Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sat May 06 2023 09:20:00 FOUS30 KWBC 060830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Sat May 06 2023 Day 1 Valid 12Z Sat May 06 2023 - 12Z Sun May 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND NORTH-CENTRAL TEXAS... Maintained the Marginal Risk area for excessive rainfall over portions of the Lower Mississippi Valley as well as portions of Texas. A quasi-stationary boundary will remain parked in the same areas...leading to another round of convection in many of the same places where thunderstorms occurred on Thursday night/Friday morning and again in the early-morning hours of today. There is a possibility that storms will traverse flood sensitive portions of LA and MS...including Baton Rouge, New Orleans, and Biloxi...as the move towards the southeast. The expectation is that some convection will be lingering at the start of the Day 1 period but taper off...followed by additional convection later in the afternoon and evening. This will keep the soils in the area primed for additional flash flooding. In Texas...the focus for convection will be just a hair farther north as compared to today. Given the antecedent conditions..the Marginal Risk area there still looks warranted. Bann Day 2 Valid 12Z Sun May 07 2023 - 12Z Mon May 08 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST... Even though there has been some convergence of ideas among the models, the forecast remains below average in confidence in placement of the heavier rainfall amounts/rates and any associated risk of excessive rainfall across the Midwest on Sunday. The trend noted on Friday afternoon of a subtle northward shift in the guidance has continued in the 06/00Z forecast cycle...although the better focus this run has been more northwest than northeast for the larger magnitude of instability and surface low pressure development with a warm front towards the east being the focus for more widespread rainfall. As a result...the inherited Marginal Risk area was nudged westward...although lower FFGs on the eastern periphery of the Marginal Risk area meant smaller adjustments there. A steady southwesterly flow adding atmospheric moisture to the area behind the warm front will help boost MUCAPE values approach 2,000 J/kg Sunday afternoon and evening, so any storms that fire will have a favorable environment to work with. How the different streams interact with each other and the timing of weak shortwave energy that can help trigger and focus convection remains far from clear, Expect continued adjustments with future updates as the details become more clear. Bann Day 3 Valid 12Z Mon May 08 2023 - 12Z Tue May 09 2023 ....THERE IS A MARGINAL RISK LINGERS OVER PORTIONS OF THE MIDWEST ON MONDAY... The threat for potentially moderate to heavy rainfall...with the risk of excessive rainfall given areas of low flash flood guidance...lingers into the early portion of the Day 3 period across the Midwest. With the mid-level flow becoming more westerly...and even west northwesterly by the end of the period...the magnitude of instability should gradually wane and be shunted southward by later Monday night and early Tuesday. Until that happens...the models were still able to generate some QPF bullseye values where 1000 to 850 mb moisture transport values are 1,5 to 2 standardized anomalies greater than climatology into a region where the precipitable water values are already above normal. NCEP models tended to be more bullish than the ECMWF/UKMET in terms of 24-hour rainfall...with the NAM still able to generate 3 to 5 inch amounts in spots. So kept a Marginal Risk into Day 3. Bann $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .