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Showing results for tags 'Flash Flooding'.
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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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Active Pacific pattern assisting downstream northeast USA blocking of two large 500MB trough passages next week. This should result in at least one Nor'easter and possibly a classic fall severe weather event if a 985MB low moves northward through central PA. Max isolated rainfall potential of ~7" somewhere in the NYC subforum (interior) would lead to isolated Flash Flooding but the dry October of generally less than 1/2" so far in NYC 'should' limit the flash flood threat. There has been more rain I84 northward so the flood threat may eventually be targeting the interior NYS/CT portion of our forum. The general non-astronomically high tide cycles should at least partially limit the potential for moderate coastal flooding. Of greater concern appears to be softening of the ground by next weeks rains and the probability for a period of 45-55MPH wind gusts on fully leaved wet tree branches in parts of the area, which would lead to a power outage problem. At this Friday October 22 608AM issuance time, it's far too early to focus on the details. The headlines of this topic will adjust a bit as we move through the weekend, including tags. For now have left SVR and FF out of the tags. 430P/Saturday the 23rd added the FF Tag. 7P/Sunday the 24th increased general rainfall to a range of 2.5-8" up from 1.5-4".
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- heavy rain
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This first event is slated mainly for Monday night-early Tuesday night but with a pulse of showers, some heavy occurring overnight Sunday night and potential for bands of high wind and or heavy rain remaining in the NYC subforum into early Wednesday, depending on the departure of the eventual nor'easter. Max gust for e LI may get close to 70 MPH with wind damage 'potential' of 50 MPH gusts s of I84 in CT into far se NYS through ne NJ. Fully leaved tree branches with potential for sodden ground uproots Tuesday due to heavy rainfall and the gusts should result in pockets of power outages from simply broken branches. If it looks extensive, we will update the topic title. Max rainfall seems to be targeting CT/se NYS/NYC metro-ne NJ but proximity of max inflow-moisture transport still showing modeling variability for the primary target. In any case... it appears likely that along and north of I78 there will be periodic showers overnight with new rainfall by sunrise ranging from a few hundredths to possibly 1". After a break Monday, bands of heavy showers should be developing north and northwest into the area Monday evening and slow their northwest to west progress Tuesday before turning southward later Tuesday night. By 8AM Wed, isolated 6" rains may have occurred with a general 1.5-3" in the subforum. FF potential while initially limited by recent dryness, the rainfall rates and amounts in banding will eventually determine where any FF potential will be realized-it's just too early to determine, as of this 720PM Sunday writing. Please follow all future NWS warnings/statements. Let's see what the OBS herein tell us about the modeling by Tuesday night. Edited title rainfall up from 1.5-4 iso 6, to 2-5, iso 8. Edited title at 1025A/26 to like 40-60KT gust focused on CT-LI.
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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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650A Tuesday Aug 31: adjusted headline with mdi-iso Major impact esp NJ/LI, Dropped PRE since no TC and therefore no criteria. 6AM Sunday August 29 thread title update. Added general 2-8" rainfall, added possible PRE, and withdrew the ? of direct or indirect. Description of increasingly likely impacts around 7AM Sunday. This issuance makes the presumption that IDA will be our next Tropical Storm and move north into the Gulf states, shearing out into the Northeast or mid Atlantic states 500MB confluence zone next week. For now the risk of all the tags above is low, but worthy of monitoring. It's possible our first rain (if indeed IDA spreads rain into our NYC subforum?) might arrive late on August 31st. Think at this point, antecedent soil saturation and streamflow conditions will assist determining whether there will be any renewed stream flooding, if indeed IDA does dump a stripe of 3-6" of rain in a portion of our subforum. Chance of 35+ KT wind gusts is also low with the main contribution of IDA, rainfall, to Septembers total. Tides are fairly low so am not anticipating any problem with tides, at least not at this Thursday August 26th thread initiation. Attached the 12z/26 GEFS 500 MB membership for 12z Wed Sept 1 to show the confluence zone and also the 12z/26 24 hr qpf from the EPS and GEFS. For now these show a glancing blow to our south with a complete subforum no rain miss still possible, but not likely. The WPC Day 6-7 forecast issued 1622z/26 confirms the potential for at least some rain.
