Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 120249
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
949 PM EST Thu Jan 11 2018

.SYNOPSIS...
High pressure will continue drifting east of the area tonight as
low pressure develops in the Tennessee Valley. This low will
pass near the area by Friday night, followed by a cold front
sweeping eastward early Saturday morning. High pressure of
Canadian origin will build across the region through early next
week.

&&

.NEAR TERM /THROUGH FRIDAY/...
High pressure continues to drift east of the area this
evening. The leading edge of light rain/low clouds is now
working across the CWA.

A 40-50 kt low level jet will result in strong theta-e
advection, which should result in steady or rising temperatures
through the night. The isentropic lift will also bring
occasional rain. Precipitable water values potentially
exceeding 1.5 inches (record territory for this time of year)
may result in locally moderate to heavy rain elements. If an
isolated rumble of thunder is possible, it`d likely be over
southern MD...but still think the chance was too low to include
in the forecast. Since dewpoints will be rising in concert with
the temperatures, foggy/soupy conditions are likely to slowly
develop, especially as rain finally falls across the region and
moistens the low levels fully. Mixing will be poor, limiting
surface wind gusts, but it has become breezy in some locations.

This first piece of energy will be lifting to the northeast
Friday morning. There may be a lingering band of rain near the
Bay before it moves off to the east. There will still be copious
moisture in deep layer southerly flow, so some showers can`t be
ruled out at any point of the day, but they will be lighter and
some dry breaks are likely. This should allow a little mixing
to occur, which will allow temperatures to rise into the mid 60s
in many locations. Have stayed on the conservative end of
guidance though since low level clouds look to stay locked in.
That said, if there is any sun at all, there will be a shot at
70.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
Guidance has continued to trend eastward with the track of low
pressure crossing the area...now directly crossing the CWA.
Shower coverage will increase during this time. A convective
line may develop ahead of the associated cold front. There will
be plenty of wind energy and perhaps a little instability.
Forecast has not changed substantially here, with a slight
chance of thunder. If the line is well-developed, there could be
strong winds associated with it. Despite a resurge in
precipitable water ahead of the front, heavy showers should be
transient. Additional hydrology thoughts are below.

An anafrontal structure will be present with this system,
resulting in crashing surface temperatures (20+ degrees in a
matter of hours) while warmer air remains aloft. The NAM and GFS
are rather quick with exiting most of the precipitation before
colder surface air arrives, while the ECMWF seems to have strong
frontogenetical forcing aloft, leading to precipitation hanging
on longer, especially in the mountains. Have continued a
conservative forecast approach for now, with an inch or two of
snow in the mountains and a chance of mix Saturday morning to
the east before ending. It will likely be windy behind the
front too...will have to keep an eye on how strong. Probably not
wind advisory, but might get close.

Some clouds may linger into Saturday afternoon, but even in the
mountains, any precipitation should be quickly ending.
Temperatures will be holding steady or falling further, which
will be below freezing in the west and in the 30s to the east.
By Saturday night, lows will be falling into the teens to lower
20s with wind chills in the single digits.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
High pressure will keep cool and dry conditions Sunday into
Monday as it tracks northeast away from our region. High
temperatures Sunday and Monday will be in the 20s and 30s...
teens at higher elevations. Lows in the teens and single
digits... some 20s Monday night. Upper level troughing will
settle over the eastern CONUS during this time.

Guidance suggests that a clipper system will be approaching our
area Monday night into Tuesday. How much moisture and how fast
it moves over our region is still the question. The GFS seems
drier and faster than the Euro... which also develops a coastal
low that eventually moves NNE away from us. Therefore the
potential for snow over our area is increasing for early next
week. Upslope snow looks more likely Monday night into Thursday.
For the rest of our CWA there is still uncertainty.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
Low clouds and light rain continue spreading north this evening.
IFR still looks likely later tonight for both cig and vsby as
moisture increases, with a period of moderate rain possible. In
addition, a low-level jet of 40-50 kt will lead to widespread
wind shear through tonight.

There should be some breaks in the showers Friday, but can`t
totally rule out rain at any time. Conditions could return to
MVFR later in the day, but there is a potential that it won`t,
especially near Baltimore. Then, a cold front crosses late
Friday night. Another round of poor flight conditions in heavy
rain (maybe even a rumble of thunder?) and gusty winds will
result.

Flow will be northwest before sunrise Saturday morning, and
will remain so through the day. Ceilings will lift, but gusty
winds (30 kt or so) will persist through the day.

VFR conditions expected between Sunday and Monday with high
pressure over our area. Sub-VFR conditions possible Monday night
into Wednesday with possible snow over our area.

&&

.MARINE...
Winds will increase over the waters as we head through the
night. Even though cold water will result in poor mixing, the
magnitude of wind available just off the deck suggests that
Small Craft Advisory criteria will still be reached. Expanded
SCA a bit given the strong LLJ just off the deck. Think the
current extent likely covers it, but might end up having to add
the Potomac at some point.

As the low level jet departs Friday, there should be a bit of a
lull Friday afternoon (at least below threshold), before winds
increase again Friday night ahead of a cold front. SCA has been
confined to the wider parts of the Bay Friday evening, expanding
to all waters later at night as the front crosses. Gales are
certainly possible behind the front into Saturday morning, but
have held off for now due to marginal nature and timing
uncertainty. SCA likely continue into Saturday night.

Winds will be below small craft criteria Sunday into Tuesday.
Therefore not expecting advisories during these days.

&&

.HYDROLOGY...
Warming conditions are expected through the end of the week,
along with periods of rain tonight through Friday night. Break
up of ice may cause localized issues, but 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. While precipitable water is well
above climatology, suggesting that flooding is possible,
relatively quick movement of individual elements and dry
antecedent conditions should preclude a widespread hydrologic
issue. And even though soils are warming, absorption will still
be limited, thus substantial ponding is possible, but not enough
issues are expected to need a Flood Watch at this time.

&&

.CLIMATE...
Meteorological winter (which began December 1st) has 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.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until noon EST Friday for ANZ530>533-
     537>542.
     Small Craft Advisory from 6 PM Friday to 6 AM EST Saturday for
     ANZ531>534-537-539>541-543.
     Small Craft Advisory from midnight Friday night to 6 AM EST
     Saturday for ANZ530-535-536-538-542.
     Small Craft Advisory until noon EST Friday for ANZ534-543.

&&

$$
SYNOPSIS...ADS
NEAR TERM...ADS/RCM
SHORT TERM...ADS/RCM
LONG TERM...IMR
AVIATION...ADS/IMR/RCM
MARINE...ADS/IMR/RCM
HYDROLOGY...ADS
CLIMATE...DHOF



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