Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
1206 am EDT Tuesday Mar 31 2015
low pressure will track across the region later tonight into
Tuesday and bring another round of precipitation. A warming trend
will arrive for the middle of the week.
Near term /through today/...
with midnight update...made minimal changes in timing for probability of precipitation
through sunrise. Still trying to determine just when rain-snow
line will be overnight into Tuesday morning...and have not made
any changes to that portion of the forecast. Previous discussion
Temperatures remain mild this evening with westerly low level flow
and weak warm advection at 850mb. Fast moving clipper type system
over the western Great Lakes this evening will race southeast
overnight and cross the region Tuesday. Adjusted temperatures
mainly ahead of system tonight to remain steady until precipitation
arrives before daybreak. With dew points in the 20s expect wet
bulbing to drop temperatures quickly...especially across the
north. 18z model runs continue to show surface low moving
southeast across The Heart of the region with rain and snow north
of the the I-80 corridor and plain rain elsewhere late tonight
into the first half of Tuesday.
Still remains to be seen how much snow will be able to accumulate
across the far north as newer runs just a bit warmer with critical
thickness. Combined with fairly warm ground...previous snow amounts
still seem a reasonable compromise with 1-2 inches for Forest
County and an inch or so elsewhere north of I-80...with a coating
possible just south of this. Plain rain elsewhere...with all
precipitation transitioning to showers in the afternoon as surface low
crosses the ridges and cold advection sweeps south.
High temperatures will be reached in the morning Tuesday with a
wide range from the middle 50s kzzv to near 40 north of I-80..then
begin a slow fall in the afternoon as north winds behind exiting
surface low increase. Tightening pressure gradient will allow for
gusts to 30 miles per hour with higher gusts possible in the ridges.
Short term /tonight through Thursday/...
precipitation should end rather abruptly Tuesday night...save for
the higher terrain...as the low pressure moves well off to the east
and northwest flow ensues. The back edge of the upper trough and some middle
level moisture should maintain some cloud cover through much of
the night before high pressure ridge builds eastward and dries out
the low-middle levels. Despite mostly sunny skies by midday
Wednesday...northwest flow should keep temperatures close to or just
below seasonal averages. A return of southwest flow is expected
ahead of the next cold front forecast to make it's approach
Thursday night. While we may decouple Wednesday night...allowing
for temperatures to fall back into the 30's...high temperatures
Thursday are expected to be well into the 60's and approaching 70
for points from Pittsburgh on south. This is in part due to strong
warm advection aloft with 850hpa temperatures approaching +10c.
In addition to the warming temperatures...a 50kt low level jet
may provide for some gusty winds out ahead of and along the slow
moving cold front. However...the intricate details of the
forecast on Thursday will be determined later as there is still a
bit of uncertainty in the frontal timing between the three
deterministic models and the ensembles. This timing would impact
the development of any showers and storms ahead of the front...and
subsequent issues with wind. Thus...have opted to stay relatively
close to the previous forecast but wanted to keep very low probability of precipitation
until late Thursday afternoon.
Long term /Thursday night through Monday/...
decent precipitation chances will continue into the weekend as the
front sags across the region and provides focus for subsequent low
pressure passage later on Friday...with an eastward exit on
Saturday. This forecast was based on a blend of GFS trends and wpc
guidance with shortwave timing and boundary placement likely to
remain questionable for a few days.
The Thursday warmth will thus be short lived...but the cool down
will not be as dramatic as this past weekend given the zonal flow
that is prognosticated aloft. Tweaked wpc guidance was used for the
seasonably cool temperature forecast.
Aviation /04z Tuesday through Saturday/...
VFR conditions will continue until low pressure spreads rain and
snow over the area toward morning. IFR...LIFR conditions are likely
in rain and snow at kfkl and kduj. Otherwise...general MVFR with
rain is anticipated for ports surrounding The Pit area.
Otherwise...as the low crosses around middle day and as surface warming
and cooling aloft increase the depth of the mixing layer...surface
wind gusts will increase to around 25 kts with a direction veer to
MVFR stratocu are expected to persist into Tuesday night as cold
advection ensues in the wake of the rapidly exiting system.
Outlook.../Wednesday through Saturday/...
more restrictions are likely with The Crossing of low pressure on
late Thursday and Friday.