Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Oct 10 2023 20:25:22 FOUS30 KWBC 102025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 424 PM EDT Tue Oct 10 2023 Day 1 Valid 16Z Tue Oct 10 2023 - 12Z Wed Oct 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTH TEXAS... ....16Z Excessive Rainfall Outlook Discussion... Few changes needed. 12Z HREF neighborhood probabilities show low-end probabilities for 2 inch per hour rate...and less than 5 percent (but non-zero) risk of rainfall exceeding 3 hour QPF late this evening or earl Wednesday morning over far southern Texas as remnant energy and moisture associated with Lidia and Max traverse the area and potentially interact with a developing wave over the western Gulf of Mexico. The previously issued ERO captured both probabilities within the Slight Risk area. 12Z soundings showed a plume of precipitable water values in excess of 2 inches from MMMZ to MMMY that was nosing into far southern Texas where values were barely sub-two inches. With southerly flow ahead of a developing wave resulting in additional moisture transport into the Texas...precipitable water values of 2.00 to 2.25 inches appears likely by later today. This deep moisture, along with favorable forcing, will raise the threat for locally heavy rain across the region. Expect the greater instability to remain near the immediate coast or just offshore, limiting rainfall rates inland.=20 These lighter rates may result in beneficial rains for most areas, however cannot rule out the threat for isolated flooding especially for urbanized areas and where soils are already relatively moist from last week's rain. Bann Day 2 Valid 12Z Wed Oct 11 2023 - 12Z Thu Oct 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF THE GULF COAST... ....Gulf Coast/Southeast... Focus for excessive rainfall will involve a wave of low pressure developing over the northwestern Gulf of Mexico and its interaction with a frontal boundary draped across the Gulf before it reaches the Southeast coast by early Thursday. Increasing southwesterly flow will feed deep moisture into a boundary lifting north ahead of the wave, with PWs increasing to over 2.25 inches (2 to 3 standard deviations above normal) along the central to eastern Gulf Coast. There remains spread in the guidance with respect to where the system tracks...with a majority of the 12Z high resolution guidance favoring a more suppressed solution similar to the old GFS and ECMWF. With those solutions being favored this cycle...shifted the axis of the excessive rainfall outlook area more towards the Florida peninsula and less across the immediate Gulf coast states. Thinking is that deeper moisture and greater instability likely to remain centered near the coast or offshore. As a result...the Slight Risk area was shifted southward over portions of the Florida Peninsula where the preferred guidance shows the greatest threat for heavy amounts, with some potential for several inches near the immediate coast...but maintained a Marginal Risk along the Gulf coast to account for the possibility of later guidance shifting back north. ....Mid Missouri and mid Mississippi Valleys... With a tightening of the deterministic WPC QPF areal coverage and a subtle downward nudge in amounts...was tempted to remove the Marginal Risk area for this outlook. Some of the global models supported with spaghetti plots from the GEFS and SREF typically showing a couple of members with QPF of 2 inches or greater and with the expectation that highest rainfall rates would come from elevated convection north of a front later in the day. The 12Z HREF...and the ARW to a lesser extent...showed widely scattered coverage of neighborhood probabilities for 3-hour QPF exceeding 3-hour flash flood guidance to maintain the Marginal Risk for the time being. Bann Day 3 Valid 12Z Thu Oct 12 2023 - 12Z Fri Oct 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF THE MID MISSOURI VALLEY... ....Northern Plains/Mid Missouri Valley/Upper Midwest... A deep cyclone developing over the central Rockies and High Plains early Thursday is forecast to drift east across the Plains and into the mid and upper Mississippi Valley by early Friday. Strong southerly winds ahead of the system will continue to transport moisture into a region of strong ascent afforded in part by favorable upper jet forcing. For most areas the combination of ample moisture and strong forcing, but little instability, will produce an extended period of moderate rains, resulting in locally heavy accumulations but limited impacts. An overall tightening of the deterministic WPC QPF did not fundamentally change the placement of the Slight Risk area compared to earlier issuances nor did a downward nudge in the amounts necessitate a downgrade in outlook category. It remains near where greater instability and deeper moisture along the axis of strongest inflow may help to increase rainfall rates and the threat for short-term heavy accumulations and rapid runoff. Training storms may also raise the threat for heavy amounts and excessive runoff across this region. ....Southeast... The preferred guidance shows the low pressure emanating from the Gulf on Days 1 and 2 moving quickly off of the Southeast coast Thursday morning. However, rich moisture and mid level energy embedded within deep southwesterly flow in its wake may continue to produce periods of heavy rainfall across portions of the Florida peninsula or north Florida/southeast Georgia. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5iKTXjBSmLgust6f71pEMkzgwk72FYDjRvNyKFBTohqg= oZ0ijDV5MbbRBqVOafG3wPWJLIZ6nzHpuIv4Kq69FAT9Uuo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5iKTXjBSmLgust6f71pEMkzgwk72FYDjRvNyKFBTohqg= oZ0ijDV5MbbRBqVOafG3wPWJLIZ6nzHpuIv4Kq69-qSFHd0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5iKTXjBSmLgust6f71pEMkzgwk72FYDjRvNyKFBTohqg= oZ0ijDV5MbbRBqVOafG3wPWJLIZ6nzHpuIv4Kq69Jx19Bu0$=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) .