Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 25 2023 00:38:34 FOUS30 KWBC 250038 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 837 PM EDT Mon Jul 24 2023 Day 1 Valid 01Z Tue Jul 25 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... Anomalous trough centered over the Hudson Bay will maintain lowered heights across the eastern CONUS, and drive a cold front slowly but steadily eastward across Upstate New York and into New England overnight. A modest shortwave is seen in water vapor satellite imagery will continue to rotate through the base of the trough this evening pushing subtle height falls and PVA into New York and which may impinge upon New England later.=20 This...combined with at least modest upper diffluence in the tail of a jet streak to drive deep layer ascent across the region...will act upon an environment favorable for additional moderate to heavy rain during the evening hours. 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. For this reason the MRGL risk was once again adjusted cosmetically based on short-term radar with the expectation that the threat tapers off by 03Z/04Z. ....Ohio Valley... Scattered convection moving through the Ohio Valley this evening was being aided by weak shortwave energy impinging on a weak low-level baroclinic zone along. This should persist into the a weak warm front/stationary front will drive additional convection eastward before it dissipates by late this evening. 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 continue to produce convection with isolated downpours before the atmosphere begins to stabilize following sunset. Where this rain falls atop more sensitive soils noted by FFG as low as 1.5"/3hrs, some isolated flash flooding is possible. Expanded the area a bit to the east given the neighborhood probabilities of 1 inch per hour late in the period. Otherwise...the forecast reasoning largely remained the same. ....Southwest/Great Basin... An expansive mid-level ridge across the area has allowed return flow of moisture to spread across portions of the Southwest U.S. which has allowed for another round of late day and evening convection. Precipitable water values approaching 1.5 inches were mainly confined to areas in proximity to the international border but dropped to values closer to 0.5 inches farther north in Nevada and Utah. Isolated to widely scattered convection was able to formn due to a combination of weak shortwave impulses interacting with the moisture plume despite subsidence beneath the core of the ridge. The HREF has been focusing the better probabilities for 1"/hr rates in the southern portion of the Marginal risk area near the highest precipitable water values. 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 rainfall rates. 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 remain possible for a few more hours before convection wanes with the loss of daytime heating. ....Florida...=20 Maintained the Marginal Risk area across portions of the western Florida peninsula as mainly unidirectional flow continues drawing moisture inland and interacts with some weak shortwave energy over the eastern Gulf of Mexico. HREF probabilities for more than 3" peak along the Nature Coast and around the Big Bend of FL earlier in the day have dropped overnight...but echoes were still occasionally building as they reach the shore either through increased surface convergence with land or due to the near surface and temperature. Either way...expect stations are that rainfall rates will be diminishing by 03Z or 04Z even if there are additional showers that come on shore. Until then...think any instances if flash flood responses would be isolated and mainly confined to urban areas.=20 Weiss/Bann 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!6nUhpB_F9qw4ttCmelgRUEIktxsdeIo3EsX7rw7a528L= UNvoMun-o1AWfxWVneHWnZetVveYGMrFnwVk_TAVa8QCIt4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6nUhpB_F9qw4ttCmelgRUEIktxsdeIo3EsX7rw7a528L= UNvoMun-o1AWfxWVneHWnZetVveYGMrFnwVk_TAV8lzL6GQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6nUhpB_F9qw4ttCmelgRUEIktxsdeIo3EsX7rw7a528L= UNvoMun-o1AWfxWVneHWnZetVveYGMrFnwVk_TAVpPUDYx0$=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) .