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FXUS61 KLWX 100226

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
926 PM EST Mon Dec 9 2019

Low pressure will move from the Great Lakes into Quebec overnight,
dragging its trailing cold front through the region Tuesday. 
High pressure will build into the region late Tuesday night  
and remain through Thursday, before shifting offshore of New 
England as low pressure develops over and approaches from the 
southeastern United States.


KLWX WSR-88D at 0130Z shows scattered light showers primarily 
east of Blue Ridge. Short-range HREF and HRRR has these
scattered showers becoming more isolated as the overnight
continues, with the overwhelming majority of the CWA staying dry
between midnight and daybreak. Temperatures are not going to
change much overnight, as low level moist advection and 
southerly flow keep pumping in warmer air. This will result in
overnight lows 5-10 deg F above average. Gusty surface winds
before midnight as evidenced by numerous surface reports
throughout the CWA at 01Z of G15-25KTS, caused by a 60KT H85 
jet which was directly overhead the CWA at 10/00Z. This low
level jet will eject northeastward towards Cape Cod by 
midnight, and the winds will slacken after midnight.


(02Z Update)...Rain showers will develop Tuesday morning, 
and will be prevalent by Noon across the entire CWA, especially
along and east of Interstate 95. The cold front will pass 
through the area on Tuesday afternoon, which will start to 
filter in colder air. The 18Z ECMWF continues to be consistent 
with its prior 4 model runs with measurable snow across much of
the CWA Tuesday night. There will be a brief lull in rainfall 
late tomorrow afternoon and early evening, before the region 
begins to feel the effects and influence of the right rear 
quadrant of a 170KT H3 jet which will be optimally positioned 
late Tuesday night providing jet induced dynamics. By 06Z Wed,
the column should be cold enough to support all snow west of a 
line from Bel Air-Damascus-Sterling-Harrisonburg. Any pcpn that
falls after 06Z west of this line will likely be all snow. East
of this line, there will be a gradual transition from rain to a
mixture to finally all snow by daybreak Wednesday. Accumulations
along and east of I-95 will be limited as the race for the cold
air to arrive in time before the precipitation departs will be

There will likely be a sweet spot in this colder region north
and west of the Balt/Wash metro areas where 2-3 inches could 
accumulate on grassy surfaces, i.e., near the Catoctins, 
northern portions of Carroll, Baltimore, and Harford Counties, 
and along the Blue Ridge. Impact-wise, I am concerned about I-70
from eastern Frederick County to Hagerstown, and the confluence
of other high-speed roads around Frederick MD, including the 
I-270 corridor from Frederick to Rockville. Motorists on the 
roads in these areas between midnight and daybreak will be prone
to slick roads and low visibilities. The one thing that is 
working in favor of lesser accumulations on roadways is that 
temps will max out at least mid-50s CWA-wide today, and along 
and east of I-95 will be hovering in the lower 60s. I do think 
that snowfall early Wednesday morning will make its way to the 
Chesapeake, although the surface temps will still be AOA
freezing, thus very unlikely to stick on roadways. I have 
updated the snow graphics reflecting grassy accumulations to 
the Bay, and slightly increased snowfall totals north and west 
of the major metro areas. Reminder...official NWS snowfall 
forecasts are for accumulations on whiteboards/grassy areas, 
and not on road surfaces./End 02Z update/

The latest model guidance has honed in on the 10:00AM to 4:00PM
window for greatest coverage of shower activity Tuesday, as low
level convergence increases along an eastward advancing cold
front and PVA associated with another mid level shortwave pivot
across. Showers will probably become a bit less widespread for a
time during the late afternoon and evening following the frontal
passage, but coverage is expected to increase again by late
evening into the overnight in concert with RRQ jet dynamics
associated with a very strong upper jet.

