Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 21 2023 19:20:34 FOUS30 KWBC 211920 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 319 PM EDT Sat Oct 21 2023 Day 1 Valid 16Z Sat Oct 21 2023 - 12Z Sun Oct 22 2023 .....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW ENGLAND... ....16Z Update... Current radar depiction shows increasing rainfall coverage over southern New England as our surface low continues to push to the north with moisture advecting into the region on the northern periphery of the SLP. 12z sounding out of KOKX showed a shallow convective potential across LI that will be advecting to the north and east as the low continues its latitudinal push from coastal NJ into coastal New England. Multiple hi-res deterministic indicated a narrow tongue of higher theta-E's moving into RI and eastern MA through the late morning and afternoon time frames before quickly shifting eastward as the flow shifts to the northwest under the beginning stages of SLP occlusion. 12z HREFbm and HREFpmm QPF forecasts were most pronounced for the short- term within that corridor with totals ranging from 1-2" over much of the area. That being said, the threat is on the lower end of potential with hourly and three-hourly probabilistic indicators not really showing much in the way of FFG exceedance probabilities (20-30% max for respective indicators), and really within a short window as the setup remains progressive within the zone above. After some coordination with the local Boston WFO, have elected to keep the aforementioned area outside any MRGL expansion. Further to the north over VT/NH, prolonged moderate rainfall continues within an inverted trof extension from western MA through central New England. Observations out of the CPV down into the Berkshires have shown rainfall totals in excess of 1.5-2" through that corridor with some minor flooding occurring within the terrain. The FFG probabilities continue to remain on the lower end, however as rates haven't been the driver of the impacts, but more of the longevity coupled with susceptible terrain influences. Hi-res ensemble and ensemble-bias corrected totals continue to run between 1.5-3" with the max located within the higher terrain in the Green and White Mtns as the deformation axis is forecast to mature and pivot overhead giving the terrain an extended period of enhanced rainfall. This will keep the area within the lower probability of flood concerns, and to keep continuity have elected to maintain the current Marginal Risk with a future assessment providing the opportunity for change. Overall, the disturbance is behaving as forecast and flash flood concerns will remain on the lower end of threshold with areal flooding more likely given the longevity of enhanced rainfall from synoptic scale dynamics.=20 Kleebauer ....New England... Expanded the southern boundary of the Marginal into Southern New England where the 21/00Z HREF showed widely scattered areas where the 40-km neighborhood probability reached 20 percent during the afternoon. This appears to be coincident where the NAM and GFS showed potential instability immediately ahead of an approaching region of synoptic-scale lift. However...the model to model placement did not line up terribly well either spatially or temporally which limits the confidence to a Marginal area. On the other hand...the one point of agreement was the focus for the developing coastal low to be farther south which also supports the subtle southward expansion. Otherwise...the changes made on Friday were largely in place this morning. The instability is still expected to be meager which limits the instantaneous rainfall rates. Total rainfall amounts of 1-3" are expected in the MRGL risk area. Localized runoff concerns, especially over urban regions and areas susceptible to flash flooding, are possible. Bann Day 2 Valid 12Z Sun Oct 22 2023 - 12Z Mon Oct 23 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann/Kleebauer Day 3 Valid 12Z Mon Oct 23 2023 - 12Z Tue Oct 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-Iml2vBV4azxSEPXaHDYGRJd_Z_Ih19KFwmK0xasNE7F= P9anpm2vrp05ItIvD2RBMEOi4lAk77UPqpVYWGJKgo8eW8w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Iml2vBV4azxSEPXaHDYGRJd_Z_Ih19KFwmK0xasNE7F= P9anpm2vrp05ItIvD2RBMEOi4lAk77UPqpVYWGJKB6Ft91E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Iml2vBV4azxSEPXaHDYGRJd_Z_Ih19KFwmK0xasNE7F= P9anpm2vrp05ItIvD2RBMEOi4lAk77UPqpVYWGJKLXu3ekI$=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) .