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Since PHI issued their FF Watch yesterday afternoon, guidance is still vulnerable to 2" one hours rains, have opened a thread for commented event info. My thinking is heaviest will occur in the interior of NJ; but NYC nor sw CT, extreme se NYS are not out of the potential. My guess is that if the region north of I78 is to FF, it will probably have occurred by 9AM Saturday with two possible bands, one late this afternoon-eve, and another near dawn Saturday. How much of this heavy convection drifts over into NYC is questionable but t's not impossible to see 1-2" in parts of the 5 boroughs. Maybe the most favored areas are near I78 and possibly the hilly areas of far northern NJ. In any case, convection could impact the evening commute and wet microburst severe wind is not out of the question late today.
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I've added the Wednesday afternoon update from SPC. My focus is basically along the warm front-triple point wherever that traverses. CAPE and wind field/shear is good enough for iso SVR up to I84 late Thu afternoon-evening but I'm thinking more favorable zone is near and south of I78 in NJ-PA. Nevertheless, started a thread due to SPC outlooks, some of modeling dumping 1.5-2" of rain in 6 hours especially in se NYS/n NJ and ne PA. I won't be able to comment much of this thread period so just stay with it patiently and keep up to date on SPC outlooks, NWS Watch/warning/stmts and of course the radar with your observed trends. Never know, there might be a later trend further North into NYS/CT. 12z/28 EC wind algorithm pegs s CT/LI with 40-50kt. -
Despite the fact that it hadn't rained for more than two weeks (Sept 10th general last significant rainfall), the upcoming pattern will probably produce two significant rainfall episodes, Tuesday and then Wednesday-early Thu. One or both of these could result in a few NWS issued Flash Flood Warnings with total 60 hour rainfall in a few spots of the NYC forum exceeding 4 inches. PWAT will be more than 1.5" most of Tue-Wed exceeding 2 inches for brief periods. Isolated SVR may occur along to east of I95 (including LI) but that is a low chance and not tagged as the primary subject matter and not outlooked by SPC this Sunday morning Sept 27 We may want to check for updates on SVR potential early this coming workweek. Have added the Sunday morning D3 WPC Excessive Risk rainfall potential for Tuesday and suspect there will be another issued tomorrow for Wednesday (maybe axised a bit further east). mPING reports will be helpful (flooding and even damaging wind). Hopefully a few of our members will have some 4+" reports by Thursday (max rainfall = very low chance could be 8" but that will require unusual overlapping heavy swaths and so a very low probability, but mentioned due to multiple models trying for this in or near our area). Otherwise, areas of showers/drizzle late today through Monday will precede the Tuesday-Wednesday events, and could produce spotty 1/4" amounts by Monday night. 832A/27
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Since PHI issued a FF watch for much of its area yesterday, and then OKX followed suit this morning for parts of NY metro; have started this topic. Yes, potential exists for Flash Flooding and a few small areas of 4+ inch rains by Noon Friday, but model guidance is not overwhelmingly favorable for our NYC forum area. Northeast NJ, because of its infrastructure, and previous soil moisture condition is most vulnerable. But, best convergence for heavy convection appears to favor I78 southward for the entire period (south of more vulnerable ne NJ), with heavy convection possible further north to FWN-HPN-BDR up there only this afternoon. (for now, it seems se NYS and much of interior CT will miss the big stuff these next several days - keep watering the flowers from time to time). More than adequate CAPE and PWAT exist for heavy rainfall, KI are not pooled excessively in our area so far, but enough exists with a frontal boundary in a fairly light wind flow aloft. SVR: Since winds aloft are light, SVR if any, should be very limited, and probably only the I78 region southward-subject to modeling change. In summary: There should be organization for 1-3, short duration episodes of very heavy rain the next several days, especially in NJ. It seems most of this will be in the southern part of our NY forum area and even though the FF guidance is not as vulnerable in Monmouth, Ocean, Mercer counties--- that may be where the predominant threat for excessive rain is greatest-per WPC's morning excessive rain outlook for today-tonight. 610A/12
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LOW chance of exceeding 4" but this is worthy of awareness, I think. Have reviewed most of the typically used 12z/6 modeling, and WPC/SPC guidance. The scenario is a weak wsw flow aloft at 700 mb, over a very moist (near 15C dew point) 850MB vorticity rich wind shift across NJ (sse-sw) with 500mb vorticity maxima passing newd within an approaching 500mb trough that doesn't swing its broad axis through our NYC forum until about 12z Saturday. Dewpoints are already generally in the mid 60s with PWAT 1.6-1.8",-not overly wet, but with slow movers, plenty to allow for potentially excessive rainfall. How it all works is not clear and odds are that more than 4" of rain won't occur but have seen 12z/6 SPC HREF MAX precip guidance nears 10" in 1 spot of east central NJ and spotty 10+ s of LI. Best axis: unsure but for now along or s of I80 through LI or southward down to near I-195 (Ocean Mercer) counties. It should light up late tonight in central or northern NJ into LI and continues into forenoon Friday,. (Could be one or 2 isolated wet microburst SEVERE's?). For nw NJ, if there is very little tonight then I could see isolated svr thunderstorms in ne PA drift into nw NJ and se NYS late in the afternoon or evening and then continue to LI as showers Friday night. Low prob of occurrence. SPC has no risk and wind fields are generally a little too light. 627P/6
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Topic posted as a cover for the possibilities but this, as of 611AM Tuesday does not look like w widespread SVR or FF event. Please see SPC D1 Marginal Risk and followup's, as well as local NWS discussions, statements and any warnings plus our own American Weather interpretations and observations of any significant storms. Enough CAPE/KI/marginal wind aloft for a couple of clusters of strong storms. Don't like the 850 westerly flow for lots of svr, and the trough appears too close for too much SVR organization. Best chance for se NYS-nw NJ, 1P-7P. Best for s CT/LI/NYC, ne NJ and s of I80 later...3P-9P, then again midnight-5AM. This secondary late night increase is when it may be most favorable for an outbreak of heavy convection and several hours of training with embedded isolated SVR/FF and estimating best location IF this is to occur, would be anywhere on Long Island overnight. (noting several models with much increased convections overnight but mostly s NJ eastward. Could be more interesting up here on LI near the leftover boundary). I wont be updating anything on this topic between 8A-330P. Hope this produces something of interest.
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After yesterdays long range failure of convection (arrived Sunday night), must look ahead. Modeling, SPC, WPC graphics all suggest opportunity for heavy convection Wednesday-Thursday (22-23). Attached the 6 hr County Average threshold FFG, with hourly guidance even lower amounts. This shows vulnerability in parts of ne NJ and se NYS, plus urban paved environments. SPC Days 1-3 is attached as issued early Tuesday. Also, added a brief 06z/21 GFS snapshot of modeled winds aloft for LGA via Bob Hart's web page (time is in UTC). Note the near 30 kt or greater westerly flow at 500 and 300 MB, showing the idea of enough wind to foster development of SVR organization and strong downdrafts. In these two days, "isolated" storm totals of 4" easily possible with 'general' two-day total amounts of 1/2-2". Warm Air Advection (WAA) instability burst (KI popping into the upper 30s from southwest to northeast by daybreak Wednesday driven by a modeled small 850MB 30 kt jet core in central PA overnight) 'should' develop some briefly heavy showers or a thunderstorm for parts of our NYC forum early Wednesday morning, especially nw NJ/se NYS. After any clusters pass, cloud debris/instability pooling and surface wind shift convergence boundaries will probably be the determinants of when and where heavy convection occurs Wednesday afternoon through Thursday evening. Heaviest should be afternoon-early evening and per modeling, think the bulk of the FF/SVR should be along and north of I80 (far northern NJ, se NYS, CT and LI) but I defer to your assessments, and those of the NWS offices/SPC/WPC, and the modeling from the HRRR and SPC HREF. 734A/21
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Modeling looks terribly slow and ineffective for two bands of convection this evening in NYS/PA. Running much faster. If this is true... parts of our NYC forum, at least se NYS and nw NJ are going to experience a brief heavy shower or gusty thunderstorm between 815PM and midnight and possibly again near 2AM Monday. That leaves us with what for Monday afternoon? I think a pretty ripe environment for big storms maybe even up just N of I80...with interestingly large mid level lapse rates, especially I78 south. Not promising big stuff near NYC but it seems to me modeling (not SPC - check their Marginal D2) is missing potential svr/ff, especially southern part of our forum Monday afternoon 2P-7P. Will rereview how things are going Monday morning around 7A, but for now... it looks like big heat has generated big storms Sunday evening just west of the forum and I think very poorly modeled. If someone has seen a model that is performing admirably in PA/NYS to the Poconos and Catskills, please let us know. 735P/19
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Wantage NJ 4 sw (8 s High Point)...since 238PM-501PM 2.5". Since the 17th 4.85" Some residents pumping water in the nw NJ hills. another couple of hours of off and on TR+. I'd be careful crossing any flooded roadways where brooks or streams are involved. NWS posted a FFW mid afternoon. We don't often flood up here in the nw NJ hills but there's a good chance of a flash flood problem. STP's look a little low to me as of 5P. Walt 506P/22
