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Area forecast discussion 
National Weather Service Pittsburgh PA
346 PM EST Sat Feb 28 2015

Synopsis...
snow will arrive late tonight and will mix with rain in some areas
on Sunday. Rain will change back to snow Sunday night. A wintry
mix on Tuesday will give way to above-freezing temperatures
Tuesday night and Wednesday before cold returns late in the week.

&&

Near term /through Sunday night/...
the rest of the afternoon and the early evening will be
quiet...with only increasing clouds in the westerly flow aloft.
Main focus centers on precipitation event late tonight through
Sunday night and resultant snow amounts.

Broad warm advection will be the main driver of precipitation
through the period...until frontal passage Sunday night. A frontal boundary
will start to organize to the northwest of the County Warning Area on Sunday...
with moist SW flow over our area. This will also push warmer air
northward. Shortwave crosses the Great Lakes Sunday night and acts
as the impetus to get the front moving...and expect it to cross by
06z Monday. Colder air then spills back in behind. Models are in
general agreement with this pattern.

Expect first flakes to arrive near zzv around midnight...and
spread east from there. Expect most points to see snow by 12z as
lift fed by warm advection and a modest isentropic contribution
becomes more widespread. Expecting rain to mix in later in the
morning as the true low-level warm punch arrives. Icing threat is
fairly low overall with not much of a warm layer aloft...but could
see a trace of icing from freezing rain/ice pellets across some of the southern
counties at the beginning of the changeover. Expect the rain/snow
mix to reach as far north as bvi/idi...with a period of all rain
at southern sites like mgw...and mostly snow to the north.
Changeover back to snow will occur after 00z as cold advection
returns behind front.

Regarding snow amounts...am still not impressed by snow ratios
given the warm intrusion. Despite expecting mostly snow across the
north...the expected low ratios...dropping to around 12 to 1
Sunday across the north and lower to the south... plus the
24-30 hour duration of the Storm... Point to the need for a high-end
advisory rather than a warning. Surrounding offices agreed to
this...and we ended up with a broad Winter Weather Advisory.
Totals range from 6-8 inches in 24-30 hours across the far
north...to 3-6 in a ribbon from zzv through pit to lbe...and 3 or
less to the southeast. Keep in mind that a mix with or change to
rain will occur for much of the area...so some of the initial
snowfall will melt on the ground in rain. Kept several southern
counties out of the advisory given lesser snow totals and no
significant icing.

Expect precipitation to start to wind down later Sunday night as
deeper moisture pulls off to the east and subsidence takes over.

&&

Short term /Monday through Tuesday night/...
snow will be winding down across the area on Monday morning as
high pressure builds into the region from the west. Snow showers
along the ridges will persist until inversion height lowers
sufficiently...likely by late Monday afternoon.

Aggressive south-southwesterly 850mb jet will strengthen Monday
night...promoting advection of anomalously potent warm and moist
air from the Gulf of Mexico. Light precipitation in this warm air
advection regime will begin to encroach on the forecast area late
Monday night and rapidly across the region Tuesday morning.
Precipitation likely will begin as light snow as overnight
temperatures will drop into the middle-upper teens.

In response to deepening low pressure in Illinois...wind will
shift to the east-southeast late Monday night...which will do
little to vacate the very cold low-level airmass despite the
strong warm air advection aloft. Historically...southerly wind
direction is needed to eradicate low-level cold air...which will
not occur until later in the day Tuesday when surface low
pressure nears. As a result...the atmosphere will be primed for
the early light snow to transition to a period of freezing rain/sleet
on Tuesday morning. Weather and ice grids were adjusted to reflect
this expectation.

Surface trough and cold front will encroach upon the area Tuesday
late afternoon and evening...with surface low pressure passing to the
northwest of the forecast area. The Ohio Valley will be solidly in
the warm sector as wind veers to southwesterly...helping to erode
the low-level cold air and change all precipitation to rain.

All models and MOS guidance offer strong evidence of measurable
precipitation occurring Tuesday night...focused in part by the
approaching boundary. Probability of precipitation were increased to 100 percent during
this time period to reflect this expectation...timed with the
passage of the boundary.

There is a good synoptic signal for an anomalous rainfall event
with this system...amid strong moisture flux /pwats approaching
maximum climatological values/ and persistent southwesterly low-
level flow. That said...models also tend to overdo quantitative precipitation forecast in warm air
advection patterns. Given that this system is an open wave and
precipitation is driven in part by warm advection...quantitative precipitation forecast on Tuesday
was toned down slightly from the more assertive GFS values.

Temperatures Tuesday night likely will climb or remain steady in
the 40s...perhaps warmer than their Tuesday daytime values.

&&

Long term /Wednesday through Saturday/...
cold air advection in the wake of the Tuesday night cold frontal
passage will return the region to near- or sub-freezing
temperatures for the remainder of the week. Upslope snow showers
likely will persist through Wednesday evening...but should see a
marked decrease in coverage by late Wednesday night.

A quick-moving shortwave trough could affect the region late in
the week with little fanfare anticipated. The dry airmass at the
surface precludes significant impacts at the moment save modest
lake-enhanced snow.

&&

Aviation /20z Saturday through Thursday/...
VFR conditions persist this evening as cloud heights drop from 12kft
to near 5kft amid strong warm air advection. Snow is expected to
develop late tonight...around 6z near zzv to around 9z near
lbe/duj. Once snow commences...conditions will deteriorate to IFR
by morning.

Snow will mix with and change to rain at most terminals /except
fkl and duj/ mid-day...with mgw seeing mostly rain through the day. A
wind shift is expected Saturday evening associated with a cold frontal
passage. Precipitation will change back to snow in the wake of
the cold front. Restrictions likely will persist through much of
Monday.

Outlook.../Saturday night through Wednesday/...
high pressure Monday night should lead to VFR conditions. Strong
warm air advection and shallow cold air likely will lead to a
wintry mix Tuesday morning before widespread rain ensues Tuesday
afternoon and night. Restrictions are likely during this time.

&&

Hydrology...
first significant river rise of the season expected Tuesday through
Thursday Mar 3 through 5...

Many of the area rivers...their tributaries and smaller streams
and creeks are ice covered. A dramatic increase in flow and stage
levels will cause ice movement starting Tuesday Mar 3.

Current forecast has a high level of uncertainty about many
critical factors...including change over to rain...rainfall
amount...temperature and snow melt. Ice movement will initially
begin in small streams and tributaries Tuesday Mar 3...with a
cascading effect when coupled with rainfall and snowmelt runoff
starting Tuesday night.

Rises on the major rivers are projected to begin late Tuesday
night over the southernmost river basins. These rises and possible
ice movement will extend farther downstream through Thursday Mar
5.

&&

Climate...
record low temperatures were set this morning. Pit tied its
previous record low of -1 set in 1934. Zzv set a new record low of
-1...breaking the old record of zero from 1947.

&&

Pbz watches/warnings/advisories...
Maryland...Winter Weather Advisory from 4 am Sunday to 7 am EST Monday
for mdz001.
Ohio...Winter Weather Advisory from 4 am Sunday to 7 am EST Monday
for ohz039>041-048>050-057>059.
PA...Winter Weather Advisory from 4 am Sunday to 7 am EST Monday
for paz007>009-013>016-020>023-029-073>076.
WV...Winter Weather Advisory from 4 am Sunday to 7 am EST Monday
for wvz001>003-023-041.

&&

$$

Cl/kramar

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