Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Nov 07 2023 08:58:00 FOUS30 KWBC 070818 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 318 AM EST Tue Nov 07 2023 Day 1 Valid 12Z Tue Nov 07 2023 - 12Z Wed Nov 08 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 Valid 12Z Wed Nov 08 2023 - 12Z Thu Nov 09 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 3 Valid 12Z Thu Nov 09 2023 - 12Z Fri Nov 10 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. A broad area of moderate to locally heavy rainfall is likely Thursday into Thursday night from TX northeast into the Arklatex. The greatest areal averaged rainfall totals are expected from northeast TX into southeast OK and southwest AR, where ~2" of rainfall is probable. This rainfall is primarily expected to be post-frontal, with overrunning of the southward dropping front and right entrance upper jet support helping drive the rainfall threat. There will be a moisture connection to the Eastern Pacific in the mid/upper levels, and some Gulf of Mexico moisture in the lower levels...so PWs will be elevated, with values starting to approach early November max values. However, mid level flow remains pretty zonal and not seeing much of a response in low level moisture transport. Instability increases Wednesday night, but is forecast to drop to minimal levels by the time rainfall expands in coverage Thursday. This lack of instability suggests rainfall rates will not be all that intense. Given the general lack of instability suspect observed rainfall rates will not be much higher than what is currently progged by the global guidance. The model consensus is for 3 hourly rainfall peaking around 0.5"-1", which is well below FFG over the region. So despite the high PWs and above average confidence in 1-2"+ rainfall, this looks more like a post-frontal lower rate type of event. Given the generally neutral soil conditions and streamflows over this area, a lower rainfall rate event is unlikely to cause much in the way of runoff concerns. For that reason we will go ahead and remove the Marginal risk. There is a better chance of surface based convection across south TX, so can not rule out a localized flash flood risk here. However even across south TX instability looks to peak around 1000 J/KG and convection that does develop should progress off to the east at a decent speed. Thus while higher rainfall rates are possible here, streamflows are running below average and it still seems like the probability of FFG exceedance is low. On top of that decent model spread remains with convective placement and magnitude. Given these factors will not carry a Marginal risk at this time...however there is some chance a Marginal will need to be reintroduced somewhere over south TX as confidence in placement increases (especially if instability forecasts were to trend upwards). Chenard $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .