Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Baltimore Maryland/Washington District of Columbia
850 PM EST sun Jan 25 2015
high pressure builds into the Middle Atlantic States briefly today.
Low pressure over Iowa will track across southern Virginia
tonight...strengthening as it moves up the coast Monday and Tuesday.
High pressure is then expected through midweek.
Near term /through Monday/...
surface lopres 1005 mb between sdf and Lex. However...the water vapor loop
depicts an impressively robust system...even cranking out a bit of
lightning across Dixie. Itll still be several hours though before the catalyst
for any significant precipitation will reach County warning forecast area. Until then...any precipitation should be real
light...ie sprinkles or flurries. Since clouds have overspread
area...temperatures not dropping much. Believe evap cooling will be the
triggering factor for that to hppn.
With that in mind...will be making a few small tweaks to the forecast.
Will be padding snow accumulate to the central shen Vly/blurdg...to acct
for dynamical cooling/precipitation production in vicinity of positive vorticity advection. That would provide
sufficient accums to add the central Virginia Blue Ridge zone to a
Winter Weather Advisory. Will also be bumping min-T upwd by a degf or two from
No changes to the synoptic evolution as described previously. Will
leave that part of the dscn below.
Concerning the sensible weather...precipitation pushes east into the
area...following the Mason Dixon line as a shortwave lifts north.
Timed the precipitation following a blend of the hires arw/nmm...which keeps
the precipitation this evening mainly north of the District of Columbia/Baltimore metros. As
the low nears overnight and the trough begins to rotate heading to
the negative tilt...the precipitation overspreads the area...coming from a
south and west position. Temperatures initially in the upper 30s/lower 40s
this evening would suggest all rain...with some snowflakes mixing
in. As the better dynamically forcing moves through...will see the
precipitation changing to all snow from a north to S path. 12z NAM and sref
BUFKIT soundings are the colder solutions...but really lag the
change over. The 12z GFS...a much warmer solution...holds onto the
rain for many portions of the area well into Monday before the cold air advection
really takes hold. Was initially impressed with 850mb temperatures a couple
days ago dropping to -7 degrees celsius...but now GFS/European model (ecmwf) and NAM
all keep around -2 degrees celsius through Monday. The thinking is it
will take the heavier quantitative precipitation forecast amts to result in an all snow solution and
as such have time the change over to correspond as the heavier precipitation
moves in. Quantitative precipitation forecast amts were tricky...with GFS on the higher end and then
the European model (ecmwf) and NAM on the lower side. Took a blend of the GFS/European model (ecmwf)
to account for the spread. When calculating snow totals...kept with
a 10:1 ratio for tonight. Would like to stress that the timing of
the changeover will be crucial to the snow totals for tonight...with
some areas maybe getting nothing...and some seeing an additional 1-2
The low pushes offshore with the upper level trough becoming negatively
tilted during the day. As the low moves east...an easterly flow set
up will keep plenty of moisture being injected in over the area
throughout the day. On going precipitation during the period as the upper level
energy shifts from the western portions of the County Warning Area and aligns along
a line parallel to the Chesapeake Bay. Have timed out the heaviest
precipitation corresponding to the better frontogenetic forcing...which has the
line of heaviest precipitation lining the Blue Ridge in the morning. 12z NAM
and GFS have captured this with the European model (ecmwf) hinting at it. Continued
to use a blend of the GFS and European model (ecmwf) for quantitative precipitation forecast totals throughout the
day Monday. As for precipitation types...expecting to see actually a little
warming in the morning...then should see decreasing temperatures heading
into the afternoon as the colder air is ushered on the backside of the
deepening low...now offshore. Thinking the typically colder areas of
Baltimore metropolitan extending to the south and west of District of Columbia metropolitan will
stay colder during the time as a northeast flow sets up. So will
keep them all snow...with rain/snow mix to the south...or possibly
even a period of all rain in the morning. As the temperatures cool in the
afternoon...expecting everywhere but southern Maryland to become snow.
Again...should be noted...any difference in p-type will change the
snow totals...especially considering that as the day progresses and we get
colder air...should expect to see increasing snow ratios.
Short term /Monday night through Tuesday night/...
offshore low pressure strengthens rapidly Monday night...while an
upper low moves across the County Warning Area into the Atlantic Ocean.
Enough moisture over the area...combined with temperatures below freezing
over most of the County Warning Area...will result in continued snow. Main
question for Monday night period will be whether an axis of
heavier snow will set up west of Chesapeake Bay before lifting off
to the northeast. A Winter Storm Watch has been issued for the
Maryland counties along the western shore of Chesapeake Bay.
Will be awaiting 00z guidance. May make some decision at that time if
sufficient consistency displayed on where the back egde of the
precipitation shield will reside.
