Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 17 2023 20:10:25 FOUS30 KWBC 172010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 PM EDT Wed May 17 2023 Day 1 Valid 16Z Wed May 17 2023 - 12Z Thu May 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHERN HIGH PLAINS, FROM THE SOUTHEAST TO THE CENTRAL GULF COAST, AND ALONG THE MOGOLLON RIM OF ARIZONA... ....Texas and Oklahoma Panhandles... A Marginal Risk area was introduced with the 16z update across much of the Texas and Oklahoma Panhandles due to the threat of a few rounds of slow-moving convection capable of containing intense rainfall rates. Upper divergence over the region associated with a strengthening jet streak over the central Plains combined with a weak 700 mb inflection over the southern Rockies set to eject eastward will provide forcing to initiate convection as early as 18z. South of the MRGL and further into west-central TX should see convection become more outflow dominant due to weak shear, but areas to the north and within the highlighted risk area will have stronger westerly 700 mb flow to allow for numerous congealing storms approaching from northern NM after 00z. PWs won't be overly impressive, but up to around 1.25"(75th-90th climatological percentile) to aid in rainfall efficiency. The 12z HREF depicts 15-30% neighborhood probabilities for 3" in 6 hours by 06z tonight over the northern TX Panhandle and border with OK. This region falls within severe to exceptional drought, but recent rainfall has allowed for near-to above normal 0-40 cm soil moisture percentiles (per NASA SPoRT-LIS). Any isolated flash flood risk is most likely to be confined to urban or low-lying regions. ....Southeast and Central Gulf Coast... The slow-moving frontal boundary draped across the Southeast will continue to spark additional rounds of unorganized, impulse convection as a steady stream of deep moisture advects northward. The PW values will generally be around 1.5-1.7" over this stalled boundary; which should be more than adequate to keep the threat for intense rainfall rates. Instability will be greatest across the central Gulf Coast (SBCAPE building to between 2000 and 3000 J/kg early this afternoon), which coincides with the greatest HREF neighborhood probabilities (30-60%) for exceeding 3" of rain by early this evening from southeast LA to the FL Panhandle/southeast AL. This area could see convection induced by a sea breeze boundary colliding with storms sinking southward ahead of the frontal boundary after 21z. The overall flash flooding threat will be low due to very high FFG and the isolated nature of intense rainfall rates up to 2"/hr, but urban and low-lying regions will be at a typical higher risk if slow-moving thunderstorms set up overhead. A small adjustment was made to the northern bounds of the Marginal Risk over AL, MS, and SC to account for recent CAM guidance and observational trends. ....Mogollon Rim of Arizona... Forecast remains on track across the Southwest and very similar to yesterday's activity, with highly localized chances for flash flooding associated with convection initiating off high terrain and weakening as it eventually progresses into lower elevations. Only adjustment to the Marginal Risk was a small eastward expansion into NM based on the latest CAMs. Highly anomalous moisture along with weak forcing associated with an upper-level low near northern Baja California will be enough to trigger and maintain convection across portions of the Desert Southwest. PW values approaching 1" (+4 sigma), as southerly flow off the Pacific and up the Gulf of California advect into the region. These PWs are anomalous for May, but would be much more common between July and September. As the trough moves east, favorable upper divergence will also remain over much of the Southwest. Convection is likely to initiate near the terrain of the Mogollon Rim of Arizona and nearby ranges, but the potential exists for storms to migrate slowly into the lower terrain before rapidly weakening given the dry air infiltration. The flash flooding threat is expected to be on the low-end and highly isolated to flood-prone regions, thus the Marginal designation. Snell Day 2 Valid 12Z Thu May 18 2023 - 12Z Fri May 19 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL/SOUTHERN PLAINS... ....Central/Southern Rockies to the Plains... A surface front sandwiched between a closed upper-level low positioned over the Upper Midwest and separate closed upper-level low over northern Baja California will focus a risk of heavy rain from the High Plains of southeast CO/northeast NM to north-central OK and south-central KS. Modest moisture is expected to help generate scattered convection growing upscale into a potential MCS further east across the Central Plains by the evening. There have been some locations across this part of the country that have had recent rains, thus lowering the local 3-hr FFG to as low as 1-1.5 inches (primarily in the higher terrain of Colorado). Also of note, there has been widespread severe to exceptional drought across western and south-central Kansas, as well as western and north-central Oklahoma. This will limit any long-duration flooding and contain the risk to any immediate flash flooding due to overlapping thunderstorms with rainfall rates up to 2"/hr. The sensitivity of the soils will be more prone to flooding across southeast Colorado and within any urban areas. The widespread drought