Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 30 2023 19:52:54 FOUS30 KWBC 301952 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 351 PM EDT Sun Jul 30 2023 Day 1 Valid 16Z Sun Jul 30 2023 - 12Z Mon Jul 31 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS TO CENTRAL PLAINS/LOWER MISSOURI VALLEY, AND SOUTHERN APPALACHIANS/SOUTHEAST... ....16Z Update... ....Southwest... The biggest change to the Day 1 outlook this morning was to remove the Marginal Risk area for the Southwest. Almost all of that area has been bone dry in recent weeks, and the monsoon, while gradually picking up, has not yet been able to overcome the semi-permanent upper level high that remains over the area. The dying convection over southwest AZ this morning, in addition to not producing any flash flooding on its own, is moving over an area that has been very dry in recent weeks, so this morning's rain is likely to soften up the soils in the area for any showers and storms that form there this afternoon, making them more apt to absorb the rainfall, rather than runoff. In similar fashion, it's also been very dry lately across southwest CO, so while an oddball flash flooding risk remains, it was determined to be under the 5% threshold for a Marginal Risk. ....Northern Plains... A fast moving shortwave disturbance riding the top of the ridge will help provide the forcing for storms to develop this evening across MT/WY and track southeastward over/around the Black Hills of SD and into the Sand Hills of NE. A 15-25kt southeasterly jet will be aligned anti-parallel to the upper level northwesterly flow, which will support both strong storms and the possibility for some backbuilding. However, given the steering flow will also be fast, think any individual cells embedded within the broader MCS will be fast-moving. Much of the northern Black Hills have been very dry over the past couple weeks, while areas south and east of the Black Hills have been quite a bit wetter. Given the fast movement of the storms and in coordination with UNR/Rapid City, SD forecast office, have opted to highlight this as a higher-end Marginal and not upgrade to a Slight at this time. The Slight upgrade may be needed with a later update once the storm trajectories, intensities, and speeds are more clear, which may take until initialization in MT/WY later today. The Marginal Risk across northeast MT and western ND remains unchanged for a second shortwave trough tracking in the mean northwesterly flow early Monday morning, with potential for training/backbuilding storms. These storms too will be moving quickly, so any resultant flash flooding will be isolated. ....Central Plains... The same front that will bring storms to portions of WY/SD will also be draped over the lower Missouri Valley tonight. This Marginal Risk area is a far lower confidence one. Ongoing storms over northern SD are tracking towards the Marginal Risk area, and any rain early this afternoon may prime some of the soils in this region. Timing will be everything, as some of the guidance suggests the MCS from the west will impact the area right around the 12Z end time for the Day 1 period. ....Southeast... No significant changes were made to the Marginal Risk area across the Southeast. Scattered strong storms with plenty of moisture to fuel them are likely to develop again today in broad northwesterly flow, however are unlikely to be able to organize too much. Isolated training/backbuilding is possible, which could result in flash flooding. Wegman ....Previous Discussion... ....Southwest... An inverted upper trough will continue to lift northeast from northern Mexico into the southwestern U.S., pushing the subtropical ridge farther to the east and allowing deeper moisture to advect across Arizona through the Four Corners region and into the central Rockies. The general consensus of the guidance continues to show PWs increasing to over 1.5-2 standard deviations above normal across southern California and along the southern Colorado Basin into the Four Corners region today. This moisture along with decreasing heights aloft is likely to support more widespread afternoon and evening thunderstorms and an increased threat for locally heavy amounts and isolated flash flooding. ....Northern to central Plains and lower Missouri Valley... Mid-level shortwaves moving off of the top of the ridge may pose a localized heavy rainfall threat as they interact with a plume of deeper moisture fed by southeasterly low level flow. While there are differences in the details, the overnight CAM guidance is highlighting two areas of storm development later today. One over southern Montana and central Wyoming, with storms spreading east into western South Dakota and Nebraska. The highest probabilities from the HREF for heavy amounts (greater than 2 inches) currently centers northeastern Wyoming into southwestern South Dakota. This may create isolated flash flooding concerns across the region before these storms move into the less flood prone Sandhills of central Nebraska. Storms that continue or develop farther downstream may also produce locally heavy rain and pose an isolated threat for flash flooding across portions of the lower Missouri Valley as well. Guidance is also showing storms developing farther to the north across northern Montana and southern Canada before moving into western North Dakota, with some of the CAMs showing localized heavy amounts across portions of northeastern Montana into western North Dakota.