Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 31 2023 00:56:30 FOUS30 KWBC 310056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Sun Jul 30 2023 Day 1 Valid 01Z Mon Jul 31 2023 - 12Z Mon Jul 31 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE DESERT SOUTHWEST...NORTHERN PLAINS TO CENTRAL PLAINS/LOWER MISSOURI VALLEY...AND SOUTHERN APPALACHIANS/SOUTHEAST... ....Southwest... Introduced two targeted Marginal Risk areas...one in fairly close proximity to the international border as mid/upper level shortwave energy seen on water vapor satellite imagery over northwest old Mexico continues to direct increasing amounts of moisture and difluent flow aloft into far southern Arizona. The 12Z run of the ARW2 and recent runs of the HRRR have a signal for up-scale growth of convection later as the dynamics increase, The University of Arizona regional runs during the day have shown a signal for MCS development...especially those with the ARW core. As usual...the dry sub-cloud air may limit initial rainfall amounts and the dearth of rainfall in recent days and weeks may also mitigate flooding concerns....00Z soundings showed approx 1000 J per kg of CAPE and in excess of 1.3 inch precipitable water values were already in place at TWC. With continued moisture transport shown into the area...and low-end neighborhood probabilities from the HREF continuing beyond 06Z...introduced a Marginal risk with just a bit of an apron to account for uncertainty. Farther north...active convection in and near the higher terrain in northern Arizona has produced isolated moderate to heavy rainfall rates over terrain which has resulted in isolated flooding and problems with run-off. The expectation is that convection here should begin to wane fairly shortly after loss of daytime heating...and some cloud top warming has already started. ....Northern/Central Plains... A fast moving shortwave disturbance that had been near the top of a broad-scale ridge has already made its turn towards the southeast...with convection growing upscale from southeast Montana into southeast Wyoming and parts of South Dakota. 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 sufficiently progressive to preclude the need for more than a Marginal risk area at this point. The same front that acts to focus the activity across the Northern Plains will be a focus for additional convection later tonight...at least according to a few runs of the HRRR and the latest run of the ARW. With the HREF showing low-end probabilities for 1+ inch per hours...1- and 3-hour QPF exceeding 1- and 3-hour Flash Flood Guidance overnight...opted to maintain the Marginal Risk area with few changes despite overall confidence being less that the area out west.=20 ....Southeast... Only change to on-going Marginal Risk area across the Southeast U.S. was to trim out the Florida panhandle portion. Neighborhood probabilities from the HREF showed the risk persisting farther east and as far south as the Florida peninsula into the late evening...but probabilities farther west drop off pretty significantly (an idea supported by warming cloud-top temperatures in the area). Bann 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 Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL ROCKIES... ....20Z Update... A few changes were made to the EROs this afternoon for the Tuesday and Tuesday night time frame. The greatest moisture across the West will gradually shift northward, focusing from the Mogollon Rim of northwestern AZ through UT and into CO. The Slight Risk area was expanded across more of the northeastern CO High Plains, in coordination with BOU/Boulder, CO forecast office. Storms developing along the Front Range/I-25 corridor will congeal as they move eastward over areas expected to get hit with rain on Day 2/Monday. The signal for heavy rain is fairly similar to Monday, so the primary factor upgrading this area is expected to be the antecedent conditions. Thus, lesser coverage of storms on Monday would reduce the threat on Tuesday as well. Meanwhile, a new Marginal Risk area was added to northern ND, far northern MN, and the northeast corner of MT. The tail end of a front draping south from a low centered over northern Manitoba and Saskatchewan will take advantage of a weak 15 kt LLJ advecting a bit more moisture in the area to support training thunderstorms tracking eastward across the Marginal Risk area. The training storms could lead to localized instances of flash flooding as the storms move roughly parallel to the front. Meanwhile across the mid-MS River Valley, the same frontal boundary will remain in place, which could lead to areas seeing their second consecutive night of rainfall as storms move southeastward along the front. This area is seeing increasing rainfall forecast in the guidance, with some suggesting areas of 3 inch rainfall totals. Thus, with much better certainty on timing and rainfall intensity, this area could be upgraded to a Slight Risk, though at this point it looks more favorable into Day 4/Wednesday. For now, it's a higher-end Marginal threat. It is worth noting that a southward trend in the guidance, as happened a week or so back, could put the same areas of extreme western KY in the path of another night of repeating thunderstorms. Unfortunately, some of the ingredients are looking to be in place, including a 25-30 kt WSWly LLJ, causing PWATs in the area to spike above 2 inches, and the boundary may get stuck in that area, which supports repeating storms. Also Corfidi Vectors in the area are weak (around 5 kt), which would also support backbuilding. However, several factors are not yet there, including extremely anomalous moisture, as opposed to just above climatological normal, a favorable upper level jet (though it becomes more favorable with time), and good agreement, which didn't occur until at maximum 24 hours before the event began. This area certainly bears watching. Wegman ....Previous Discussion... ....Southwest to the northern High Plains... While drier air is expected to begin limiting the potential for heavy rainfall and flash flooding concerns across southern Arizona and southern California, deep monsoon moisture and shortwave energy will continue to support the threat farther northeast through the Great Basin and into the central Rockies and High Plains. By Tuesday, some of the greater PWs anomalies (2+ standard deviations) are forecast to move into eastern Wyoming and Colorado. This includes recent burn scars and areas with relatively low flash flood guidance values. A Slight Risk was added to highlight the greater flash flood threat for this area.=20 Additional upgrades may be needed, especially if confidence in heavy amounts across other vulnerable areas increases.=20 ....Mid Missouri to the lower Ohio Valley... The pattern from Day 2 into Day 3 remains much of the same, with a similar threat for southeastward propagating storms, producing heavy amounts along much the same axis. Deeper moisture (PWs reaching above 2 inches) bolstered by strong westerly inflow into the low level front is expected to increase the threat for heavy amounts this period. Upgrades above a Marginal Risk may be forthcoming, especially if the models start to show a stronger signal for repeating heavy amounts across the same area. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6YIePpnY4k9XAolc-tyri95g9iMbenze3NZ13Yc_Uzep= UokuyL94_p1voOGb8Yw_roKusuCODrT6dyYj-6_cRBpng28$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6YIePpnY4k9XAolc-tyri95g9iMbenze3NZ13Yc_Uzep= UokuyL94_p1voOGb8Yw_roKusuCODrT6dyYj-6_cdiR1ZyQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6YIePpnY4k9XAolc-tyri95g9iMbenze3NZ13Yc_Uzep= UokuyL94_p1voOGb8Yw_roKusuCODrT6dyYj-6_ckgqD12o$=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) .