Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 31 2023 20:24:44 FOUS30 KWBC 312024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Mon Jul 31 2023 Day 1 Valid 16Z Mon Jul 31 2023 - 12Z Tue Aug 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE CENTRAL HIGH PLAINS, THE LOWER MISSOURI RIVER VALLEY AND PORTIONS OF THE OZARKS... ....16Z Update... ....Front Range and Plains of CO/WY... An easterly LLJ combined with an impulse of shortwave energy in the upper levels will allow organized convect to develop along the Front Range this afternoon, generally shifting E to NE across the Slight Risk area across the CO and extreme western NE. HREF probabilities have come up to a 25% chance of 3 inches of rain over the 24 hours ending at 12Z Tuesday morning a bit east of the I-25 corridor. In collaboration with BOU/Boulder, CO forecast office, the Slight Risk area was introduced with this update. ....Much of Missouri and southwestern IA... CAMS guidance agreement is improving that a rather narrow line of training/repeating storms is likely to develop tonight somewhere in the Slight Risk area. The dying area of convection over northern Missouri this morning is helping prime the soils for tonight's storms, with several hours break in the activity during the afternoon. Around sunset, a WSWly LLJ is expected to develop across OK/KS/western MO, which will be bumping up against the nearly stationary frontal boundary over central MO, which the present convection is moving along. In the upper levels, the right entrance region of a 90 kt northwesterly jet streak will add further lift to tonight's storms. The influx of moisture associated with the LLJ will allow for repeating storms as the flow is roughly orthogonal to the axis of the front. There is excellent agreement between the CAMs that the aforementioned narrow line of repeating storms will develop, with individual cells tracking southeastward along the front. There is considerably poorer agreement as to where that line of storms will form, with the most likely area across central Missouri around the Columbia/Jefferson City area, or perhaps just a bit east of there. The FV3 is the easternmost outlier, showing the line close to St Louis to the western bank of the Mississippi west of Cape Girardeau. 12Z HREF probabilities for 3 inches or more of rain central MO have been increasing, now to 50%, 25% chance for 5 inches or more, and 15% for 8 inches or more. The Slight Risk area represents a considerably wider east-west area than the line of storms is expected to impact, largely to account for the aforementioned uncertainty with where the storms will form. ....Florida... HREF probabilities also continue to increase for today's round of afternoon and evening storms over almost all of the FL Peninsula. 24-hr Neighborhood probabilities are up to 90% along the Treasure and Space Coasts, but only a 15% chance for 5 or more inches. Of course, as the convection is diurnally driven, these amounts of rain are mostly focused into this afternoon and evening. The inherited Marginal Risk was expanded to include much of the Peninsula, though heavier rains over an urban area could result in some isolated considerable impacts since the storms will have ample moisture to work with and will be very slow-moving. ....Southwest... Increasing monsoonal moisture will team up with the upper level ridge and transiting shortwave impulses of upper level energy to increase coverage of storms this afternoon and evening. The Marginal risk was expanded to include much of central and southern Utah with this update. The Mogollon Rim of northwestern AZ continues to be a focus of heavier storm activity in many of the CAMs for this afternoon. Wegman ....Southwest to the central High Plains... Above normal moisture will continue to be present across portions of the Southwest, particularly portions of Arizona today but also extending northward toward portions of the Central High Plains/Central Rockies. First, moisture anomalies peak near 2-3 standard deviations above normal today across portions of Arizona which combined with expected daytime heating/instability should produce higher coverage of storms and intensity compared to recent days. This is supported by the various CAMs and the 00Z HREF which shows peak 0.5" hourly total probabilities this afternoon at 60-80 percent from northern AZ into far southern UT. This raises the concern for isolated flash flooding for both urbanized areas and complex terrain. Across portions of Colorado and Wyoming, shower/thunderstorm activity is expected to be enhanced with approaching shortwave energy timing during the peak of the afternoon. Localized hourly totals up to 1-1.5" will be possible, especially by early evening as storms move east off the terrain into the foothills and Plains. ....Mid Missouri to the Mid Mississippi valley... Current convection across portions of the Plains early this morning will continue to move southeast in the northwest flow regime, rounding the periphery of the strong upper ridge axis centered over Texas. This activity will move along/northeast of a stalled frontal boundary in the region and act on the southwesterly low-level jet with upwards of 30-35 kts of inflow at 850 mb. As the main complex of storms moves southeast, additional convection is likely to develop ahead of it across portions of far eastern KS and much of Missouri. Hourly totals exceeding 1" will be possible with the strongest cores this morning, resulting in a localized flash flood threat. Additional convection is expected to develop late this evening into tonight (early Tuesday morning) across portions of the region, as the low-level jet increases, and some of this rainfall may overlap, particularly across portions of northern MO as supported by the 24-hr HREF probabilities of 3"+ which reach 30-40 percent. The environment will be certainly conducive for training, highly efficient rain producing thunderstorms that could produce localized but higher totals. If the trends toward a higher end heavy rainfall threat continue to grow for tonight across central Missouri, an upgrade to Slight Risk could be needed. ....Eastern Florida Peninsula... Continued deep moisture present across Florida, characterized by precipitable water values exceeding 2" (near 1.5-2 standard deviations above normal) will interact with a stalled frontal boundary to the north to enhance the daily afternoon/evening thunderstorm coverage and intensity today. Hourly totals between 1-2" will be possible during the peak heating this afternoon/evening, supported by the 00Z HREF