conditions remain the limiting factor within much of the Slight Risk area over Kansas and Oklahoma, however model consensus shows the potential for 1 to 3.5" with potential local maximums nearing 6 inches. The greatest excessive rainfall risk will likely be near a northern bookend vortex associated with an organized MCS. The most likely location for this within recent model guidance is along and just south of the western/central KS-OK border, as depicted by 12z HREF neighborhood probabilities of 30-60% for more than 3" of rain in 6 hours Thursday night. The Slight Risk was trimmed out of central KS due to a southward trend in most of the guidance. The Marginal Risk area was trimmed out of eastern KS, but expanded southward across central and western TX to account for slow-moving convection initiating from height falls ejecting out of the Southwest and a pronounced EML from the northern Mexican Plateau. ....Southeast... Showers and thunderstorms will near the stalled frontal boundary in the Southeast and around a newly forming weak upper-level low near the Southeast coastline. Convection should remain widely scattered and unorganized, while also occurring over parts of the country where soils can handle brief periods of heavy rain. Although this part of the country has FFG generally in the 3-5" range, urban areas and typically prone low-lying regions will be most at risk to the isolated flash flood threat. The latest guidance once again is indicating an upper-level low and associated surface low to remain closer to the GA and SC coasts, with 24-hour HREF neighborhood probabilities now showing a concerning 60-80% chance for 5" of rain across coastal SC. High FFG due to sandy soils and lingering uncertainty regarding the surface low placement remains the limiting factor for a potential upgrade across this region, but the Marginal Risk was expanded to incorporate where convection could become more organized and within an airmass of 1.5" precipitable water. Elsewhere, the Marginal Risk was also expanded into Middle Tennessee on the western periphery of the upper-level low as instability (1000-2000 j/kg of sfc-based CAPE) nose northward in response to surface ridging east of the southern Appalachians. Developing convection will have up to 1.25" of PW to work with along with weak surface-mid level flow, while backing as well. This could result in locally slow-moving thunderstorms capable of producing at least 2" of rain, potentially exceeding the regions 3-hour FFGs. Snell/Campbell Day 3 Valid 12Z Fri May 19 2023 - 12Z Sat May 20 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL/SOUTHERN ROCKIES AND CENTRAL ARIZONA, AS WELL AS THE SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY... ....Southern/Central Rockies into Central Arizona... During this period a cold front is expected to slow/hang in the vicinity of the Central/Southern Rockies on Friday, with upslope flow on its backside in an environment with anomalous moisture. Wet antecedent conditions along with high streamflows will maintain increased soil sensitivity across parts of the region, especially with additional rainfall anticipated over these wet areas. Up to 2" of rainfall is expected, with locally higher amounts, resulting in localized flooding and exaggerated flooding where snowmelt comes into play. The Marginal Risk area was expanded westward into central AZ and the Mogollon Rim. Here, PWs between 0.75"-1.00" (over the 95th percentile compared to climatology), and renewed convection around an upper-level low lifting north from the Gulf of California could pose a localized flash flooding threat. ....Lower Mississippi Valley to Southern Plains... Farther east, ongoing convection from early Friday morning across eastern OK and southern KS may contain elevated rain rates continuing to push into parts of far eastern OK, AR, and southern MO, leading to the Marginal Risk area. The progressive nature of the front should not allow for too much training of storms, precluding any higher risk level. Storms along the cold front extending into the Ohio Valley and the Great Lakes should also be progressive enough to limit the flash flood potential. The Marginal risk was expanded into central TX, where better instability and PWs intersect, along with an area currently experiencing 0-40 cm soil moisture percentiles over 80% (per NASA SPoRT-LIS). ....Coastal North Carolina.. There remains large spread regarding a compact surface low pressure system lifting north on Friday near far eastern North Carolina. PWs potentially exceeding 1.5" combined with frontogenesis associated with the surface low and associated warm front could support a narrow corridor of intense rainfall possibly exceeding 4-5", but this rain could very well occur offshore or over areas with very high FFGs along the immediate coastline. Will continue to monitor the excessive rainfall potential should trends shift the heavy rainfall amounts further inland. Snell/Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9pkjDEyDtvUglUC1e7bktmk7JI0Vav4yKuGKs9bxceDt= wNv08tTkqgMSBHujpvk-9Cm1fHUoFgb3V9vLT7ehseJAvFk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9pkjDEyDtvUglUC1e7bktmk7JI0Vav4yKuGKs9bxceDt= wNv08tTkqgMSBHujpvk-9Cm1fHUoFgb3V9vLT7eh88VYWi8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9pkjDEyDtvUglUC1e7bktmk7JI0Vav4yKuGKs9bxceDt= wNv08tTkqgMSBHujpvk-9Cm1fHUoFgb3V9vLT7ehgN8aTZQ$=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) .