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Area forecast discussion 
National Weather Service Pittsburgh PA
604 PM EST Friday Feb 27 2015

after a cold night clouds will increase Saturday followed by rain
and snow Sunday through Monday morning. A significant warm-up is
possible Tuesday into Wednesday before cold air returns.


Near term /through Saturday/...
adjusted temperatures and sky grids based on current
readings...Sat pictures...and 18z guidance. Clear skies followed by
some cirrus as surface ridge sits on top of region overnight. Snow
cover and light winds will allow for good radiational cooling
and lows near or well below zero.

Surface ridge will move east during the day Saturday with cirrus
and middle deck increasing toward evening. Highs will recover into
the 20s.


Short term /Saturday night through Monday/...
model guidance continues to be in good agreement in the low
pressure system that is forecast to make its approach late
Saturday night/Sunday morning. This low will move northeastward
through the upper Ohio Valley with the support of a digging upper
level trough during the day on Sunday. With the lows placement
just a bit to the north...warm advection looks to support a mix
of rain and snow for part of the region. Although confidence on the
exact placement of the rain-snow line and it's movement on Sunday
still remains in question...the last several model runs have
trended warmer with near/above freezing surface temperatures and
have pushed critical thickness values further north. Thus...while
all of the deterministic runs suggest 0.75 inches of quantitative precipitation forecast area-
wide...warm air aloft and at the surface should cut down on
snowfall accumulations for the locations that remain in the area
with greatest uncertainty. At this point...the highest confidence
in accumulating snow remains for points north of I-80 and a Winter
Storm Watch has been issued for those counties. With these
locations remaining coldest through the entire day on
Friday...even modest snow ratios should bring values close to 6
or more inches. Further south...the changeover to rain should cut
down on accumulation and points south of I-70 are most likely to
see the longest period of rain at this point.

A changeover back to snow is expected everywhere overnight Sunday
as cold advection returns with the departure of the low.
Subsidence will quickly end snowfall for most of the area by Monday
morning...with lingering snow possible in the ridges.


Long term /Monday night through Friday/...
a warm up in temperatures expected early on in the extended as
troughing deepens over the western US and southwesterly flow sets
up across the eastern states. A closed-off area of low pressure
ejecting out of the Desert Southwest early in the week will start
to transition east Monday and help drag in moisture off the socal
coast into the strong southwesterly flow...with additional
moisture pooling in from the Gulf. Ensemble members are showing
minimal spread in this setup...which may result in a significant
amount of precipitation through the middle of the week across the
region. Forecast precipitable water values are around the 99th percentile by
Wednesday over the area and ivt values are showing a bulls eye
over the region. This...combined with the additional snow across
the north and a riper snowpack to the south raises flooding
concerns. We will be following this closely in upcoming model
runs. See hydrology section below.


Aviation /00z Saturday through Wednesday/...
VFR conditions will persist overnight amid light wind...and
through Saturday as the center of high pressure moves east of
the area.

Outlook.../Saturday night through Wednesday/... restrictions are
likely Sunday into early Monday...and again Tuesday night and
Wednesday as low pressure systems cross the region.


..first significant river rise of the season expected Tuesday and
Wednesday Mar 3 and 4...

Given the fact that many of the area rivers... tributaries and
smaller streams and creeks are covered over with ice... 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 factors...including rain amount...temperature...snow
melt and ice movement. Small streams and tributaries will be first
affected Tuesday Mar 3. Rises on the major rivers are projected to
occur starting Tuesday night and continue through Wednesday Mar 4.


Pbz watches/warnings/advisories...
PA...Winter Storm Watch from late Saturday night through Monday
morning for paz007>009-013>016.



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