=20 ....Southern Appalachians/Southeast... Plenty of moisture (PWs around 1.75 inches or higher) is expected to remain along and south of a frontal boundary as it continues to dip south through the Carolinas today. This moisture interacting with energy associated with overnight convection over the Ohio and Tennessee valley may produce some locally heavy amounts as it moves across the southern Appalachians and into the Southeast later today. This deep moisture will extend south to the Gulf Coast and across the Florida peninsula, bolstering the potential for heavy downpours and localized runoff concerns as the more typical sea breeze convection develops during the afternoon. Pereira Day 2 Valid 12Z Mon Jul 31 2023 - 12Z Tue Aug 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHWEST TO THE CENTRAL HIGH PLAINS, THE MID MISSOURI TO THE MID MISSISSIPPI VALLEYS, AND OVER PORTIONS OF FAR SOUTHEASTERN GEORGIA AND FLORIDA... ....20Z Update... Only very minor changes were needed to the Day 2/Monday ERO. There will be a bit more moisture available in Arizona for Monday afternoon's convection as compared with today/Sunday. Thus, think the chances for flash flooding are a bit higher, particularly along the Mogollon Rim and in south-central AZ. The additional moisture should allow for added coverage and organization to the storms as they track northwestward across AZ. The persistence of the rain should allow for a better opportunity for isolated flash flooding to develop. A second area of storms is likely across CO, though those will be moving northeastward with time. In between, there is much less signal for storms in the CAMs guidance, so the Marginal Risk was dropped from UT and portions of far western CO. Southeastward moving storms will track down the lower MO River Valley, in many of the same areas as storms will move through later tonight. Once again they should be fast-moving enough that flash flooding should remain isolated, but the second consecutive day featuring heavy rains should mean antecedent conditions will be more favorable for thunderstorm development. Wegman ....Previous Discussion... ....Southwest to the central High Plains... A favorable monsoon pattern will continue to support storms across portions of the Southwest, especially for Arizona and interior southern California, while expanding farther north and west across the Great Basin and the central Rockies. Deepening moisture and energy moving along the western periphery of an eastward-shifting upper high is likely to support the increasing coverage of showers and thunderstorms across southern Nevada, as well as much of Utah, Colorado, and Wyoming. PWs of 1-2 standard deviations above normal are expected across much of this area. Storms that develop across the region will have the potential to produce intense rainfall rates and raise flash flooding concerns, especially across urbanized and areas of complex terrain. ....Mid Missouri to the Mid Mississippi valley... Mid-level energy embedded within northwesterly flow interacting with a deep moisture pool associated with a low level front may produce rounds of convection with locally heavy amounts. The 00Z guidance continued to show a fair amount of spread with regard to the magnitude of amounts, but those that did show heavy amounts generally agreed on an axis extending from southwestern Iowa through Missouri. ....Southeast Georgia and northern to central Florida... Deep moisture (PWs around 2 inches) associated with a stalled frontal boundary is likely to enhance shower and thunderstorm coverage and the potential for heavy rainfall amounts across the area. Pereira Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_nIRLPiFh8-1KRMs3k9AzyaO-Fx-G_qxFqJ7k4obKR_W= _Mo8f4LfKnN1ZoT97BgimG6rdstWUz_zoe9tXlpBSt9OwYs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_nIRLPiFh8-1KRMs3k9AzyaO-Fx-G_qxFqJ7k4obKR_W= _Mo8f4LfKnN1ZoT97BgimG6rdstWUz_zoe9tXlpBaTWDjo0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_nIRLPiFh8-1KRMs3k9AzyaO-Fx-G_qxFqJ7k4obKR_W= _Mo8f4LfKnN1ZoT97BgimG6rdstWUz_zoe9tXlpBrnxz5LU$=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) .