Since the low level front will have passed by Tuesday night, 
temperatures will begin to cool. As such, rain is expected to  
change to snow. There is still uncertainty in the timing of 
temperatures becoming cold enough for snow, especially east of 
the Blue Ridge where downsloping and compressional warming 
effects tend to be most notable. Even when precipitation changes
to snow, surface temperatures will likely be near or above 
freezing, so snow may have trouble sticking in the lower 
elevations. Overall, the setup remains largely unchanged, with
typical run to run and model to model variability noted. The
highest confidence in snowfall accumulations will be over the
higher terrain (above about 1000 feet), with the higher
elevations of north-central/northwestern Virginia likely to see
the highest totals.

Even though boundary layer temperatures are expected to be
marginal in the lower elevations and especially in the I-95
corridor, the timing of wintry precipitation lines up with the
Wednesday morning commute, and the powerful mid/upper jet
dynamics may result in localized heavier bands of precipitation,
beneath which snowfall rates could overcome surface temps and
result in accumulation even on paved surfaces.

Precipitation should come to an end by midday Wednesday, with
gusty northwest winds ushering in an Arctic airmass in its wake.


Guidance overall is in good agreement with the synoptic pattern 
concerning the long term. A strong area of high pressure moves 
eastward out of the Great Lakes region and extends down the 
majority of the Eastern Seaboard. The resultant is dry yet 
chilly conditions expected for Thursday. At the same time, low 
pressure develops over the Gulf of Mexico and will begin to lift
northeastward late Thursday night into Friday.

While guidance is in better agreement about the timing of precip, 
there is still some spread in regards to timing, track and
temperatures. The GFS/GEFS bring precipitation into the CWA by 
Friday morning, while the ECMWF/EPS delay precip onset by about 
6 hours. Interesting to note that today's 12Z ECMWF suite 
advanced the precip by about 6 hours, compared to the 12 hour 
difference (from the GFS) it showed yesterday. Therefore, there
is better confidence that precip onset will occur by Friday 
afternoon. With the chilly air mass in place, some wintry 
precipitation (freezing rain), will be possible at the onset, 
mainly near and west of the Blue Ridge. Elsewhere, mainly plain
rain is most likely.

The low will exit the region by early Saturday morning with 
some partial clearing expected for the afternoon. Dry weather 
persists through Monday as temperatures remain slightly on the 
cooler side of normal.


IFR CIGS will prevail through 05Z/midnight then improve to LIFR
and remain through daybreak. Winds should start to relax too
after 05Z as the H85 ejects northeastward.

Latest guidance pegs the 15-21Z window Tuesday for most likely 
showers and sub-VFR CIGs/VSBYs. A brief respite is anticipated 
Tuesday evening, before precipitation re-develops and changes to
snow after midnight. The most likely terminal to see 
accumulation is MRB, least likely at CHO/DCA, though there could
be a quick coating even on paved surfaces at any terminal if a 
localized heavier band of snow moves overhead, overcoming 
marginal surface temperatures.

VFR returns by midday Wednesday with gusty NW winds. Another 
round of rain seems definite Friday afternoon into Saturday as 
coastal low pressure develops to the southwest and lifts 
northeast. Expect IFR conditions in moderate rain Fri night and 
Saturday. Mixed pcpn possible mainly near MRB.


Winds will continue to blow SCA overnight as the H85 jet impacts
continue. There will likely be a lull in the winds Tuesday, so 
the headline drops to just the lower most Maryland portion of 
the Chesapeake Bay and lower tidal Potomac. Winds likely 
increase in NW flow behind a cold front Tuesday night and 
especially Wednesday.

Winds lighten Wednesday night through Thursday night as high
pressure builds overhead.

SCA conditions possible Friday into Saturday as pressure 
gradient tightens in advance of a coastal low pressure lifting 


MARINE...Small Craft Advisory until 10 AM EST Tuesday for ANZ531>533-
     Small Craft Advisory until 6 PM EST Tuesday for ANZ534-537-543.
     Small Craft Advisory until 6 AM EST Tuesday for ANZ530.
     Small Craft Advisory until 6 AM EST Tuesday for ANZ535-536.



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