Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
331 am EST Tuesday Feb 9 2016
multiple low pressures impact the region through Wednesday before
pushing off to the northeast. High pressure builds in the wake
through Friday. A quick moving cold front swings through Friday
night with Canadian high pressure building in for the weekend.
Near term /through tonight/...
a few adjustments being made this morning with the latest model
runs converging on dcrsg potential of the higher snow totals.
Overall...have dropped snow totals anywhere between 1-3 inches
across the area. As such...have enough confidence to cancel the
Winter Storm Warning...replacing with a Winter Weather Advisory through
this evening. The Winter Weather Advisory already in place remains the
same except have cancelled for District of Columbia/Arlington/Fairfax and Prince
William with the increased confidence now of less than 2 inches...
advisory criteria. Coastal flood advisories are in effect today as
well...please refer to the coastal flood section for more details.
The light rain has been slow to transition to snow or even a
rain/snow mix tonight outside of the Potomac Highlands. Upper level
trough across a good portion of the eastern US the main player in
precipitation development early this morning with one surface low well north over
the eastern Great Lakes...and a second low organizing off the coast
of Virginia. As this second low deepens...pushing off to the NE...
expecting an inverted trough extending back across Maryland into PA to
act as a trigger for further precipitation development over the eastern
half of the County Warning Area this morning and the afternoon. With the upper level
trough in place...a potent shortwave moves into western Virginia...which
will act as the trigger for snow showers for the western half of
the County Warning Area. As it moves over Delaware-Maryland-Virginia this evening...the precipitation area
will transition...leading to drying over central Virginia.
While there is higher confidence for the precipitation location/timing...
there is now less confidence of the higher quantitative precipitation forecast amts...and the snow
potential leading to higher snow amts. As previously mentioned...
backed off of snow totals between 1-3 inches for a couple of
reasons. The precipitation was slow to transition to snow tonight with a
lack of heavier bands setting up. Quantitative precipitation forecast totals on the 00z run are
also lower. A question that we had been discussing for the past
few days was the thermal profiles and the resulting precipitation. While
there is plenty of cold air aloft...the lowest layers were
struggling to drop below freezing. Had there been heavier precipitation...
had thought wet bulbing would occur...transitioning to the rain to
snow. However...temperatures are still hovering in the middle to upper
30s...and even dew points are above freezing...suggesting it may still
be some time before most of the County Warning Area changes over. Thinking this
should occur between 10-15z as winds become more northerly...injecting
in lower dew points. BUFKIT profiles show both with the NAM and GFS
that southern Maryland and areas of the Piedmont may have a Snowflake or
two...but no accumulate. Moving north...Culpeper to District of Columbia and south will be
no more than an inch. The highest amts are still the northeast
corner of Maryland...topping off under 5 inches...but these may still be
overdone...with 3-5 inches being more sparse than widespread. Also
to note...that with the warmer temperatures...the snow may fall for a
good while before actually accumulating.
Short term /Wednesday through Thursday night/...
higher confidence in the middle of the week forecast as the focus
turns from precipitation threat and more into the much colder temperatures. With
the departing low...expecting northwest flow to persist through this
time...keeping upslope snow showers in the forecast through Thursday.
The Winter Weather Advisory for the Allegheny Front is in effect through
Thursday with between 6 and 10 inches possible from now till then.
Outside of the upslope snow...as the upper level trough pivots
east...enough energy aloft could trigger an isolated snow shower along
the Mason-Dixon...though with the much drier airmass in place...it
will be difficult and it may end up being more just flurries.
For the temperatures...Wednesday will be slightly cooler than today...with the
first surge of cold air advection coming in during the day...dropping lows Wednesday
night into the single digits and teens. Wind chill advisories may be
needed for the Allegheny Front with wind chills dropping to below
-10 degrees. The threat will be possible again Thursday night.
Long term /Friday through Monday/...
