in St. Mars county), Annapolis and SW DC Waterfront today
through Wednesday.




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