Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180206
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
906 PM EST Fri Nov 17 2017

.SYNOPSIS...
High pressure will move offshore tonight through Saturday. A
strong cold front will cross the area late Saturday night. High
pressure will build to the south of the region late Sunday
through Monday before moving offshore Tuesday. Another cold
front will pass through Tuesday night into Wednesday and
Canadian High pressure is likely to build overhead for
Thanksgiving Day.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
High pressure is centered over eastern North Carolina, with
winds nearly calm across much of the area. However, warm
advection has already begun in advance of the approaching system
to our west, and high level moisture is already moving overhead
in the form of cirrus. So far the cirrus has done little to
stop temperatures from dropping rapidly since sunset to around
freezing. Just pushed a forecast update with an adjustment from
current trends. The forecast is tricky though, since it`s
possible temperatures level off as clouds thicken, or even rise
in some spots as a light southerly wind develops late.

Guidance has sped up leading edge of showers a touch quicker,
reaching the Potomac Highlands around dawn. Initially this
should be very light and falling from mid-level clouds. Wouldn`t
be surprised if there were a few sleet pellets due to cold/dry
low levels. There`s also some concern with how we`ve radiated if
those temperatures will recover above freezing before
precipitation arrives. At this time, don`t think there is an
appreciable threat for any freezing rain though.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/...
System developing in the central plains at present will bring
some active weather to us this weekend, with the main focus
being near and after cold frontal passage late Saturday night.

To start Saturday, high pressure will be to the southeast, and a
warm front will be approaching from the south as low pressure
strengthens and heads northeast across the Corn Belt. As the
warm front lifts north through the day, clouds will continue to
thicken and lower, and some rain may break out across the area.
Best chance is northwest of I-95 corridor, up in western MD and
eastern WV, but guidance is shifting a bit further south as
well. PoPs have been adjusted upward slightly across the
northern half of the area, but may ultimately need to go higher
along the US 50 corridor. That having been said, rain still
looks likely to hold off until midday in the metro (if it makes
it that far). Temps should rise above freezing everywhere by
then, but there is a small chance a little ice could occur in a
sheltered valley near Cumberland early Saturday. Otherwise, the
low level southerly flow will be increasing through the day,
and a late day or evening high looks more likely than not. In
fact, temps could spike several degrees warmer than our forecast
towards midnight, but given uncertainty, held temps down for
now. One caveat: if steadier precipitation does develop across
the north during the morning, an in-situ wedge/cold pool could
develop, resulting in highs stuck in the 40s. Then this cold
pool may not break, especially in the North Branch Potomac River
area, until the cold frontal passage, as depicted by the NAM
3km nest (and supported by climatology).

Warm front should lift north of the CWA late Saturday/early
Saturday night, and the rain chances look to diminish for a time
late Saturday afternoon and Saturday evening as it does so. Cold
front then pushes east across the region late Saturday night
into early Sunday morning, possibly accompanied by a low-topped
squall line or squall segments. There is a lot of low level wind
ahead of the front, but given the southerly flow and likely low
level inversion, do not expect widespread wind advisory criteria
ahead of the front - however, some ridges may touch wind
advisory before the front passes Saturday night. Highs will also
likely be reached predawn Sunday across most of the region, with
temps possibly spiking into the 60s, but our forecast highs do
not reflect this given they focus on the daylight hours Sunday
when temps will be more or less steady, perhaps even falling.
Winds with the actual front could touch wind advisory late
Saturday night/early Sunday, especially if there is a band of
showers with it.

