Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 20 2024 20:20:32 FOUS30 KWBC 202020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Tue Aug 20 2024 Day 1 Valid 16Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST U.S... ....16Z Update... No significant changes were needed. The latest guidance suggest the potential for training severe storms to occur over more of southern Colorado, particularly the Plains of southeast Colorado, so the Marginal was expanded to the north and east. More of the slot canyons associated with the Utah Mighty 5 were included in the Marginal due to their susceptibility to flash flooding with limited rainfall. Wegman ....Previous Discussion... Current upper air analysis across the CONUS depicts a sprawling mid-level ridge positioned over much of the Southern Plains into the Southwest. The ridge itself will maintain its orientation with some modest strengthening heading through the afternoon with the center of the ridge positioned across the Southern Rockies. Enough diurnal destabilization and moisture trapped under the ridge will offer a threat of widely scattered convection across the terrain in AZ, extending into Southern CO and Northern NM. The overall QPF footprint is fairly lackluster in the areal average, but a few pockets of heavier precip can be found within the CAMs indicating a low-end potential for heavy rain and potential flash flooding. The primary concerns will remain tied to the complex terrain and adjacent towns that could be impacted by rapid onset flooding. Slot canyons and remnant burn scars located across the Southwest will be under the threat as well, one of the main reasons for continuity from the previous forecast, as well as an expansion eastward into Southern CO and Northern NM where multiple burn scars are located. This remains the primary area of interest for the period. A small area across east-central SD will also experience a threat of locally heavy rainfall as remnants of a complex of thunderstorms migrates eastward along the leading edge of favorable mid-level ascent as a shortwave ejects out of the High Plains. Favorable instability axis across the Northern Plains favors points further west with the storm complex likely to enter a less favorable convective regime to maintain a strong enough intensity to induce flooding prospects. LLJ will also be on the decline leaving behind a relatively mundane setup for prolonged convective enhancement that would be necessary for flash flood prospects. Considering the above variables, and the addition of drier soil moisture located within the forecasted zone of impact, a nil risk area was maintained, however the threat is non-zero (<5%). Kleebauer Day 2 Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF=20 ARIZONA AND SOUTHWESTERN UTAH... ....2030Z Update... ....Southwest... In coordination with all of the impacted forecast offices, a Slight Risk expansion was introduced for portions of Arizona and far southern Utah on Wednesday. An approaching upper level trough will begin to draw monsoonal moisture north from the Gulf of California. An upper high will be centered over New Mexico, resulting in light easterly flow into Arizona, while a shortwave over Nevada races eastward across northern Utah through the day. The combination of all of these different features will result in an area of scattered convection Wednesday afternoon from southern Arizona NNW across the heart of Arizona to southwestern Utah.=20 With the greatest moisture availability over southeastern Arizona, in and around Tucson, the heaviest convection is expected over that region as PWATs rise to near 1.5 inches. Slow-moving training convection is expected over the area, which could result in widely scattered instances of flash flooding, especially in urban and flood-sensitive areas.=20 Further north into Arizona, the heaviest convection on Wednesday is likely to be more closely tied to the Mogollon Rim escarpment. Here too the convection is likely to be slower moving, but with a bit more limited moisture, the storms may not be quite as prolific rainfall producers than the storms further south. However, the more significant change in elevation with the Mogollon Rim will make up for that due to localized reductions in FFGs. The rapid changes in elevations and the greater prevalence of canyons in the area will be the storms don't have to produce as much rain to result in impactful flash flooding.=20 Along the Arizona/Utah border, more significant southwesterly flow as well as closer proximity to the aforementioned shortwave trough will result in a flareup of convection from the Grand Canyon north through Zion and Bryce Canyon N.Ps. Easily floodable slot canyons and burn scars in that area will make impactful flash flooding that much more likely Wednesday afternoon, but the rainfall is likely to continue along the Mogollon Rim northwest to southern Utah well into the overnight hours, resulting in a prolonged threat for flash flooding even after the diurnally forced convection has long dissipated. The overnight persistence of showers and storms is due to the rapid approach of a longwave trough digging south along the Pacific Northwest, and increasing southerly flow ahead of that trough funneling Pacific moisture north into the region. ....New York and Northern New England... A Marginal risk area was introduced with this update. A slow-moving closed low will be over the region, resulting in likely training showers and potential thunderstorms. The thunderstorms are most likely during the day during peak heating when instability between the warm ground and the cold upper atmosphere is maximized. Moisture availability will be the primary limiting factor, but given the history of slow-moving closed lows in that region over performing this summer and the susceptibility of the northern Greens and Whites to flash flooding, it seems a Marginal risk was needed. That said, given the relative lack of moisture, it seems unlikely that highly impactful flooding will occur at this point.