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Western PA/Pittsburgh Summer Discussion 2021


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2 minutes ago, TimB84 said:

I tend to agree. Not necessarily because I think we’ll see a half foot of rain in the immediate Pittsburgh area - that’s probably an outlier, but because there’s a solid chance that some location in the PBZ forecast area (MGW, perhaps?) will see 6”. And that possibility covers a solid number of counties under their jurisdiction.

Yep, and I think it depends on who is writing it. There was one a day or two that was excellent, but this dude who wrote this just is straight awful and frankly lazy. Here is the detailed one.

 

Quote
Tuesday Night and Wednesday... here comes Ida.

The weakening tropical system Ida will begin to move northward
across the TN Valley Tuesday night and accelerate in response to a
northern stream trough digging southeastward from the Midwest. Rain
from the northern side of Ida will overspread northern West Virginia
and SW PA (and perhaps barely into SE Ohio close to the Ohio River)
overnight and continue through much of Wednesday as Ida moves
northeastward through Appalachia. This will pose a significant flood
threat to locations south and east of Pittsburgh and anyone in that
area needs to remain weather aware for the next few days.

How much rain and where exactly? That`s the biggest question. Recent
model guidance has trended slightly farther SE, painting the axis of
heaviest rainfall somewhere between SW PA and the higher terrain of
WV, with Morgantown being near the center axis in several models.
Across the heaviest rainband, look for amounts between 3 and 6 inches
with localized higher amounts. Farther north, amounts should drop
off precipitously w/ areas north of Pittsburgh likely receiving less
than an inch. Where does this put the immediate Pittsburgh metro? In
an area of high uncertainty. Model guidance ranges from roughly just
1 inch upwards to about 5 inches for Allegheny County and
surrounding area, all dependent on the exact track of the system. We
should be able to hone in on these details by the evening suite of
model guidance.

Rain will come to an end by dusk or shortly thereafter on Wednesday
as the remnants of Ida depart eastward, with much cooler, drier air
filtering in behind the system.

 

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32 minutes ago, TimB84 said:

12z HRRR (those darker yellows are still a little too close for comfort):

3391655A-1C5C-4775-A74E-84023C17467E.jpeg.b30137a102d79b667b8c72c56963d548.jpeg

I mean how accurate are models this far out to the point where 50 miles could be the difference between 1inch and 6inches. We've seen this before in winter.

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1 hour ago, TimB84 said:

Feel like the 12z models are shifting back south (GFS is an exception, but are we relying on the GFS inside 24 hours?).

I was thinking the opposite after reviewing 12z models, most seemed to have nudged a bit North.

 

NWS put out a solid discussion pertaining the upcoming situation. 

.SYNOPSIS...
The remnants of Hurricane Ida will bring heavy rainfall and
potentially significant flood impacts to the Upper Ohio Valley
tonight into Wednesday. Cooler, drier air is expected after the
system passes Wednesday night.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
Regional radar imagery shows a line of showers developing and
strengthening in area of modest CAPE (~500 J/kg) and along/near a
diffuse frontal boundary stretching along and just north of I-70.
With rainfall rates of up to 2 inches per hour possible, any
training segments will pose a flash flood risk. Total amounts in the
highest areas will likely be in the vicinity of 2 inches per HREF
probabilities, effectively saturating the ground in those areas
before tomorrow`s rainfall. This convection should wane towards
dusk.

Attention then turns to the remnants of Ida and a potentially
significant flood event for the Upper Ohio Valley into the Allegheny
Mountains.

Ida is forecast to move into Appalachia Tuesday night before heading
off the Mid-Atlantic coast on Wednesday. Very high moisture content
associated with Ida will surge northward into the Upper Ohio Valley
along a strengthening frontal/baroclinic zone that will lie right
across the forecast area. Strong frontogenetical and isentropic lift
in the frontal zone in the right entrance region of the northern
stream jet will allow for a band of moderate-to-heavy rainfall to
develop and persist across the area. Run-over-run model guidance has
been consistent with placing the axis of heaviest rainfall in an area
stretching from the Mon Valley to areas northeastward such as
Uniontown and Connellsville and then into the Laurel Highlands. 24-
hour totals ending 8PM Wednesday look to be around or possibly in
excess of 6 inches in this heaviest axis, with rainfall totals
dropping off fairly precipitously to north and west.

