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fxus61 klwx 121515 

Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
1015 am EST Thu Dec 12 2019

high pressure over the region this morning will head off the
coast by tonight. Low pressure will start developing along the
Gulf Coast tonight and then head northward up the East Coast
Friday and Saturday. High pressure will return briefly on Sunday
before another system potentially affects the region Monday and
Monday night. Cold high pressure will then follow for the middle
of next week.


Near term /through Friday/...
strong high pressure (1040mb) sits just northwest of the region
presently and will continue sliding east toward the coast
throughout the day. Winds will be light and skies mostly clear
today. After a cold start, the weak winter sun will only allow
modest warming, with most places failing to reach 40 degrees
today. Not bitterly cold for mid-December, but below normal for
sure. Winds will be light, however, and with the sunshine, it
won't feel as bad as it could.

Previous discussion:
tonight will start out mostly clear, allowing for some
radiational cooling. However, developing system to the south
will quickly send some warm advection our way, resulting in
increasing clouds. Guidance almost universally has precipitation
overspreading the I-64 corridor by 7 am, and many models have
precip approaching District of Columbia itself by then. Have sped up timing of
precip arrival in the forecast somewhat, but not as far as
latest guidance would suggest. That said, it remains a known
bias for models to be too slow with warm advection precip, so
remain concerned that precip may arrive sooner than forecast.

The reason this is a concern, of course, is that with warm air
aloft but cold surface temps, the most likely precip type before
and just after dawn is freezing rain. Have added ice
accumulations, though relatively modest ones, extending into the
District of Columbia western suburbs and north up the I-81 corridor, but would not
be shocked to see ice edge even closer to District of Columbia itself if precip
arrives as early as some guidance suggests. That said, the
initial burst of precip should be fairly light, so not expecting
any Winter Storm Warning type stuff here... a tenth of an inch
is probably the most anyone will get. However, the time of day
makes this highly impactful, being that we will likely have cold
Road surfaces and we will be entering rush hour. All that having
been said, its still late second period, so opted to wait a bit
longer before issuing any winter weather headlines.

The light rain may actually end for a time in the afternoon
Friday, as the initial burst of warm advection wanes. Highs east
of the Blue Ridge should reach the lower 40s with the flow
turning easterly, but west of the Blue Ridge, temps likely
struggle to get much above freezing.


Short term /Friday night through Saturday night/...
the main low pressure system will move north across the region
Friday night. By this time, expect nearly everyone if not
everyone to have risen above freezing, so rain is expected, and
its possible it could be moderate to borderline heavy for a
time. That said, timing and placement are still uncertain, and
it hasn't been excessively wet lately, so don't currently expect
any Hydro issues.

Low pushes north on Saturday, and rain should taper to showers.
After cold advection starts in the afternoon, upslope showers
will develop and change to snow in the higher elevations by
evening. However, only light accumulations are expected, with no
winter weather advisories for snow currently anticipated.
Further east, Saturday afternoon and evening will be slowly
drying out, with relatively mild temps near 50 in the afternoon
dropping into the 30s at night.


Long term /Sunday through Wednesday/...
as a potent low pressure system over New England departs to the
northeast into the eastern Canadian Maritimes Sunday, a broad
area of high pressure over the Midwest U.S. Will quickly build
into the region late Sunday into Sunday night. A few inches of
upslope snow in the Potomac Highlands should come to an end
around midday Sunday with the building high pressure. A dry and
cold period will be brief late Sunday and Sunday night.

The next low pressure system will approach from the southwest
late Sunday night through Monday night. As of now, there is a
noticeable discrepancy or difference between the GFS and Euro
deterministic models. The GFS makes the track of the low from
the middle Mississippi Valley to the lower Great Lakes. The Euro
deterministic model makes the track of the low from the
Tennessee Valley to the Delmarva Peninsula and then offshore.
The GFS is the warmer solution. One thing to note that could be
the determining factor as to whether we see accumulating
snowfall or a mix of snow, rain or even all freezing rain is
both models indicate a modest to strong southerly flow at 850
mb. This ultimately would warm and moisten the upper boundary
layer to support a wintry mix or freezing rain over all snow. It
is still too early to tell exactly what will happen but the
threat is there for wintry precipitation.

The low pressure system is expected to move to the east and
offshore Tuesday and Tuesday night. Some rain showers east of
the mountains and some snow showers in the mountains could
linger or remain from the low Tuesday afternoon and evening.

High pressure will build into the region Wednesday and
Thursday. Drier and cold conditions will ensue. Expect below
average temperatures each day.


Aviation /14z Thursday through Monday/...
VFR through this evening for all terminals. Lowering cigs and
encroaching rain/freezing rain then become a concern late tonight and
Friday. Freezing rain most likely cho and mrb, with much less (but still
some) risk iad. Not particularly concerned at dca/BWI/mtn for
freezing rain given later arrival of precip and warming easterly flow.
Low cigs and vsby likely Friday night with rain. Improving to
MVFR/VFR on Saturday as low lifts north and rain ends, though
scattered showers may linger.

VFR conditions at all terminals Sunday and Sunday night. MVFR
conditions possible at cho and mrb on Monday with approaching
light snow or a wintry mix. MVFR conditions possible at other
terminals late Monday and Monday night. MVFR conditions could
linger Tuesday with lingering rain or snow showers.


Small Craft Advisory conditions continue in the central Bay, but that will end
over the next couple of hours as high pressure builds to the
north. The high pressure will slide east as low pressure moves
up from the south tonight through Friday night, but not yet
particularly concerned about Small Craft Advisory issues given stable air mass in
place. Small Craft Advisory could become an issue as low pressure pulls away
Saturday into Sunday.

Small Craft Advisory conditions possible Sunday as high
pressure over the Great Lakes interacts with low pressure over
New England. No marine hazards Sunday night through Monday


Lwx watches/warnings/advisories...
District of Columbia...none.
Marine...Small Craft Advisory until noon EST today for anz534-543.


near term...rcm/cjl

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