=20 Wegman ....Previous Discussion... A more robust moisture advection pattern will ensue across the Southwest U.S with some Pacific moisture being pulled northward thanks to the combination of the Southern ridge shifting eastward and weakening, along with a shortwave trough quickly moving inland through CA leading to a pathway for elevated moisture and embedded shortwaves pivoting out of Sonora and nearby Baja to move northward. Ensemble QPF is much greater compared to the D1 time frame thanks to the poleward advancement of anomalous moisture (+2 to +3 deviation PWATs based off the latest NAEFS) and subsequent instability increase. The ascent pattern across the Great Basin is the driver for a better signal further north in-of AZ and UT thanks to diffluent flow out ahead of the shortwave. Ensemble QPF has increased since the last series of runs with a more pronounced convective depiction signaling some totals between 1-1.5" now introduced in the means. The back end of the 00z HREF is already showing the increased convective pattern at the beginning of the diurnal period with the remainder of the evening still to go. Ensemble bias corrected QPF is running between 0.75-1.25" for an areal average over a good portion of AZ with the focal points situated over the terrain in Southern AZ and across the Mogollon Rim and Grand Canyon areas of Central and Northern AZ. The prospects are certainly favorable for more localized flash flood concerns just based off the anomalous PWAT advection anticipated. After collaboration with offices out west, a targeted SLGT risk was added across Northern AZ encompassing places like Flagstaff to the Grand Canyon, representative of the strongest heavy precip signals expected during the D2 period. Additional upgrades to a SLGT risk are also possible within the next series of updates, especially within Southern AZ and Southern UT pending the run-to-run consistency within guidance. Kleebauer Day 3 Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR CORNERS REGION... ....Southwest... A digging longwave trough tracking south down the West Coast will interact with plentiful available moisture along the Four Corners region, advecting instability to 1,200 J/kg, and associated jet energy. The plume of available moisture will shift a bit to to the east, focused around the Four Corners. Increasing forcing associated with the approaching trough will support training and backbuilding convection across much of northern Arizona and southern Utah. The storms will then turn northeastward with the southwesterly flow and move into western Colorado. Numerous burn scars, slot canyons, and a few urban areas will all locally add to the susceptibility of flash flooding, as widespread 1 to 2 inch rainfall totals are expected. Locally higher amounts are probable, especially into southern Utah, where the forcing will be the greatest. Thus, portions of southern Utah are considered in a higher-end Slight risk. Elsewhere, much of Arizona will be dealing with the second consecutive day of training convection, so antecedent conditions along the Mogollon Rim will also be a concern. The greatest uncertainty and potential for a downgrade will be across western New Mexico, where lesser topography, greater distance from the upper low, and greatest storm motions will all work to lower the flash flooding risk. All of the remaining moisture and instability will track east into western Colorado, where additional burn scars and topographic influences will locally raise the flash flooding threat. For most areas, Thursday will be the more active of the two days (Wednesday/Thursday), and therefore for those areas with active convection Wednesday, Wednesday night, and for some areas continuing into the day Thursday, antecedent conditions will become an increasing factor to potential flash flooding. PWATs will increase above 1 inch into Utah and Colorado, which is about 2 sigma above normal. The anomalously high moisture content will fuel repeating storms throughout the period. For southern Arizona, the increasing forcing to the north will result in a net decrease in convection on Thursday as compared with Wednesday. Thus, the flash flooding threat on Thursday is lower in that area. ....Maine... Lingering moisture with a slow moving cut-off low will result in continued showers and a few storms across northern Maine. The low will increase in speed as it moves into the Canadian Maritimes, so the flash flooding threat should decrease through the day. Wegman ....Previous Discussion... A continuation of the elevated moisture pattern across the Southwest will transpire in D3 with an axis of scattered to widespread convection likely across much of AZ through the Inter Mountain West. Small mid-level perturbations are still forecast to rotate northward around the western flank of the ridge positioned across the Southern Plains. A digging upper low off the PAC Northwest coast will allow the flow to become even more meridional across the West allowing for the more anomalous PWAT advection to occur further in the interior leading to the best heavy rain axis to likely situate over Northern AZ through much of UT and Western CO. The growing consensus on the specifics of where the heaviest precip will focus will likely correlate to an upgrade at some point in the future, but timing of the mid and upper pattern still has some points to hash out before going into more detail. As of now, the MRGL from the previous forecast was maintained but expanded on all sides as the 00z ensemble QPF footprint expands with agreement from the recent ML output on the precip placement. Across Florida, a stalled frontal boundary will become the focal for scattered convection on Thursday afternoon with the areal QPF average relatively elevated, but still not depicting a substantial enough signal to warrant a risk addition to the area. This will be a period to monitor for perhaps a targeted risk along either coast, pending sea breeze pattern evolution where heavy rain could affect the the urban corridors either side of the Peninsula, or an area inland like Orlando proper. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-nPNzLKjXjHDrozGTM7YgQlZQk8W4x7a21Y3JJw_tQpD= Ayml3NO06hMl3WrsyiKzGYjE6A1aF83RegQOfLyf4bHHq4I$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-nPNzLKjXjHDrozGTM7YgQlZQk8W4x7a21Y3JJw_tQpD= Ayml3NO06hMl3WrsyiKzGYjE6A1aF83RegQOfLyfGaIq5kA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-nPNzLKjXjHDrozGTM7YgQlZQk8W4x7a21Y3JJw_tQpD= Ayml3NO06hMl3WrsyiKzGYjE6A1aF83RegQOfLyfrFra6z0$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .