Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Oct 23 2023 20:31:10 FOUS30 KWBC 232031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 PM EDT Mon Oct 23 2023 Day 1 Valid 16Z Mon Oct 23 2023 - 12Z Tue Oct 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTH OF THE BIG BEND OF TEXAS TO NORTHWEST TEXAS... 16Z update... Current observations and the latest model guidance show a broader area of convection with embedded areas with heavy rainfall lifting through Texas. This is expected to persist through the day while expanding into Oklahoma later in the period. Rainfall rates of up to 2/2.5 inches per hour are forecast to streak north-northeast across west-central Texas into southwest Oklahoma with some training of storms possible along the southwest to northeast axis. This may quickly reach or exceed local soil saturation leading to areas of flash flooding. Campbell Previous Discussion... PW values are expected to become very anomalous...2 to 3 standard deviations above the mean...in a southwest to northeast axis across portions of Texas from the Big Bend into Northwest Texas during the upcoming day 1 period. This is in response to strengthening south southeasterly low level flow off the western Gulf, northwestward through the Lower Rio Grande and into the Southern Plains ahead of overall amplifying upper trof over Southern California and the northeastward ejection of mid level energy associated with the remnants of Norma across north central Mexico and into western Texas. The latest hi res model consensus is for increasing precip coverage Monday afternoon, with an organized southwest to northeast oriented convective line sometime after 0000 UTC 10/24, with potential for training of cells in a southwest to northeasterly direction from the Big Bend, northeastward into Northwest TX Monday night into early Tuesday. In areas of training, hourly rainfall amounts of 1-2" are possible. The slight risk area matches well with where HREF neighborhood probabilities are high, 40-90% for 1"+ 2"+ totals day 1. This area also corresponds well with the area of high 1"+ EAS probabilities, 40-90% and a smaller area of 2"+ EAS probabilities, 10-50%. Much of this area has been dry over the past several weeks, but the expected rainfall rates will still pose a threat of runoff issues. The latest CSU first guess fields show a fairly large slight risk area, larger than the day 1. The latest CSU first guess is a significant increase in coverage from the old day 2. Oravec=20=20 Day 2 Valid 12Z Tue Oct 24 2023 - 12Z Wed Oct 25 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....Southern to Central Plains into the Lower Missouri Valley... 21Z update... Widespread convection will persist across the Southern Plains during this period with the focus of some of the higher amounts shifting northeast, spanning from central Texas to southeast Kansas. While parts of this region have been much drier of late and can handle a few inches of rain before becoming problematic, there is potential for isolated areas of excessive rainfall, especially if it falls in a short time frame. The latest guidance supports a shifting/expanding the Slight Risk area to span from portions of the Texas Hill Country to southeast Kansas. Areal averages of 1 to 2 inches are forecast with isolated maxes of 3 to 4 inches possible. Campbell The closed low moving across far northern Baja California late day 1 will continue to press eastward across northern Mexico day 2.=20 This will maintain broad southwesterly mid level flow across the Southern to Central Plains into the Upper MS Valley and allow for the northeastward expansion of anomalous PW values from the Southern Plains into the Central Plains and into the Upper Mississippi Valley, with values 2 to 3 standard deviations above the mean pushing into those areas. Heavy rains will likely spread northeastward day 2 into eastern portions of the Central Plains and portions of the Lower MO Valley. Similar to areas farther to the southwest on day 1, these areas farther to the northeast have also seen little precip over past few weeks, with low soil moisture values and below average stream flows. This should be a detriment to widespread runoff issues, although with potential for training of precip in a southwest to northeasterly direction, can not rule out isolated runoff issues from northeast OK, southeast KS and northwest MO where a marginal risk is denoted. We did trim the previous slight risk across this area and narrowed up the marginal.