Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 200230
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
930 PM EST Thu Jan 19 2017

.SYNOPSIS...
High pressure will move off to the east tonight. A warm front
will move into the area Friday and stall out nearby through
Saturday. Low pressure will affect the area Sunday and Monday.
Weak high pressure may approach the area Tuesday before a cold
front approaches Wednesday.

&&

.NEAR TERM /THROUGH FRIDAY/...
High pressure is currently located along the eastern seaboard.
With thin spots in the cirrus, some locations have already fallen
to their dewpoints (lower to mid 30s), necessitating lowering the
low temperatures in some locations. However, with thicker clouds
arriving from the southwest, the temperatures should hold steady
or rise a few degrees the second half of the night. Any rainfall
arriving in the southwest corner of the CWA late tonight will be
very light.

Warm advection, albeit rather weak, starts late tonight and
continues on Friday as the closed low opens into a shortwave and
lifts northeast across the region. With this overall being a
weakening feature, rainfall is not expected to be particularly
significant on Monday, generally less than a quarter of an inch.
Rain may move through as several bands as suggested by short-range
high resolution guidance, so went with "periods of rain" given
potential for dry intervals. It`s possible the first band is
little more than sprinkles, with the second band providing most of
the measurable rainfall. The gap between any such bands remains
uncertain. That all having been said, in general we expect rain to
arrive across metro DC 8-10 AM, with light rain likely occurring
at swearing in, and the rain should mostly depart 2-4 PM. However,
the low clouds, fog and drizzle may persist given light southeast
flow and no frontal passage expected. With the rain expected on
Friday, expect highs to stay below guidance, mid to upper 40s
generally.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
Friday night into Saturday night, the region is more or less
between systems, but with the warm front stalling to our south and
nothing to disturb the air mass, low clouds, mist, drizzle and
perhaps fog look likely to remain over the region, at least
through early Saturday. Some insolation may reach the ground
enough to dissipate the drizzle, mist and fog on Saturday
afternoon, but it will be short lived. Warm advection precip with
the next complex low pressure system will start Saturday evening,
with rain likely to overspread much of the region late Saturday
night.

With the clouds, mist, drizzle and patchy fog, lows on Friday
night should not drop much, with readings generally in the 40s.
Highs on Saturday will be dependent on the clouds thinning a bit
with a break in the fog and drizzle. Assuming this occurs, we will
likely rise into the low-mid 50s. Saturday night lows should stay
in the 40s with the clouds and rain arriving late.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
A storm system will develop over the Mississippi Valley and
deepen as it moves toward the east-northeast Sunday and Sunday
night. This low will send energy into our region Sunday into
Sunday night, bringing rain to the region.

On Monday, the main low will move to the northeast a little
further. The threat for rain continue Monday into early Monday
evening. Temperatures will remain mild for January but cooler air
quickly lines up on the doorstep of the region to move in Monday
night. Some guidance suggests enough cold air may arrive before
precipitation ends to allow a change to snow, particularly across
higher elevations well northwest of I-95 corridor. Will need to
watch this potential. Other concern Monday is for gusty winds, and
some guidance is hinting at wind advisory gusts as the low passes
just to the east. Will need to look at this harder in coming day
or two.

High pressure will usher in drier air Tuesday and Tuesday night.
Clouds will break for sun, except for clouds and upslope rain
and/or snow showers in the Potomac Highlands possible.

An upper level trough of low pressure will sag from the Great
Lakes region toward the mid-Atlantic Wednesday. A return of mild
air from the south will evolve ahead of the trough with some
sunshine giving way to some clouds.

Temperatures will be seasonable with some sunshine Thursday. An
upper level trough of low pressure will be just to the north but
dry conditions should prevail. There is a chance of upslope rain
or snow showers in the Potomac Highlands.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
VFR through late tonight with high pressure overhead sliding to
the east. With the approaching warm front, clouds will thicken and
lower, with intervals of rain starting early-mid morning Friday
and continuing into the afternoon. Visibility and cigs will lower
at least to MVFR. IFR is much more uncertain, with statistical
guidance seeming to drop conditions too quickly given light rain.
Have coded the lower cigs as "sct" to indicate the uncertainty.
While rain ends in the afternoon, low clouds, mist and drizzle
with a better chance of IFR conditions likely to persist through
Friday night into Saturday morning before improving perhaps to
MVFR Saturday afternoon and evening. Conditions may drop again
late Saturday night as rain moves back in.

Sub-VFR conditions Sunday through Monday with gusty winds
developing Monday. Improving conditions Tuesday.

&&

.MARINE...
Winds should remain below SCA through Saturday night, becoming
southeasterly. Rain, drizzle and fog/mist likely to arrive Friday
morning and persist through Saturday morning. Improved conditions
seem likely Saturday afternoon and evening before rain returns
late at night.

Approaching system looks to bring increasing winds Sunday and
Monday as it strengthens and moves northeast along the coast. SCA
looks likely with gales possible Monday. Winds begin to diminish
Tuesday.

&&

.TIDES/COASTAL FLOODING...
Tidal anomalies have risen considerably today. However, tide
levels look to remain below flood for this high tide cycle. With
little impetus for draining the water, levels will need to be
monitored over the weekend, although guidance does not suggest
flooding at this time.

With low pressure moving up the coast later Sunday into Monday,
the set-up will be favorable for increasing water levels in the
Bay.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

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


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