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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
215 am EST Wednesday Mar 4 2015

Synopsis...
a cold front will move into the region Wednesday night followed
by high pressure for Thursday night lasting into the weekend.

&&

Near term /through today/...
had to issue a short fuse Freezing Rain Advisory for northern
Baltimore and Harford counties as light rain is moving through
with multiple mesonets at 31-32 degrees. Trace amounts of ice on
cold surfaces.

Previous...

A complex frontal system will begin to impact the middle Atlantic
region on Wednesday. Ahead of the front...milder air will push
north into the forecast area...though not to the extent that
previous guidance suggested. Much of the area will be shrouded in
cloud cover and periods of light rain. Temperatures will likely
top out in the 40s in most locations with perhaps the lower to
middle 50s central Virginia to southern Maryland. Kept probability of precipitation categorical all
day Wednesday. Precipitation should remain as liquid in all areas
through 00z Thursday.

&&

Short term /tonight through Thursday night/...
guidance continues to paint an anomalous picture...draining cold air south
Wednesday night in wake of cold front. While theres not a strong low or positive vorticity advection...
theres plenty of forcing available thx to strong baroclinic zone and rrq
of upper jetmax. A stream of ample moisture will be overrunning the cold
air...sourced from the western Gulf of Mexico and Pacific. Therefore...it
seems well within reason to have quantitative precipitation forecast continuing while temperatures drop below
freezing. That supports a scenario where rain will change to snow.

There are still timing questions as well as whether the entire
column will cool in unison. If not...then ptype may be more pl and
less snow. That is more of a concern across central Virginia and southern Maryland.
In addition...there is still a range of solutions with respect to how much quantitative precipitation forecast
will fall as something frozen. Solutions seem to be coming in cooler
and wetter. Given the range of possibilities...do not wish to
underestimate potential. Forecast accums have been raised a bit across
the north...where steady snow seems apparent. Did not do likewise
across the S due to the pl potential. Regardless...at the 50 percent
confidence level...see enough evidence to raise a watch for the entire
area.

Precipitation will continue into Thursday afternoon. Cold air continues to stream southward Thursday night
aftr precipitation ends. Stuck close to a prvs forecast/MOS consensus for
min-T Thursday night. Have plenty of lows in the single digits.

&&

Long term /Friday through Tuesday/...
after a very cold start Friday morning...temperatures will struggle
to rise during the day in the continued weak cold air advection...with highs
remaining below freezing over much of the area. Despite the return
of southerly winds overnight Friday...temperatures will still be
very cold. Went a little warmer than guidance for Friday night and
Saturday due to the increased weak warm air advection.

Weak cold front associated with low pressure well to our north
slides through the area Saturday afternoon/evening with little
effect on sensible weather...except for a wind shift. There is a
chance of some upslope snow showers in the far west...but left probability of precipitation
out of forecast until a clearer signal appears. Beyond Saturday
model guidance becomes noisier with varying model solutions...so
went with a blend of HPC and model guidance.

&&

Aviation /07z Wednesday through Sunday/...
sub VFR conditions expected at all terminal sites through
Wednesday. Precipitation will transition to plain rain later this evening
except later tonight near the Mason Dixon line. Patchy fog...MVFR
to IFR ceilings and visibilities expected into Wednesday with periods of
light rain expected.

At or below IFR will continue Wednesday night-Thursday due to mainly snow. Imprvg conds Thursday
night.

Do not anticipate any aviation concerns this weekend.

&&

Marine...
Small Craft Advisory conditions may be possible over a portion of the waters on
Wednesday. Visibilities will be reduced due to rain Wednesday.

Rain at the beginning of Wednesday evening will transition to snow and possibly some
pl. As cold air drains southward...mixing will imprv. Have raised a Small Craft Advisory Wednesday
night to acct for that...which likely will need to continue into at least Thursday.

With light and variable winds through much of the period... do
not anticipate any marine concerns Friday through Sunday.

&&

Hydrology...
through Wednesday afternoon...around a half to three quarters of an
inch of rain is expected...except for heavier amounts that could
approach 1.0 - 1.5 inches along the Allegheny Front. Temperatures
will rise well into the 40s everywhere /and 50s in the Rappahannock
and James River basins/...promoting snowmelt. The snowpack is
currently estimated to be holding anywhere from a half inch of water
in the cacapon/Lost River basin to around an inch and a half in the
South Branch Potomac basin. Modeling indicates not all of this snow
water will be released...especially given a total lack of sunshine.

Given this...the combination of snowmelt and rain will promote rises
on area rivers...but at this time streams are likely to remain
within-bank.

Some complicating factors to keep in mind...if the changeover from
rain to wintry precipitation on Wednesday evening occurs later than
expected...concerns may become heightened. Also many of our area
streams are covered with ice or even partially frozen. These effects
will have to be monitored for a potential role in affecting runoff.

Lastly...as we saw with the recent rain/ice/minor snowmelt...any
amount of liquid precipitation stands a good chance of causing minor
poor drainage flooding...as normal drainage patterns may be blocked
by plowed snow.

&&

Climate...
as everyone can attest this was one of the coldest febs on
record. West/ the District of Columbia Cherry Blossom festival planned for 3/28-4/12 I
took a look at past cold febs and when peak bloom occurred. Going
back to 1921...it appears that 2015 (avg temperature 30.3 at dca) had the
4th coldest Feb for the period.

The five coldest....and the peak Blossom dates (fm nps.Gov web
site):

Yr Feb average temperature Mar average temperature peak bloom date

1934 24.6 41.2 4/15
1979 28.4 51.5 4/2
1936 29.6 50.3 4/7
2015 30.3 ?? ??
2007 30.9 47.7 4/1

&&

Lwx watches/warnings/advisories...
District of Columbia...Winter Storm Watch from this evening through Thursday evening
for dcz001.
Maryland...Winter Storm Watch from this evening through Thursday evening
for mdz003>006-011-013-014-501>508.
Winter Storm Watch from late tonight through Thursday evening
for mdz016>018.
Freezing Rain Advisory until 5 am EST early this morning for
mdz006-507-508.
Virginia...Winter Storm Watch from this evening through Thursday evening
for vaz026>031-040-052>054-501-503>507.
Winter Storm Watch from late tonight through Thursday evening
for vaz025-036>039-050-051-055>057-502-508.
WV...Winter Storm Watch from this evening through Thursday evening
for wvz050>053-055-501>506.
Marine...Small Craft Advisory until 6 am EST early this morning for
anz530>543.
Small Craft Advisory from 1 am Thursday to 6 am EST Friday for
anz530>543.

&&

$$

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