Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 10 2024 20:43:03 FOUS30 KWBC 102042 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 442 PM EDT Tue Sep 10 2024 Day 1 Valid 16Z Tue Sep 10 2024 - 12Z Wed Sep 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ....16Z Update... ....Central Gulf Coast and Florida... Rainfall along the far south Texas Gulf coast should come to an end by early afternoon, with the expectation that the risk for any additional heavier rainfall should remain along the immediate central and upper Texas Gulf Coast as Francine tracks to the north- northeast. The greater threat will be with bands of convection already focused along the central Gulf coast far to the northeast of Francine, with additional bands expected later this evening and overnight as the center of the system approaches from the southwest. Updated 12Z hi-res guidance shows remarkably good agreement on the heaviest rainfall footprint along the central Gulf Coast, stretching from near Cameron, LA east through southeastern LA and to the immediate coast of MS. Areal average totals through early tomorrow morning are on the order of 2-3" with locally higher amounts of 3-6" possible with training bands. Additional storms are expected further east in vicinity of a quasi-stationary frontal boundary draped through central Florida. Enhanced moisture in the area from Francine will help contribute to some locally heavy downpours with rain rates of 2-3" per hour, which could lead to some isolated flooding/ponding issues in urban areas. ....Intermountain West... General footprint for the current Marginal Risk remains the same with a small northwestward extension into southwestern Wyoming to cover additional areas of scattered convection expected this afternoon. Latest hi-res guidance indicates the potential for some locally heavy storm total rainfall of 0.5-1", possibly as high as 1.5", as well as some low-end HREF probabilities (20-30%) of exceeding 2-year ARIs. These totals may be enough to lead to some isolated concerns for flash flooding in the usual more terrain sensitive areas such as slot canyons, dry washes, and any burn scars. Putnam ....Previous Discussion... ....Gulf Coast... Tropical Storm Francine will continue making headway to the north and northeast towards Louisiana, with NHC forecasting the system to become a hurricane. The combination of dry/cool air to the west and northwest of the storm will lead to a tight rainfall gradient on its west side. Further enhancing the western gradient will be the increasing vertical wind shear with time out of the west- southwest, which will attempt to shift convection downshear and downshear left of its center (to its northeast). With QPF continuing to trend downward, the Slight Risk for the TX coast was removed this cycle. Across the immediate Brownsville and Port Isabel area, recent heavy rains could make them more sensitive to additional rainfall this morning. Even though the risk area was reduced to Marginal, Slight Risk impacts cannot be ruled out. Prospects increase gradually along the central Gulf Coast for heavy rainfall. While initial bands from the periphery of tropical cyclones can quickly erode existing instability ashore, portions of Acadiana and Southeast Louisiana along with far southern MS have had an exceeding wet week from a cyclone that ultimately merged in with Francine, so keeping the inherited Slight Risk makes sense. Also, guidance tends to bias slow with tropical cyclone recurvature, and this Slight Risk area can account for some possible track acceleration between now and its coastal approach. As with any warm core cyclone, the low-level inflow will exceed the mean flow, aiding precipitation efficiency. Plentiful effective bulk shear will organize convection into curved bands within its cyclonic circulation. Hourly rain totals to 3" will be possible with any bands that manage to train. ....Florida... An unusually far south synoptic front for Mid-September focuses very high moisture east of Francine where, at times, effective bulk shear could flirt with 25 kts. Where semi- organized and organized convection manages to form, hourly rain totals to 3" and local amounts to 6" are considered possible where storms merge, train, or get tied to the boundary. The 00z NAM and 00z Canadian were especially bullish with local maxima, both remained widely scattered to isolated in coverage of the heaviest rainfall. This kept the risk at the Marginal level. Another area with sufficient instability and moisture is behind the front near the northeast FL coast. Adding to possible issues there is low level and upper level flow trying to zero out. Short bands of efficient convection, which could lead to hourly totals to 2" and local amounts to 4", are possible. Kept this area in the Marginal Risk to help support their flood watch. ....Inter-Mountain West and Southern Rockies... A scattered convective footprint is anticipated across portions of the West with the main threat residing over the Southern Rockies into the Wasatch of UT. The threat is on the lower end of probabilities considering the middling signal within the latest HREF for >1" chances over the region. However, with precipitable water values near 0.75" and sufficient instability, localized flash flooding can't be ruled out in any dry washes/arroyos, burn scars, slot canyons, or steep topography where hourly rain totals manage to reach 1.5". The Marginal Risk area maintained showed good continuity. Roth Day 2 Valid 12Z Wed Sep 11 2024 - 12Z Thu Sep 12 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF LOUISIANA AND SOUTHERN MISSISSIPPI... ....2030Z Update... ....Southeast... Minimal adjustments were made to the current Moderate Risk area=20 focused mainly on a narrowing corridor of heavier rain totals along and to the east of the forecast path of Francine, which has been=20 extended a bit further northeastward into Mississippi. This region=20 covers an area with moderate to high (50-90%) probabilities of=20 rainfall totals exceeding 5" and rain rates in excess of 2-3" per=20 hour. The majority of this rainfall is expected to fall within 6-12 hours, exacerbating the potential for scattered to numerous=20 instances of flash and urban flooding. Storm total rainfall in the=20 broader Slight Risk area is expected to be on the order of 2-4".