Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 12 2023 01:01:22 FOUS30 KWBC 120101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Fri Aug 11 2023 Day 1 Valid 01Z Sat Aug 12 2023 - 12Z Sat Aug 12 2023 ....THERE ARE SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF FAR SOUTHEAST KANSAS, FAR NORTHEAST OKLAHOMA, SOUTHWEST MISSOURI, AND NORTHWEST ARKANSAS... ....Ozarks and Mid-Mississippi Valley into the Southeast... 01z Update: Have introduced a targeted Slight risk area for portions of far southeast KS, far northeast OK, southwest MO, and northwest AR. While the threat is rather conditional (given that there isn't high confidence in proliferating/training convection), there is enough of a signal from CAM guidance to support a low-end Slight risk (with 18z HREF neighborhood probabilities for 3-hr FFG exceedance of ~20%). In addition, this area largely overlaps with fairly wet antecedent conditions (per NASA SPoRT-LIS 0-40 cm soil moisture anomalies of up to the 70-90th percentile for portions of AR/MO). Should convection proliferate in association with a weak shortwave impulse, 2-3" short-term localized totals seem likely (given very high levels of instability and anomalous tropospheric moisture with PWATs of 1.8-2.2"). Effective bulk shear of 35-45 kts is also plenty to organize convection, and SPC RAP analysis also indicates very high MCS maintenance values of 90%+. Outside of the Slight risk area (where the Marginal risk encompasses), antecedent conditions are not nearly as wet (and the better signal for convection farther north looks to be more progressive with less of a chance of localized training). Previous discussion: Unlike further north, this area will have plenty of moisture and instability, with PWATs exceeding 2 inches in some areas, and instability exceeding 4,000 J/kg of MUCAPE, which is extremely unstable. Thus, what will be missing in this area is a strong enough source of forcing to allow the storms that will form over this area to organize. Scattered storms are already over portions of northern AL this morning. They will track southeastward, following the greatest moisture and instability. However, other than localized outflow boundaries, the forcing will be limited. Thus, expect the storms to form and reform as the broader area of scattered storms moves southeastward, but the storms will be largely independent of each other. While the plentiful moisture and instability will support individual storms becoming strong enough to produce heavy rainfall, the storms will be moving enough and will be widely scattered enough that the flash flooding threat will be largely confined to urban and poor drainage areas. High FFGs common across the Deep South will also limit the flooding threat. ....Southeast Michigan, Far Northeast Indiana, Northern Ohio... 01z Update: Convection is ongoing across the region for at least a few more hours, and have adjusted the inherited Marginal risk accordingly. Previous Discussion: A 15-25 kt LLJ will transport a bit of moisture from the Midwest that will be lacking a connection with the Gulf into portions of the Midwest. Convection is likely to break out along this front, especially during peak heating, and continuing into the evening. CAMs guidance is in decent agreement that multiple rounds of showers with some embedded storms will form along the southern boundary between steadier rain and forcing to the north, and isolated convection with little forcing to the south. Thus, this region is likely to be in the middle, and thus subject to multiple rounds of storms. The southwesterly flow will also transport air with MUCAPE around 1,500 J/kg, and PWATs to around 1.5 inches. This is far from impressive for this time of year, but will be enough moisture availability for the storms. ....Eastern Arizona and Western New Mexico... 01z Update: Maintained the Marginal risk area for ongoing convection. The bulk of this precipitation will likely end by 06z, with the localized flash flood threat gradually diminishing through the night. Previous Discussion: A very diffuse monsoonal signature is the best the guidance is showing as far as convective coverage goes this morning for the return of storms again this afternoon. With limited forcing and capping aloft, the convection may not be quite as strong today as they were yesterday evening. Nonetheless given the sensitivity of the area to flash flooding, particularly over burn scars, slot canyons, dry washes, and any urban areas, the Marginal Risk was maintained with this update. ....Central Sierras into Nevada... 01z Update: Maintained the Marginal risk area for ongoing convection. Diurnally-driven is likely to come to an end between 03-06z, ending the localized flash flood threat. Previous Discussion: The guidance continues to indicate the potential for isolated flash flooding from the Central Sierra Nevada eastward into central NV. No significant changes were noted with this update, as any storms capable of flash flooding will only be able to form if there is enough break in the cloud cover to allow instability to increase. Churchill/Wegman Day 2 Valid 12Z Sat Aug 12 2023 - 12Z Sun Aug 13 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD AREA... ....Northeast, to Ohio Valley, to Central Plains, to Four Corners... The large Marginal Risk area was only changed a small amount from the previous outlook. An expansive front will be draped across much of the country on Saturday, with primarily diurnally driven clusters and lines of convection peaking in the afternoon and evening hours. The 12Z HREF shows a scattering of low 40km neighborhood probabilities of flash flood guidance (FFG) exceedance across much of the Marginal Risk area between 18 UTC and 08 UTC, generally below 15 percent. This indicates that while hi-res models are showing some localized intense rainfall, particularly on very short time scales (30-60 minutes), it is not very organized or sustained, and the models do not agree much on placement. The run-to-run consistency has also been lacking. Nevertheless, the hi-res models are also showing maximum rainfall rates of 1-2 inches per hour over much of the Marginal Risk area as well, fairly consistent with an isolated threat of excessive rainfall and flash flooding. The Marginal Risk was fine-tuned based on the HREF probabilities, as well as updated CSU machine learning guidance, and ensemble CAPE and PW probabilities to identify the most supportive environment. A Slight Risk upgrade was considered in a couple areas, generally where HREF EAS (ensemble agreement) probabilities suggested more agreement on a sub-regional concentration of intense rainfall. One such area was in NE PA, N NJ, S NY (including NYC), CT, and W MA. HREF neighborhood probabilities of 1 inch per hour rain rates were higher in this area, including as high as 35 percent in NYC. However, there has been a lack of run-to-run consistency, so confidence in placement that precise is generally lower. Another area considered was the central High Plains, eastward into the Ozarks and Mid Mississippi Valley. Hi-res models do have a signal for streaks of higher QPF in that area, but in the Plains FFG is relatively high, and further east there is lower confidence on consistent placement. However, both these areas will be monitored for potential future upgrades. ...South Dakota and Vicinity... 