Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 230108

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
908 PM EDT Thu Jun 22 2017

A stalled boundary will return north as a warm front toward the
area tonight. A cold front will cross the area late Friday
night into Saturday, with tropical moisture streaming into the
Mid Atlantic ahead of the cold front. High pressure will return
Sunday and persist through early next week.


A line of showers and thunderstorms will continue to move
southeast across central PA and will likely brush northeast MD
tonight. Warm and moist conditions continue across the region
tonight. Isolated showers and thunderstorms have been popping up
across the eastern panhandle of WV. Shear has been weak across
this region and activity has been of the garden variety.

Previous discussion...

A surge of isentropic lift will be approaching the area tonight.
While this should be quick moving, precipitable waters will be
increasing in excess of 2 inches, so locally heavy rain will be
possible. The best opportunity for showers will be in the
Potomac Highlands. Instability will be limited, so will be
confining that to slight chance at best.


The first round of showers will be exiting Maryland in the
morning. There is substantial uncertainty what will transpire
during the midday-afternoon hours. The latest indications are
that mid-levels will be warm enough that the atmosphere will be
capped. Have reduced PoPs to indicate a break in the warm
sector, but not eliminated the mention entirely due to the
uncertainty. There will still be a 850-500 mb jet on the order
of 40-50 kt, so if the cap is broken, strong storms will result.

Heights lower Friday night as the cold front approaches, with
the remnant low of Cindy intertwined. Model guidance remains
consistent that another mid level speed max...this time up to 60
kt...will be shooting northeast in advance of the low. While
instability will be meager, shear will be plentiful, especially
east of the track of the low. At this time, that track seems to
be from Staunton/Waynesboro across metro DC/Baltimore. There
will be ample convergent forcing for another round of showers
and a few thunderstorms. Given precipitable waters above 2
inches (possibly up to 2.5 inches), high confidence that locally
heavy rain will result. The question remains if shear can root
in the boundary layer. If so, then an overnight severe risk
would be realized as well.

The front/low will clear the area Saturday...with spread
remaining in how quickly that will transpire. Grids depict
improvement over the course of the day, but held onto showers
southeast of DC during the afternoon. Will have clearing skies
by evening.


Below normal temperatures are expected much of next week as a large-
scale trough slowly pivots across the northeastern CONUS. Models are
in above average agreement across our area during this time,
increasing confidence in the forecast (highs generally upper
70s/lower 80s with lows several degrees on either side of 60, about
5-10 degrees below normal).

A secondary (moisture-starved) cool front should cross the area late
Monday. Again, models/ensembles cluster reasonably well for this
time range. An increasing in high and mid-level clouds and a few
showers are likely during this time.

A gradually warming trend should commence by the end of next week as
high pressure moves offshore, resulting in a southwesterly return


Overall, VFR conditions are expected at the terminals through
Friday. However, A round of showers/thunderstorms will be
crossing the area late tonight, some of which may contain
moderate to locally heavy rain. The issue remains confidence in
impacts with respect to a point forecast. If storms do reach an
airfield, then restrictions would result.

There will be a lull on Friday midday-afternoon. A few
showers/thunderstorms may be around (more conditional). Again,
any storm would result in brief restrictions, but the likelihood
of that will be lower in the afternoon than early morning.

A second round will cross the terminals late Friday night into
Saturday morning. Areal coverage should be better. Again, local
restrictions likely within storms.

Gradual improvement will come during the day on Saturday. Mainly
VFR Sun-Mon with WLY flow AOB 10 kts.


Southwest flow will increase tonight, with flow aloft
substantive through Friday night. The complication is poor
mixing. Am keeping overnight Small Craft for the mid Bay/lower
Potomac, and for all waters Friday-Friday night. Given the wind
flow aloft, higher winds likely in vicinity of
showers/thunderstorms. The best chance for storms over the
waters will be late Friday into early Saturday.

Mixing will improve during the day on Saturday in the wake of
the cold front. The gradient should subside Saturday night.

Gusts should remain below Small Craft Advisory levels Sunday
into Monday as high pressure builds over the waters.


Given brief nature of the two round of moderate to heavy rain
and scattered showers/storms in between, areal average storm
total (tonight through Saturday) rainfall are generally in the
1-2 inch range, Gusts should remain below Small Craft Advisory
levels Sunday into Monday as high pressure builds over the
waters.possibly less to the southeast. If anywhere exceeds two
inches areal average, it would be the Potomac Highlands. Thus
MMEFS indicates little to no potential of mainstem river
flooding. The main concern will be flash flooding in any areas
which receive repeated and/or prolonged periods of heavy rain.
Again, that would be the mountains, primarily Friday night into
Saturday morning, with the preceding events priming/saturating
the soils.


Water levels remain near astronomical normals. While an increase
is anticipated through Friday, flow will remain southwest. As
such, flooding is not anticipated at this time. However,
Caution/Action stages likely will be reached by Friday.


MARINE...Small Craft Advisory from 10 AM Friday to 6 AM EDT Saturday
     for ANZ530-531-535-536-538>540.
     Small Craft Advisory from 2 AM Friday to 6 AM EDT Saturday for


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