The greatest uncertainty in the rainfall forecast will be across the
rainfall gradient , which will encompass the immediate Pittsburgh
metro area. HREF probabilities suggest rainfall amounts may be
upwards of 4 inches towards the southern portion of Allegheny
County, dropping significantly to perhaps just around 1 inch towards
the Beaver County border.

No changes are currently planned for the flash flood watch.
Officially the watch begins at 2am, though rain is expected to begin
before that. Wouldn`t rule out a warning or two today with afternoon
convection or this evening as the initiate wave of Ida rainfall
approaches the area.

As far as impacts, we`re anticipating a somewhat similar scenario to
significant tropical systems of the past such as Gordon (2018) or
even Francis/Ivan (2014). The similarities exist within rainfall
totals and duration, though the axis of heaviest rainfall will
likely be slightly farther southeast than it occurred with Francis
or Ivan. This will cause significant rises in the Monongahela River
and its tributaries such as the Youghiogheny and Cheat Rivers.
Additionally, flash flooding is likely in those surrounding areas
with many smaller streams likely going to exceed bankful. If you live
in a flood prone area, please have an emergency plan in place and
methods for receiving warning information.

Rainfall should begin to exit eastern Ohio and NW PA by noon
Wednesday, eventually exiting the entire Pittsburgh forecast area by
tomorrow evening once Ida shifts off and cool, drier air ensues.
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13 minutes ago, RitualOfTheTrout said:

I was thinking the opposite after reviewing 12z models, most seemed to have nudged a bit North.

 

NWS put out a solid discussion pertaining the upcoming situation. 

 

Yeah, early returns from the NAM and earlier runs of the HRRR looked promising for less rain. Everything else has made a decisive shift north. Actually, not sure where I got that. Maybe from the other convective models, or from haste. Wasn’t the NAM or HRRR.

Other thing is these ongoing slow-moving storms this afternoon ahead of the main event. Flash flood warnings in effect this afternoon.

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8 minutes ago, TimB84 said:

Yeah, early returns from the NAM and earlier runs of the HRRR looked promising for less rain. Everything else has made a decisive shift north.

Other thing is these ongoing slow-moving storms this afternoon ahead of the main event. Flash flood warnings in effect this afternoon.

Gotcha, yeah I didn't take into account the timing of your post vs the models I was looking at. The worst is still SE of Pittsburgh and although it seems like whatever the trend is in the last 12-18 hours continues until game time I don't see guidance being the far off but if there are any convective components to the rainfall that will through another variable into the mix. That line moving through now will saturate the ground before the main event for sure. I was hoping to have some clarity, my commute in takes me through various flood prone areas so it would have been nice to make the call on whether to just stay home tomorrow. Last thing I want is to get detoured or stuck on Washington Blvd.

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Just now, RitualOfTheTrout said:

Gotcha, yeah I didn't take into account the timing of your post vs the models I was looking at. The worst is still SE of Pittsburgh and although it seems like whatever the trend is in the last 12-18 hours continues until game time I don't see guidance being the far off but if there are any convective components to the rainfall that will through another variable into the mix. That line moving through now will saturate the ground before the main event for sure. I was hoping to have some clarity, my commute in takes me through various flood prone areas so it would have been nice to make the call on whether to just stay home tomorrow. Last thing I want is to get detoured or stuck on Washington Blvd.

I edited the above post. I think it was the other convective models that wanted to push it south, as much as you can trust those. NAM and HRRR both had a slight increase in qpf in the immediate area.

That’s the thing here, it’s just like winter. No clarity about anything. At least we know the p-type this time.

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7 minutes ago, RitualOfTheTrout said:

Gotcha, yeah I didn't take into account the timing of your post vs the models I was looking at. The worst is still SE of Pittsburgh and although it seems like whatever the trend is in the last 12-18 hours continues until game time I don't see guidance being the far off but if there are any convective components to the rainfall that will through another variable into the mix. That line moving through now will saturate the ground before the main event for sure. I was hoping to have some clarity, my commute in takes me through various flood prone areas so it would have been nice to make the call on whether to just stay home tomorrow. Last thing I want is to get detoured or stuck on Washington Blvd.

Flooding on the 10th street bypass already. It's starting early  

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