Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170106

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
906 PM EDT Fri Mar 16 2018

High pressure will build across the region tonight. A weak low
pressure will pass just south of the region on Saturday. High
pressure will build back into the region for Sunday and Monday.
Another complex low pressure system may impact the region
Tuesday and Wednesday.


Tonight will be the quietest of the three forecast periods. High
pressure is nudging into the area from the northwest, and winds
have started their downward trend with sunset. While cirrus is
moving in from the southwest, it should remain relatively thin
until later in the night. With a fresh batch of Canadian air in
place, lows will once again be below freezing across the
region, but with diminishing winds, wind chills will be less of
an issue. Will explore lowering temperatures in some of the more
sheltered locations. Dew points remain in the teens, so a dip
of temperatures down into that range is possible. Temperatures
may climb a bit toward dawn, especially on the ridges, as the
atmosphere moistens and clouds increase.


For the evening update, have not seen enough in the latest
guidance to make wholesale changes for the Saturday event.
There are still a range of solutions regarding precipitation
placement and type, although the greatest chance for notable
accumulations of snow or ice remains somewhere near the US 48
corridor in the Potomac Highlands, where the coldest air
overlaps the highest QPF potential.

Previous discussion:

A fresh dose of Canadian air with some radiational cooling to
boot posts a threat to the region Saturday as a weak low
pressure crosses just south of our region. Latest guidance has
precip moving in after sunrise, but before noon, for portions of
the region west of the Blue Ridge, and with clouds moving in,
temperatures may remain at or below freezing as precipitation
starts. This poses a risk of freezing rain if thermal profiles
are conducive, so have issued a winter weather advisory for the
zones most at risk. While the main freezing rain threat would
likely be in the morning, higher elevations could see the threat
persist through the day, and freezing rain is not the only

Across the region, the soundings progged for Saturday are quite
cold. While surface temps are likely to warm above freezing
region-wide as we head through Saturday, with temps so cold
aloft, rain may not be the only precipitation type, and
soundings in fact suggest sleet and snow may occur across much
of the region at times, especially near and north of the
I-66/US 50 corridor. Guidance also suggests some sort of fgen
band may set up over this region, though there is by no means
unanimous agreement with this. Given the great uncertainty, have
allowed a small accumulation of snow/sleet in much of the
region, and have mixed p-types for much of the day. Its possible
that for areas such as the Washington DC metro, the precip could
start as rain, change to sleet and snow if intensity gets high
enough, then go back to light rain or drizzle as it ends. This
could end up reminiscent of the February 17th snow band (also a
Saturday) when a burst of heavy wet snow briefly coated many
areas roads before tapering off and changing back to rain.
However, other models show much less intensity of precip, so its
also possible that mainly rain falls and there is little to no
accumulation. Given this, will not yet highlight anything in the
HWO east of the Blue Ridge.

Latest guidance has the system being suppressed mainly southwest
of the Baltimore area thanks to stronger confluence aloft north
and east of the region, so odds favor precip failing to reach
northeast Maryland or even Baltimore proper. Elsewhere, the
precip and clouds are likely to keep temps chilly Saturday, with
high 30s to low 40s expected for highs.

As the system moves away Saturday night, precip should end and
clouds should break as high pressure returns. After another cold
night, Sunday should see warmer temperatures with more sunshine
as high pressure builds overhead. Highs will be mostly in the
50s. Despite the warmer day, another night of radiational
cooling should put temps back below freezing in most areas
Sunday night.


Monday starts dry with high pressure in control. During this
time an occluded low pressure system will be moving east over
the Tennessee Valley towards the Appalachians with precipitation
associated to it starting as early as Monday night. The low
pressure will transfer its energy off of the Mid-Atlantic coast
Tuesday night into Wednesday and then slowly move NE away from
us. Guidance is still in some disagreement varying between snow
and rain, but suggesting precipitation to last from Monday night
into Wednesday night. The track and speed of this system play
an important role on the local impacts we might see. We will
continue to monitor the forecast, but confidence is increasing
of a coastal storm impacting our area next week.


VFR conditions are expected tonight as northwest winds continue
to slacken and become southwesterly late. Sub-VFR possible at
all terminals Saturday (mainly late morning onward) as a system
with some rain and perhaps some sleet and snow moves across the
terminals. The overall probability of IFR is low, and the amount
of impact otherwise could depend on the intensity of the
precipitation. Precip may stay just south of MRB and BWI, which
would significant impacts at those terminals. However, it would
be more likely that these locations see visibility reductions in
snow should precipitation make it that far north. VFR should
return behind the system later Saturday night and persist
through Sunday night.

Expecting mainly VFR conditions Monday with high pressure in
control. Sub-VFR conditions expected Monday night into Wednesday
with coastal storm possibly moving near by during this time.
Breezy conditions on Tuesday and Wednesday, some gusts up to 20
kts possible.


Northwest winds have started to subside with sunset, now below
25 kt. Some areas will likely be dropped from the SCA early,
while it`s possible some areas on the open bay could hold on to
SCA conditions a few hours after midnight. We may be near SCA
gusts in the northern bay on Saturday, but confidence was not
quite there for an SCA. If precipitation stays south of
Baltimore, a few 20 kt gusts are more plausible. Winds should
remain relatively light Sunday under high pressure.

Wind gusts will be below the Small Craft Advisory criteria on
Monday. Winds will increase Monday night and remain above
criteria into Wednesday, which means that small craft advisory
is likely these days.


Winds will continue to relax and humidity will start to
increase tonight and especially during the day on Saturday as a
system with rain and snow moves into parts of the region. Heavy
precip is possible Tuesday and Wednesday which would moisten
fuels significantly as well.


MD...Winter Weather Advisory from 8 AM to 5 PM EDT Saturday for
VA...Winter Weather Advisory from 8 AM to 5 PM EDT Saturday for
WV...Winter Weather Advisory from 8 AM to 5 PM EDT Saturday for
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>543.


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