Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 klwx 100226 

Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
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.


Near term /through Tuesday/...
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 County Warning Area 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 County Warning Area at 01z of g15-25kts, caused by a 60kt 850 mb
jet which was directly overhead the County Warning Area at 10/00z. This low
level jet will eject northeastward towards Cape Cod by
midnight, and the winds will slacken after midnight.


Short term /Tuesday night through Wednesday night/...
(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 European model (ecmwf) continues to be consistent
with its prior 4 model runs with measurable snow across much of
the County Warning Area 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 300 mb 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 County Warning Area-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 at or above
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 National Weather Service 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 positive vorticity advection 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.


Long term /Thursday through Monday/...
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 County Warning Area by
Friday morning, while the European model (ecmwf)/eps delay precip onset by about
6 hours. Interesting to note that today's 12z European model (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.


Aviation /02z Tuesday through Saturday/...
IFR ceilings will prevail through 05z/midnight then improve to LIFR
and remain through daybreak. Winds should start to relax too
after 05z as the 850 mb 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 northwest 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 Small Craft Advisory overnight as the 850 mb 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 northwest flow behind a cold front Tuesday night and
especially Wednesday.

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

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


Lwx watches/warnings/advisories...
District of Columbia...none.
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.



National Weather Service Glossary of Abbreviations