Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 101936

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

High pressure will continue shifting eastward into the Atlantic
tonight into Thursday. 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.


The center of the surface high has shifted offshore of the
northeastern US, but an elongated ridge axis remains entrenched over
the region and will remain in place through tonight and into
Thursday morning. This will provide for mainly dry weather and light

Stratus deck continues to hold firmly in place this afternoon from
central and eastern Maryland southwestward into central Virginia.
Breaks of sun have developed north/west of there. Some continual
erosion of this deck is expected through the remainder of the
afternoon into the evening.

Overnight, some clearing may take place initially, but a reformation
and expansion of the stratus deck is likely, especially towards
morning as warm air advection continues aloft atop a cooler wedge of
air near the surface. Some very light rain or drizzle is also
possible towards morning in the southern Shenandoah Valley, along
the Blue Ridge, and into Highland County. In addition, model
guidance is insistent on bringing in fog to portions of the region,
and while widespread dense fog is currently not expected due to the
forecast stratus, areas of light fog are certainly possible with the
strong low level inversion. Lows tonight will be generally in the


The surface ridging will shift further east away from the region
during the day Thursday giving way to southerly flow and increasing
moisture ahead of a system digging through the central/southern
Plains states. Expecting an overcast day across the region with some
morning fog and possibly areas of light rain/drizzle southwest
giving way to increasing chances of more steady rain starting during
the early to mid afternoon across central/western VA and spreading
northeastward towards the metro areas towards late afternoon and
evening. This rain will then overspread the entire region Thursday
night. A rumble of thunder is also possible, but coverage/confidence
too low for inclusion at this time. Highs should reach the upper 40s
to low 50s on Thursday and likely remain steady or potentially even
rise Thursday night.

The first wave of rain will taper off Friday morning, giving way to
a break in the precipitation Friday afternoon. This combined with
increasing southerly flow should allow for temperatures to reach the
60s for most locations.

The low pressure system will move northeastward near the
Appalachians Friday/Friday night and into southern New England
by Saturday morning. This will drag a cold front across the
region Friday night and early Saturday morning. An additional
period of rain is expected, along with the potential for another
rumble of thunder or even a convective fine line with some gusty
winds. As the front passes, rain may change over to some wintry
precip across the higher elevations of the Potomac Highlands by
Saturday morning. Total areal- average liquid precipitation
amounts for the entirety of the storm from Thursday through
Saturday morning are currently projected in the 1-1.5" range.
While dry antecedent conditions should preclude any widespread
hydrological concerns, ice break up due to increasing warmth and
rain may create localized ice jam issues. Cold ground reducing
absorption may also lead to some ponding.


A cold front will be over or just east of our CWA Saturday
morning. Some showers possible for part of the day as the front
moves away with upslope snow showers lingering maybe a little
longer. The upper level pattern behind this system will have an
upper trough over the eastern CONUS that will remain through at
least Tuesday.

The early cold front passage on Saturday will allow for the high
temperatures to be reached sometime between midnight and early
Saturday. Temperatures will be falling through the day due to strong
cold air advection behind the front. High pressure builds in on
Saturday night and remains into Monday, bringing dry and cooler
conditions to our area. Temperatures will be below normal but not as
drastic as recent weeks.

The upper level trough will be pushing east Monday night into
Wednesday. Guidance is in disagreement as to what -if any-
precipitation will approach the area Tuesday into Wednesday either
associated to a cold front or upper level disturbance.


VFR is expected for the remainder of the day and evening with a
stratus deck in place from roughly the Blue Ridge mountains
eastward. Ceilings are generally in the FL040-FL060 range and should
remain at or above these levels through at least midnight.
Overnight, stratus deck is likely to reform and expand in coverage,
in addition to lowering in height. Development of MVFR and
potentially IFR ceilings will be the main aviation concern late
tonight and into Thursday. Some fog is also possible, especially
at MTN/BWI. MRB is least at risk for any fog or low ceilings.

Widespread rain is then expected to overspread the area late
Thursday afternoon/evening and last into Friday morning. This will
bring MVFR/IFR conditions in low ceilings and visibilities. In
addition, low level southerly winds will be increasing and low level
wind shear will become an issue.

Some improvement in visibilities/ceilings are possible Friday
afternoon before another wave of rain moves in Friday night.

Sub-VFR conditions possible early on Saturday as a
front moves away. Conditions should improve to VFR later on Saturday
and remain into Monday.


Sub-SCA winds are expected through at least Thursday with high
pressure overhead. Southerly flow will then increase through the day
Thursday night, however mixing will be very limited as warmer air
aloft moves over the cooler waters. SCA will be possible Thursday
night, but for now kept below criteria as confidence not high enough
for issuance. SCA conditions also possible Friday and Friday night.

Breezy conditions expected Saturday into part of
the day on Sunday that could reach advisory criteria. Winds should
diminish gradually through the day Sunday and remain below criteria
into Monday.


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 issues. Confidence is currently too low and
threat too isolated 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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