20 UTC Update: The only change was to shift the Marginal Risk area slightly to the west to better match new guidance. The 12Z HREF, now extending into the Day 2 period more fully, did not show very high probabilities of FFG exceedance, so it is possible this Marginal Risk could be removed in the future. Previous Discussion: Much of the guidance shows showers and storms forming along a 20 kt LLJ tracking up the Missouri Valley and into eastern SD Saturday night. The increased moisture along with upper level support from a strong shortwave trough diving southeastward out of the Canadian Prairies should allow for locally heavy rain to develop over this area. Soils are wetter than normal after recent rains from a few days ago in this area, and while the area is notoriously hard to flood, think the antecedent conditions may allow for isolated instances of flash flooding due to the more organized nature of the storms over a rather confined area of eastern SD. A new Marginal Risk was introduced with this forecast update. ....Central Sierra Nevada... A Marginal Risk area was added in the Central Sierra Nevada for the second consecutive day. ECENS and GEFS precipitable water values are both forecast to be near or above the 95th percentile in this area, and hi-res model guidance indicated CAPE values reaching close to 1000 j/kg again in the afternoon hours. This should be sufficient for some localized rainfall rates around 1 inch per hour, which could lead to isolated flash flooding. Lamers/Wegman Day 3 Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EASTERN KANSAS, SOUTHERN AND CENTRAL MISSOURI, AND THE LOWER OHIO RIVER VALLEY... ....Eastern Kansas, Missouri, and Ohio Valley... There is growing concern for a more organized episode of significant rainfall in the Day 3 period, particularly from Sunday evening into Sunday Night. A warm front will begin to lift northward as southwesterly low-level flow begins to increase in response to a digging trough into the Great Lakes and mid-upper level height falls overspreading much of the Northern Plains and Midwest. The environment along and south of the warm front should be characterized by strong instability, with CAPE in excess of 3000 j/kg likely, and deep moisture, with PWs well in excess of 2 inches. In fact, ECENS mean PWs are in excess of the 99th percentile, with similarly anomalous integrated vapor transport, over southern Missouri by 00-06 UTC Monday. Furthermore, the deep layer mean wind should be oriented nearly parallel to the warm front, and veered southwesterly low-level inflow, generally supportive of backbuilding and training convection. Overall, it seems like an ideal setup for a training convective complex and a swath of very heavy rainfall from KS-MO eastward into the Ohio Valley. CSU machine learning guidance for ERO probabilities have increased into the upper end of the Slight Risk range, and indeed this seems like a candidate for a future upgrade to a Moderate Risk. Right now, we would tend to place the highest probabilities from south-central Missouri eastward into southern Illinois, northwest Kentucky, and far southern Indiana. Interests in this area should monitor the forecast and the potential for future ERO upgrades. Of particular concern is that a large portion of the higher probability area has just experienced a very wet 30-day period, with rainfall two to four times the average. That should lead to a greater risk overall, even if rainfall is not truly exceptional. ....Upper Midwest... The broad Marginal Risk to the north of the KS-MO-Ohio Valley Slight Risk was reduced in size to fit the higher ensemble probabilities from the 12Z guidance suite. In general, the probabilities of 1+ inch of QPF are highest from the eastern Dakotas into Minnesota and Wisconsin. We had considered removing NE/IA from the Marginal Risk due to the weaker signal, but for now the overall size of the area was reduced. Rainfall rates should be much lower in this region given a general lack of instability, and it seems unlikely that they would exceed 1 inch per hour on more than a localized basis. Therefore, any flash flooding would likely be due more to persistence of moderate to occasionally heavy rain over a longer period of time. 6-hour FFG in this region is generally between 2 and 3.5 inches, although higher in some places. This does seem possible with the strong forcing associated with the digging trough, and even the global ensembles such as the ECENS do show small probabilities of 2+ inches of rain in 6 hours. Therefore, the Marginal Risk was maintained, but it may need to be further reduced in size in future outlooks. ....Central Sierra Nevada... A Marginal Risk was added to the Central Sierra Nevada due to expected persistence of the same conditions from Day 1 and 2 -- precipitable water values above the 90th percentile, weak flow near a stalling mid-upper level low, and SBCAPE values in the afternoon on the order of 500-1000 j/kg. That could support localized rain rates to around 1 inch per hour, and some localized flash flooding. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8tFVro3l1pPi9LyfumRU3NkeB3eB12wt7O2ShcCnSU9L= -bBSYBNl-waJ6nG7OTsEmSYUXXZQuJ4Vz_xu3jodCmBgUes$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8tFVro3l1pPi9LyfumRU3NkeB3eB12wt7O2ShcCnSU9L= -bBSYBNl-waJ6nG7OTsEmSYUXXZQuJ4Vz_xu3jodzbfrnIM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8tFVro3l1pPi9LyfumRU3NkeB3eB12wt7O2ShcCnSU9L= -bBSYBNl-waJ6nG7OTsEmSYUXXZQuJ4Vz_xu3jodaXCKE7A$=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) .