Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 150233

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
933 PM EST Fri Dec 14 2018

A large area of low pressure will slowly move over the Mid-
Atlantic this weekend, then move offshore Monday as a cold front
dives south from the Great Lakes. High pressure will follow
through the middle part of next week.


Forecast adjustments through Saturday mainly consist of minor
tweaks to temperatures and PoPs. Initial arm of isentropic lift
has pushed north through the area, with pockets of drying
working north from southern Virginia. Some reports of around
two thirds of an inch or rain have come in from central
Virginia. Combined with the melting snow, this would likely be
the first area to experience any flooding, with perhaps isolated
issues as soon as late tonight, depending on the coverage of
additional rain. Although not explicitly forecast by guidance,
may need to keep an eye on dense fog potential in central
Virginia, especially as higher dew points advect over the snow
pack late tonight.

Previous discussion:

Cutoff low pressure remains adrift over the lower Mississippi
River Valley. Deep southerly return flow is advecting in
plentiful moisture from the Gulf of Mexico and Atlantic Ocean.
This low-level warm/moist advection will strengthen overnight
through midday Saturday as southeasterly 850 hPa flow increases
to 40 kts on the north side of a warm front, coincident with
increasing mid-level PVA and right entrance upper jet dynamics.
This will result in strong synoptic lift, resulting in
widespread moderate rain with a few heavier elements possible.
Of note, the 12Z ECMWF keeps the heaviest precipitation to our
south (south of I-64) tonight into Saturday, likely due to
convection "robbing" moisture off the Carolina coast and a
slightly slower/further south evolution of frontal dynamics.
While this is possible, believe the strong onshore low- level
flow will result in strong enough moisture advection to
compensate, with QPF more in line with the rest of the 12Z model
suite. The ECMWF does end up with the same storm total QPF
amounts (through Sunday) but it just brings it in later.

As a mid level dry slot pivots over the area mid to late
Saturday morning into Saturday afternoon, there will likely be a
lull in the most widespread steadier rain, though light rain and
drizzle will likely continue. Lapse rates increase, however, as
the upper level low begins to move overhead by Saturday night,
reinvigorating steadier rain especially north of US-50 in tandem
with best frontogenesis on the northeast side of the upper low.


Although low-level frontogenesis weakens notably by Saturday
evening, mid- level frontogenesis in the 700-400 hPa layer
increases on the north/northeast side of the upper low. With
continued moisture advection beneath this (at least in the
evening), a second area of steady rain with possible heavier
downpours seems likely, especially given the potential for
embedded convective elements (mid-level lapse rates 7-8 C/Km).

The latest 12Z EPS data offers pretty convincing evidence that
this second round will be heavy and steady enough to pose
potential flooding issues. Also, the majority of any flooding is
likely to be delayed until after daybreak across central
Virginia to southern Maryland, and around midday Saturday across
much of the rest of the area. As such, have adjusted the Flood
Watch start/end times to begin Saturday (6 AM south, noon north)
and end 6 AM Sunday.

Another note for Sunday is cooling temperatures aloft beneath
the upper low/on the northwest side of the surface low moving
off the SE VA coast. Particularly the 12Z NAM/ECMWF suggest
temperatures marginally cold enough for snow, sleet and/or
freezing rain first across the ridges then possibly down into
the valleys of eastern WV/western MD later Sunday into Sunday
night (can`t rule out a few conversational flakes further east,
but should be inconsequential with no accumulation).

Low pressure pulls out to sea Monday, with a cold front/northern
stream shortwave diving south across the area via the Great
Lakes by the end of the day. This will kick up northwesterly
breezes and bring in another (brief) shot of cold air, and may
be accompanied by some upslope snow showers (or freezing drizzle
if moisture is shallow) over the Allegheny highlands Monday


A high pressure ridge will cross the area Tuesday and Wednesday.
Expect mostly sunny skies and near normal temperatures through
the period. It`s days 6 and 7 that will need to be watched.
Shortwave energy will carve out another amplified trough axis,
which will be sweeping across the eastern United States. The
amplification of the trough axis varies amongst guidance
solutions, but these differences do not have a big tangible
difference on the forecast from this vantage point, as PoPs will
return to the forecast regardless. Area will be on the warm
side of the system, therefore most if not all of this
precipitation would fall as rain.


