Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 07 2023 08:31:08 FOUS30 KWBC 070831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Fri Apr 07 2023 Day 1 Valid 12Z Fri Apr 07 2023 - 12Z Sat Apr 08 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE EXTREME UPPER TEXAS COAST ACROSS CENTRAL AND SOUTHERN LOUISIANA TO SOUTHERN MISSISSIPPI... Another round of heavy rainfall appears to be on-tap for portions of Lousiana on Friday as moisture continues to stream northward from the Gulf and interact with mid- and upper-level trough lingering over the area and a quasi-stationary front that is forecast to extend from the western Gulf of Mexico across Louisiana into central/southern Mississippi. Given the amount of organization shown by the convection that persisted well into the overnight hours...it is expected that some moderate to heavy rainfall will be on-going at the start of the Day 1 period at 12Z before tapering off towards the time of convective minimum followed by heavy to potentially excessive rainfall redeveloping later in the day. How much the on-going convection impacts the convective initiation and evolution later in the day introduces uncertainty into outlook but the RAP forecast soundings still show 0-3km lapse rates around 7 degrees per km...precipitable water values approaching 1.75 inches over southeast Louisiana by 08/00Z and a moisture flux convergence maximum along the front at least builds some confidence that additional heavy rainfall will redevelop. Areal average 24-hour deterministic WPC QPF amounts on the order of 2 to 4 inches are depicted over southeast Louisiana with locally higher rainfall amounts implied. 40 km neighborhood probabilities shown by the 00Z HREF for rates in excess of 2 inches per hour were generally focused between the Louisiana Gulf coast and Lake Ponchatrain in the period from 19Z to 22Z...but at least some 15 to 25 percent probabilities are shown for 1+ inch per hour rates extend north of Lake Ponchatrain. These kind of rainfall rates pose a threat of excessive rainfall...but would be greatly impactful if those rainfall rates or rainfall amounts comparable to those observed on Thurday happen to occur in a highly urbanized area where the amount of runoff is maximized due to the amount of surfaces impervious to water. Given the uncertainty as to how things develop later...maintained the Slight Risk for now.=20 Opted to expand the Slight Risk areas farther west than shown in yesterdays Day 2 ERO over portions of southwest Louisiana and nearby Texas given the amount of rain observed/flash flooding observed and where additional rainfall is expected today. Given the latest QPF from the models and the deterministic WPC QPF...maintained the Marginal Risk area extending northeastward along portions of the Appalachians and far western Carolinas due to some post-frontal storms. The main limiting concern here being the progressive nature of the convection. Bann Day 2 Valid 12Z Sat Apr 08 2023 - 12Z Sun Apr 09 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE FLORIDA PANHANDLE TO THE CENTRAL CAROLINAS... ....2030Z Update... Only very minimal adjustments were made to the on-going Marginal Risk as the previous Day 3 outlook becomes today's Day 2 outlook.=20 Global guidance largely remained consistent...although a bit of disagreement on the axis storm coverage and thus heavy rainfall threat lingered across the Southeast. Given southeasterly low-level flow setting up over the Atlantic and over the boundary will help to continue to support the threat for areal average rainfall totals of 1-2", with locally higher amounts prompting an isolated threat for flash flooding, especially given the 500 mb to 250 mb mean trough and weak shortwave energy embedded within the broader trough to provide some focus and support while the flow aloft is at least modestly difluent. Bann Day 3 Valid 12Z Sun Apr 09 2023 - 12Z Mon Apr 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST... On-shore flow of deep-layered moisture will once again develop across portions of the Pacific Northwest on Sunday that persists into Sunday night/early Monday morning...especially in the coastal ranges from Washington into northern Oregon. Precipitable water values in the 1.00 to 1.25 inch range will be drawn from the eastern North Pacific to the coastline by low level winds of 30 to 45 kts in the 925 to 850 mb layer.. Global model QPF values were typically in the 1.5 to 2.75 inch range...most of that being focused in a 12 to 15 hour period. Given potential for the flow to interact to terrain...will highlight the excessive potential with a Marginal and reevaluate again in subsequent outlooks. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6vCzkWO-M8kV7sEKIEGyiaWFbi2B63une0-5FE8XYmoj= Dva9ipzqc77g_bdbyfqJpHhvT7A9AWb7Sdwq89Igj8uuveE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6vCzkWO-M8kV7sEKIEGyiaWFbi2B63une0-5FE8XYmoj= Dva9ipzqc77g_bdbyfqJpHhvT7A9AWb7Sdwq89Ig8kZW03g$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6vCzkWO-M8kV7sEKIEGyiaWFbi2B63une0-5FE8XYmoj= Dva9ipzqc77g_bdbyfqJpHhvT7A9AWb7Sdwq89Igw9J5dp0$=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) .