Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Baltimore Maryland/Washington District of Columbia
943 PM EDT Sat Mar 15 2014
high pressure will build into the region from the north tonight
into Sunday. Low pressure will track along a boundary to our south
Sunday and pass to our south Sunday night into Monday. A wedge of
high pressure will persist over the area through Tuesday...with a
cold front crossing the region on Wednesday.
Near term /until 6 am Sunday morning/...
near term headline update...small craft adzy dropped for the evening
for the northern-Middle Bay.
No headline changes planned for the evening update west/ the winter
hazards Sun afternoon-Monday afternoon. See the update section in the short
term for more details.
Arctic high pressure will build toward the region from the Great
Lakes tonight. A northerly flow ahead of the high will allow for
chillier conditions. Temperatures will drop into the 30s by Sunday
morning. Low pressure will drop out of the Front Range tonight and
and plenty of warm and moist air from the Gulf of Mexico will
begin to overrun the low-level cold air in place. Increasing
clouds are expected as a result...but dry conditions will
Short term /6 am Sunday morning through Monday/...
low pressure will track through the Gulf Coast states Sunday before
emerging off the middle-Atlantic coast Sunday night. Arctic high
pressure will continue to build over New England and the surface
ridge axis will dip south into the mid-Atlantic...supplying plenty
of cold air.
the 18z versions of the GFS/NAM showing some internal consistency
but still divergent from each other west/ the NAM more aggressive west/
the snow/quantitative precipitation forecast amounts - as it is west/ a possible Tuesday/Wednesday storm
system. The GFS and its ensembles are in good agreement and also
spreading a good 0.8-1.3" quantitative precipitation forecast from the bulls-eye outward - over the
southwestern quadrant of the County Warning Area...but the first 6hr block of quantitative precipitation forecast west/
0.2-0.4" will be mainly liquid sun aftn/eve...espec east of the Blue
Ridge. Some of the higher terrain will start to accumulate snow earlier
on and shortly after precipitation onset...but elvns below 2kft will take
longer to drop near and below freezing. Even west/ middle-30s at the surface
- snow will be falling but will take a little time to start
accumulating after a several hour period earlier in the day west/ temperatures
in the u30s/l40s.
The outlier in this equation is the Euro west/ a more minor spread of
quantitative precipitation forecast across the southern periphery of the County Warning Area - a maximum of 0.5" liquid
that straddles the southern tier counties and tapers down to barely a
tenth of an inch along the Mason-Dixon line. This model too has
been consistent as well...which lends to waiting until the 00z
appears to see if the past 12hrs of dynamic weather over the Southern Plains
has helped to shape the model's newer run any differently.
From previous disc...
warm and moist air from the Gulf of Mexico will overrun low-level
cold air in place...causing precipitation to overspread from
southwest to northeast Sunday afternoon into Sunday evening. There
may be just enough warm air near the surface Sunday for
precipitation to start off as rain or a rain/snow mix. Little snow
accumulation is expected Sunday afternoon in the valleys and
metropolitan areas due to the warm layer near the surface.
The Winter Storm Watch remains in effect for most of the area...
and the west.S. Warning remains in effect for the central Shenandoah
Valley into the Potomac Highlands where certainty for a significant
snow is highest.
Surface high pressure persists over the northern middle-Atlantic Monday...and
over New England Tuesday. This will maintain The Wedge of cold air
across the central middle-Atlantic trough this time.
The coastal low will be already well off Hatteras Monday morning and
be quickly moving northeast. However...the upper trough and presence
of a cold air wedge will maintain clouds and slow the tapering off
of snow. Light snow will persist over the area essentially through
the morning and taper off completely or to flurries through midday.
Additional accumulate of around an inch for the southern half of the County Warning Area after
12z where forcing is best. A few tenths to half inch for northern half...
depends on where the back edge is. Headlines continue until 2pm and
as usual will probably be able to be cancelled early...particularly
from the northwest.
Long term /Monday night through Saturday/...
surface ridge axis looks to hold steady through Tuesday which keeps a NE
flow and moisture advection. Chance to likely probability of precipitation for Piedmont and
ridges. Precipitation from The Wedge looks to be light with a saturated
layer less than 6 kft thick. Perhaps this will be more of a
sprinkle/flurry. Warm air advection (along with the strong March
sun) should allow Crystal formation to end by Tuesday afternoon.
With cold air damming...will need to watch out for pockets of
freezing rain/drizzle up along the Blue Ridge. For now went with a
snow to rain/snow to rain transition. Details can be made once the
snowfall from Sunday night/Monday is handled better.
Weakening short wave in southern stream aloft along with southerly
flow above surface cold damming wedge look to provide some lift for
light precipitation Tuesday night. Still a bit far out but GFS
forecast soundings showing lower saturation with dry air aloft. With
temperatures falling to around freezing and lack of moisture in ice
growth region aloft...freezing drizzle will be a possibility.
Confidence is not high at this time and will go with rain or snow in
the forecast and continue to monitor potential. Northern stream low
approaching the Great Lakes will push trailing surface cold front
through the region late Wednesday/Wednesday night. Should see some rain
showers with prefrontal trough. Temperatures should moderate then through
the end of the work week as storm track looks to remain north with
zonal flow aloft.
Aviation /01z Sunday through Thursday/...
winds have already dropped off to at or below 10kt across the
region...though a light northwesterly breeze will remain over northern sections
of the middle-Atlantic - dropping off toward central Virginia. High pressure will
roll down toward the region overnight through Sunday night.
Clouds will increase Sunday and snow will overspread the terminals
late Sunday through Sunday night. Ceilings/visibilities will deteriorate to
IFR/subifr levels during this time.
Ongoing IFR conds Monday morning become MVFR as snow tapers to light
and ceilings raise a bit. MVFR/IFR conds and northeasterly flow east of
I-95...Ely west continue through Tuesday as cold air damming
MVFR/IFR conditions will be possible Tuesday night and
Wednesday due to increasing low level moisture and cold front.
winds will diminish later tonight as high pressure builds toward
the waters. However...low pressure will pass by to the south of
the waters Sunday night and a Small Craft Advisory is in effect
for the Maryland Chesapeake Bay and lower tidal Potomac River due
to an increasing pressure gradient.
NE flow over the waters from surface pressure ridge that drapes across
the Chesapeake Bay through Tuesday. Small Craft Advisory likely continues for Monday
and Tuesday...particularly for southern Maryland waters.
Small Craft Advisory conditions may be met on Thursday behind a departing
District of Columbia...Winter Storm Watch from Sunday afternoon through Monday
afternoon for dcz001.
Maryland...Winter Storm Watch from Sunday afternoon through Monday
afternoon for mdz003>007-009>011-013-014-016>018-501-502.
Virginia...Winter Storm Watch from Sunday afternoon through Monday
afternoon for vaz027>031-039-040-042-050>057-501-502.
Winter Storm Warning from 2 PM Sunday to 2 PM EDT Monday for
WV...Winter Storm Watch from Sunday afternoon through Monday
afternoon for wvz050>053-055-501>504.
Winter Storm Warning from 2 PM Sunday to 2 PM EDT Monday for
Marine...Small Craft Advisory from 6 PM Sunday to 6 am EDT Monday for