Initial band of rainfall is resulting in some IFR vsby. IFR cigs
are on its heels, working north from central VA. Would expect
IFR if not LIFR conditions at all terminals by later tonight. Low
pressure approaching from the lower Mississippi River Valley
is bringing fairly steady rainfall, though it may taper to
drizzle or even stop at times through the mid section of the
night. Some lulls are also likely late morning Saturday through
the afternoon before another round Saturday night into Sunday
morning. MVFR possible during the day Saturday, though most
guidance indicates cigs will remain IFR. IFR or lower
conditions continue Saturday night, gradually improving later
Sunday. Easterly surface winds AOB 10 kts expected, but LLWS
possible late tonight through Saturday with increasing LLJ (850
hPa around 40 kts).

VFR expected Monday through Wednesday in NW flow, gusty at times
Monday into Tuesday.


Fog is likely over the waters with visibility 1 nm or less at
times as warm moist air moves over the cool waters through at
least Saturday night. Periods of rain will likely prevent
widespread dense fog from forming.

Easterly flow generally 10 kts or less expected into Sunday on
the north side of low pressure passing through southern
Virginia. There may be a brief period of near SCA winds near the
Tangier Sound waters just after midnight tonight as the gradient
starts to increase, but it will also be fighting a strengthening
low-level inversion. This situation may repeat Saturday
afternoon as a surface low approaches from the southwest.

SCA more likely in NW flow Sunday night into Monday, possibly
extending into Tuesday before becoming lighter as high pressure
builds in Wednesday.


PWATs of 1-1.5" are advecting into the region ahead of a large,
slow-moving area of low pressure. Low-level warm/moist advection
beneath mid/upper jet forcing should bring one round of soaking
rain tonight into Saturday morning, with mid-level frontogenesis
on the north and northeast side of an upper low likely bringing
another round Saturday night into Sunday. Rainfall totals
through Sunday are expected to average 1.5-2.5 inches. In urban
areas (if rainfall rates are higher) and areas that still have a
snowpack, this may result in minor flooding issues. Depending on
exactly where the heaviest rain falls, mainstem river flooding
could linger into early next week.


An onshore flow has caused tidal anomalies to increase to around one
foot above normal. The onshore flow will continue through Saturday
night before slowly turning offshore Sunday. Winds will increase a
little later tonight and Saturday, causing anomalies to increase a
bit more. However, the astronomical norms are lower due to an
unfavorable lunar cycle. Therefore, confidence for minor tidal
flooding is lower. Latest forecast leans closer to the bulk of the
guidance which shows water levels cresting in caution stage for
sensitive areas through Sunday morning. However, it will be close to
minor flood levels, so anomalies will have to be monitored closely
as the onshore flow increases.


Rainfall totals continue to creep upward, with Baltimore
setting the annual record already. Here are the current rankings
for wettest year on record (through 4PM December 14th):

Washington DC area (DCA)
1. 61.33 inches (1889)
2. 60.83 inches (2003)
3. 60.78 inches (2018)
4. 60.09 inches (1878)

Weather records for the Washington DC area have been kept at
what is now Ronald Reagan Washington National Airport (DCA)
since 1945. Precipitation records observed downtown extend the
period of record back to 1871.

Baltimore MD area (BWI)
1. 65.67 inches (2018)
2. 62.66 inches (2003)

Weather records for the Baltimore MD area have been kept at
what is now Baltimore-Washington International Thurgood Marshall
Airport (BWI) since 1950. Precipitation records observed
downtown extend the period of record back to 1871.

Dulles VA area (IAD)
1. 65.67 inches (2003)
2. 61.30 inches (2018)
3. 59.05 inches (1972)

Weather records have been kept at what is now Washington Dulles
International Airport (IAD) since 1960.

NOTE: All climate data are considered preliminary until
reviewed by the National Centers for Environmental Information


DC...Flood Watch from Saturday afternoon through late Saturday
     night for DCZ001.
MD...Flood Watch from 6 AM EST Saturday through late Saturday night
     for MDZ016-017.
     Flood Watch from Saturday afternoon through late Saturday
     night for MDZ003>006-011-013-014-018-501>508.
VA...Flood Watch from 6 AM EST Saturday through late Saturday night
     for VAZ025-036>038-050-055>057-503-504-508.
     Flood Watch from Saturday afternoon through late Saturday
     night for VAZ026>031-039-040-051>054-501-502-505>507.
WV...Flood Watch from Saturday afternoon through late Saturday
     night for WVZ050>053-055-501>506.


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