Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1125 am EST sun Feb 1 2015
low pressure will approach and cross the region through early
Monday. Much colder air is expected early in the week.
Near term /through Monday/...
snow continues across the region as an initial shortwave advns across the area
ahead of developing low pressure. Will maintain advisories/warnings as is at this time but will
reevaluate this afternoon. Critical thicknesses/morning radiosonde observations show snow will
be the main precipitation type until warm advection changes the precipitation over to rain S
of I 70 this afternoon. Nudged temperatures to the latest NAM numbers for the lt
Previous disc...second warm punch accompanies approach of h850 low
tonight...which will track across Northern Ohio to Erie by 12z
Monday. Increasing SW flow will bring a renewed feed of
moisture...and also the h850 zero line to just south of I-80.
Thus...snow will mix with rain and freezing rain...before turning
to all rain for the majority of the County Warning Area by 06z Monday. Surface low
passes near or just south of pit late tonight...which will turn
winds to more of a westerly component and start to bring colder
air back in. Expect most of Ohio to turn back to snow by 12z
Monday...with rest of County Warning Area following in quick order later Monday
morning. Probability of precipitation decrease as well with deeper moisture being shunted
off to the East. Forest/Venango/Mercer still pictured as remaining
mostly snow...although a brief mix will still be possible tonight.
Wintry mix and rain will hold back accumulations south of there.
Short term /Monday night through Tuesday/...
dry slot will begin pushing east of the County Warning Area line by Monday morning as
we transition to cold advection in the wake of the low. 12z Monday
Marks a good transition point in the system as most precipitation will have
fully changed over to snow as the surface low moves east through PA.
Stratiform snow will still be present through middle-morning as the
middle-level deformation band evident on the latest NAM swings
through...despite a Stout 500mb jet that will inject dry air into
axis of the upper low. Copious moisture still exists from the surface to
700mb and remains through the early evening as the bottom drops out
on the middle-level temperatures.
Very strong cold air advection will bring 850mb temperatures down nearly 20c in the matter
of 12hrs. The dendritic growth zone will drop like an anchor to the
surface allowing every bit of moisture to precipitate out in the form of
snow. Rapid cooling of the middle-levels will translate to the surface
causing additional problems as surface temperatures are expected to decrease
through the day. This will mean that untreated pavement surfaces
could see flash freezing...and that slick spots could exist even in
the absence of earlier heavy snow or mixed precipitation.
Snow showers in cold air behind the upper low could produce around
an inch of fluffy accumulation across much of the lowland
counties...with a little more in the ridges of PA/MD/WV and up
north. Expecting fluffy snow showers to finally taper down to
flurries and then fully cease by midnight Monday as the moisture dries
up. Temperature forecast Tuesday morning is tricky...as soon as the
cold air arrives...its gone and warm air advection begins to kick in by day break
Tuesday. Will remain below guidance Tuesday morning as we decouple under
high pressure with clearing skies and a lingering snowpack. Calm
surface winds may preclude the need for a Wind Chill Advisory along the
north Tuesday morning...but it will be downright cold regardless.
Long term /Tuesday night through Saturday/...
the temporary exodus of high pressure Marks the beginning of the
extended as a weak low moves through the southern Great Lakes by
mid-week. This relatively dry system should produce light snow along
the northern periphery of the County Warning Area Wednesday into Thursday...but just as
important...the trailing cold front will reinforce the upper level
trough as cold air pours in towards the end of the week. High pressure
will ensure a dry start to the weekend...but another low is poised
to impact the area by next sun.
Aviation /16z Sunday through Thursday/...
initial band of light snow in place...with mostly MVFR ceiling and
IFR visibility. Expect this band to lift out by midday. Fkl/duj should
remain in the snow for the most party with low MVFR/IFR. Low
confidence forecast for the afternoon. Renewed moisture push
tonight as surface low approaches...with all sites expected to
drop down to IFR and perhaps LIFR. Fkl/duj will continue with
snow...while other sites change to rain or have a period of wintry
mix. Did not include icy precipitation types yet...hoping for more
clarity in future forecasts. Colder air returns to zzv just before
the end of the taf period...with rain changing back to snow.
Outlook.../Monday through Thursday/...
widespread restrictions can be expected into Monday as a low
pressure system crosses the region. Next chance for widespread
restrictions will be with a Wednesday cold front.
Ohio...Winter Weather Advisory until noon EST Monday for ohz039>041-
PA...Winter Weather Advisory until noon EST Monday for paz020-021-
Winter Storm Warning until noon EST Monday for paz007>009-
WV...Winter Weather Advisory until noon EST Monday for wvz001>003.