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fxus61 klwx 091956 

Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
256 PM EST Mon Dec 9 2019

low pressure will move from the Great Lakes into southeastern Canada
tonight, dragging its trailing cold front through the region
Tuesday into Wednesday. High pressure will follow Wednesday
night through Thursday, then shift offshore of New England as
low pressure develops over and approaches from the southeastern
United States.


Near term /through tonight/...
the steadiest rain was located over the I-95 corridor as of
2:30pm Monday afternoon, as a lead shortwave impulse in the
700-500 hpa layer overrides a surface warm front lifting north
across the area. In the wake of this trough, the warm front
should lift north, and forcing will weaken. There's abundant
moisture in the column, and this couple with height falls ahead
of the approaching upper trough is expected to result in
isolated to widely scattered showers overnight. There are much
higher probabilities over the Allegheny Highlands where
orographic enhancement results in more widespread shower
activity re-developing after midnight, but warm temperatures
will keep precipitation all rain even at the highest elevations.


Short term /Tuesday through Wednesday night/...
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 /19z Monday through Saturday/...
IFR cigs should begin to lift as a warm front lifts north and
low level inversion mixes out between 21-00z. Low level wind shear will remain
prevalent through about 03z, before better mixing allows
surface winds to increase, reducing speed differences between
the surface and 2 kft above ground level.

Guidance is almost unanimous on VFR returning by or shortly
after midnight as low level mixing increases, but sub-VFR could
linger a bit given climatological stubbornness of warm fronts
overnight in our area (they usually progress a touch slower).
Surface gusts out of the SW to around 20 kts are likely
overnight at times as mixing increases.

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 have not been mixing down very well this afternoon, but
that is expected to change this evening as low level stability
decreases, allowing strong winds at a few thousand feet above
the surface to mix down. Therefore, left the headlines as is for
this afternoon, and expanded to include all waters for a time
overnight. 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 from 6 PM this evening to 10 am EST
Tuesday for anz531-532-538>540-542.
Small Craft Advisory until 6 PM EST Tuesday for anz534-537-543.
Small Craft Advisory until 10 am EST Tuesday for anz533-541.
Small Craft Advisory from 6 PM this evening to 6 am EST
Tuesday for anz530.
Small Craft Advisory from 9 PM this evening to 6 am EST
Tuesday for anz535-536.


near term...dhof

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