Scientific Forecaster Discussion
fxus61 kpbz 271143
Area forecast discussion
National Weather Service Pittsburgh PA
743 am EDT Thu Jul 27 2017
showers and thunderstorms return to the forecast today, with
rain chances continuing into Saturday. Heavy rainfall is
possible, especially southeast of Pittsburgh. Dry and
seasonable weather develops by Sunday.
Near term /through tonight/...
adjustments to hourly forecast trends based on observations and
near term models for the Post dawn update.
Previous...a frontal boundary will sink southward across the
region during this period, ending up in the general vicinity of
the Mason- Dixon line by 12z Friday. Precipitation along the
boundary itself will be relatively scant, with much of the deep
moisture remaining off to our southwest during the day as low
level flow remains weak. Have continued with a north/south pop
gradient, backing off a little more from previous values. This
keeps likely pops confined to southwest of hlg through 00z.
Severe risk this afternoon and evening is looking small, as
instability will be hard to come by. Still, steep low-level
lapse rates and dry mid levels could promote a damaging wind
gust or two in the strongest storms.
Precipitation will taper in coverage during the evening as the
front presses south and heating is lost. However, activity will
start to tick up again towards morning as a closed upper low
drops across Michigan, helping to draw more moisture up into the
southern County Warning Area. This process will continue in a more significant
fashion on Friday.
Temperatures will be fairly seasonable today, although dewpoints
will be on the increase. Clouds and humidity will keep
temperatures warm tonight.
Short term /Friday through Saturday night/...
the upper low continues its trip southeast this period, crossing
western PA Friday night and heading in the direction of the
Delmarva area thereafter. A surface low will also strengthen off
of the mid-Atlantic coast. The departure of the low continues
to trend slower on the models, with impacts continuing to linger
into Saturday at least.
Likely pops continue across the region Friday and into Friday
evening as the upper low crosses, with a slow taper towards the
southeast during the night. Little to no severe risk exists
during this period. The main concern will likely be heavy to
excessive levels of rainfall, especially southeast of
Pittsburgh. Strong 850-700 mb frontogenesis across southern PA
may couple with moist easterly flow to support some downpours.
Nearly saturated low levels along with a deep warm cloud layer
also support the heavy rain idea. Cannot rule out storm totals
in excess of 2 inches in parts of the southeast cwa, perhaps
locally higher, which could cause localized flooding. Still too
early to pinpoint an area with a Flood Watch, but will mention
threat in severe weather potential statement.
Have slowed down the pop decrease through Saturday in response
to the slower model trends with the system, and have even added
some shower chances Saturday night to the southeast County Warning Area. For
now, expect precipitation to finally clear the ridges by 12z
Long term /Sunday through Wednesday/...
Great Lakes high pressure should provide mostly dry weather
during this period. The trend with the midweek front appears
slower, and have stuck with a dry forecast for Wednesday for
now. Temperatures will be fairly seasonable overall.
Aviation /12z Thursday through Monday/...
after patchy morning MVFR fog dissipates, mainly VFR conditions
are expected today. Shower and thunderstorm chances are expected
to increase this afternoon, mainly S of I 70 with the approach
of a cold front. Maintained thunderstorms in the vicinity mentions for zzv and mgw, with
some tempo restrictions at zzv where tstm occurrence is more
probable. Precip should diminish this evening with exiting
shortwave support, though increasing low level moisture should
result in MVFR cigs for Ohio ports overnight.
restrictions are likely Fri and Sat with slow moving low