Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 22 2022 20:24:49 FOUS30 KWBC 222024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 323 PM EST Tue Nov 22 2022 Day 1 Valid 16Z Tue Nov 22 2022 - 12Z Wed Nov 23 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE EAST CENTRAL COAST OF FLORIDA... ....16Z Update... Very few changes made to the previous forecast. A small westward expansion was made to coordinate better with the CAMs in this area. The convection is very slow moving into the coast so far, but individual cells so far have not been very strong. Daytime heating should help to intensify the convection this afternoon. Several of the CAMs are suggesting convection developing along a line from the Marco Island, FL area to the southern end of Lake Okeechobee, but with this area being fairly rural, FFG values are not expected to be met by any convection that does form. The latest HREF guidances suggests a 60 percent chance of achieving 2 year ARI values in the Fort Pierce and Vero Beach areas, and even a 25 percent chance of reaching 10 year ARI values in the same area. The EC and GFS both agree on PWAT values remaining 2.5 standard deviations above normal with values approaching 2". Wegman ....Previous Discussion... The axis of above average PW values..2+ standard deviations above the mean...will persist across central Florida day 1. Onshore easterly to east southeasterly low level flow expected to persist in this high PW...supporting an area of defined boundary layer convergence along the east central Florida coast day 1, with continued potential for showers and isolated heavy totals. The 0000 UTC hi res guidance has trended mostly offshore for day 1 with any heavy precip potential. However, several of the global models (GFS, EC, CMC) along with a few of the latest hrrr runs show continued potential for locally heavy rainfall amounts.=20 Confidence is not great with respect to where heaviest totals will occur, but there is potential for slow moving or nearly stationary cells along the east central coastal sections of Florida that may produce isolated runoff issues, especially over more urbanized regions. With respect to changes to the previous outlook for this period, the north side was trimmed south out of JAX WFO to the northern border of WFO MLB. The southern portion was extended slightly farther to the south to the southern border of WFO MLB. Oravec Day 2 Valid 12Z Wed Nov 23 2022 - 12Z Thu Nov 24 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Thu Nov 24 2022 - 12Z Fri Nov 25 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY... ....2030Z Update... The models remain in good agreement on an area of heavy rain developing on Thanksgiving Day into Thursday night across a swath of the country from east TX through northern MS. A nearly stationary frontal boundary situated under a 130 kt southwesterly jet will draw plentiful Gulf moisture northward into the Slight Risk area, where the front will act as a focal point for convection that is likely to either train over the same areas or result in multiple rounds of heavy rain over the area. While there remains some uncertainty on where the heaviest rainfall will set up, it appears likely that with PWATS of 1.5 to 1.75", which are over 2 standard deviations above normal, and a steady supply of additional Gulf moisture over these same areas should overcome antecedent dry conditions and result in localized flash flooding. Further, there will be a bit of rainfall Wednesday night over northern areas that may help to saturate the soils. Instability will be greatest across western areas in east TX, where the heaviest rainfall rates will be, but then those storms will quickly move northeastward across the rest of the Slight Risk area. In coordination with the FWD/Dallas-Fort Worth, TX and HGX/Houston, TX forecast offices, a Slight Risk was introduced with this update. Wegman ....Previous Discussion... Model guidance has come into better agreement with the developing closed low over the Southern Plains Thursday into Friday. There continues to be some timing differences with the GFS and ECMWF slightly more progressive than the NAM, CMC and UKMET. Despite these timing differences, there is model consensus for an expanding area of heavy precipitation late day 2 continuing into day 3 from northeast Texas, eastward into the Lower Mississippi as strengthening southerly low level flow off the western Gulf late day 2 will raise PW values to 1.5-1.75"+ (2+ standard deviations above the mean) from eastern Texas, eastward into the Lower Mississippi Valley. Much of this region has had below average precip totals over the past several weeks, save for upper coastal TX into southwest coastal LA. This is resulting in stream flows below average across much of this region. For this reason, only a marginal risk area is denoted day 3. This may be upgraded to a slight risk in subsequent outlooks if model consensus for the heaviest axis remains consistent. There will be potential for training of cells in a general south southwest to north northeasterly direction across the marginal risk area, supporting potential for totals in the 3-5"+ range where training occurs. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-CK4l_E9yP9Nqx17d_udxiGZp2jaRAQqDCbNbJOucBvn= xuL_jr5npTq09eC9b_11e-IqwDzUgncmc3XIUH7VtiKkItw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-CK4l_E9yP9Nqx17d_udxiGZp2jaRAQqDCbNbJOucBvn= xuL_jr5npTq09eC9b_11e-IqwDzUgncmc3XIUH7VvZ0HGio$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-CK4l_E9yP9Nqx17d_udxiGZp2jaRAQqDCbNbJOucBvn= xuL_jr5npTq09eC9b_11e-IqwDzUgncmc3XIUH7VR1XUuNA$=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. 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