Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 16 2023 00:57:09 FOUS30 KWBC 160057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Mon May 15 2023 Day 1 Valid 01Z Tue May 16 2023 - 12Z Tue May 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID MISSISSIPPI VALLEY... ....Ozarks Region and Parts of the Mid MS Valley... 0100 UTC Update -- Pared the western portions of both the Slight and Marginal Risk areas across eastern KS-OK and northern MO based on the latest observational and guidance trends. Latest HRRRs along with HREF probabilistic guidance supports a more compact Slight Risk across south-central and southeast MO into southern IL and far western KY-northwest TN. ....Washington-Oregon.. 0100 UTC Update -- Based on the latest satellite, radar, and mesoanalysis trends, have expanded the Marginal Risk area a little to include more of northeast and southern OR. Previous discussion, issued at 1930Z... In coordination with SEW/Seattle, WA, PQR/Portland, OR, PDT/Pendleton, OR, and MFR/Medford, CA forecast offices, the ERO was updated to include a Marginal Risk for portions of the Pacific Northwest this afternoon through this evening. Abundant moisture with PWATs around 1 inch, or 2.5 sigma above normal, MUCAPE values over 1,000 J/kg in some spots and very weak steering flow has amplified the potential for strong but very slow moving storms along the various mountain ranges in the Marginal Risk area. 12Z CAMs are hinting at some of that storm development, but may be underdone given all the above favorable conditions for heavy rain. This combined with some snowmelt will support flash flooding, particularly on the various burn scars in the area where the flooding threat is greatest. ....Texas... 0100 UTC Update -- Short range CAM trends, including recent HRRRs, show more spotty/less organized convection than organized overnight, consistent with the current radar/satellite trends along with the weak/diffuse large-scale forcing. Thus we were able to drop the Slight Risk and maintain the broad Marginal Risk area. ....Gulf Coast... Thermodynamic profiles remain decent just after sunset (mixed-layer CAPEs 1000-2000 J/Kg and PWs between 1.75-2.00"). While not very organized, the stronger clusters of slow-moving, mainly pulse-type storms will have the potential to cause localized flash flooding until midnight/around 06Z when the deep-layer instability will have been sufficiently exhausted. Hurley Day 2 Valid 12Z Tue May 16 2023 - 12Z Wed May 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY TO THE CENTRAL AND SOUTHERN APPALACHIANS... ....20Z Update... No significant changes were needed to any of the risk areas for Tuesday. The front over the OH Valley has been in place in nearly all of the guidance for several runs now, with little movement noted. Small portions of the TN Valley were removed from the Slight as the main forcing looks to be closer to the OH Valley. These storms tracking east into the southern Appalachians may result in locally higher rainfall totals as the mountains act as additional forcing, though their impact is not expected to be major. The storms may weaken and dissipate some in the immediate lee of the mountains of southern VA, but are likely to reintensify as they move across the VA Tidewater. Instability will be a bit lower in southeast VA, so the flash flooding threat is a little bit lower. Significantly higher FFGs are also present in VA as compared with points in the Appalachians and west across the OH Valley, so the Marginal remains in place, though potential for an upgrade to a Slight is highest in this area. The threat and the meteorology supporting isolated flash flooding both in AZ and Intermountain West along the Canadian border remain the same, with no changes to the forecast reasoning. Wegman ....Previous Discussion... ....Ohio and Tennessee Valleys and the Central and Southern Appalachians... With plenty of instability (over 2,000 J/kg, and PWATs to 1.5 inches to the south of the front, there will be plenty of moisture, instability and forcing for heavy rain. Although the nature of this boundary will be more on the progressive side, there will be some upper jet support to work with by this time, with the right entrance region moving across the region. This enhanced lift should result in robust convective development over wet antecedent conditions. Most of the 3-hr FFGs from eastern Missouri to the Mid-Atlantic Coast is in the 1.0 to 2.5 inch range, with the lowest being near the areas of higher terrain. The inherited Slight Risk continues to highlight the areas were the threat for areas of excessive rainfall and flash flooding are more elevated so no adjustments were needed at this time. ....Mogollon Rim of Arizona... Recent runs of the guidance has depicted abnormally high amounts of atmospheric moisture, as high as 4 sigma above normal in this region, though there was initially no forcing to help get storms to form. However they are hinting that weak forcing will be present during this period that could be sufficient in triggering convective activity that could produce heavy rain and isolated flash flooding, especially on any old burn scars, slot canyons, and dry washes. The Marginal Risk area was maintained for this period. ....Intermountain West... A Marginal Risk area remains in effect from the Okanogan Highlands of northeast Washington east to the western Montana Hi-Line. A strong shortwave trough will skim well northeast of the area, but the combination of plenty of antecedent moisture, topography helping with forcing, and 500-1,000 J/kg of CAPE should be enough for some isolated flash flooding, especially along the mountains. The area around Great Falls, Montana is currently in the bullseye for the most rainfall, but that is subject to change. Campbell Day 3 Valid 12Z Wed May 17 2023 - 12Z Thu May 18 2023 ....THERE IS A MARGINAL RISK OF FLASH FLOODING FOR PORTIONS OF THE COASTAL SOUTHEAST AND ALONG THE MOGOLLON RIM OF ARIZONA... ....20Z Update... ....Mogollon Rim of Arizona... A semi-mon... pattern will remain in place across Arizona on Wednesday. A weak, but still notable upper level trough will track southeast across northwestern Mexico on Wednesday. Ahead of the low, southerly flow off the Pacific and up the Gulf of California will continue to pump unseasonably high amounts of Pacific moisture into the Desert Southwest. With PWATs approaching 1 inch, which is over 4 sigma above normal in some spots, the same southwesterly flow will favor shower and thunderstorm development along the Mogollon Rim again on Wednesday afternoon. As the trough moves east, the signal for potential heavy rain will do the same...so the Marginal Risk area was trimmed a bit to the east as compared with the Day 2/Tuesday Marginal Risk area. Unfortunately, even without the upper level forcing, the moisture will remain in place in this region for at least a couple days beyond Wednesday, so the thunderstorm chances will remain elevated through the rest of the week. ....Coastal Southeast... The slow-moving front responsible for the rain across portions of the Mid-Atlantic on Tuesday will settle across the Southeast by Wednesday. The front will have plenty of moisture and instability to work with as it stalls inland but roughly parallel to the Gulf and Atlantic coasts respectively. Thus, as with previous days, the threat for thunderstorms remains high, though the flash flooding threat will be low, as FFGs are very high over this region despite somewhat wetter than normal soils. The main flash flooding threat will be in the urban areas of the Marginal Risk, including New Orleans, Gulfport, Mobile, Pensacola, Tallahassee, Valdosta, and Savannah. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!61GLR8WfvVxy5ljNIemHH0hiSoypCaOw9ylS7Z4dKMsO= VEwhROhQbeMbRSEMiLRbi7h2aEeD8ztXk766KYhaJGd4JJg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!61GLR8WfvVxy5ljNIemHH0hiSoypCaOw9ylS7Z4dKMsO= VEwhROhQbeMbRSEMiLRbi7h2aEeD8ztXk766KYhatjiruKk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!61GLR8WfvVxy5ljNIemHH0hiSoypCaOw9ylS7Z4dKMsO= VEwhROhQbeMbRSEMiLRbi7h2aEeD8ztXk766KYhafyPclm8$=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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