=20 The western edge of the Slight Risk has been trimmed eastward as=20 increasing shear will limit convection and subsequent rainfall=20 along the western side of the system track. ....Intermountain West... Still expect post-frontal, moist upslope flow (PWs 1.0-2.0=20 standard deviations above the mean) will lead to periods of locally heavy rainfall across portions of western Montana into central=20 Idaho. Newly available hi-res guidance shows storm total rainfall=20 of 1-3", backed by HREF guidance indicating high (70-90%)=20 probabilities of totals over 1".=20 Putnam ....Previous Discussion... ....Southeast... Francine should begin extratropical transition and become highly sheared around, if not before, the time of landfall Wednesday afternoon/evening, with a developing dry slot possible. This should lead to a convective comma head and an inflow band/baroclinic trough/developing front becoming increasing removed to the east of its center which would both be foci for heavy/excessive rainfall. The cities of Lafayette, Baton Rouge, and New Orleans (along with their suburbs) would be most susceptible for flash flooding as hourly rain totals to 4" (if not more) should be possible, particularly within the trailing band/forming front to its southeast. Acadiana and Southeast Louisiana are more susceptible due to heavy rainfall from this past week caused by the cyclone which merged in with Francine, so the soils will be more primed relative to what is typical to the area. Continuity was generally maintained in the risk areas. ....Intermountain West... A deep layer cyclone moving into MT brings moderate to strong upslope flow into western portions of the state, along with MU CAPE of ~750 J/kg. Precipitable water values are sufficient for heavy rain-related issues, so increased the risk area to Slight for portions of western MT. Hourly rain totals to 1.5" with local amounts to 3" are expected, which would be most impactful in burn scars. Some of the QPF would be in the form of snow at the highest elevations of the northern Continental Divide. Roth Day 3 Valid 12Z Thu Sep 12 2024 - 12Z Fri Sep 13 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR THE MID-SOUTH... ....2030Z Update... ....Mid-South/Southeast... Francine will continue into the Mid-South on Thursday. Increasing=20 advection of dry air into the system should begin to limit both=20 rainfall coverage and totals. However, totals of 2-4", locally=20 higher, can still be expected throughout the Slight Risk area, with the highest potential for these amounts over northern Mississippi=20 and western Tennessee. Similar to Wednesday, the Slight Risk has=20 been trimmed on the western edge given an increasingly eastward=20 shift of convective coverage compared to the track of the now=20 extratropical system. Given high FFGs and dry antecedent conditions in the area, some further refinement/consolidation of the Slight=20 Risk may be necessary with greater confidence in the overall=20 coverage of the more significant rainfall totals, especially as hi- res guidance becomes available. Further east, guidance differences remain with respect to potential rainfall along Francine's cold=20 front, with both the Euro and ECens mean more bullish on totals in=20 the 2-4"+ range that could be Slight Risk worthy. However, given=20 this uncertainty and the expected progressive nature of the front=20 more likely to limit these higher end amounts, have left the=20 Marginal Risk as is for now.=20 ....Intermountain West... Similar to day 2 (Wednesday), moist, upslope flow (PWs 1.5-2.5=20 standard deviations above the mean) will lead to periods of locally heavy rainfall over portions of western Montana. A Slight Risk has been introduced given better guidance agreement and increasing QPF totals. Areal average rainfall of over 1" and locally higher=20 totals of 3"+ may lead to some scattered instances of flash=20 flooding, especially given significant overlap with rainfall the=20 day before. Increasingly cooler air as the upper-trough passes=20 overhead will lead to lowering snow levels, especially by Friday=20 morning, but expect they should remain relatively high enough=20 through Thursday for the additional rainfall to pose a similar=20 flooding risk to Wednesday. Putnam ....Previous Discussion... ....Mid-South/Southeast... Francine is expected to be fully extratropical, with a well- developed comma head and trailing front. Guidance suggests that MU CAPE will be modest, 250-500 J/kg. Effective bulk shear should continue to be sufficient for convective organization where enough instability develops/exists. Precipitable water values remain near 2", so hourly rain totals up to 1-2" should be possible, which would be most problematic in urban areas and initially beneficial elsewhere. This region has been quite dry lately. With the volume of rain expected, and the concept of this being on the cusp of a flash flood and longer duration flood event, kept the Slight Risk area which showed minimal change from continuity. Across the Southeast, there is a risk of training convection along Francine's cold front, but its progression is expected to limit issues. Still, hourly rain totals to 2" and local amounts to 4" should be possible. Western MT, Idaho Stovepipe & Vicinity... One or two surface lows are expected to lift northward this period through MT, drawing in sufficient upslope flow, moisture, and instability from the east to be concerned about the potential for organized convection. The guidance signal is not cohesive as of yet, so left the risk across the area as Marginal. Should the guidance get more agreeable or confidence increase, a Slight Risk could become necessary in later updates. Hourly rain totals to 2" and local amounts to 4" appear possible. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9RWvWp18r0co0ALnExbadDYHsiarnfHhCTSxBYPOBUYN= -SDuJpSDBXE-zhybpJPDAaYP2nQ3Ih8DqL3Su6JI0NRAnGE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9RWvWp18r0co0ALnExbadDYHsiarnfHhCTSxBYPOBUYN= -SDuJpSDBXE-zhybpJPDAaYP2nQ3Ih8DqL3Su6JINkW_W1I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9RWvWp18r0co0ALnExbadDYHsiarnfHhCTSxBYPOBUYN= -SDuJpSDBXE-zhybpJPDAaYP2nQ3Ih8DqL3Su6JIjWLHe-Y$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .