Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
820 PM EDT Monday Jul 6 2015
while showers will diminish in coverage this evening...unsettled
weather will continue through much of the week as a cold front
crosses the region Tuesday night and then stalls across the
Near term /through Tuesday/...
a weakening upper low continues to fill overhead and ever-so-
slowly lift northeastward. This has allowed for a fairly prime
environment to develop that facilitates heavy rainfall late this
Precipitable water values per the NAM have run up toward 1.8 inches this
afternoon...and given surface dewpoints running in the middle 60s to
lower 70s across the area...these types of precipitable waters are not at all
surprising. Modest instability of a few hundred j/kg can be noted
on model soundings...however layer shear is exceedingly weak.
This has allowed for some rather copious rain producers to form
this afternoon...and move ridiculously slowly. This is
particularly true where low level convergence seems to be
maximized in the decaying deformation zone around the filling 850
mb low. As such...weak northeasterly flow has allowed for a rather
stationary band of convergence in the low levels that has forced
rather continuous convective initiation over eastern Ohio through
the afternoon. However...as this convergence weakens progressively
into this evening...the heavy rain threat should gradually subside.
High resolutions models indicate a strong downward turn in
activity after 00z...and that seems to be occuring as we speak on
the regional radar mosaic.
Otherwise...somewhat lower precipitable waters and leaner 700 mb relative humidity values move
into the region overnight...which should slowly wind down the rain
chances from west to east. However...high boundary layer relative humidity values
mean low temperatures are likely to stay up with humid conditions
continuingand patchy fog becoming an issue. Fries
Previous discussion for Tuesday...a cold front will approach
from the west during the day. Effective shear appears to be quite
weak with only modest instability present...and Storm Prediction Center has put nearly
all locations in a marginal risk for severe weather. Compared to
recent days...tomorrow will be on the warm side. The projected
high in Pittsburgh is 86 degrees...which has not been reached in
about 3 weeks...even though the normal high for this date is 83
Short term /Tuesday night through Thursday/...
the front will cross the region tomorrow night...bringing
widespread rainfall. As has been the case recently...the front
appears likely to stall out across the region. Latest models show
fairly good agreement that the front will stall across southern
portions of the forecast area Wednesday into Wednesday...but have
a bit more disagreement on Thursday. 12z NAM has the bulk of the
rainfall from Pittsburgh north into New York...while the 12z European model (ecmwf)
shows rainfall primarily south of Pittsburgh into Kentucky. Until
models come into better agreement...will simply keep chance probability of precipitation
for Thursday. With considerable cloud cover expected through the
period...temperatures should drop a few degrees below normal
Long term /Thursday night through Monday/...
frontal boundary will waver across the region through the period
as shortwaves move east in relatively zonal middle level flow. With
individual shortwaves hard to time have kept chance probability of precipitation through
the period. Temperatures close to wpc forecasts which seemed a good
Aviation /00z Tuesday through Saturday/...
scattered showers and thunderstorms continue to diminish late this
evening...with all terminals VFR. Should keep some of the middle-
level clouds overnight...but some clearing could lead to some MVFR
fog in the pre-dawn hours Tuesday. High clouds will begin to move
in by daybreak Tuesday in advance of the front that will slowly
approach from the west. Unstable conditions ahead of the front
means that restrictions are again possible in showers and
thunderstorms that develop Tuesday afternoon.
Outlook.../Tuesday through Saturday/...
a stalled boundary across the area will lead to periodic
restrictions through the latter part of the week.