Further west...another inch or so of snow is likely across north
central Maryland...the eastern WV Panhandle...the Washington District of Columbia metropolitan
and northern Virginia. Refrained from extending the Winter Weather Advisory
at this time due to the likelihood that additional snow would take
place after evening rush hour...and end before the Tuesday morning
The low will continue to push to the northeast Tuesday...although
GFS and European model (ecmwf) differ on whether it progresses farther than Long
Island. Best chance for additional snow accumulation continues to
be along the western shore of Chesapeake Bay...with snow otherwise
ending from west to east across the County Warning Area. Cold air will advect into
the region with northerly flow around the back side of the
low...with gusty winds likely, high temperatures Tuesday with be in the
Combination of strong high pressure to the west and the offshore
low east of New England will continue bring in very cold air from
Canada Tuesday night. Low temperatures will drop into the teens over much
of the County Warning Area...with low 20s along and east of I-95. Any remaining
precipitation comes to an end Tuesday night...except potential upslope
snow over the Potomac Highlands.
Long term /Wednesday through Sunday/...
high pressure will build across the region Wednesday and Wednesday
night. Dry conditions expected with high and low temperatures about
10 degrees below normal.
High pressure will move east Thursday to allow for a Great Lakes low
pressure system to drag a cold front across the region late Thursday
and Thursday night. There will likely be a period of light rain and
light snow along the low's warm front and ahead of the low pressure
system. As we venture into Thursday night and the low passes
overhead with a trailing cold front...snow showers are expected and
could lead to a small accumulation. Additional and higher
accumulations are expected along the western ridges in the Potomac
Highlands due to upslope wind. Temperatures will remain below normal
by about 3 to 5 degrees.
There is a chance for lingering snow showers Friday...mainly in the
west and north as an upper level trough of low pressure pivots
across the region. A downsloping wind off of the Potomac Highlands
and Blue Ridge should suppress a lot of the snow showers.
High pressure will build into the region late Friday through
Sunday...allowing for drier and more peaceful weather conditions.
Temperatures should remain below normal.
Aviation /02z Monday through Friday/...
VFR conds prevail across the terminals at this time. Anticipate flgt conds
dropping before morning push as precipitation arrives. Have pushed LIFR back
into middle morning. Am concerned that precipitation prior to that will either be
light or have a healthy contribution from rain /outside of mrb/. Once at or below
IFR conds arrive...they will likely last through Monday morning with periods of
heavier snow. Precipitation type questionable as possible rain/snow mix or all
rain at kdca/kbwi/kcho at first...then becoming all snow aftr 12z.
Other terminals more likely only a brief period of rain/snow mix
becoming all snow overnight.
Ongoing precipitation Monday with snow...IFR or lower conditions likely.
Another round of heavier precipitation (all snow) Monday night into Tuesday keeps
IFR or lower conditions in place.
Sub-VFR conditions likely with light snow Monday night. Heavier
may develop Monday night or Tuesday morning near kbwi/kmtn with
north-northwest winds gusting 20-25kt. Snow may continue at kbwi/kmtn
Tuesday. Any leftover snow at the other terminals should end.
Gust northwest winds expected across the region...around 20-25kt...with
some 30kt gusts possible. VFR Tuesday night. Gusty northwest winds continue.
VFR conditions Wednesday. However...northwest winds will be
gusting 20 to 30 knots.
IFR or perhaps LIFR conditions could develop Thursday with periods
of light rain and light snow near the terminals with an approaching
low pressure system.
sub-Small Craft Advisory conditions continues this evening. Winds will be incrsg
overnight...however the best chance for gusts to 20 kts looks to be toward
dawn...and especially then during the day Monday.
Gusty north-northwest winds expected across the waters Monday night as low
pressure strengthens offshore. Will expand glw to include the entire
chesepeake Bay. Middle-upper ptmc a bit too far west...and am not certain
how far west the p-gradient would extend. Gale may need to be expanded
by Tuesday morning as colder air aloft allows better mixing.
Small craft advisories are likely Wednesday morning as northwest
winds gust 20 to 30 knots.
Small craft advisories may be needed Wednesday afternoon...depending
on how fast northwest winds diminish into the overnight.
District of Columbia...Winter Weather Advisory from 2 am to 6 PM EST Monday for
Maryland...Winter Weather Advisory until 6 PM EST Monday for mdz004>006-
Winter Storm Warning until 6 PM EST Monday for mdz003.
Winter Weather Advisory from 2 am to 6 PM EST Monday for
Winter Storm Watch from Monday evening through Tuesday
afternoon for mdz006-011-014-017-018-507-508.
Winter Storm Warning until 6 PM EST Monday for mdz501-502.
Virginia...Winter Weather Advisory until 6 PM EST Monday for vaz025>027-
Winter Storm Warning until 6 PM EST Monday for vaz028.
Winter Weather Advisory from 2 am to 6 PM EST Monday for
WV...Winter Weather Advisory until 6 PM EST Monday for wvz505-506.
Winter Storm Warning until 6 PM EST Monday for wvz051>053.
Winter Storm Warning until 6 PM EST Monday for wvz050-055-
Marine...Small Craft Advisory until 6 PM EST Monday for anz530>534-
Gale Warning from 6 PM Monday to 6 am EST Tuesday for
Small Craft Advisory until 6 am EST Tuesday for anz535-536.