Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 182212

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
612 PM EDT WED MAY 18 2016

High pressure over the upper Great Lakes will slowly build over
the region Friday. Low pressure will approach the area Friday
night from the Tennessee Valley and track to our south Saturday.
High pressure will build overhead early next week...but an upper-
level low will remain close by just off the Mid-Atlantic coast.


Upper-level low pressure over the western Great Lakes will remain
nearly stationary this evening. Low pressure is reflecting at the
lower levels of the atmosphere near the Appalachian Mountains and
this will also remain nearly stationary during this time. A northeast
flow will continue to usher in unseasonably cool conditions for
this time of year. Plenty of clouds are also expected due to
moisture being trapped underneath the subsidence inversion...
thanks to the low-level easterly flow.

Periods of rain will continue across the Allegheny and Potomac
Highlands as well as portions of the Shenandoah Valley...eastern
West Virginia and western Maryland this evening...closer to the
upper- level low. However...the rest of the area will remain far
enough away from this system to be dry...but a few sprinkles
cannot be ruled out due to the moisture trapped underneath the
subsidence inversion.

The upper-level low will shift to the east tonight...and the 850mb
low associated with this system will also shift east from the
Appalachian Mountains into our area. However...this system will
weaken as the upper-level support shifts into New
England...outrunning this system. The eastward shift will cause
light rain to move east through the Shenandoah Valley toward the
Blue Ridge Mountains this evening and overnight. Light rain may
even push toward the Washington and Baltimore Metropolitan areas
into the Virginia Piedmont tonight...but it will have a tough
time since the low will be weakening. Points farther east should
remain mainly dry with just a sprinkle or two. Rainfall tonight
will be light...under one tenth of an inch with the best chances
near and west of the Blue Ridge Mountains...closer to the
dissipating low. Min temps will range from the 40s west of the
Blue Ridge Mountains to the lower and middle 50s near Washington
and Baltimore.

The upper-level low will remain over northern New England Thursday
and this will carve out an upper-level trough over our region.
Surface high pressure will build slowly build in from the north
and west...allowing for drier air to work its way into the area. will be slow to do so so clouds may hang around
through midday before giving way to some sunshine. A couple
showers cannot be ruled out...especially near the higher terrain
but coverage will be isolated so most areas will be dry. Highs
will be a bit warmer due to some afternoon sunshine...but still
below climo. Highs will range from the 50s in the mountains to the
upper 60s/near 70 around Washington and Baltimore.


High pressure will continue to build over the area for Thursday
night into Friday...bringing dry conditions. Friday will likely be
the best day of the week with some sunshine and max temps ranging
from the 60s in the mountains to the lower and middle 70s for most
other locations. High clouds will slowly increase throughout the
day ahead of the next system.

Low pressure will track into the Tennessee Valley Friday night and
it will begin to transfer its energy to a coastal low near the
Carolina coast. Warm and moist air ahead of this system will
overrun the cooler air in place...causing clouds to quickly lower
and thicken and rain to overspread the area from southwest to
northeast. The rain may be locally heavy overnight due to copious
amounts of moisture associated with this system along with
significant forcing from an increasing low-level wind field and
forcing from the upper-level system.


Low pressure will move off the Carolina coast and out to sea
Saturday. Rain will slowly come to an end from west to east as the
day progresses, but the upper-level trough driving the surface low
will linger over the area through the weekend. There is still a lot
of spread in the ensemble guidance regarding exact rainfall amounts.
This is due to the convective nature of the forcing driving the
system which is not easily resolved at this time range. The
placement of heavier bands of precipitation and exact amounts will
likely remain in question until about 48 hours out, but a general 1
to 2 inches looks likely given the synoptic setup. Of note, guidance
has trended a bit slower with the whole progression (which again
makes sense given the blocky nature of the pattern) which means much
of Saturday may end up being a washout.

Sunday will be the better of the two weekend days, but intermittent
lighter rain and drizzle will remain possible as the upper-level
low cuts off and lingers over the area.

Showers may pop up during the afternoon hours Monday and possibly
Tuesday as the upper low spirals slowly off to the east. A second
area of surface low pressure may try to spin up off the east coast
early next week but details are very fuzzy this far out especially
given the erratic nature of the cutoff upper low that would be
forcing it.

High pressure should begin establish itself off the southeast
coast during the middle to latter portions of next week leading
to a general warming trend.


VFR conditions this evening for most of the terminals as some
drier air near the surface moves in on a northeast flow. Low
pressure will weaken as it moves toward the area tonight. This may
bring some light rain...especially to KMRB and KCHO. Light rain
may approach the other terminals as well. Cigs will lower a bit
toward MVFR levels. Will keep cigs around 3kft for most of the
terminals...but went with MVFR conditions for KCHO. Cigs will
gradually lift Thursday as high pressure builds overhead.

High pressure will build overhead Thursday night through Friday.
Areas of fog are possible Thursday night into early Friday. Low
pressure will approach the terminals Friday night. Rain will
overspread the area along with lowering cigs/vsbys.

Sub-VFR conditions are expected Saturday with low
ceilings and visibilities in rain. Sub-VFR conditions will remain
possible into Sunday with low clouds and drizzle or intermittent
light rain lingering across the area. Winds will be easterly 10 to
15 knots Saturday...becoming northerly 10 to 15 knots Sunday.


High pressure will gradually build toward the waters tonight
through Friday. Winds should remain below SCA criteria during
this time. Low pressure will approach the waters Friday
night...bringing rain to the waters late. Winds will strengthen a
bit due to a strengthening gradient. A small craft advisory may be
needed for portions of the waters overnight Friday.

A tightening pressure gradient between high pressure
departing to the northeast and low pressure passing to the south
will lead to easterly winds gusting well into Small Craft Advisory
criteria on Saturday (20 to 30 knots). Winds will gradually diminish
Sunday but likely gust in excess of 20 knots for at least part of
the day as winds pivot to the north and channel down the Chesapeake




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