Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
244 PM EDT sun Mar 29 2015
a weak system will bring rain and snow overnight into Monday.
Near term /through Monday/...
current satellite imagery showing clear skies across the region as
high pressure begins to push southeast of the area. Southerly flow
is expected to increase through the evening as the center of the
high moves east and the pressure gradient tightens. This should
help accelerate further warming during the late afternoon/early
Tonight...a relatively dry and weak system will move through the
western Great Lakes and into our west. As the main trough axis
enters...a weak warm front will lift just into the southern
extent of the forecast area...keeping temperatures across the
southern half of our counties elevated. Timing across model
guidance has been fairly consistent...but the latest runs push
timing back an hour or two as it fights the dry air firmly
Expecting a suppressed diurnal temperature trend...as warm
air tries to push northward through the overnight into the early
hours Monday. Predominant precipitation type near Pittsburgh and
south is expected to be rain...but areas north of Pittsburgh will
cool enough to transition the precipitation from rain to a
rain/snow mix. Model soundings north of I-80 indicate this area
has the best chance to see all snow. Regardless of precipitation
type...with only modest forcing and overrunning of middle- and upper-
level moisture...precipitation rates should be low. Little...if
any...snow accumulation is expected through Monday morning.
Stratiform precipitation will quickly dwindle quickly west to east
after sunrise Monday. This is due to the deeper moisture
evacuating the area...leaving only modest residual low-level
moisture. Forecast soundings indicate the potential for patchy
drizzle through midday Monday...but confidence not high enough for
inclusion at this time.
Deeper moisture will linger along the north keeping light
rain/snow potential in through much of the day. Chances across the
north will increase again in the afternoon as colder middle-level air
associated with an upper-level wave arrives. This will subsequently
steepen the lapse rates and develop convectively driven rain/snow
showers. Elsewhere...clouds cover should scatter enough for
pockets of sunshine and warming. Expecting a pronounced gradient
of high temperatures Monday from near 50 across the south to upper
Short term /Monday night through Wednesday/...
unstable northwesterly flow will briefly turn west-southwesterly
by late Monday evening to allow for the County Warning Area to dry out and at
least partially clear out. However...an active pattern looks to
continue thereafter. Rather disturbing model trends seem to have
taken hold on the 06z and 12z cycles today...with the models
largely falling into two camps. The NAM and Canadian favor a more
northern track of a system through the area on Tuesday...while the
GFS and 00z European model (ecmwf) favor a more southerly track.
Unsurprisingly...the sref basically split the difference and
allowed for a middle ground forecast...which was largely in line
with continuity...so forecast changes really were not all that
large as a result. It does seem that the GFS was suffering from
some grid scale issues...while the NAM was likely suffering from
its habitual over development issues. These problems further lend
credence to favoring an ensemble mean approach such as the sref
A compromise approach still favors relatively rapid encroachment
of warm advection isentropic ascent over the forecast area late
Monday night into Tuesday. This occurs as a surface low crosses
the area with at least the southern half of the area likely to
enter the warm sector for a portion of that time and probably halt
rainfall there for a time. At the onset...some precipitation type
issues may be present if the southern solutions verify...however
any frozen precipitation should quickly change to rain as warm
advection quickly looks to erode any cold air. This is especially
true given the lack of antecedent dry air as this system rolls
into the area...minimizing the potential for wet bulb cooling as
A relatively quick departure of the system looks to occur Tuesday
night with much drier air flooding into the County Warning Area on Wednesday. Some
temporary cold advection on Wednesday will likely still hold
temperatures back. However...drastic pattern amplification of the
pattern to our west looks to develop a strong thermal ridge that
will be poised to push into the area later in the week. Fries
Long term /Wednesday night through Sunday/...
while the GFS has slowed the arrival of a front on Thursday from
previous model runs...00z European model (ecmwf) is even slower than that.
Given that the GFS is the most open wave solution relative to the
Canadian...gefs...and European model (ecmwf)...timing slower than the GFS is likely
more optimal for the arrival of showers into the County Warning Area. As a
result...warm advection will rule The Roost on Thursday...likely
allowing for the warmest day of the season to this juncture.
However once precipitation arrives...low pressure forming along
the cold front will keep precipitation in the forecast Friday
until it moves into New England Saturday. Temperatures will drop after
Thursday...leading back to yet another period of below normal
Aviation /19z Sunday through Friday/...
VFR conditions will persist with nearly clear conditions. High
cirrus beginning to push in through central Ohio.
A weak system entering the region late this evening is expected to
bring precipitation to all sites. South and west of Pittsburgh
should remain all rain...with some snow mixing in elsewhere.
Ceilings with precipitation should initially begin as VFR...but
are expected to quickly drop to MVFR. Some marginal low level wind
shear may also develop overnight if winds decouple...especially
west of Pittsburgh...but did not have enough confidence to include
in tafs at this time.
Some improvement will occur Monday after daybreak with most
locations returning back to VFR as the front pushes east.
However...deeper moisture will linger along our north and continue
to impact fkl/duj. Lingering rain/snow showers will be
reinvigorated by the upper-level low a it swings through Monday
afternoon near the end of the taf period.
Outlook.../Monday through Thursday/...
restrictions are possible once again with crossing low pressure
Tuesday and Thursday.