Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 24 2023 19:56:59 FOUS30 KWBC 241956 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 PM EDT Mon Jul 24 2023 Day 1 Valid 16Z Mon Jul 24 2023 - 12Z Tue Jul 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST/MID-ATLANTIC, MID-MISSISSIPPI VALLEY AND OHIO VALLEY, FLORIDA, AND THE SOUTHWEST/GREAT BASIN... ....Northeast/Mid-Atlantic... Anomalous trough centered over the Hudson Bay will maintain lowered heights across the eastern CONUS, and drive a cold front slowly eastward across the Great Lakes and towards Upstate NY today. A modest shortwave is progged to rotate through the base of the trough this aftn/eve causing subtle height falls and PVA into the Mid-Atlantic and Northeast, combining with at least modest upper diffluence in the tail of a jet streak to drive deep layer ascent across the region. This lift will act upon an environment favorable for heavy rain noted by PWs of 1.25-1.5 inches, around the 75th percentile according to the SPC sounding climatology, and MUCAPE of 1000-1500 J/kg to produce convection which the HREF and HRRR both indicate will have rain rates of 1-2"/hr due to weak mid-level lapse rates in deep warm cloud depths. The simulated reflectivity from the high-res today generally features scattered convection, but the presence of 30-40 kts of effective bull shear may help drive multi-cell organization in some areas, especially Upstate NY into western New England. In general these storms will move progressively to the E/NE on 0-6km mean winds of 20-25 kts, but some short term training is possible as Corfidi vectors align to this flow downstream of the primary trough axis. The HREF probabilities for 1" and 3" of rain are relatively modest, so despite compromised FFG due to 14-day rainfall that is 150-300% of normal, the FFG exceedance probabilities are low. For this reason the MRGL risk was just adjusted cosmetically, but should more organized convection develop and train, especially from eastern PA northward through the Hudson Valley and into western New England, which has at least a subtle signal in the HRRR and experimental RRFS, some scattered instances of flash flooding could be possible. ....MS VLY/Ohio Valley... An MCS moving through Missouri this morning is being driven by a weak shortwave dropping through NW flow on the upwind side of a large trough across the eastern CONUS. This MCS should weaken by the aftn in response to the veering and waning LLJ, but the accompanying shortwave impinging the weak low-level baroclinic zone along a weak warm front/stationary front will drive additional convection to the east this aftn/eve. Subtle height falls within the mid-level trough will additional aid ascent, which when acting upon an environment with PWs of 1.25-1.5 inches and a ribbon of MUCAPE exceeding 2000 J/kg will produce convection with heavy rain rates of 1-2"/hr. Although mean 0-6km winds will be progressive at 15-20 kts, these should remain aligned generally to the front, suggesting some short duration training, with effective bulk shear driving multi-cell clusters across the region. Where repeated rounds of thunderstorms can occur, rainfall could reach 2-3" as reflected by HREF exceedance probabilities. Where this rain falls atop more sensitive soils noted by FFG as low as 1.5"/3hrs, some isolated flash flooding is possible. ....Florida... Deep trough centered over the Hudson Bay will extend into the Gulf Coast today, leaving generally flat zonal flow across the Gulf of Mexico and Southeast. A weak impulse rotating through the flow will combine with the tail of a jet streak positioned offshore and an approaching cold front which will stall in the vicinity to produce scattered convection today. Nearly unidirectional flow over the Gulf of Mexico will drive convection west to east into the northern FL Peninsula today, driven by this ascent working over an airmass characterized by a ribbon of 2" PWs and MUCAPE above 1000 J/kg. Flow parallel to the approaching front will allow for training of cells into FL, while Corfidi vectors becoming increasingly aligned anti-parallel to the mean wind suggests backbuilding to lengthen rainfall duration. HREF probabilities for more than 3" peak along the Nature Coast and around the Big Bend of FL, with rainfall rates likely reaching 2"/hr at times. Although this area has generally been dry recently, training of these rates could produce isolated flash flood responses, especially in any urban areas. ....Southwest/Great Basin... Expansive mid-level ridge will become entrenched across the Southwest/Four Corners area, anchoring over New Mexico today. The placement of this ridge will allow for return flow to spread across the western portions of the Southwest and into the Great Basin, noted by PW anomalies on the NAEFS ensemble tables reaching +1 to +2 sigma from western Arizona northward through NV/UT and into ID/WY. While subsidence beneath the core of the ridge will suppress much in the way of convection due to a +18C 700mb cap, this PW plume around its periphery combined with weak shortwave impulses lifting around the ridge will likely result in scattered convection, primarily during peak heating as MUCAPE rises to 750-1000 J/kg. The inherited MRGL risk was adjusted via an expansion northward to account for the higher PW plume and at least modest signals in the HREF for 1"/hr rates. Storms are likely to be moving pretty quickly on 0-6km mean winds to the northeast at 15-20 kts, but could still produce 15-min rainfall totals of 0.25-0.5 inches as reflected by the HRRR sub-hourly. In general this will be less than FFG, but where storms can move a bit slower (primarily southern AZ) or where this rain can occur atop more sensitive terrain features like washes and slot canyons, isolated flash flood instances are possible. Weiss Day 2 Valid 12Z Tue Jul 25 2023 - 12Z Wed Jul 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC... ....Mid-Atlantic/Northeast... A longwave trough will persist across the eastern CONUS extending from a closed low near the Hudson Bay. Within this trough, an embedded shortwave will advect eastward while amplifying, moving across the coastal Mid-Atlantic and into New England