Behind the front, most areas will dry out as low pressure lifts
northeast into southern Canada, but upslope flow will get going
along the Allegheny Front, so snow showers are likely most of
the day Sunday before diminishing Sunday evening. Advisory level
accumulations are possible along the Allegheny Front. Further
east, the main story will be the wind, which is likely to gust
close to advisory criteria even in the low elevations of the
metro, though odds are better in northern areas (Maryland) than
further south (Virginia). Right now best gradient and wind aloft
combined with mixing appears to maximize gusts during the
morning Sunday, so this is when we peak them in the forecast,
but its possible it peaks with the actual front overnight, or
later in the day when mixing increases (but winds aloft and
gradient diminish a bit). Given how far out in time it is, have
held off on wind advisory for now, but it still looks like a
decent bet especially near the Mason-Dixon line.

By Sunday night, the low will be continuing to move away and the
gradient, winds aloft and mixing will all decrease as heights
start to rise. This should both cut off the snow in the upslope
regions as well as allow gusts to fall off. Temps will be chilly
Sunday night, but not as cold as we experienced a week ago.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
High pressure will build into the region Monday and Monday
night. Dry conditions expected with temperatures 5 to 10 degrees
below average each period.

The high pressure will move offshore Tuesday to allow for a
strong cold front to plunge southeastward into the region
Tuesday night. The front should be a dry front with the lack of
moisture present. However, we can`t rule out a couple of upslope
snow showers Tuesday night.

High pressure will build into the region Wednesday through
Friday. Temperatures will remain chilly for this time in
November with dry conditions and some sunshine.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
VFR through at least Saturday morning, though cigs will be
lowering and vis may briefly be restricted in showers during the
afternoon Saturday (earlier at MRB, where there`s also a chance
of MVFR cigs by midday). There may also be some low level wind
shear Saturday morning, particularly at MRB, but also possibly
at the other terminals. As usual, NAM is much lower with cigs by
Saturday evening, but have played conservatively at low end VFR
for now.

Brief IFR possible in showers with winds possibly gusting to 40
knots late Saturday night as cold front plows through. Going
quickly back to VFR Sunday, but winds may continue to gust, with
possibly 40 knot gusts at times Sunday, with best odds being at
BWI/MTN. Winds diminish with continued VFR Sunday night.

VFR conditions expected Monday through Tuesday night. Winds
west around 10 knots gusts 15 knots Monday. Winds southwest 5 to
10 knots Monday night through Tuesday night.

&&

.MARINE...
Winds diminishing at present as high pressure moves overhead.
Most of night should be sub SCA, but southerly flow on back side
of high will begin after midnight. Have pushed back Small Craft
Advisory to 3 AM for the wider Bay waters, where channeling
should be maximized.

Winds above the surface look quite strong as Saturday wears on,
but think this southerly flow over somewhat cooled waters
should keep gales from happening Saturday and most of Saturday
night, so just extended SCA through the night. We may need a
gale late Saturday night with the cold frontal passage and
accompanying gusty showers, and will likely need one in the wake
of the front on Sunday for NW winds gusting perhaps as high as
40 knots. Winds diminish Sunday night as the front and storm
moves further away and high pressure starts building in.

Small craft advisories may still be needed Monday. Winds west
10 to 15 knots gusts up to 20 knots. No marine hazards Monday
night through Tuesday night.

&&

.TIDES/COASTAL FLOODING...
Anomalies remain around a third of a foot, but will likely
rebound as the lingering anomalies to our south push back north
with increasing southerly flow later tonight and Saturday. Minor
flooding is anticipated at our sensitive sites, perhaps even
some of the less sensitive ones, during the high tides later
Saturday into early Sunday. After that, cold front will bring
northwest winds back and should drop the anomalies enough to
prevent minor flooding by midday Sunday at latest, probably
earlier.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 AM Saturday to 6 AM EST Sunday for
     ANZ530-531-535>542.
     Small Craft Advisory from 3 AM Saturday to 6 AM EST Sunday for
     ANZ532>534-543.

&&

$$
SYNOPSIS...RCM
NEAR TERM...ADS/RCM
SHORT TERM...ADS/RCM
LONG TERM...KLW
AVIATION...ADS/RCM/KLW
MARINE...ADS/RCM/KLW
TIDES/COASTAL FLOODING...RCM



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