Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180119
AFDLWX

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

.SYNOPSIS...
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.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Most of the convection has dissipated due to the loss of daytime
heating. However...the remnants of an outflow boundary remain
over northern and central Virginia...and latest mesoanalysis
shows some instability along with a weak capping inversion.
Isolated showers and thunderstorms are possible across these
areas late this evening.

A southerly flow will cause more warm and humid conditions
overnight. A stray shower cannot be ruled out along a weak
surface trough near and west of the Blue Ridge and Catoctin
Mountains...but most areas will be dry.

Low-level moisture will get trapped underneath the nocturnal
inversion...resulting in areas of low clouds and fog. Fog may be
locally dense...especially in areas that received heavier
rainfall and also rural areas and sheltered valleys. Min temps
will range from the lower to middle 60s in the Potomac Highlands
to the middle 70s in downtown Washington and Baltimore.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/...
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.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
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
thunderstorms.

&&

.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
Areas of low clouds and fog are expected late tonight into
Tuesday morning. IFR conditions are possible...especially with
CIGS in the eastern terminals and possibly from VSBYS in the
western terminals. Scattered showers/thunderstorms are expected
Tuesday afternoon/evening which may bring brief gusty winds and
heavy rain resulting in reductions to ceiling/visibility.

Patchy low clouds and fog are possible overnight Tuesday into
early Wednesday as well. A thunderstorm cannot be ruled out
Wednesday afternoon and evening...but coverage should be
isolated.

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.

&&

.MARINE...
Southerly winds will channel up the lower and middle portion of
the Maryland Chesapeake Bay tonight and the lower Tidal Potomac
River. A Small Craft Advisory is in effect for these areas until
2 am. However...it will be marginal with gusts around 18 knots.
Otherwise...most of the time winds should remain below SCA
criteria through Wednesday night. Main weather concern will be
for thunderstorms, which could pose a risk for Tuesday afternoon
and evening. Gusty winds are possible. Thunderstorms are
possible Wednesday afternoon and evening...but coverage should
be isolated.

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.

&&

.TIDES/COASTAL FLOODING...
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.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 2 AM EDT Tuesday for ANZ532>534-537-
     540-541-543.

&&

$$
SYNOPSIS...MM
NEAR TERM...BJL/MM
SHORT TERM...MM
LONG TERM...ADS
AVIATION...BJL/ADS/MM
MARINE...BJL/ADS/MM
TIDES/COASTAL FLOODING...MM



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