Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 21 2023 22:05:42 FOUS30 KWBC 212205 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 604 PM EDT Sun May 21 2023 Day 1 Valid 2204Z Sun May 21 2023 - 12Z Mon May 22 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTH-CENTRAL ROCKIES & HIGH PLAINS, THE NORTHERN ROCKIES, AND THE SOUTHEAST... ....2200Z Special... Expanded the previous Marginal Risk area in California southeastward from the Sierra Nevada to include the California deserts, southern Nevada, and western Arizona. Anomalous moisture, along with sufficient instability, and weak flow aloft are supporting the development of slow-moving storms across the region. These storms have the potential for produce heavy rainfall rates and localized flash flooding. Refer to the WPC MPD #320 for additional information regarding the threat for heavy rainfall and flooding across this region. Elsewhere, made some smaller scale changes based on recent radar and satellite trends, as well as the latest hi-res guidance. Pereira Previous discussion... ....Northern Rockies... A slowly digging neutral to slightly negatively tilted trough will track east from the Pacific, moving onshore across western Washington late this afternoon, then continuing to push inland through tonight. Shower and thunderstorm activity will break out over the Marginal risk area during the early afternoon hours per recent CAM guidance, but the stronger storms are expected to hold off until the late afternoon/early evening along the OR/ID border, then track northeast across ID and into western MT. Expect at least 2 rounds of storms to be following this northeastward-oriented conveyor belt that develops ahead of the strong trough. The storms may weaken some with nightfall, but are likely to persist through much of the overnight hours tonight across far northern ID and northwestern MT. Relatively wet soil conditions remain in place from recent rainfall in this area, and while the individual cells will be rather progressive, the multiple rounds of storms interacting with the terrain will make isolated flash flooding a possibility, especially over burn scars, canyons, and other poor-drainage areas. Except for a slight southwestward expansion of the Marginal Risk area over eastern Oregon, no other changes were necessary with the 16Z update. ....South-Central Rockies and High Plains... A rather weak but still significant upper level disturbance crossing through CO today will tap into a bit of Gulf moisture that is being advected via a 25 kt southerly LLJ tracking up across the TX & OK Panhandles and north into KS. Showers and thunderstorms will develop as early as midday today across CO and NM, and that convection will organize a bit as they encounter the LLJ over the High Plains late this afternoon through this evening. There is some potential for training, but the primary threat is slow-moving storms. Southern portions of the outlook have areas of nearly saturated soils, and these areas will be more susceptible to additional heavy rainfall. The dry line will play a role in the development of the strong storms expected. Localized flooding concerns persist, and the Marginal Risk remains from west-central TX through west-central KS. For the 16Z update, the Marginal Risk area was extended southward to include south-central Texas and portions of the Rio Grande, where a better model signal for strong convection exists in the CAM guidance for some slow moving cells, mainly before 00Z.=20 ....Coastal Southeast... A slow moving front approaching from the north and west will team up with sea breeze boundaries to result in a broken line of showers and thunderstorms, and this will likely be nearly stationary across much of the central/northern FL Peninsula and up into GA. The Marginal Risk highlights the areas where the highest rainfall rates are expected, mainly along the front in GA and far northern FL, and any sea breeze convection over the northern FL Peninsula through Orlando and possibly into the greater Tampa Bay area. With wet soils already in place, any storms that form over urbanized or other poor drainage areas will be strong enough given the ample moisture in the area to produce isolated instances of flash flooding. The Marginal Risk area was extended slightly southward across interior portions of the central Florida Peninsula with this update where the CAM guidance depicts some slow moving cells later this afternoon/evening owing to sea breeze convergence. ....Sierra Nevada... Low-mid level easterly flow underneath the western U.S. ridge will advect enough moisture westward to produce mainly scattered afternoon/evening showers and storms across the Sierra Nevada, mainly in the 20Z to 4Z time period. Although the rainfall is not expected to be all that impressive with amounts generally under half an inch in most cases, the combination of additional rain and rapid snowmelt will tend to result in enhanced run-off concerns, particularly across any burn scar areas.=20 Wegman/Hamrick Day 2 Valid 12Z Mon May 22 2023 - 12Z Tue May 23 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL/SOUTHERN PLAINS AND FOR PORTIONS OF THE SOUTHEAST... ....Northwest Texas to South-Central Kansas... Dry-line induced showers and thunderstorms are expected once again on Monday from the TX Panhandle northeast into south-central KS. The upper level energy will continue slowly tracking across the region, likely developing and redeveloping over many of the same areas. Meanwhile, the southerly LLJ will remain in place across the Plains, but will not be overly strong, generally between 20 and 30 kts. These three features will all work in tandem to cause renewed showers and thunderstorms over many of the same areas that have seen rain in the recent past, and are likely to pick up rainfall again today. Thus, by Monday the soils will still be nearly saturated for many of these areas, which will support isolated instances of flash flooding as Monday afternoon's round of storms moves through. The model guidance is in general agreement for general 1-2 inch rainfall totals, with some scattered 2-4 inch totals per recent 12Z CAM guidance, with the ARW2 and the NAM conest have the greatest totals. No major changes were warranted for the 20Z issuance. ....Southeast... The nearly stationary frontal boundary may lift northward a little across western GA and into AL on