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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPBZ 171746
AFDPBZ

Area Forecast Discussion
National Weather Service Pittsburgh PA
146 PM EDT Wed Apr 17 2024

.SYNOPSIS...
Two waves of showers and thunderstorms are expected today, with
a marginal to slight risk for severe weather with the second
wave. Additional showers with isolated thunderstorms are likely
Friday with a cold frontal passage, which will cause a shift to
a drier and cooler weather pattern into next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
KEY MESSAGES:

-  Two rounds of showers and thunderstorms are expected today,
   with the second round offering severe potential.
-  Damaging wind and large hail will be the primary threats,
   with a more limited tornadic threat.
------------------------------------------------------------------

500am update... Light rain showers/sprinkles are beginning to
move in from the southwest in association with a 700mb moisture
axis. The dry airmass preceding these showers means radar
returns will initially oversell the likelihood of measurable
precipitation reaching the ground. More consistent rain will
likely arrive after 8am in association with another 700mb
moisture axis that is better co-located with vort advection.
Forecast was updated to capture some of these trends, but does
little to change the overall messaging for the day.

Rest of the discussion...

Column warm, moist advection ahead of a west-moving warm front
associated with an occluding surface low and its negatively-
tilted upper trough will promote morning showers with embedded
thunder. Initial precipitation will generally be light given the
prior dry airmass in place, with ensembles favoring this first
round exiting the region by 18z (2pm EDT). The second round will
be associated with the approach and passage of a weakening
surface cold front (weakening due to low center lifting to
northern Great Lakes and upper trough beginning to phase farther
to the NW). Thunderstorms are likely along and ahead of this
boundary which is favored to reach eastern OH between 19z-21z
(3-5pm) and western PA between 22z-01z (7pm-9pm). Drier air and
subsidence will quickly erode showers post-frontal passage
overnight, with cold advection more limited as height rises
quickly develop.

Ensemble spread has narrowed on the convective environment
between the cold front and morning warm front, increasing
confidence in developing strong to severe storms. Mean HREF
SBCAPE values between 1000-1500 J/kg is forecasted for eastern
OH to the I-79 corridor ahead of the cold front with 30-40kts
shear and around 90th percentile PWAT vales lead credence to a
severe threat. Though convection firing ahead of the surface
boundary could develop supercell characteristics, frontal speed
and coalescing of cold pools should foster a QLCS storm mode.
The environmental setup and storm mode favors damaging wind and
large hail as the primary hazardous weather threats. Tornadic
potential is likely to be limited, but acute attention should be
placed on any right movers that deviate to the ESE (from
expected E motion) based on modeled hodographs. Though faster
storm motion should limit flash flood concerns, it must be noted
that thunderstorms producing heavy rain could still create flood
impacts based solely on the saturated ground conditions for much
of the region.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
KEY MESSAGES:

- Above normal temperature and dry weather expected for
  transitory day Thursday.
- Another round of showers and low probability thunderstorms
  expected Friday with cold front passage.
-----------------------------------------------------------------

Shortwave ridging and surface high pressure will slide across
the Upper Ohio River Valley Thursday in the wake of Wednesday`s
system and ahead of the main upper trough axis over the Central
Plains. Though height rises aren`t expected to match similar
readings as yesterday (Tuesday), they will be enough to maintain
above normal temperature with generally mostly sunny skies.

A fairly progressive shortwave embedded within the broad
southern Canada trough will race across the Great Lakes Friday
as a surface low lifts northeast through Lake Erie. Ensemble
models are trending toward a mid-morning to early afternoon
arrival of the showers and embedded thunderstorms ahead of the
surface cold front. Little to no surface based instability is
expected to develop (resulting in limited lightning activity,
let alone severe threat) while total accumulations aren`t
expected to be impactful (less than 30% probability of 24-hr
rainfall exceeding 0.5").

Dry slotting within the upper trough as the axis sits to the
west will quickly erode precipitation chances after the frontal
passage. Temperature will more acutely fall with stronger cold
advection, dropping area temperature to seasonal average values
Friday night.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
KEY MESSAGES:

- Dry but cooler conditions expected for the weekend.
- Periodic shortwave passages may offer brief precipitation
  windows through the middle of next week.
-------------------------------------------------------------------

Dry and seasonable conditions amid mostly sunny skies are
favored to kick-start the weekend. However, ensembles favor the
passage of the upper trough axis later Saturday into Sunday
morning, pushing through a reinforcing shot of colder air to the
region. Very limited moisture available means the passing wave
and associated cold front are likely to be dry. With clearing
skies and potentially light winds, this setup will offer the
coolest morning temperature since April 7th as ensemble ranges
of temperature suggests lower to upper 30s. It will bear
monitoring for potential frost and/or freeze issues given the
active growing conditions for much of the region.

Long range ensemble outlooks suggest a return to a more active
synoptic pattern featuring various shortwave trough movements
within broad eastern CONUS troughing. This pattern would favor
periodic shower (limited thunderstorm) chances and seasonable
temperature.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
-- Changed Discussion --
Scattered to broken clouds with mixed levels (predominantly
5-10kft) and a few scattered light showers are ongoing across
the area to start the TAF period. Expect this to generally
continue through the afternoon before thunderstorm activity
ramps up and a line of thunderstorms - possibly severe and
capable of producing damaging winds and large hail - moves
through the area from west to east between roughly 22Z and 03Z.
These storms may also bring brief reductions in ceilings and
visibilities to area terminals as they push through. Hi-res
ensemble guidance continues to indicate high probabilities
(>60%) of MVFR or lower ceilings settling into much of the area
overnight in the wake of these storms. If this occurs,
conditions would improve during the late morning or early
afternoon hours on Thursday, right at the tail end of the
current TAF period.

Winds are generally out of the south or southwest around 10
knots to start the TAF period. Expecting more frequent gusts to
20-25 knots to begin later this afternoon owing to deeper
boundary layer mixing. Wind settle again overnight in the wake
of the storms and veer to more westerly by the end of the
period.

.Outlook...
Restrictions remain possible Thursday night into Friday under
another passing upper trough and associated chances for rain.
Prevailing VFR returns over the weekend.
-- End Changed Discussion --

&&

.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.

&&

$$

SYNOPSIS...Frazier
NEAR TERM...Frazier
SHORT TERM...Frazier
LONG TERM...Frazier
AVIATION...Cermak