PBZ issues Area Forecast Discussion (AFD) at Jul 16, 1:50 PM EDT708
FXUS61 KPBZ 161750
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
150 PM EDT Tue Jul 16 2024
.SYNOPSIS...
Severe potential this afternoon and evening will be followed by
storms late tonight in the I-80 corridor. Thereafter, a cold
front will swing through, allowing a third and final chance of
severe weather tomorrow. Damaging winds remain the primary
threat. Seasonable weather will prevail Thursday and Friday
before a weekend warm-up.
&&
.NEAR TERM /THROUGH WEDNESDAY/...
KEY MESSAGES:
- Heat risk is elevated today, primarily in river valleys.
- Three independent chance of severe weather are possible this
afternoon/evening (primarily south of the Pittsburgh metro),
late tonight (most likely in the I-80 corridor), and tomorrow
afternoon (through Pittsburgh and points south and east).
- The primary threat remains damaging winds.
__________________________________________________________________
For todays forecast, dew points were matched to observations and
adjusted down, whereby temperates were nudged up with mixing
likely poorly modeled based on models guidance compared to the
12Z sounding and other area observations. First, this has
resulted in an uptick in winds for today, with gust of 25mph to
35mph possible. Second, heat risk remains today for much of the
area, primarily confined to the valleys and urban areas with
cumulative effects of warm overnight lows and heat from Monday.
High temperatures into the 90s appear likely (60% to 80%) for
most with the exception of areas north of I-80 and higher
elevations. Heat indices top out into the upper 90s for valley
locations, but still a hair below heat advisory criteria, thus
we have opted to not issue for today. Regardless, it is
encouraged that sensitive groups minimize time outdoors. If you
must go outside, ensure that you are hydrated and prepared for
the elements.
The main, but conditional, threat for today will be severe
thunderstorms. First, we will talk about the environment. There
is roughly a 60% to 90% chance of SBCAPE south of I-70 exceeding
1000 J/kg. In Pittsburgh and points north, that drops to 30% to
40%. There is less than a 30% chance of any area exceeding 30kts
of bulk shear, however chances of 20kts or greater are 50% to
80% in Pittsburgh and south, whereby chances drop to 20% to 40%
for points north. A surface DCAPE minima will gradually move
east of us into today with values of 800-1000 J/kg favored by
afternoon. With all of these factors considered, the environment
for severe storms appears best south of Pittsburgh.
Lets talk secondarily about forcing and initiation. A weak mid-
level wave will pass this afternoon that does not guarantee a
slam dunk for initiation, especially with mid-level dry air
hindering initial development. Should anything develop and
overcome dry air, the probability of severe in the environment
would be high, particularly for downbursts. The most likely
timeframe is 2pm to 8pm. Damaging winds would be the primary
threat in downbursts. Hail remains a low probability secondary
threat, but it will need storms topping 35kft to 40kft to be
realized with such warm low levels. Tornado probability has
decreased due to anticipated high cloud bases and more dry air
than anticipated, signaling that storms may be cold-pool
dominant.
Save a brief reprieve behind the passing wave late this evening
through the early overnight, another embedded weak low-to-mid
level wave will pass in the flow overnight. This time,
probabilities of at least 20kts of bulk shear are 20% to 40% in
the I-80 corridor, and less than 20% elsewhere. CAPE will likely
be in the hundreds and mostly elevated, but should any mature
updraft develop, it will again be possible at collapsing into
gusty winds at the surface, pending breaking a shallow surface
inversion. The most likely time for this event will be around
4am to 8am. This remains the lowest probability of the three
severe chances.
Last, but not least, severe probabilities increase into the day
tomorrow along and ahead of the passing cold front. With better
synoptic forcing and an increasing gradient within the trough
itself, the confidence in initiation is the highest in this
period. Consequently, bulk shear probabilities increase to 50%
to 60% of greater than 30kts, and CAPE will be in the 500 to
1500 J/kg range, but mid level dry air will drop off, decreasing
downburst threats. Convective nature will resort to higher
confidence in storms, but lower confidence in intensity. Current
thinking reflects that damaging winds are still the primary
threat from noon to 8pm with a secondary threat of hail.
&&
.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/...
KEY MESSAGES:
- A drier and cooler period will ensue through Thursday and
Friday.
_________________________________________________________________
Dependent on front progression speed, some storms may linger
into the mountains of northern West Virginia in the early
overnight period, but thereafter, dry conditions are favored.
Low-level cold and dry advection will keep temperatures near or
even a few degrees below average Thursday and Thursday night
with valley fog chances.
Stout mixing into the day on Friday under high pressure will
result in temperatures near normal, and fairly low humidity.
This will leave the area feeling comfortable for the last day of
the work-week. Radiative cooling will also drop temperatures
near normal.
&&
.LONG TERM /SATURDAY THROUGH TUESDAY/...
KEY MESSAGES:
- Diurnal shower/storm chances increase slightly with increased
moisture.
-------------------------------------------------------------------
Rising heights Saturday/Sunday in response to increased
troughing over the Central Plains may bring moisture back from
the south, leading to increased diurnal precipitation chances,
mainly southeast of Pittsburgh. This will also result in a
warming trend through the weekend.
&&
.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
A cu field has formed over much of the area this afternoon with
cigs gradually raising to around 5-7kft, while efficient heating
and deep mixing have allowed southwest to westerly winds
increase to 10-15 knots with gusts as high as 30-35 knots in
some locations. These conditions continue through the rest of
the afternoon. A few showers and storms could develop in the
19Z-00Z timeframe, with the environment favorable for downbursts
capable of producing strong, erratic wind gusts if any terminals
were to be impacted.
Convection wanes after 00Z and winds relax with the loss of
daytime heating/mixing. Another wave of showers and possibly a
few embedded thunderstorms will move into the area around
daybreak Wednesday as the remnants of a storm system over
IL/IN/KY moves northeast through the local area. Hi-res
ensembles show high probabilities (60-90%) for MVFR cigs
building into the area with this morning convection, lasting
throughout the morning before very gradually lifting towards the
tail end of the current TAF period. A few showers and storms may
linger throughout the morning as well, but the better chances
will remain just beyond the current TAF period as a cold front
sweeps through the area Wednesday afternoon.
.Outlook...
Showers and thunderstorms, and associated restrictions, are
likely with the passage of a Wednesday cold front. VFR returns
Thursday through Saturday under building high pressure.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...None.
&&
$$
SYNOPSIS...Milcarek
NEAR TERM...Milcarek
SHORT TERM...Milcarek
LONG TERM...CL/Milcarek/88
AVIATION...Cermak
Source:
PBZ issues Area Forecast Discussion (AFD) at Jul 16, 1:50 PM EDT---------------
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