Arctic air will be reinforced Friday-Sat by another disturbance within
500 mb trough. Guidance suggesting that the leading edge will track north of the
area Friday...allowing shsn to be caught in the mountains...followed by a
greater push late Friday night/Ely Sat morning. Northwest winds and significant cold air advection will
follow that. Latest guidance suggesting that maxt Sat will be in the teens
ptmc Highlands/20s elsewhere. Wind chills will be a concern then...and will
only be amplified Sat night as temperatures fall into the single digits--even
in the cities of DC/balt. Subzero temperatures forecast in the mountains will have a
gradual moderating trends by the start of the new week...perhaps
with another round of potl precipitation.
Aviation /09z Tuesday through Saturday/...
IFR/MVFR ceilings currently being observed across the taf sites with
light rain impacting all but kcho. As the precipitation transitions to all
snow...expecting ceilings to become IFR or LIFR and visible to diminish to
1sm or less. The time frame for transitioning is still low-moderate
confidence...generally 11-14z. Snow persists this morning before
transitioning back to rain at kdca and rain/snow mix at
kiad/kbwi/kmtn. Ceilings/visible will improve to MVFR this afternoon with this
transition. Snow amts will be less than one inch kcho/kdca...1-3
kiad/kbwi...and 2-4 kmrb/kmtn.
The precipitation will taper off completely by tonight with VFR conditions
then expected tonight through Thursday. Winds during the taf period
will be NE less than 10 kts...becoming northwest and incrsg this afternoon...with
gusts possible tonight. Gusty northwest winds will continue Wed-thurs.
VFR most likely Fri-Sat. Winds Sat could gust northwest 20-25 knots.
sub-Small Craft Advisory conditions on all waters through today. Incrsg northwest
winds tonight will produce occasional Small Craft Advisory gusts on the Chesapeake
Bay...becoming more persistent and widespread Thursday and Thursday
Winds Friday will be on the light side...as we/ll be in a weak hipres ridge
ahead of another trough axis. That axis/reinforcing fnt will cross waters
before Sat am. Small Craft Advisory conds expeceted during the day...and most likely
lingering through the night.
water levels shot up to 2.5 feet above astro normals overnight...
which is significantly above any guidance forecast...except perhaps
cbofs. Its on pace to exceed minor flood thresholds at just about
all tide sites on the western shore of the Bay. Havre Delaware Grace will
be tough to achieve...but correlations suggest that minor flooding
should occur south of there near Edgewood. Therefore...have expanded
cf advisory to the entire shoreline. Have also added Charles Colorado to
account for potential effects on/near Cobb Island. Piney Point and
Annapolis may each come close to MDT /wrng/ level...but will hold off
Winds will gradually be swinging around to the northwest this afternoon and
eventually west tonight. However am not sure how strong they will
be...and its therefore in doubt whether they will have a blowout
effect. In addition...the PM tide cycle is the higher of the two
astronomically...so even if there is a pinch of relief...it will be
compensated. For that reason...have extended these advisories for a
2nd tide cycle.
Latest hec-Ras guidance suggests that minor threshold will be
achieved at SW District of Columbia/Alexandria as well. Its unclear whether the big
slug will make it all the way up. Wasd2 seems to be tracking well so
far...but there are the first indications at Alexandria that the low
tide wont be nearly as low as modeled. That could play into the rest
of the Potomac shoreline...which is something that will be monitored.
District of Columbia...coastal Flood Advisory until 10 am EST this morning for dcz001.
Maryland...Winter Weather Advisory until 6 PM EST Wednesday for mdz501.
Winter Weather Advisory until midnight EST tonight for
Coastal Flood Advisory until midnight EST tonight for mdz011-
Virginia...Winter Weather Advisory until 6 PM EST Wednesday for vaz503.
Winter Weather Advisory until midnight EST tonight for vaz027-
Coastal Flood Advisory until 10 am EST this morning for vaz054.
WV...Winter Weather Advisory until 6 PM EST Wednesday for wvz501-
Winter Weather Advisory until midnight EST tonight for
Marine...Small Craft Advisory from midnight tonight to 6 am EST
Wednesday for anz533-534-537-541-543.