neighborhood probabilities for 2" in 1-hr peaking at 40-70 percent across the eastern Florida Peninsula. This supports the potential for localized flash flooding for mainly the urban locations. Taylor Day 2 Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF EASTERN MISSOURI AS WELL AS THE CENTRAL ROCKIES & PLAINS... ....2030Z Update... ....Eastern Missouri... Concerns continue to increase across this region for the potential for multiple nights of training thunderstorms. Expect that the first night's (tonight) training thunderstorms will be ongoing at the start of the Day 2/Tue period from the overnight. Each night's storms will have a similar cause. A southwesterly to westerly LLJ, which may peak as strong as 40 kts, will advect plentiful Gulf moisture into eastern MO the next couple nights. The LLJ will bump up against a slowly retreating front, effectively becoming a warm front. The front and associated lift from the right entrance region of the jet streak will support additional lift, causing stronger and more widespread storms. However, the storms are likely to be relegated to the front itself, so any areas of training convection will be very spatially limited. That said, the areas caught under those training storms could see rainfall rates to 2 inches per hour, which would likely overwhelm any local creeks and streams, despite the ongoing drought. The biggest question is where the storms will form, and perhaps of similar importance: Will the storms that form Tuesday night overlap areas hit by the storms tonight? Should the answer to that question be yes, then any flooding caused by tonight's rains will be made worse by Tuesday night. Much of the guidance suggests there will at least be some overlap...i.e. the same area seeing consecutive nights of heavy rain. Tuesday night's rains are agreed to likely be the heaviest and most widespread of the next 3 nights. In addition, St. Louis itself may be one of the areas in the cross-hairs for heavy rain, especially Tuesday night. A Moderate Risk was considered for areas just west of St. Louis, as this area looks to be the most likely to see heavy rains, with the highest likelihood for overlap from tonight's rains. It was determined through coordination with LSX/St. Louis, MO and EAX/Pleasant Hill, MO forecast offices to hold off on that upgrade until there's a better idea on how much overlap there will be, and if there remains good consistency in the guidance that 3 inches or more of rain will fall in this area. Obviously this portion of the Slight is considered a higher-end Slight. 12Z HREF probabilities of 60-70% chance of 3 inches of rain and a 25-30% chance of 8 inches of rain in this region certainly point to the necessity to eventually upgrade a portion of the area. The good news is that the area that will be most impacted will both be gradually shifting east with time, and will be very localized, perhaps to only a 30-50 mile stretch, east-west. Thus, areas close to the rain will see minimal impacts other than the rapidly increased runoff. Thus, given the Slight Risk for this region covers a larger range of possibilities based on 12Z CAMs guidance, the area covered by the light may need to shrink with time despite the likelihood for localized Moderate level impacts embedded within. ....Plains of CO to southwest NE and northwest KS... Shortwaves rounding the broad ridge over TX will track northeastward across this region once again, with abundant moisture still available from the overnight. Thus, expect a second round of afternoon showers and thunderstorms, which may cause flash flooding, especially in areas hit by convection from today. ....North Dakota and northern Minnesota... Very few changes other than some minor tweaks based on the latest guidance were made. The signal for heavy rains remains low in this area, but since the front helping force the storms will be stalling out, there is localized potential for training/backbuilding storms. Wegman ....Previous Discussion... ....Southwest to the northern High Plains... The anomalous deep monsoonal moisture begins to move out of the Southwest during the period, reaching further north into northern Utah, Wyoming, and northern Colorado. This will begin to limit the threat of flash flooding for portions of Arizona and Utah. However, shortwave energy lifting through the flow will bring another day of unsettled/active weather and the threat of intense rain rates and heavy rainfall, particularly for portions of Colorado and southern Wyoming where rain rates of 1-2"/hr may fall over recent burn scars and areas of relatively low flash flood guidance. ....Northern Plains... Localized heavy rainfall will be possible during the period as an axis of higher moisture surges ahead of an approaching front combined with favorable forcing and enough instability to produce localized intense thunderstorms that may lead to a few instances of training thunderstorms that could produce flash flooding. ....Mid Missouri to the lower Ohio Valley... Repeating shortwave energy rounding the periphery of the strong upper ridge axis centered over Texas will keep an active pattern in place from portions of the Plains through the Mid Missouri to Mid Mississippi Valley. In the northwest flow regime, there is greater than normal uncertainty with how convection will develop and evolve but the guidance continues to suggest activity will be ongoing at the start of the period (Tuesday morning) across portions of southern Iowa into Missouri. Depending on how that plays out, additional convection may develop later in the period, followed by another round of overnight convection into early Wednesday morning. Each round could produce locally heavy rainfall and intense rain rates, and the nighttime convection has the synoptic setup that bears watching for localized intense rain rates and training convection that could produce localized higher rainfall totals. As a result, with growing support from the 00Z deterministic and ensemble guidance, a Slight Risk was introduced this cycle. Taylor 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!5RfFzPPDF6Y6wPcjFiUETQA0nBJz0LuvFckf2wN8JdWB= ZbcZGD2OqMH9ZjXC-tQKSK23UBgI1rvUc_QDNVDT_lYGMdo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5RfFzPPDF6Y6wPcjFiUETQA0nBJz0LuvFckf2wN8JdWB= ZbcZGD2OqMH9ZjXC-tQKSK23UBgI1rvUc_QDNVDTxLZr3yQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5RfFzPPDF6Y6wPcjFiUETQA0nBJz0LuvFckf2wN8JdWB= ZbcZGD2OqMH9ZjXC-tQKSK23UBgI1rvUc_QDNVDTuJvtkfI$=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. 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