Monday. This will allow deeper Gulf moisture to advect a bit further north and west into AL on Monday. Meanwhile, multiple weak disturbances will track westward across the region in the mean flow aloft, which in addition to the front, residual cold pool boundaries from previous convection Sunday, and simple daytime heating will spur a renewed round of showers and thunderstorms over much of the Southeast, with a focus on central and northern FL. Once again for southern FL the primary forcing will be from sea breeze convergence during the late afternoon and early evening hours. With ample moisture and instability in place, once again any storms will be capable of heavy rain, which poses the greatest risk of flash flooding over urban and other poor-drainage areas. The CAM guidance once again has scattered 2-4 inch rainfall maxima throughout much of the Marginal Risk area, and it is possible that a Slight Risk area may eventually be needed in future updates once the main axis of intense convection becomes better resolved in the guidance, but for now no major changes are needed at this time. Wegman/Hamrick Day 3 Valid 12Z Tue May 23 2023 - 12Z Wed May 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTH FLORIDA AND SOUTHERN GEORGIA... ....Southeast... The same stalled out front that has been acting as the forcing for storms both today and on Monday will still be draped across the Southeast on Tuesday. The easterly flow off the Atlantic will increase ahead of the front on Tuesday as an area of high pressure builds southward over the Mid-Atlantic. This will advect PWATs of 1.75 inches or more over much of FL. Upper level energy will move over the area from west to east. Westerly flow will be prevalent in the upper levels. Thus, while easterly flow will be prevalent in the lower and mid-levels, the steering flow will be westerly. The stationary front adding forcing means all the ingredients will come together for either slow-moving or training convection to develop along the FL/GA border on Tuesday, which have the potential to cause locally very heavy rains that persist for extended duration as the storms barely move or train over the same areas. In addition, the corridor of heaviest rain is likely to be over or near Jacksonville, Valdosta, and possibly Tallahassee. Some uncertainty exists as to how far west the convection will develop. Regardless, this is a near ideal setup for locally extreme rainfall totals. Thus, the inherited Marginal Risk was expanded over much of FL, south GA and the southeast corner of AL, with an embedded Slight Risk area introduced with this update along and on either side of the FL/GA border. Certainly this axis of heaviest rainfall may change, but given the prevailing weather pattern, stubborn front, and reinforcing features holding the front in place, large changes in placement are unlikely. Finally, soils in this area after multiple days of repeated rains are likely to be even more saturated than they are now, which further increases the flash flooding potential. Depending on the consistency of this pattern and how much rain is picked up in this area in the intervening days, it's possible a Moderate Risk may be needed in this area given the very favorable setup for heavy training thunderstorms.=20 For the 20Z update, the Slight Risk area was trimmed back on the western flank since it now appears the heaviest rainfall will be primarily confined to southeastern GA and into northeastern Florida based on the latest 12Z guidance suite. However, there is the potential for several inches of rainfall in some of these locations, and this would further increase the flood potential owing to heavy rainfall prior to this time period. ....Southwest OK through North Central TX... Convection is likely to redevelop once again across this region on Tuesday, though they may be a bit displaced further east than previous days, which should help with any flash flooding threat. The same LLJ will still be pumping Gulf moisture northward, so plenty of energy and moisture will be available for storms capable of producing heavy rains. Since soils in eastern areas will be a bit drier than areas further west over the Panhandles, expect the flash flooding threat to remain isolated. Unlike previous days, the primary rain threat will be focused in the Tuesday night period over the Marginal Risk area, with less activity during the day Tuesday. No major changes were necessary with the 20Z update. ....Intermountain West... A very slow moving longwave trough over the West Coast will direct a large stream of moisture northeastward in advance of the trough. Embedded within the trough is an upper level low that will be over the BC/AT border north of WA/ID. A shortwave disturbance pinwheeling south of the low will approach the intermountain west Tuesday. This will locally increase the forcing over the Marginal Risk area over northeast NV, northern UT and southeastern ID. Anomalous moisture will still be in place, with PWATs up to 0.75 inches are over 3 sigma above normal. This will support training storms tracking northeast from NV through UT and into ID. The storms that are forced by the local mountains may stay more stationary, which will locally increase the flash flooding potential. The Marginal Risk area was slightly increased for this update. ....Northern Plains... A period of scattered to numerous showers and storms is likely to develop across much of southern ND and adjacent portions of neighboring states Tuesday evening as a cold front approaches from Canada and intercepts a low level southeasterly jet of 25-30 knots that will be advecting deeper moisture into the region. Although the areal coverage of heavy rainfall is not expected to be all that high, the storms that do develop will likely have rainfall rates high enough to cause some isolated flooding concerns, so a Marginal Risk area has been introduced. Wegman/Hamrick Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8s2ZPwB9B6MBcm7Vsmt149XSfDS8yg9Zz4CgxFrTnPeB= HEXsBIBCB2vepRkuvsBEzInKSfjaFGh37i7lHzCYfLkpxGE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8s2ZPwB9B6MBcm7Vsmt149XSfDS8yg9Zz4CgxFrTnPeB= HEXsBIBCB2vepRkuvsBEzInKSfjaFGh37i7lHzCY0heIXCE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8s2ZPwB9B6MBcm7Vsmt149XSfDS8yg9Zz4CgxFrTnPeB= HEXsBIBCB2vepRkuvsBEzInKSfjaFGh37i7lHzCYIMth1-o$=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) .