Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170805

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

An upper level trough will affect the weather today 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.


Closed low over southern Ontario and Quebec is sending a
shortwave trough in our direction. At present it extends
southwest from the low across Ohio and Indiana. It should
gradually move southeast and move over our region this evening.
While the feature weakens as it moves into the region, the
cooling aloft should promote plenty of instability, so showers
and thunderstorms are likely to develop. Main limiting factor
today will be a mid level cap which looks more likely to hold
near and east of I-95 than west of it, so POPs are highest in
the mountains and much lower near the bay, but still have at
least isolated coverage CWA-wide. Given CAPE values which exceed
2000 j/kg (and in some guidance, 3000 j/kg), but almost
negligible shear, main threat would be pulse-type severe, but
there could be some isolated flash flooding as well if storm
motions prove very slow (which they well could). However, we
have mostly dried out from our wet weather early in the month,
so conditions aren`t ripe. Thus, only expect isolated reports of
severe and flash flooding (a handful of warnings at best), if
any, and do not believe watches will be needed at this time.
Today will be a bit more uncomfortable with highs in the upper
80s to near 90 in most spots, and dew points returning to around
70, resulting in heat indices in the low-mid 90s.

Given lack of strong forcing, most convection should quickly
disispate this evening, and the balance of tonight will be dry
with lingering clouds. Patchy valley fog is also possible,
especially where it rains today. Lows will be a bit milder, with
70s becoming more common.


Upper trough slowly weakens and moves out of the area Tuesday
and Wednesday as high to the southeast continues to bring in
more warmth and humidity. Dew points will rise a bit more and so
will temps, but right now, expect heat indices to peak in the
mid-upper 90s Tuesday and upper 90s-low 100s Wednesday... just
below criteria for a heat advisory. Conditions will not be quite
as unstable overall on Tuesday as we are expecting today, but a
lee side trough combined with less of a cap means that the
chance of showers and t-storms along the I-95 corridor actually
looks higher on Tuesday than it does today. By Wednesday, pops
fall everywhere as what remains of the trough aloft is moving
away and there is less of a lee side trough. Highs will
generally be low-mid 90s with lows in the 70s.


An upper trough present through the first half of the week over
the eastern states will shift east off the coast by Thursday. A
sprawling high centered over the southern Great Plains begins
to control area weather in the wake of this trough.

A shortwave trough crosses to the north on Thursday with a weak
cold front entering the LWX CWA later in the day. This front
looks to stall over the area and provide a focus for diurnally
driven convective storms Thursday through the weekend.

Heat will be a concern later in the week, particularly Thursday
ahead of the cold front and Sunday after the stalled front
moves out. As of now values around 100F seem reasonable for
Thursday in urban areas.


VFR overall through the TAF period and beyond. Main cig/vis
concern is patchy early morning fog, especially Tuesday and
Wednesday mornings and particularly where any rain falls the
previous afternoon/evening. Other than fog, the main concern
will be scattered thunderstorms during the next several
afternoons, with best coverage west of I-95 today, and near I-95
on Tuesday. Will likely insert VCTS into TAF`s shortly for later

A weak cold front stalls over the area later Thursday into the
weekend which may allow sub-vfr conds during thunderstorms.


Generally sub-SCA today and Tuesday, though southerly channeling
could result in near SCA conditions Wednesday. Other than that,
main concern is for thunderstorms, which look more likely on the
waters on Tuesday, but are possible both today and Wednesday
(just not as likely). Any storm could result in gusty winds over
the next couple of days.

Generally light NWly flow Thursday into the weekend behind a weak
cold front.


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.


Meteorological summer - June 1 to Aug 31, 92 days. Yesterday
was day 46 of that, so we`re at the midpoint. Here are a few
temperature stats on thIS summer so far.

Site   Average Temp  Rank  Record year (through yesterday)
DCA        79.1        6     2010
Balt       75.7        35    2010
IAD        74.9        10    1994

I was curious about why DC was ranked so high, given that it
has not felt (to me) even remotely close to as hot as 2010, so I
looked at max and min temps for the site to see how this year
ranks:  11th for max temps, 7th for min temps.




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