Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 28 2023 19:58:15 FOUS30 KWBC 281958 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 PM EDT Thu Sep 28 2023 Day 1 Valid 16Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER PORTIONS OF THE MID-ATLANTIC AND NORTHEAST... ....16Z Update... In coordination with the local WFO offices in Mount Holly and Upton, have upgraded the D1 ERO to a Slight Risk given the trends in short term guidance in terms of placement and magnitude of heavy rainfall this evening. General synoptic scale pattern remains fairly similar to previous forecast package, but more emphasis on heavy rain potential has arose given the signals on the latest ensemble means and deterministic QPF distribution. 12z HREF neighborhood probabilities of >1" and >2" totals across northern and central NJ, as well as the NYC metro into the southern Catskills was sufficient to upgrade the area to the SLGT risk. There is still some discrepancy on the exact placement of the heaviest QPF footprint, however the highly anomalous u-vector wind fields across northern NJ and adjacent areas are all prevalent across all guidance which would generate a robust upslope component typically found in these types of heavy rain events. Convective potential is still on the low side, but non-zero given the theta-E advection regime likely overnight into early tomorrow morning as indicated by all deterministic, global or hi-res base. Total QPF between 1-2" with locally as high as 4" are being depicted within the HREF blended mean QPF which only lends credence to the higher potential with the SLGT. Main areas of concern will be the higher terrain in NNJ and southern NY state, as well as the urban areas in-of NYC/Newark/Jersey City. Kleebauer ....Mid-Atlantic into Northeast... We added a Marginal risk across portions of eastern PA into NJ and southeast NY with this update. Lower confidence than normal for a day 1 forecast as an area of low pressure develops offshore and attempts to push an inverted trough/coastal front inland. The extent of the flash flood risk really comes down to the positioning of this front/convergence axis later tonight into Friday morning. If this axis stays offshore then any instability will also remain offshore, resulting in just some stratiform rainfall moving inland. However if the axis is along the coast or inland, then some weak instability should get ashore as well, and set the stage for the potential of shallow efficient convection. Most of the HREF members do bring some heavier rain onshore, with the highest probabilities focused over NJ. However each HREF member has a slightly different location, and most of the 00z global models keep the bulk of the heaviest rain offshore through 12z Friday. Given this spread and uncertainty opted to stick with a Marginal risk for now. The main threat for day 1 is the last 6 hours (06z-12z Fri), so will let the day shift take a look at the 12z HREF and make a call on whether to upgrade to a Slight or not at that time. ....Ohio Valley... A Marginal risk was maintained across portions of TN/KY into southern OH. Convection will be ongoing at 12z this morning, with some brief training/backbuilding remaining possible. We might end up with a downward trend in convective activity by mid to late morning...but with an elongated area of vorticity continuing to traverse the area through the day good synoptic ascent will remain in place. Thus once instability recovers from the morning activity we could see some additional convective development across the region this afternoon/evening. The magnitude/organization of this additional activity is conditional on the amount of recovery we get....but if we area able to ramp instability back up then some additional brief training could occur given the persistent low level inflow and convergence in place. Isolated flash flooding will be possible, both this morning and potentially again this afternoon/evening. ....Florida... Only minimal changes to the inherited Marginal risk area over FL. Anomalous moisture will remain in place, with a stationary front and stronger upper level flow helping trigger above average convective coverage again today. High rainfall rates will drive a localized flood risk over any more susceptible urban locations. Chenard Day 2 Valid 12Z Fri Sep 29 2023 - 12Z Sat Sep 30 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN NEW JERSEY, FAR SOUTHEASTERN NEW YORK, AND THE NEW YORK CITY METROPLEX... ....20Z Update... Increasing consensus in a higher impact event across portions of the Northeast U.S, including the NYC metro area has lead to an upgrade to a Moderate Risk over the aforementioned area. After coordination with the local WFOs over portions of the Mid-Atlantic and Northeast, confidence was high enough to introduce the Moderate given the signals across much of the 12z deterministic suite and ensemble guidance. More on the setup below... Only minor adjustments were made to the MRGL over the Upper Midwest to account for convective signals within the means and 10 year ARI exceedance probabilities over the western Michigan U.P. There were no changes to the MRGL in FL as the convective environment remains favorable for locally heavy rainfall within an elevated PWAT regime thanks to persistence in the southern stream jet and location of a slow-moving front over the northern sections of the FL Peninsula. ....New York/Northern New Jersey... 12z suite of guidance has narrowed the corridor of expected heavy rainfall that will transpire thanks to a prominent easterly fetch event expected on the northern periphery of an inverted trof in-of the northern Mid-Atlantic coast. The signal for a prolonged heavy rain event has increased markedly over the past 24 hrs with the current deterministic and ensemble mean guidance now pinpointing a swath of 2-4+" rainfall footprint across the NNJ and the adjacent NYC metro and southern NY state towards Poughkeepsie. 12z HREF mean is approaching 3-4" for just the NYC metro which would cause considerable problems due to urban sprawl and inability for rainfall to penetrate the impervious surface. 12z HREF probabilities are also depicting an upper threshold event within some of the probabilistic signatures. 3"/3 hr rainfall probability is exceeding 70% for the NYC area up through the exo-burbs outside the metroplex. 1-2"/hr rainfall rate probabilities are also into the 70-80% territory which is typically found in the most robust setups, usually within a tropical moisture field which is what is expected given the enhanced IVT signatures on global deterministic correlating well with the axis of heavy rainfall. GFS/ECMWF PWAT anomalies are both approaching the 1.5-2 deviations above normal range focused in the corridor of higher forecast QPF. Instability will be lackluster for the most part, but some theta-E advection into coastal NJ, NYC, and Long Island will pose a lower-end convective threat across the region which would only exacerbate flash flooding concerns. The biggest change outside the magnitude of rainfall is the propagation of the main QPF field further inland thanks to the advection of low-mid level moisture being forecast due to very anomalous U-vector wind fields generated from our inverted trof. Enhancement on the NW side of the precip field will be possible Friday morning and early afternoon as the shortwave trough over the Great Lakes pivots into the northern Mid-Atlantic and closes off across northern PA. This would allow for a more dynamic signature within the northern and eastern fringes of the upper-level circulation creating localized banding structures over the terrain as depicted by a few hi-res deterministic. This is one of the reasons we see secondary and tertiary maxes within the general QPF forecast from deterministic as banding structures will be found during the event evolution. Local maxes between 6-8" will be plausible across the outlined Moderate area and even within the confines of the SLGT risk. This has evolved into a higher impact potential and will be monitored closely for future updates to the orientation of the MOD risk with coordination from the local FO's involved. Kleebauer ....Upper Midwest... Convection should be ongoing Friday morning across portions of SD/ND/MN, although do think this activity will be pretty quick moving. The better chance of isolated flash flood issues appears to be Friday night as the front slows and low level moisture transport sees a nocturnal uptick in intensity. This should be enough to result in another convective round, with propagation vectors supporting some backbuilding potential by this time. With PWs running near to above the climatological 90th percentile any training/backbuilding of cells could pose a localized flash flood risk ....The Florida Peninsula... Pretty much a persistence forecast across FL...with 2"+ PWs and a favorable environment for good convective coverage in the area of a slow moving frontal boundary. High rainfall rates over urban locations may again result in some localized flood concerns. Chenard Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6-kmsJsDBvU5jAOy4XpBh8L-HDcZVs2oHU7Nb-IucMFr= kn54K1DBI4U9PMwF0kHXzouCZ38ibHjMZC5EHrr3s3KVHwA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6-kmsJsDBvU5jAOy4XpBh8L-HDcZVs2oHU7Nb-IucMFr= kn54K1DBI4U9PMwF0kHXzouCZ38ibHjMZC5EHrr3IU-yfsc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6-kmsJsDBvU5jAOy4XpBh8L-HDcZVs2oHU7Nb-IucMFr= kn54K1DBI4U9PMwF0kHXzouCZ38ibHjMZC5EHrr36M15U_g$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .