Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 17 2023 08:23:43 FOUS30 KWBC 170823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 AM EDT Wed May 17 2023 Day 1 Valid 12Z Wed May 17 2023 - 12Z Thu May 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHEAST TO THE CENTRAL GULF COAST AND ALONG THE MOGOLLON RIM OF ARIZONA... ....Mogollon Rim of Arizona... Highly anomalous moisture along with weak forcing 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 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. The Marginal Risk was expanded somewhat to include the broader threat for isolated convection capable of intense rainfall within flood-prone regions. 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, thus the Marginal designation. ....Southeast and Central Gulf Coast... The slow-moving frontal boundary draped across the Southeast will continue to have additional rounds of unorganized, impulse convection develop as a steady stream of deep moisture advects northward. The PW values will generally be around 1.5" over this stalled boundary; which should be more than adequate to keep the threat for intense rainfall rates, though the flash flooding threat will be low due to very high FFG and sandy 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. A small adjustment was made to the northern bounds of the Marginal Risk over central South Carolina. Campbell/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 will be 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, the 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 inches with one hinting at very local maximums nearing 6 inches. The Slight Risk was nudged a little further east and south across Oklahoma and the northern bounds of the Marginal Risk area was trimmed back across northern Kansas. ....Southeast... Showers and thunderstorms will near the stalled frontal boundary in the Southeast and near 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 terrain can handle brief periods of heavy rain. Although this part of the country has FFG generally in the 3-5 inch 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. If that pans out than there will be increased heavy rainfall for coastal South Carolina therefore a Marginal Risk remains in effect and was expanded to include the South Carolina Lowcountry. Campbell/Snell Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8cSJaysqqdde0IusVJyJCo-0Qgd7ZUkTDofg7C8r1soN= 9my3z21KUZQbaxcNNOjfOU9P4dZF4kjAF2C8JnE80ilVgLs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8cSJaysqqdde0IusVJyJCo-0Qgd7ZUkTDofg7C8r1soN= 9my3z21KUZQbaxcNNOjfOU9P4dZF4kjAF2C8JnE8pgJpnXY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8cSJaysqqdde0IusVJyJCo-0Qgd7ZUkTDofg7C8r1soN= 9my3z21KUZQbaxcNNOjfOU9P4dZF4kjAF2C8JnE8T7K9rYo$=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) .