Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 160825

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
325 AM EST Fri Feb 16 2018

A cold front will drop through the area today. High pressure will
briefly build north of the region tonight, then a wave of low
pressure will pass to the south Saturday. High pressure will return
briefly again on Sunday before another storm system approaches from
the west early next week.


A subtle surface trough out ahead of the main cold front will drop
south across the area bringing showers this morning. There will
likely be a brief lull before another round of showers with the main
cold front early this afternoon. Temperatures will start out mild
(50s to around 60), but then fall through the day as cold air moves
into the region on gusty NW winds (gusts to 30+ mph late this
afternoon and early evening). A few brief downpours are possible.

Tonight will be much colder with lows dropping below freezing just
about everywhere. Gusty winds in the evening will make it feel
several degrees cooler, but the wind will abate pretty quickly by
late evening as high pressure builds over/just north of the area.


Surface low pressure will develop over the Tennessee Valley Saturday
morning, driven by a digging upper trough flicking across the Great
Lakes/Ohio Valley. Transient cold air in place will result in a
myriad of precipitation types Saturday afternoon and evening. The
latest guidance has trended a little cooler with a little more
precipitation, with appreciable snow/sleet amounts of 2-4 inches
seeming more likely along the western ridges and the Mason-Dixon
line, tapering to an inch or less south of US 50/E of the Blue
Ridge, and mainly rain for southern Maryland. Snow-to-liquid ratios
are still expected to be below climatology even in areas of all snow
due to most of the column where precipitation is developing being
above -10 C, though upper jet dynamics could compensate for this
somewhat especially over northern Maryland. A light glaze of
freezing rain also appears likely for much of the area except
northern MD (mainly snow) and extreme southern MD (plain rain).
Slightly higher amounts around a tenth of an inch are possible over
the central Shenandoah Valley where shallow cold air would be more
easily trapped. Surface ridge axis/weak wedge orients itself more or
less along I-95 so believe the metros could see a touch of freezing
rain as well (usually along this ridge axis is where lower dew
points/cooler air pools).

Latest guidance is in very good agreement on timing, with little or
no precipitation entering the CWA before noon. Clouds and
precipitation quickly increase from SW to NE between 1 and 4pm, with
the entire area seeing some sort of precipitation by 6 or 7pm.
Conditions clear just as quickly around or shortly after midnight,
perhaps lingering a few extra hours over the western slopes.

Temperatures are expected fall to around freezing late Saturday
night and early Sunday morning. Models are slightly deeper and
stronger with the low pressure and resultant gradient in its wake,
but still believe winds will be light enough to result in a little
patchy ice formation/re-formation early Sunday morning.


As high pressure moves offshore Monday, isentropic lift from the
return flow will permit showers to overspread the area. Am sticking
with likely PoPs for the period. Temperatures should not be a
problem-- we`ll start the day above freezing, and reach the lower-
mid 50s.

The forecast area be in the warm sector Tue-Wed. And with 850 mb
temps around +12 or +13C, that could mean highs in the 70s
again...without necessarily ample sunshine. Then the cold front
drops southeast Wed night into Thu. There are position/timing
uncertainties with the front, which will affect PoPs. Tuesday-
Tuesday night should be holding onto a slight chance mainly
Blue Ridge west. Wednesday is a little less certain, as the GFS is a
bit faster with frontal progression. Believe the threat would
increase late in the day (as opposed to the start).

Temperatures will drop Thursday due to frontal passage. Again, there
are timing/strength differences to resolve which will affect
precipitation, but regardless of solution, highs should be in the
50s...not the 70s. Lows, in the mid 30s-lower 40s, will still be
above climo too.


LLWS at times early this morning with strong LLJ just ahead of a
cold front. Patchy MVFR or even IFR in isolated heavier downpours,
but overall minimal restrictions expected in occasional -SHRA this
morning. Patchy MVFR possible along cold front early this afternoon.
Winds will switch to NW late this morning into early afternoon,
becoming gusty (to around 30 knots) by mid to late afternoon.

VFR expected tonight. Winds go light late this evening.

IFR likely Sat aft-eve with a wintry mix expected. Winds light NE
becoming SE. VFR returns late Sat night as winds become NW.

Flight restrictions likely Monday as a warm front and associated
precip lifts across the terminals. Tuesday should be better in terms
of a much lower chance of showers. Am not as certain with respect to
cloud bases though.


There may be a lull in SCA level gusts mid to late morning right as
a cold front drops through. Winds will increase markedly out of the
NW this afternoon and persist into this evening, with a few gusts to
around 30 knots possible, especially in any heavier showers right
along the cold front. Winds go light late tonight and remain light
through Saturday.

South winds will affect the waters Mon-Tue. Winds Monday will be
less (approx 10 kt) due to poor mixing associated with anticipated
rain. However, this forcing will be north of the waters by Tuesday.
The biggest hindrance will be warm air over cold waters. For that
reason, forecast winds will not exceed 15 kt.


MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ530-
     Small Craft Advisory until 3 AM EST Saturday for ANZ532-533-
     Small Craft Advisory until 6 AM EST Saturday for ANZ534-543.


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