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0.2-3.5" rainfall and OBS thread Wed-Thu Sept 8-9, 2021 may cause renewed flooding along and northwest of I95


wdrag
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123PM Tuesday (7th):  Raised iso max amount to 3.5".  This thread will serve as OBS for this event as well.

 

528A Tuesday (7th): added possible iso SVR to the tags. This per some of the guidance with CAPE & decent wind aloft plus the newly added SPC MARGINAL risk across our area. No other changes at this update issuance. 

 

Added some information graphics from the NWS issued prior to dawn this Labor Day 2021.  The Marginal risk for Wednesday-ear;ly Thursday and it's discussion,  plus the Mid Atlantic River Forecast Center 6 hour Flash Flood Guidance (use legend for 6 hour amounts needed to begin flooding). Have checked the available River Stage ensembles and they do not seem to respond with rises, but undoubtedly a few small streams will respond with uncertain to predict rises. 00z/6 EPS rainfall is higher (almost twice) than the GEFS. The EPS does not go into the automated River Stage ensemble response guidance. NWS modifies this guidance for their official river stage forecasts, which generally officially publish daily around 10A-11A. 

Periods of showers Wednesday into Thursday, with an uncertain timing end to the showers and uncertain location of the max rainfall axis, which could reach 2". Just too uncertain at this issuance time to confidently narrow the range. 

Intensity will determine amount and runoff response.  This WPC discussion from predawn Labor Day was added below. 

 

...Northeast...

 

The progressive shortwave trough and its attendant surface low and associated fronts mentioned in the day 2 period will be tracking 

through the Great lakes and Ohio Valley region into the Northeast. Additionally, there will be a warm front lifting through the 

Mid-Atlantic states. PW values will be increasing from as the front approaches from the West, reaching 1.25 inches (+1 standard 

deviation from the mean) by 06/12Z on the 8th. Much of the Northeast will be in an area with broad forcing for ascent. 

Initially, low-level winds will be westerly near 15-20 kts but will then become southwesterly, increasing to 35 to 45 kts by 

6/12Z as the cold front approaches the region. A fairly narrow axis of QPF is expected to align along/ahead of the cold front 

from West Virginia to Maine, over a large portion of the region that was hit hard by torrential rains from Ida within the past 

week. These areas are still recovering therefore any measurable rainfall will have the potential to aggravate ongoing flooding. A 

Marginal Risk area for excessive rainfall and flash flooding was raised, spanning from eastern Pennsylvania/northern New Jersey to 

north-central Vermont.

 

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  • wdrag changed the title to 0.2-3.5" rainfall Wed-Thu Sept 8-9, 2021 may cause renewed flooding along and northwest of I95

130PM:  A couple of notes. SPC HREF from the 12z cycle added.  It's possible parts of our area will escape with 0.1" rainfall but am staying with the vast majority of our NYC subforum receiving a minimum of 0.2" with two areas of max rainfall... just northeast of I95 and also possibly e LI and the CT River.  Max rainfall appears headed for e PA, nw NJ or se NYS with max possible 3.5". SPC HREF mean and max  rainfall appended as well as 12z/7 HRRR operational rainfall total. Again a narrow stripe over little may be found vicinity NYC- Oxford CT, but I wouldn't count on it being less than 0.1". 

Looks to me like some spots in e PA/w NJ and possibly se NYS will have a tornado threat 6PM-10P.  Value above 1 should be a cue.  Guidance appended.  

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  • wdrag changed the title to 0.2-3.5" rainfall and OBS thread Wed-Thu Sept 8-9, 2021 may cause renewed flooding along and northwest of I95
1 hour ago, Brian5671 said:

seems a bit much for flood watches-models seems to have mostly less than an inch for many outside of isolated pockets.

12z Euro as an example:

1631217600-k3wSXu07E8g.png

EC was misleading last event, remember it displaced north and there was talk of NYC missing out, as late as mid morning Wednesday? Quite a distraction from USA modeling.  Think we need to stay with the NAM3K, HRRR, and SPC HREF.  SPC has slight risk out for tomorrow western portion of the subforum. 

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54 minutes ago, wdrag said:

EC was misleading last event, remember it displaced north and there was talk of NYC missing out, as late as mid morning Wednesday? Quite a distraction from USA modeling.  Think we need to stay with the NAM3K, HRRR, and SPC HREF.  SPC has slight risk out for tomorrow western portion of the subforum. 

Yes. The EC did among the worst jobs. It largely missed the focus of where the heaviest rainfall wound up. It was much too dry. 

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1 hour ago, donsutherland1 said:

Yes. The EC did among the worst jobs. It largely missed the focus of where the heaviest rainfall wound up. It was much too dry. 

Thanks Don... it is my opinion that the EC is fantastic synoptically (cool season especially), but that it lacks on convective qpf (warm season) which results in differing frontal positions and max axis of qpf.  What I'm seeing in some of the guidance is dew points (strong northward moisture transport) Wednesday afternoon-night into the front with surface dews likely to exceed 70 in NYC and most of NJ. Probably briefly rises to near 73 in NYC.  The TOR threat is still particularly large. Rechecked with no change.  I'll update overnight and tomorrow morning by 730AM.

 

Don't know if anyone has noticed the numerous reports in the midwest of both hail/wind including 25 reports of Large hail. 

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00z/8 HRRR and 3K NAM basically no changes through Wed night.  EC is probably going to be correct about a second batch of rain Thursday, esp I95 east. HRRR and 3KNAM  looking for a problem svr storm - poss tor-supercell w hail in ne PA.

HRRR STP

3KNAM STP for about 00z/9.

2-5KMaxc updraft, note the large hail track identical that follows

then HRRR total rain

and NAM3K total rain

 

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Here's some guidance to consider: available le at 530AM Sept 8. most of the rain today... NJ/NYS/PA, then tomorrow (after midnight, most of the rain LI/CT as a secondary surge is expected. 

Below all from 9/8/21 00z-06z modeling cycle

SPC D1 outlook

WPC D1 outlook

WPC D1-2 qpf

06Z HRRR max 2-5KM updraft showing potential till axis of strong thunderstorms, the tmay produce svr wx, including a TOR- the latter which still looks plausible over eastern PA this eve. (also shows something up near ALB)

08Z WPC D! excessive rainfall discussion

06Z SPC SVR discussion

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2 hours ago, LibertyBell said:

Where did this horrible wind come from? I didn't see anyone say prepare for windy conditions.  It took my power out!

I highly doubt the wind caused the outage. Just run of the mill SW wind with sea breeze enhancement. And to answer your other comment, Long Island is much more prone to surge flooding then freshwater rainfall flooding. 

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2 hours ago, LongBeachSurfFreak said:

I highly doubt the wind caused the outage. Just run of the mill SW wind with sea breeze enhancement. And to answer your other comment, Long Island is much more prone to surge flooding then freshwater rainfall flooding. 

But if we had 10 inches of rain in 2 hours a lot of basements would be be flooded (including mine, because I've had water in there with less rain than that- fortunately I have a drain in the floor of the basement)

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2 hours ago, jconsor said:

Great updates Walt @wdrag - thank you!  Just finished the Rosh Hashana holiday so I am just now catching up on this threat, which looks much more significant than two days ago before the holiday when I last looked.

Check out this entire thread I posted - just posting some snippets to not overload forum:
 

 

How is it possible to get 3-5 inches of rain from just a typical late summer frontal passage?

 

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