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Monitor NWS products including NHC and local offices from OKX and PHI. This Topic doesn't guarantee anything but serves as a platform for comment. Suggest starting with actual 5+" rainfalls, or unusual flooding. Note: WPC is already bullish on Tuesday's rainfall. Tropical Debby remnants-east coast trough-RRQ upper level jet related pattern this week had WPC intermittently forecasting 7+" of rain in NJ and possibly coastal CT/LI. PA and se NYS may also be involved in big rain. PRE conditions may??? exist late this week (Fri-Sat?) but uncertain in our NYC subforum. Added a few graphics. WPC overall has been advertising 5+ in its graphics for our area since 06z/4. Below is the WPC 7 day rainfall forecast based on ensembles etc. Sampler past 3 days rainfall via CoCoRaHs. Machine ECMWF EFI, Machine GEFS ARI and UFHS, WPC D2 excessive and their D5. You'll note the GEFS machine tools extend beyond D5. This weeklong accumulative event may be over by Sunday the 11th. FFG is very dry south of I80, so far. This could. change radically by Friday night. Ensemble river stage products right now are conservative but highlight I95 corridor in our area. FFG for now is most vulnerable I80 northward in NJ. What to do? Just think of what you may??? need to alter in your plans due to flooded roads/streams, cellars. Potential doesn't mean it occurs. 727A/5 Initial title Potential POCKETS 5-12" of rain by 18z Sunday. Suggest OBS Tue-Sunday only when exceeding 5".123411
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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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