during peak afternoon heating. This ascent will work upon a favorable thermodynamic environment characterized by PWs reaching above 1.5" collocated with MUCAPE eclipsing 1500 J/kg. The result of this will rapid development of thunderstorms during the aftn/eve, with cells organizing into clusters through 25-35 kts of effective bulk shear. Storm motions of these cells are likely to remain progressive to the east/northeast, but some short duration training is possible. Rain rates within convection will likely exceed 1"/hr, and may reach 2"/hr as shown by HREF probabilities and the HRRR sub-hourly fields. This will result in bands of 1-3" of rainfall, with locally heavier totals possible as reflected by modest exceedance probabilities in both the HREF and experimental RRFSe. The greatest risk area for this heavier rainfall in both ensembles is along the I-95 corridor from around Fredericksburg, VA to just west of New York City. While confidence here is modest due to spread in the placement of more organized convection and a possible MCS in the high-res simulated reflectivity, the greatest overlap of exceedance probabilities with saturated soils is across this same region. This prompted the coordination of a small SLGT risk embedded within the larger MRGL risk extending all the way to the Canadian border. Scattered instances of flash flooding are possible, especially where these intense rates track across urban areas, with a more isolated potential existing elsewhere across the Northeast. ....Southwest/Four Corners... Mid-level monsoon ridge will remain entrenched across the Four Corners/Southwest Tuesday with only slight reduction in amplitude as weak shortwaves traverse the periphery and a more amplified shortwave trough digs out of the Pacific Northwest. This will produce slightly greater deep layer ascent, which when combined with 500-750 J/kg MUCAPE should result in scattered showers and isolated thunderstorms Tuesday. The PW plume from Monday is progged to weaken D2 noted by NAEFS ensemble tables showing just near normal PWs, but this will still be sufficient to produce rainfall rates of 0.5"/hr, with local rates as high as 1"/hr as progged by HREF neighborhood probabilities. The guidance has backed off slightly on total coverage and rainfall amounts, but the high-res simulated reflectivity still suggests storms will rotate SW to NE at around 15 kts, with some multi-cell organization possible within effective bulk shear of 15-25 kts. The flash flood threat appears modest overall, but if any of these more intense rain rates can occur atop sensitive features like burn scars, washes, or urban areas, isolated instances of flash flooding could be the result. Weiss Day 3 Valid 12Z Wed Jul 26 2023 - 12Z Thu Jul 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE FOUR CORNERS, AND IN THE VICINITY OF THE GREAT LAKES... ....Upper Midwest through the Ohio Valley... Dual shortwaves moving across the region on Wednesday will advect more easterly through the period in response to flattening flow north of an expanding/elongating mid-level ridge centered near the Four Corners. The first of these is progged to move from the Arrowhead of MN early to Upstate NY late, with a secondary impulse trailing it to move across a similar area the latter half of the day. Each of these will be accompanied by a modest wave of low pressure with associated fronts to enhance ascent from west to east. While uncertainty continues into the timing and latitude of these features, each one will likely produce scattered convection through PWs that approach +2 standard deviations overlapped with MUCAPE that may surge above 3000 J/kg ahead of the fronts. The simulated reflectivity during this time indicates that convection may organize into clusters or even MCSs through 30-40 kts of effective bulk shear, driving rain rates that may reach 2"/hr at times. With the mid-level flow becoming increasingly zonal, mean flow will become aligned to the warm fronts suggesting an increasing likelihood for training from west to east, with some backbuilding into the greater instability also possible. This could result in multiple rounds of heavy rain producing thunderstorms moving across the region Wednesday, and the MRGL risk has been expanded from MN east through OH/NY. This was done to best match the ECENS/GEFS/SREF probabilities, and to account for spatial spread in the guidance over soils that have compromised FFG as low as 1.5-2"/3hrs. ....Four Corners... The monsoon ridge will remain amplified near the Four Corners on Wednesday, but some elongation is noted in the mass fields from SW to NE which should allow higher PW anomalies to drape farther northeast than on previous days. This evolution of the mid-level ridge is due to a shortwave advecting east from the Pacific which will drive height falls from the NW, resulting in this elongation of the ridge. PW anomalies are again progged to be normal to slightly below according to the NAEFS ensemble tables, but MUCAPE may reach as high as 1000 J/kg beneath the ridge, especially since convective debris cloudiness is expected to be minimal on Wednesday. This results in still favorable thermodynamics for heavy rainfall, and as forcing increases through subtle height falls and any weak impulses rotating around the periphery of the ridge, scattered showers and thunderstorms are likely during the aftn/eve. Rainfall rates of up to 1"/hr are probable, which despite generally fast forward motion to the E/NE could produce locally 0.5-1" of rain as shown by modest ECENS/GEFS probabilities. Overall the flash flood risk on Wednesday appears isolated, but a few instances are possible especially over the most vulnerable soils or terrain features. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-GCTq4mqe0986koU4_W2dOdL7WlkCwFf5Snvm_pMt_Zg= 8LGFsgsAHRlqRJTtY6cwHx29WroAdiUPFh-P4iP192huMek$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-GCTq4mqe0986koU4_W2dOdL7WlkCwFf5Snvm_pMt_Zg= 8LGFsgsAHRlqRJTtY6cwHx29WroAdiUPFh-P4iP17Xw23Tk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-GCTq4mqe0986koU4_W2dOdL7WlkCwFf5Snvm_pMt_Zg= 8LGFsgsAHRlqRJTtY6cwHx29WroAdiUPFh-P4iP1JvPegtU$=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) .