Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 120826

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
326 AM EST Fri Jan 12 2018

Low pressure developing in the Tennessee Valley today will pass
near the area tonight, dragging a cold front eastward early
Saturday morning. High pressure of Canadian origin will build
across the region through early next week.


Forecast area entrenched within mild south flow. Although there is a
40-50 kt low level jet (at 925 mb), mixing has been remarkably poor
due to a steep inversion in the boundary layer. Still, surface
temperatures have slid little since Thursday afternoon, and remain
in/near the 50s areawide (at 3am in January!). Showers have been
spotty and light, but a tongue of moisture has been fed by the
upglide between 925 and 700 mb; these showers have been entering
central Virginia. Am running with likely or occasional PoPs. The
more solid line of showers, with much more substantial rainfall
rates, can be found in the upper Ohio Valley in advance of the cold
front. However, this moisture lifting north as well. It is not
anticipated that the forecast area will see any of it.

Occasional light showers should continue through the morning hours.
By midday, there should be a lull in the precip as the low-level jet
lifts northeast. That said, there is more than enough upglide within
the theta-e ridge for a shower at just about any time, so am backing
off to chance PoPs at best. Do not expect breaks in the overcast,
although its possible. In spite of this, temperatures should rise to
the mid-upper 60s.

Minimal instability (a couple hundred j/kg MUCAPE) should be able to
develop during this lull during the afternoon. As the surface front
and negatively tilting trough axis approach tonight, and a low
pressure system tracks north along the frontal boundary, a more
solid line of showers should be able to develop...especially south
from the low center. Due to the instability and intense shear
present, would not rule out a rumble of thunder. More plausible,
although still not likely, would be for the 40-50 kt of wind
available around 850 mb to mix down...even in the absence of
lightning. Am maintaining a slight chance of thunder to account for
this risk.

Guidance maintaining its signal that low-level cold air will come
rushing east behind this system. Its a question of whether the
precip will exit before the cold air arrives. Through sunrise,
believe that risk will remain west of the Blue Ridge, although
possibly along the Mason-Dixon line too. Thermally, the column would
support a change to a mix (sleet or perhaps freezing rain) if there
were still precip present. Am not completely certain what type will
dominate, and there is still time to figure it out. Do have a likely
wintry mix in the aforementioned areas, and precip changing to
snow in the mtns (with a couple of inches accum possible).


The precip type challenges that dominate northern/western
sections tonight spread east to the rest of the area Saturday
morning. At the same time, precip will be exiting to the
northeast. The ECMWF remains about the slowest solution with
regard to the qpf and thus its the coldest solution. Am leaning
upon NAM/NAMnest/HiResW-ARW, which is a bit faster than the
Euro wrt qpf timing, but not nearly as fast as the GFS. This
means that PoPs drop off to about 20-30%, but with the same type
thoughts (possibly a wintry mix before ending, but low

Even in the mountains, precip shuts off during Saturday morning as
the Great Lakes (especially Erie) is frozen. Additional snow
accumulation will be minimal.

For the rest of the weekend, the story will be the cold. Saturday
will be windy too (20-30 mph), so wind chills will be an issue. Am
not straying far from blended temperature guidance. That suggests
that temperatures will fall most of the day Saturday, to a low in
the teens Saturday night. Sunday nights lows will be similar.
Temperatures likely won`t get above freezing Sunday.  The lowest
wind chills likely will be Saturday night, as the gradient relaxes
as high pressure nears on Sunday.


Arctic high pressure will begin to retreat Monday as an Alberta
clipper tracks across the midwest Mon into the eastern Great Lks by
mid next week. Clipper will be generally weakening as it moves east
into an area of strong high pressure over Atlantic Canada but it may
still be able to bring a period of very light snow (less than 0.1
inches liquid equivalent) Tue-Tue night as it crosses the area. A
new surge of modified Arctic air will follow behind it Wed. A
moderating trend in temperatures is set to begin next weekend Jan 20-
21 as east coast trough lifts out and heights rise.


The window of VFR around the hubs at 06Z TAF release has filled in.
Although specific cigs/vsbys will be variable, will be sliding to
IFR (or lower) by morning push, and should generally be staying
there through the day. There will be a lull midday-afternoon, but am
quite uncertain if that will translate to better conditions. Also,
the 40-50 kt low-level jet will continue to be an issue through mid
morning before pulling out by noon.

A cold front/low pressure system will track across the terminals
tonight, providing a better opportunity for showers with brief LIFR
conditions and perhaps with gusty winds. After that, winds will veer
northwest, and increase to 15-25 kt. Timing will be predawn. There
is a slight chance that cold air will arrive before all precip
departs. If this occurs, then rain would change over to a wintry mix
before ending. Low confidence on specifics, but worth mentioning.

The weekend and beginning of next week will feature VFR
conditions and northwest winds. The strongest winds will be
Saturday into Saturday evening.


The cold water having an impact early this morning. In spite of
strong winds just off the water, winds reaching the surface have
only sporadically be achieving SCA criteria. But it is happening, so
will continue with the Advisory through noon.

There will be a lull this afternoon before the gradient increases
tonight. Will be maintaining tonight`s Advisory, and am extending it
into Saturday...when mixing profiles much more favorable behind a
cold front. This likely will need to be continued into Saturday
night, and perhaps Sunday.

It remains possible to get brief gales just behind the cold front
early Saturday morning. Will continue to include this potential in
the synopsis due to continued uncertainties.


Warm conditions are expected again today-tonight, along with
periods of rain. 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

Around one to one and a half inches of rainfall is anticipated
through tonight. 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.
Limited rainfall up to this point has helped. 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.


MARINE...Small Craft Advisory until noon EST today for ANZ530>534-
     Small Craft Advisory from 6 PM this evening to 6 AM EST
     Saturday for ANZ531>534-537-539>541-543.
     Small Craft Advisory from midnight tonight to 6 AM EST
     Saturday for ANZ530-535-536-538-542.


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