Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181921

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
321 PM EDT Sun Mar 18 2018

High pressure will build overhead through tonight. Low pressure
will pass through the Tennessee Valley on Monday, with a coastal
low developing off the mid-Atlantic coast Tuesday. A third low
pressure system will follow the coastal low and pass our region
to the south Wednesday. High pressure will return for late next


Current temperatures are on track to reach our high forecasted
temperatures for the day. Currently temperatures are widespread
low to middle 50s. Dewpoint temperatures had to be brought down
5 to 7 degrees as there are widespread dewpoint temperatures in
the upper teens to low 20s. High pressure will continue to build
overhead through tonight and provide dry conditions to the region.
Plenty of sunshine the rest of the afternoon. Seasonably chilly
conditions with clear skies most of tonight before clouds increase
at the mid-levels late.


High pressure, providing dry and seasonable conditions, will
move from near the coast out to sea Monday to allow for a
developing coastal low to begin its formation Monday night.
Temperatures will start off near or slightly below freezing
Monday morning as clouds continue to increase, as well as
dewpoint temperatures. High temperatures Monday should reach 50
degrees or better once again across the region.

Through the period of Monday night through Tuesday night, a low
pressure system will reach the Appalachian Front and transfer
its energy to the East Coast to form the coastal low that is
expected to intensify as it moves to the northeast. Still, there
are variations in the timing of the formation of this coastal
low, the track of this low, and the precipitation type.

The NAM, GFS and European model do not agree on a lot of these
mentioned factors prior to 18z Tuesday. The NAM during the past
couple model runs has been flipping back and forth with
significant amounts of snow to a small amount of a mix of rain
and snow after some rain. The GFS has been consistent with snow
amounts across the region with an average of 2 to 4 inches of
snow after a period of rain. The latest European model should be
used with a little caution, considering it is showing most areas
receiving several inches of snow Tuesday into Tuesday night. A
couple of days ago, it was revealing the same scenario, but the
last model run or two, it backed off on the amounts. Lately, and
possibly to this minute, our forecast has been a blend of the
GFS and European model with a lean toward the GFS model.

Where the three models tend to agree is around the 18z time of
Tuesday. All three models have the newly-formed coastal low on
Tuesday about 50 miles to the east of Norfolk, Virginia with a
trailing trough of low pressure to the southwest then northwest
into southwest Virginia. At least with this position, we have a
potential of seeing several inches of snow, especially north and
west of the I-95 corridor.

As the coastal low passes by to our east then northeast later
Tuesday morning and Tuesday afternoon, colder air will get
drawn in from the north. The colder air will move into place
shortly ahead of the next shortwave that will be on the heels of
the first coastal low. A lull in the precipitation is most
likely later Tuesday in between the two lows.

Precipitation will fill in as the second coastal low develops
Tuesday night. Guidance still diverges on how strong the low
will be and consequently how much precipitation fills in. It
does appears that the best chance for precipitation will be
across the west and south...closer to the track of the low.
Thermal profiles will be colder due to northerly winds and this
will cause the p-type to be snow for most areas. Accumulating
snow is possible.


A secondary low pressure system will be near our region on
Wednesday morning and will transfer its energy off of the Mid-
Atlantic coast. This system will intensify offshore as
precipitation continues over our area for part of the day
Wednesday before the system moves away later that day. Cold air
aloft and near the surface supports a p-type of snow over the
region with model ensemble average QPF amounts between a tenth
to three tenths of a inch. A high pressure system builds over
our region Thursday into Friday bringing dry conditions over the
area before another low pressure system and its frontal
boundary affect us for next weekend.


VFR conditions through Monday evening for all terminals. MVFR
conditions will develop overnight Monday into early Tuesday.
There is a chance that IFR conditions could evolve as early as
daybreak Tuesday and continue through Tuesday night depending on
the coverage and intensity of the wintry mix or snow. Winds
becoming northeast then southeast Monday, then back around to
the northeast then north as the coastal low develops and moves
up the coast.

Sub-VFR conditions expected for some part of the day
on Wednesday with a low pressure system near our region by during
this time. Breezy conditions expected on Wednesday, some gusts up to
20 kts possible. VFR conditions expected on Thursday and Friday with
high pressure in control.


No marine hazards through Monday. Small craft advisory
conditions possible Monday as the coastal low starts to develop
along the coast. Gale warnings possible Tuesday into Tuesday
night, mainly for the central Chesapeake Bay. Afterwards, small
craft advisories are possible as the gusty winds diminish.

Wind gusts will be above small craft criteria on Wednesday as
low pressure system moves near our region... therefore a small
craft advisory is likely. Winds should remain below criteria on
Thursday and Friday.




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