=20 Farther to the southwest from southwest OK into Northwest TX, a slight risk area was maintained from the previous outlook. This is the region of potential overlap of day 1 heavy rains and day 2 heavy rains, where 2 day totals could run as high as 2-4"+. ....Upper MS Valley into the Upper Great Lakes... 21Z update... The latest guidance suggests a broader west-east axis of precipitation along the frontal boundary which increases QPF and the potential for excessive rainfall. The inherited Marginal Risk was expanded further west across southern Minnesota and further south across the northern "Mitt" of Michigan. Campbell A strengthening west southwest to east northeast oriented frontal boundary expected to stretch from southern MN into north central WI and the eastern U.P. of MI. The expected strengthening west southwesterly to southwesterly low level flow into this front will support increasing overrunning convective precip along and north of the boundary where PW values will be increasing to 2 to 3 standard deviations above the mean. There will be potential for training of cells parallel to the frontal boundary, supporting short term precip amounts of 1-2"+. Much of the area in the vicinity of this front has been dry over the past week, although heavier totals have occurred over the past few weeks from southern MN into southern WI. The risk level was kept at marginal for the time being given low soil moisture levels and below to near normal stream flows. Oravec=20 Day 3 Valid 12Z Wed Oct 25 2023 - 12Z Thu Oct 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....Southern Plains... 21Z update... Soil moisture will likely have increased across the Southern and Central Plains in response to the multi-day rain event. Areal averages over 72 hour period from central Texas to far southern Kansas will be in the 3 to 6 inch range with higher maxes. The latest guidance keeps the axis orientation and amounts similar to that of the previous forecast therefore only minor adjustments were made to the Marginal and Slight Risk areas for this period. Campbell The multi day heavy rain event for portions of the Southern Plains will continue as the strong upper trof moving across northern Mexico day 2, ejects northeastward across the Southern Plains.=20 The above average PW values across the Southern Plains day 2, will increase a bit day 3 as the upstream strong dynamics approach this area. There is fairly good model consensus for additional widespread heavy rains in this anomalous PW axis from the Big Bend, northeastward into Northwest Texas and large portions of Oklahoma, save for the Panhandle and southeast OK. While there are some differences in the model qpf axes, there is a strong signal for another day of potential overlap of heavy rains from the days 1 and 2 time periods. At the moment, we have kept the risk level at slight, but showing a more expansive risk area than on the day 2 outlook. The new day 3 slight risk area is more narrow than the old day 4, to better match the model consensus qpf axis. With the potential for multiple days of heavy rain overlap, an upgrade to moderate is possible with later issuances, depending upon where the heavy axes occur days 1 and 2. The day 3 slight area does match fairly well with CSU first guess fields, although having a slightly different orientation to the axis to match the latest day 3 qpf. ....Upper Mississippi Valley... 21Z update... Still looking like another round of convection could materialize across the north-central U.S. A minor adjustment of the Marginal Risk was made on the southwest side to extend further across western Minnesota and northwest Iowa. Campbell The surface frontal boundary stretching west southwest to east northeast from portions of the Upper MS Valley into the Upper Great Lakes is not expected to move much during day 3. Another round of strengthening low level southwesterly flow possible into this boundary Wednesday night into early Thursday. This may support another round of overrunning convection moving along this front and potential for training of cells parallel to the front.=20 There is some latitude spread with respect to any potential heavy qpf axis, with the marginal risk drawn to encompass this spread.=20 Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4LHwaV1wVciv9NA7xcJ45g_PI-SM9RQ5A7YzONDr1JsZ= OupDaEnzpL-Fa3-TDF1ji1Lb1ZmBtQ8C6ws0QF3BpxIfpQY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4LHwaV1wVciv9NA7xcJ45g_PI-SM9RQ5A7YzONDr1JsZ= OupDaEnzpL-Fa3-TDF1ji1Lb1ZmBtQ8C6ws0QF3BSCaJorU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4LHwaV1wVciv9NA7xcJ45g_PI-SM9RQ5A7YzONDr1JsZ= OupDaEnzpL-Fa3-TDF1ji1Lb1ZmBtQ8C6ws0QF3BEl3AjP8$=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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