Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 221557 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
1157 AM EDT Fri Jun 22 2018

A warm front will slowly return north through Saturday. Low
pressure will move from the Ohio Valley to the eastern Great Lakes
during this time, then eastward across New England Sunday bringing
a cold front across the region. High pressure will then build in
from the eastern Great Lakes and remain in control through the
middle of next week.


A Flash Flood Watch has been issued for been issued for late
this afternoon through tonight for portions of central and
northern Virginia, and southern portions of eastern West
Virginia. Showers and thunderstorms are expected to develop
later this afternoon, bringing additional heavy rains and the
potential for flooding across these areas.

Previous Discussion...

A surface boundary bisects the southern half of Virginia this
morning with a moist east northeast flow from the Atlantic
across our CWA. With the ample cloud cover and ongoing rain
along and east of the Blue Ridge this morning, temperatures are
holding in the lower 60s where rain has been more prevalent to
near 70 degrees to the north and east. Speaking of rain, we
have seen a flood warnings abound this morning across portions
of the Shenandoah Valley, and central and northern Virginia,
where several inches of rainfall has resulted in flooded
roadways and some rivers/streams exceeding their banks. Latest
meso analysis indicates a ridge of 2+ inch precipitable water
values extending southeast to northwest across this area,
coupled with dewpoints in the mid-upper 60s east of the Blue
Ridge, ample moisture is prevalent across the region.

Continued thinking in this area of rain will continue to weaken
this morning and towards midday as it enters a more stable
airmass. The aforementioned boundary will lift northward today,
with the area to the south of it encountering sufficient
instability this afternoon with the expectation of additional
showers to develop. As with previous forecast thoughts, these
showers and thunderstorms will trend a bit more progressive, but
combined with the rainfall already received over the past 24
hours, there is concern with an additional flooding threat
through the overnight hours. As such a Flash Flood Watch has
been issued late this afternoon through tonight for portions of
central and northern Virginia, and southern portions of eastern
West Virginia.


By Saturday morning, the front will be draped near the Potomac
River, with mid-level low pressure gradually filling over the
Great Lakes. As the front lifts north, warm and unstable air
will be able to overspread the entire forecast area. Meanwhile,
a belt of shear will overspread from the mid-level low and
associated vorticity. Believe that combination will result in at
least scattered thunderstorm development...some of which could
be strong, primarily due to winds. A trough axis will cross by
evening, which should provide some stabilization.

The stalled surface boundary and a warm, humid air mass will
remain in place Sunday. One differing factor will be westerly
flow aloft. However there will be sufficient support for more
scattered showers and thunderstorms of at least a diurnal
nature. A second point will be that the stalled boundary likely
be become a cold front by afternoon and evening. Any transient
shortwaves could enhance the thunderstorm activity, as would the
cold frontal passage.


Canadian high pressure will build overhead for Monday and
Tuesday, causing drier and cooler conditions. The high will move
offshore during the middle portion of the week, causing hot and
humid conditions to return.


Mostly VFR conditions this morning at all terminals, with the
exception of CHO where MVFR to IFR CIGs have prevailed. Shower
activity will move northward today, with conditions slowly
deteriorating, but will maintain mainly MVFR conditions today
with the possibility of IFR conditions as well.

Once conditions drop, will be maintaining restrictions through
tonight. There likely will be a break with brief improvement,
but lack confidence in timing such a limited lull.

Additional rounds of showers and thunderstorms expected this
weekend. May have periods of VFR in between, but flight
restrictions will be pervasive.

A cold front will cross the terminals Sunday night. VFR will
follow for early next week.


A boundary south of the waters will slowly lift north through
Saturday. East flow in advance of the front supports the Small
Craft Advisory today into this evening.

Winds should subside a bit as the gradient weakens late tonight
into Saturday. However, scattered thunderstorms Saturday into
Saturday night will likely produce local gusty winds.

A cold front will approach the waters Sunday before passing
through Sunday night into Monday. A Small Craft Advisory may be
needed for portions of the waters both ahead of and behind the
cold front through Monday night. High pressure will build
overhead Tuesday.


Water levels running between a half foot and one foot above
astronomical normals today. With east winds today that should
be maintained a minimum, if not rise slightly. Minor inundation
may result at the most sensitive locations, especially for the
PM tide cycle, which is the higher astronomically. Action stage
will be more widespread.

A similar situation will likely continue into Saturday.
Although winds will be light south, the excess water will be
unable to drain, and spotty minor inundation will once again be
a possibility.

Winds will become west Sunday and northwest Monday.


VA...Flash Flood Watch from 4 PM EDT this afternoon through late
     tonight for VAZ025-026-036-037-503-504-508.
     Flash Flood Watch from 6 PM EDT this evening through late
     tonight for VAZ029-038>040-050-051-055>057-502-507.
WV...Flash Flood Watch from 4 PM EDT this afternoon through late
     tonight for WVZ505-506.
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>543.


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