Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 07 2023 19:52:40 FOUS30 KWBC 071952 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 251 PM EST Tue Nov 07 2023 Day 1 Valid 16Z Tue Nov 07 2023 - 12Z Wed Nov 08 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Putnam 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. Putnam 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. ....2030Z Update... Previous forecast reasoning remains valid for the ArkLaTex based on updated guidance. Portions of central/south Texas will see an influx of higher moisture from the Pacific as well as some southerly flow from the Gulf with notable PW values 2-2.5 standard deviations above the mean. A broad SW-NE rainfall axis looks to be tied at least partially to post-frontal rainfall as a cold front pushes south/eastward through the region towards the Gulf Coast by Friday morning. Despite the available moisture, guidance differences in the coverage/location of surface based convection along and ahead of the front noted overnight remain (central versus south/coastal Texas), and the front should be progressive enough that any more notable downpours don't look to be long enough duration for flash flooding concerns at this time. Thus, after a review of the updated guidance, a Marginal Risk was not reintroduced for now. Putnam ....Previous Discussion... 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 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8grP1Uh6CmunwRa-w8anb2OmKdTedD7phtZA_2c5W1Ce= ymvF5bPgvOYZVFUEmf8edTARD7DgFC9H3D0rYJPL9SRgQN8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8grP1Uh6CmunwRa-w8anb2OmKdTedD7phtZA_2c5W1Ce= ymvF5bPgvOYZVFUEmf8edTARD7DgFC9H3D0rYJPLXr-7ToQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8grP1Uh6CmunwRa-w8anb2OmKdTedD7phtZA_2c5W1Ce= ymvF5bPgvOYZVFUEmf8edTARD7DgFC9H3D0rYJPLG5Xi6wk$=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) .