Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 190759
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
359 AM EDT Sat May 19 2018

.SYNOPSIS...
A stationary front over central Virginia will slowly lift
northward today. A cold front approaching from the Ohio Valley
will cross the region from west to east Sunday, then stall to
the south early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Doppler radar shows a broad area of moderate rain extending
from western Maryland and eastern West Virginia Panhandle to the
I-95 corridor. Also, radar shows a thin band of heavier
downpours extending from northeast Virginia southwest into
south-central Virginia. All of this rain is moving from south-
southwest to north- northeast and is producing rainfall rates of
one quarter to one half inch per hour with isolated spots
receiving about one inch per hour. We received nearly two-
tenths of an inch in each of the past two hours here at the
National Weather Service office. A continuous flow of rain is
expected through daybreak before we start seeing trends of
heavier rainfall shifting eastward.

More rain is expected most of the day today with the heavier
stream of rain expected to shift toward the Chesapeake Bay and
eventually across to the Delmarva Peninsula. We could see a few
prolonged periods of breaks in the rain in the Shenandoah
Valley and parts of the Potomac Highland during the afternoon.

Rain showers and perhaps a rumble of thunder could revisit our
western zones from the Potomac Highlands to the Shenandoah
Valley tonight. Of course, additional rain amounts would just
exacerbate the flooding in parts of the region, but the
duration of any rain showers shouldn`t be nearly as long as the
past couple of days.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
A cold front will approach the region from the northwest Sunday.
A few showers and a thunderstorm should accompany the front in
the Potomac Highlands Sunday morning and midday, while the
chance pushes east and south into northeast Maryland and eastern
Virginia Sunday afternoon.

The front pushes into central Virginia Sunday night and stalls
across the state Sunday night and Monday.

Showers and afternoon and evening thunderstorms on Monday could
develop and move from west to east across our CWA. The highest
rain amounts should be generally along the eastern slopes of the
Blue Ridge Monday afternoon and Monday night. Temperatures on
Monday should reach the lower 80s.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
A chance of showers and thunderstorms will continue Tuesday with
a front across our southern zones. The forecast for Wednesday
into Thursday is very uncertain at this point. What is known is
that a stronger low pressure system will pass well to our north,
dragging a cold front through our area in its wake. The timing
of this front is highly variable at this time, so will keep
chance PoPs in the forecast for Wednesday, moving to slight
chance for Thursday. The key thing to note here is that, upon
passage, this front should bring in some much drier air behind
it. Temperature forecasts for this scenario will almost
certainly change, but for now, highs should be in the low 80`s
on Wednesday, followed by a slight cool down on Thursday in the
wake of the aforementioned cold front, with temps in the upper
70`s.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
IFR/LIFR conditions overnight before improving MVFR this
afternoon. VFR conditions expected Sunday into the first half
of next week, except for brief lower conditions in showers and
thunderstorms.

Mainly VFR conditions forecast Monday and Tuesday, with the only
threat of sub-VFR conditions being a passing shower over the
terminals. Winds will be light and favor a southerly trajectory.
A frontal boundary will drop southward through the terminals late
Monday, before stalling south of the terminals through mid week. A
moist easterly flow north of the aforementioned boundary will bring
the potential for sub VFR VIS/CIGs through mid week.

&&

.MARINE...
Small craft advisory conditions through Sunday for all waters
due to gusty southeast flow.

Generally sub-SCA conditions expected Monday through Tuesday, with
winds remaining light. A frontal boundary will cross the waters late
Monday which will help increase shower activity, resulting in the
potential for periodic gusty winds.

&&

.HYDROLOGY...
Steadier rain continues to pivot northward across central
Virginia. Rain has a little more organization than in previous
days due to better frontogenetical forcing and moist advection
around the 850 mb level. Easterly flow has also strengthened
which has increased humidity levels, so even with a lack of
deeper convection rainfall rates could easily exceed half an
inch to near one inch per hour (especially near/east of I-95).

Of note, St. Mary`s River near Great Mills reached its highest
stage since Sandy in 2012, and more rain is expected overnight,
causing river levels to rise further.

&&

.TIDES/COASTAL FLOODING...
Water levels are running around a foot to 1.5 ft above
astronomical predictions on the Bay and lower tidal Potomac. On
the upper tidal Potomac, however, those departures increase to 2
ft or more. For the upcoming tide cycle, that should equate to
minor flooding on both sides of the river. Coastal Flood
Advisories remain in place.

By today, it gets more complicated as there will be freshwater
input as well. This will affect Georgetown more than southwest
DC or Alexandria.
-For Alexandria: have extended the Advisory for another two tide
 cycles (late tonight). It could be necessary for a third
 cycle, but there is lesser confidence in the duration.
-For Washington DC: Georgetown will be the driver due to the
 freshwater contribution, where moderate flooding is forecast
 for three cycles (with levels not dropping below minor). At the
 southwest waterfront, best confidence for moderate flooding
 will be during the day today; subsequent cycles may only be
 minor flooding. Similar to Alexandria, have duration
 uncertainties. Therefore, have issued a Warning for today and
 tonight tide cycles, and left the Watch in place for Sunday.
Elsewhere
 on the Bay, minor flooding will be a concern this weekend
 during the times of high tides. Will be assessing upcoming
 guidance and could be making Advisory decisions overnight.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flood Watch through this evening for DCZ001.
     Coastal Flood Warning until 6 PM EDT Sunday for DCZ001.
MD...Flood Watch until 8 AM EDT this morning for MDZ501-502.
     Flood Watch through this evening for MDZ003>006-011-013-014-
     016>018-503>508.
     Coastal Flood Advisory until 9 AM EDT this morning for MDZ016.
     Coastal Flood Advisory until 8 AM EDT this morning for MDZ017.
VA...Flood Watch until 8 AM EDT this morning for VAZ503-504.
     Flood Watch through this evening for VAZ025>031-036>040-
     050>057-501-502-505>508.
     Coastal Flood Advisory until 9 AM EDT this morning for VAZ057.
     Coastal Flood Advisory until 8 AM EDT Sunday for VAZ054.
WV...Flood Watch until 8 AM EDT this morning for WVZ501>506.
     Flood Watch through this evening for WVZ050>053-055.
MARINE...Small Craft Advisory until 6 PM EDT Sunday for ANZ530>543.

&&

$$
SYNOPSIS...KLW
NEAR TERM...KLW
SHORT TERM...KLW/DHOF
LONG TERM...KJP
AVIATION...KLW/KJP
MARINE...KLW/KJP
HYDROLOGY...DHOF
TIDES/COASTAL FLOODING...HTS



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