Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
811 PM EDT sun may 24 2015
warmth and humidity will return to the region...but so will shower
and thunderstorm chances as the Holiday weekend draws to a close.
Near term /through Monday/...
minor changes made to previous forecast...with the most
substantial made to the sky field. Current visible satellite
imagery...nearing sunset...shows high clouds streaming northwest to southeast
across the northern 1/3 of the forecast area. With the axis of the
upper ridge poised to pass overhead tonight through the early
morning hours...upper-level flow will back from west to
southwesterly...pushing bulk of the high clouds north of the
area. Have trimmed back clouds to a much clearer Memorial Day
As southwesterly flow increases tonight through Memorial
Day...temperatures and moisture will increase as well. Lows
tonight will be 5-8 degrees above normal...and highs will be
closer to 10 degrees above the normal values.
Short term /Monday night through Wednesday/...
recurring shortwave impulses are forecast to erode the ridge over
the region through middle week with moisture depth increasing in the
developing warm southwest flow. Shower and thunderstorm probabilities
will thus be on the increase beginning along a Zanesville to Franklin
line late on Monday. Given the lack of confidence in model
boundary layer moisture magnitude and resulting instability under
warm advection aloft...along with poorly resolved convection
tendencies...have reserved likely probability of precipitation for the Wednesday time
frame given the NAM and GFS propensity for a stronger shortwave
emerging from the parent trough...which is projected over the
Mississippi Valley by then.
As for severe potential...middle level warmth will limit instability
and NAM/GFS deep layer shear projections are marginal at best...at
least until the upper trough approach on Wednesday. That same
warmth would support heavy rainfall via warm cloud processes...and
precipitable water values will be appreciably higher under this
train support pattern...but convection potential is expected to be
diurnally dependant and sustained moisture convergence absent
until the aforementioned shortwave crosses and a cold front
slides out of the Midwest on Wednesday.
The bottom line is that a hazardous weather outlook is not needed at
this time...although precursor rainfall and timing of the approaching
middle/upper support may alter that diagnosis as middle week approaches.
Long term /Wednesday night through Sunday/...
the latest model runs agree on scenario of eastern Continental U.S. Ridge
erosion...with a weak cold front encroaching on the upper Ohio
region on Thursday. Chance probability of precipitation were maintained with that
timing...but eliminated for Friday given expectations of shortwave
ridging. A deeper shortwave trough with a stronger cold front
accompanied by increasing precipitation chances are projected for the
weekend. Temperatures are expected to average around the normals
for the period.
Aviation /00z Monday through Friday/...
high pressure will support VFR with light southerly wind into
Monday. Gusts will pick up out of the southwest by late Monday
Outlook.../Monday through Friday/...
abundant moisture and disturbances in southwest flow are likely
to spawn periodic restrictions through Thursday.