Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
930 PM EDT Monday Apr 20 2015
showers will diminish through late evening as a cold front swings
east of the ridges. Unseasonably cool temperatures can be expected
the remainder of the week.
Near term /through Tuesday/...
no major changes to previous thinking as showers just ahead of
advancing shortwave aloft will work across areas mainly I-79 and
east next 2-3 hours. Previous discussion follows.
Wind shift has reached the ridges and temperatures have dropped
off nicely this evening. Cold front will continue to push east
with much cooler air working in overnight. Probability of precipitation may still be
overdone for later tonight but scattered showers still lingering back
across southeast Ohio and northern West Virginia...and may be able to
hold together as upper level shortwave passes. Temperatures and
winds appear on track with no changes.
An upper-level low will pin-wheel into the northern Great Lakes
Tuesday...setting up a broad trough over the area that will include
several weak disturbances cycling through. The first such
disturbance will bring the chance of rain showers back to the area
Tuesday evening. With the broad trough overhead...windy cloudy and
cool conditions will exist Tuesday.
Short term /Tuesday night through Thursday/...
the long wave pattern looks to full transition into troughing over
our region on Tuesday night. Model guidance has been hinting at
this for about a week in this interlude...and it looks to come to
full fruition by this time. As our region sits in the base of the
long wave trough...with the main jet axis to our south...this will
result in temperatures dropping significantly from our recent warm
period...as well as with a few very fast moving systems affecting
the area through the short term. As these systems will be embedded
in high speed upper flow...their timeliness is a bit more
difficult to pin down. Additionally...as layer temperatures fall
progressively as troughing becomes more deeply entrenched...both
low level stability and the freezing level start to fall. This
means predicting dry weather becomes more tenuous...as do sunny
skies...and even precipitation types start to become questionable
All that said...the model consensus favors the strongest short
wave passage for Wednesday afternoon. Probability of precipitation were adjusted to
likely for the bulk of the area for this time frame.
However...given the speed of the middle-level flow...it will likely
exit by Wednesday night...with just instability driven showers
remaining under the middle-level cold pool across the north...as well
as a few upslope showers. BUFKIT soundings suggest that by
Wednesday night...a few of these could transition to snow showers
with boundary layer cooling and a vastly decreased melting depth
in the lowest layers. This window for this will be short as
moisture peels eastward rather rapidly and the jet core shifts
toward the Atlantic Seaboard by the end of the short term. This
places US in the left entrance region of the upper jet
streak...which would become decreasingly favorable for any
organized precipitation development. The forecast was thus
maintained dry into the latter half of the work week.
While the wet weather will likely be curtail by the latter half of
the week...cyclonic flow through the atmosphere will reinfcorce
colder temperatures at the surface. Model consensus 850 hpa
temperatures fail to rise above 0c after Wednesday morning
through the remainder of the short term. Because of this...highs
will likely struggle mightily to reach 50f the rest of the week.
Additionally...overnight lows will flirt with or drop below
freezing several times. This will seem cold...but it is in no way
abnormal as the growing season climatologically does not start
until may for any of our forecast area. Thus...we are still quite
a bit from the start of the frost/freeze program for the year.
Long term /Thursday night through Monday/...
some isolated showers will remain Thursday morning...but will
diminish through the day. GFS/European model (ecmwf) generally show one system in
the region through the extended forecast...but the European model (ecmwf) is slower
with the system and also much farther to the south. Considering
ensembles show good support for the European solution...have
removed the low probability of precipitation that were in the forecast over the weekend.
However...models do show agreement on broad upper level low across
northeastern North America...and this will keep temperatures 5-10
degrees below normal through the period.
Aviation /02z Tuesday through Saturday/...
a cold front was exiting the area into central PA...though some Post
frontal showers will remain possible through this evening.
Otherwise...VFR stratocu is expected through Tuesday. A broad upper
trough is expected to set up across the Great Lakes Tuesday...and
could bring scattered showers for areas from pit north. Included a
vcsh mention in the tafs for this. In addition...west winds could
gusts to around 30 knots Tuesday with a tight pressure gradient
Outlook.../Tuesday night through Saturday/...
periodic restrictions a are possible much of the period mainly
north of pit as the upper trough persists over the Great Lakes.