PBZ issues Area Forecast Discussion (AFD) at Jul 18, 8:56 PM EDT021
FXUS61 KPBZ 190056
AFDPBZ
Area Forecast Discussion
National Weather Service Pittsburgh PA
856 PM EDT Fri Jul 18 2025
.SYNOPSIS...
A stalled boundary across the southern part of the area will
continue to provide a heavy rainfall threat this evening, mainly
south of I-70. Additional chances for flash flooding, as well as
at least isolated severe storms, will occur this weekend as a
warm front lifts through Saturday, and a cold front passes
Sunday.
&&
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
KEY MESSAGES:
- A stalled boundary will be a focus of a heavy rainfall threat,
mainly south of I-70, with lighter rain to the north
- Fog development is likely overnight in areas that get heavy
rain today.
---------------------------------------------------------------
A boundary remains stalled in the general vicinity of the
Mason-Dixon Line through tonight. Crossing perturbations in the
quasi-zonal mid-level flow have fostered more of a stratiform
rain to the north of the boundary, and more of a convective
nature to the south, in an area with 1000-2000 J/kg of surface-
based CAPE and precipitable water values in the 1.9 to 2.0 inch
range. There has already been some training of showers in the
area of the ongoing Flood Watch, and this potential will
continue into this evening. HREF maximum rainfall potential
suggest pockets of 2-3 inches of rainfall through this evening
is possible, which represents a flooding risk, especially if it
falls on ground that is more saturated from previous rain.
In contrast, the risk of severe thunderstorms appears quite low.
The high PWAT and skinny CAPE profiles suggests that updrafts
will be relatively weak, and DCAPE levels remain relatively low.
Cannot totally rule out a wind gust or two in Tucker County and
vicinity, where higher CAPE and effective shear may support
enough storm organization for a wind threat.
Much of the activity will begin to weaken after sunset, with
only scattered/isolated activity in the ridges by 06Z as
instability diurnally decreases. Lingering low-level moisture
and daytime rain will led to some fog develop tonight,
especially over the ridges and locations in WV that received
the heavier totals.
&&
.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/...
KEY MESSAGES:
- The front likely returns north on Saturday, bringing a risk of
heavy rain and flooding at least to areas south and west of
Pittsburgh.
- A more isolated flooding threat is possible Sunday with a cold
frontal passage.
- Severe potential for Saturday and Sunday, with damaging wind
gusts being the most likely threat.
----------------------------------------------------------------
The boundary will begin to lift northward across the region again on
Saturday, as a shortwave trough crosses the Middle/Upper Ohio
Valley, and as surface low pressure crosses the Upper Midwest into
Lower Michigan by 00Z Sunday.
Trends in the CAMs suggest later timing for the bulk of the
precipitation, more towards the very late afternoon and
evening. Model soundings are suggesting some mid-level warm
air/capping that may inhibit convection until the shortwave
arrives later in the day. This late-day convection may arrive in
the form of a weakening MCS in Ohio, at a time when instability
may be diurnally decreasing. SPC has pulled the day 2 slight
risk a bit further west, essentially out of the forecast area,
while maintaining a marginal risk over areas north and west of
MGW. This seems reasonable, as increasing low- level flow and
20-30 knots of deep shear may support storm clusters capable of
damaging wind gusts mainly, with a lesser risk of large hail.
The flooding risk still exists as well given the likelihood of PWATs
rising above the 90th percentile once again by later
Saturday/Saturday night, with soundings supporting warm rain
processes especially in eastern Ohio. Interestingly, HREF
probabilities of greater than an inch of rain over 24 hours are
fairly suppressed through Saturday night, maximizing in the
30-50% range west of the Ohio River and south of I-70. While the
best flood risk may exist to the southwest of Pittsburgh
through tomorrow night, think that these probabilities may be a
bit underdone. While storm motions will be higher, the
potential for heavy rates and an already saturated ground will
keep the flood risk elevated.
Sunday will feature a cold frontal passage as the Michigan
surface low tracks into New England. Ensembles suggest a severe
threat perhaps as early as the late morning hours and into the
afternoon, with potential for 1000-1500 J/kg of surface-based
CAPE overlapping with 30-40 knots of deep shear, mainly from
Pittsburgh on south. This supports the day 3 marginal risk in
this area from SPC for mainly a wind threat. The flooding risk
will likely be confined to areas where east-west training may
occur ahead of the advancing front.
&&
.LONG TERM /MONDAY THROUGH FRIDAY/...
KEY MESSAGES:
- Building high pressure aloft may provide relatively dry
weather early next week.
- Heat and humidity begin to increase during the middle part of
the week.
- Shower/thunderstorm chances increase Thursday and Friday.
-------------------------------------------------------------------
There is good confidence among the ensembles that strong ridging
builds over the Mississippi Valley and into the eastern CONUS next
week. Along with crossing high pressure to our north, a mainly
dry pattern appears to extend into Tuesday, and possibly into
Wednesday, with the potential for subsidence/warm air aloft to
suppress convection. Of course, this will also lead to above-
normal temperatures. NBM probabilities of 90+ highs increase to
40-70% over much of the area by Wednesday, and then 50-80% on
Thursday. Combined with high dewpoints, heat risk is set to
rise by the end of the forecast period.
Shortwaves riding the top of the ridge may begin to suppress heights
slightly by the Thursday/Friday period. This could lead to better
rain chances during this period. Longer-range machine learning
guidance suggests an uptick in severe storm potential as well, which
makes sense given the amount of instability/moisture that may be
available.
&&
.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
Nearly all area ports experienced at least some rain today,
which will be followed by only a sct/bkn deck and allow some
cooling overnight. With calm winds and moisture, at least every
port has the chance to see fog or low stratus. While point
probabilities of dense fog are generally 20% to 40%, these may
be under-doing potential given the amount of daytime rain. The
lowest visibility was noted for the most susceptible ports, and ones
that saw the most rain today.
Some clearing is expected beyond daybreak, through cig lifting may
be slower for southern ports (notably MGW, ZZV, and HLG) since
clouds forecast to clear from north to south. By the afternoon, most
locations will observe VFR conditions. Another round of afternoon
storms is possible as a warm front lifts north, but coverage should
remain isolated. Due to the low probability of daytime
showers/thunderstorms, they are not mentioned in the TAFs.
Outlook...
A chance of showers/storms is possible overnight Saturday on a
prefrontal trough, however, the probability for gusty winds have
decreased. The actual cold front passage Sunday will allow
additional late-morning to afternoon storm chances including, and
south of, PIT. Dry weather returns early next week before more rain
chances mid-to-late week.
&&
.PBZ WATCHES/WARNINGS/ADVISORIES...
PA...None.
OH...None.
WV...Flood Watch until 1 AM EDT Saturday for WVZ513-514.
&&
$$
SYNOPSIS...Rackley
NEAR TERM...Rackley/CL
SHORT TERM...CL
LONG TERM...CL
AVIATION...Milcarek/Lupo
Source:
PBZ issues Area Forecast Discussion (AFD) at Jul 18, 8:56 PM EDT---------------
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