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Started a topic that may not bear as much fruit as the recent ELSA of this past week but with the attached 6 hr FFG fairly low now in the immediate NYC suburbs, thought it might be good to heads up some of the potential. Sun afternoon-Tuesday---warm frontal residuals and moisture pooling in weak low lvl WAA and halfway decent winds aloft that diminish considerably on Tue, I think there will be pockets of FF producing thunderstorms that could yield spot 5" amounts in that 60 hour period (slow movers Tuesday). Instability is related to considerable CAPE over 2000J in parts of the area Mon and Tue. Warm fronts can be big rainfall producers and so my thinking is the biggest problems will be I80 south, though some of the modeling gets things cranking up to I84. Suspect a few SVR Wet microbursts each day, especially Monday and Tuesday. Additionally the heat wave should begin Tuesday in the non-marine wind influenced coastal plain with the HI 95-100 Tuesday, Wednesday, Friday and possibly right through the weekend if the 12az/9 EC op is incorrect and the GFS as of 18z/9 is correct on 576 thickness in our area next weekend. My guess, once the heat wave starts, the debate shifts to it's big break... right now I favor longer than the EC and quite possibly lasting through next weekend. Hottest day(s)?: I think Fri when some of modeling has HI 100-maybe 105 near NYC. SVR seems likely to me on Wednesday the 14th due to increasing wind fields, then a drying out Thu-probably nothing that day, with fairly widespread SVR looming either next Fri or Sat when not only CAPE seems pretty large, but wind fields increase quite a bit and more easily sustain a big outbreak. TT toward next weekend increase a bit toward 48-50..not a huge svr signal but together with the wind fields and CAPE, should be interesting. In Summary: Lower FFG, high CAPE suggest pockets of excessive rainfall coming this week, along with SVR storms, esp Mon, Wed, FRI. Who gets the worst and when?? It does appear this will be more than the routine summer week, once again, with the big 500MB ridge along the east coast and troughing hugging the Great Lakes.
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- severewx
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The evolving pattern this coming week is continuing to model high CAPE (Heat Index NYC metro non-marine influenced near 100F) Tue-Wed and even Thursday afternoons with enough wind aloft and instability to permit one or 2 days of SEVERE storms in our area. Tuesday SVR appears relegated North of I80 (mainly NYS/CT). Wednesday to me looks like a potentially big day, especially I80 region northward with TT in the lower 50s, big CAPE. Thursday into Friday: Uncertainty on evolution but big CAPE and instability probably remain in the vicinity of our subforum. It too could be a severe day, however of interest is the eventual position of the frontal boundary. IF, it drifts down into our subforum with light north-northeast flow north of it, then it seems to me the projected path of decayed ELSA wind fields and moisture plume - convergence (925-850MB southerly flow overrunning a cool frontal boundary, ranging up to the 200MB RRQ of jet located in se Canada) would promote the potential for a Predecessor Rainfall Event (PRE) as documented in research by NYS associated well known mets. Have added a couple of their composites. These 4+" events occur 24-36 hours in advance of the TC, located hundreds of miles north or northeast of the TC even though the TC itself can miss to the south. If it doesn't and tracks over the PRE then that magnifies the potential sub-basin (mesoscale) trouble. As we are aware, we're starting to pile up the water across our subform so this potential result-NOT guaranteed, should be monitored and we'll see if it can occur, fritter, or displace north of our subforum. I'm doubtful if the potential displaces south of us. My guess is we'll know by Tuesday or Wednesday. In the meantime, I think we can expect severe storm wind/hail in parts of our area during midweek, along with possible storm cluster FF in PW air of 1.6", and probably should monitor the evolution of ELSAs ensembled wind fields up this way. Composites are added, as well as 00z/3 EC PWAT for late Thu (showing the TC well south but PWAT axis up near here). This is imperfectly modeled, the speed and location of ELSA and associated wind fields will adjust and affect the results. Let's see how this may breakdown and whether or not we can pick up ISOLATED 4-6" new rainfall between Tue-Fri in parts of our area-ISOLATED. WPC went to press this morning with a general 1"+ for our area, which supports embedded isolated heavier. You probably recall around the 28th-29th they had our entire area 1.5" for the current ongoing event with general 2"+ central and S NJ. Appears they did quite well. 638PM Monday July 5- EDITED Title to include ELSA- Too much modeling is drifting its track within range of coastal NJ and LI for Tropical Storm considerations Thu night-Friday AM. While it's not certain to make a direct second landfall across LI/southeastern New England, think it best to open the door a little more, at the least for 4+" rains this week combined convective outbursts and whatever Elsa.
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Uncertainty as always but SPC has expanded their D1 into our area, and the attached 12z HRRR suggests iso 50-55KT gusts into PA/NJ/LI, mirroring the 00z/19 EC OP. Looks to me like 90-95F today (non marine), decent CAPE. The severe risk suppresses south tomorrow, but WAA begins late Sunday and then both Monday and Tuesday look steamy 90-95F inland, depending on sunshine which also implies cirrus from Claudette possibly tempering heat/cape. This should result in pockets of heavy convection, and where training, could see a ISOLATED max of 4" by 6PM Tuesday. Otherwise, the necessarily more conservative early Saturday WPC outlook works. So while not a big deal (yet) for heavy rain, the convection later today, later Monday and Tuesday could provide some news (damage) footage, including lightning related. Walt
