Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 100836

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
336 AM EST Wed Jan 10 2018

High pressure overhead this morning will move offshore this
afternoon. A broad area of low pressure will approach the area
late Thursday into Friday, with a cold front expected to pass
through the region Saturday morning. High pressure of Canadian
origin builds across the region through early next week.


High pressure overhead has resulted in calm winds across our
area with temperatures down in the 20s for most, with lower 30s
being observed in the city centers. Some areas were able to
radiate quickly last evening with clear skies and calm winds.
High clouds continue to stream overhead from the west, which
will aid in keeping temperatures dropping much more than they
already have. Areas across southern MD and central VA has seen
some reduced visibilities due to fog, with low level moisture in
place, and these areas remain under clearer skies. Have not
seen the visibility restrictions observed like yesterday
morning, but patchy fog with some areas down to 1/4 of a mile
will be possible early this morning.

With warm air advection occurring aloft, and a weak undulation
passing by in the mid-levels, could see some light rain/drizzle
or freezing rain/drizzle is possible for locations west of the
Blue Ridge. Model guidance still depicting some very light
precipitation possible in these areas early this morning, and
with temperatures in the 20s, could be some icy spots.

High pressure slides east of our area by this afternoon and
resides off the coast through tonight. This will result in
continuing warm air advection with a light southerly flow.
Conditions will remain mostly dry through tonight, with a
possibility of fog/drizzle overnight. Temperatures today will
be a bit cooler than yesterday thanks to increasing clouds, but
still remain slightly above normal in the low to mid 40s. Expect
above normal temperatures tonight in the low to mid 30s.


A broad area of low pressure will be responsible for increasing
rain chances for the second half of Thursday across the region
as it approaches from the south and an upper level trough tracks
toward our area from the west. Locations in the southern third
of the CWA could see some rain late Thursday morning, but
generally it will be an afternoon and overnight event for most
of area.

We could some break in the rain Friday morning as the first wave
moves off to our northeast, and a low pressure area and
associated cold front rides along or just west of the
Appalachians Friday afternoon and Friday night. Ample moisture
will remain over the region with continued warm air advection
and precipitable waters values ranging between 1 to 1.5 inches.
The heaviest rain will occur Friday night as the low pressure
crosses our area. While this rain is very much needed with the
dry conditions that persisted so far this winter, with the cold
ground underneath us, this could cause some drainage issues (See
Hydrology section). However, with high temperatures in the 50s
on Thursday and around 60 on Friday, this could help fight that
battle. Something we will continue to monitor as this rain event


By the start of the day Saturday, the cold front would have passed
through the forecast area, and strong cold advection will be
ensuing. This might be a case of a midnight or early day highs, with
temperatures falling/leveling off during daylight hours. However,
won`t go into that level of depth in the database just yet. Any
showers would be the lingering beginning of the day variety. Will
hold a chance in the forecast. An exception would be for upslope
areas, where scattered snow showers should last most of the day.

For Saturday night through Monday night, building high pressure will
keep cool and dry conditions in place. Temperatures at 850 mb will
be in familiar territory: dropping below freezing. However, this
time a reasonable average would be near -10C, so temperatures should
be below normal but not drastic...highs upper 20s-lower 30s, lows
teens-lower 20s. Am not certain how sunny it will be either since
the jet will be overhead.

It could become a bit unsettled by Tuesday as a northern stream
trough axis swings through the northeast/Mid Atlantic. There would
be only limited temperature modification before this disturbance
arrives, so precipitation type would need to be determined, along
with event timing.


MVFR or lower VIS possible early this morning at IAD/DCA/BWI
with the development of patchy fog. High pressure overhead and
calm winds, along with just enough low level moisture, it is a
possibility early this morning. As of 08z we have not seen any
reductions at the terminals, likely being fought off by high
clouds moving overhead but we will continue to monitor

High pressure will shift offshore today through tonight,
resulting in a light southerly flow across the area and dry
conditions. Warm air advection will promote low to mid level
clouds, with bases AOA 040-050, but conditions expected to
remain VFR and dry. Rain will overspread the area Thursday
afternoon into Friday, with another wave Friday night. This will
result in periodic MVFR/IFR conditions.

If there were any flight restrictions this weekend, it would be at
the start of the day Saturday. VFR likely thereafter. Gusty winds
Saturday may be the only operational consideration.


High pressure will migrate east of the area through Thursday
resulting in light southerly winds across our waters, thus no
headlines expected. This flow is expected to increase Thursday
night, bringing the possibility of SCA conditions across
portions of our waters through Friday night. How quickly will
winds reach this threshold Thursday night is in question as
warmer air above the cooler water, mixing may be an issue, even
into Friday/Friday night as a low pressure area approaches from
the west.

A cold frontal passage will lead to a period of favorable momentum
transfer Saturday into Saturday evening. Small Craft Advisory
conditions likely in gusts 20-25 kt. Winds will gradually diminish
late Saturday night into Sunday.


Warming conditions are expected through the end of the week, along
with periods of rain Thursday night through Friday night. While dry
antecedent conditions should preclude any widespread hydrological
concerns, ice break up due to increasing warmth and rain may create
localized ice jam concerns. Confidence is currently too low and
localized to include in the HWO at the moment, but we will
continue to monitor. Around one to one and a half inches of
rainfall is anticipated during this period, subject to change
based on finer scale details yet to be ironed out.


The recent brutal cold stretch has finally snapped. The cold peaked
during the first week of January, making it one of the coldest on
record for our area (and much of the eastern United States). Below
is a list of record coldest first weeks of January (1st through 7th).

Washington DC area (DCA)
1. 16.8 degrees (1918)
   16.8 degrees (1879)
3. 18.4 degrees (1877)
4. 19.0 degrees (2018)
5. 21.1 degrees (1877)
Temperature records date back to 1872

Baltimore MD area (BWI)
1. 15.2 degrees (2018)
2. 18.4 degrees (1918)
Temperature records date back to 1872

Dulles VA area (IAD)
1. 15.3 degrees (2018)
2. 19.5 degrees (1968)
Temperature records date back to 1960

Meteorological winter (which began December 1st) has also been very
dry so far. Below is a list of the top 5 driest meteorological
winters (December 1st through February 28th/29th) on record.

Washington DC area (DCA)
1. 2.60 inches (1871-72)
2. 3.32 inches (2001-02)
3. 3.85 inches (1980-81)
4. 4.15 inches (1976-77)
5. 4.76 inches (1873-74)

Notes: The winter of 1870-71 had 4.16 inches, but 31 days of data
are missing (no data for December 1870). So far, Winter 2017-18 has
had 0.66 inches through January 8th. Precipitation records date back
to 1871.

Baltimore MD area (BWI)
1. 4.03 inches (1976-77)
2. 4.12 inches (1980-81)
3. 4.28 inches (2001-02)
4. 4.30 inches (1871-72)
5. 4.51 inches (1979-80)

Notes: The winter of 1870-71 had 2.93 inches, but 31 days of data
are missing (no data for December 1870). So far, Winter 2017-18 has
had 1.07 inches through January 8th. Precipitation records date back
to 1871.

Dulles VA area (IAD)
1. 3.24 inches (2001-02)
2. 3.37 inches (1976-77)
3. 4.83 inches (1979-80)
4. 5.18 inches (1980-81)
5. 5.55 inches (2010-11)

Notes: The winter of 1962-63 had 0.00 inches, but 38 days of data
are missing. The winter of 1961-62 had 5.34 inches, but 34 days of
data are missing. So far, Winter 2017-18 has had 0.80 inches through
January 8th. Precipitation records date back to 1960.




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