Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Oct 24 2023 00:50:12 FOUS30 KWBC 240050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Mon Oct 23 2023 Day 1 Valid 01Z Tue Oct 24 2023 - 12Z Tue Oct 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTH OF THE BIG BEND OF TEXAS INTO SOUTHWESTERN OKLAHOMA... Rainfall across Texas through 00Z has been largely stratiform but with embedded occasional bursts of convection containing MRMS-derived rainfall rates in excess of 1 in/hr. An anomalous plume of moisture (approximately 2 to 3 standard deviations above the mean) extended from just east of Big Bend N.P. into northwestern Texas and western Oklahoma. This moisture is a combination of a low level feed from the Gulf of Mexico and mid to upper level moisture streaming northeastward related to the remnants of Norma. Despite the anomalous moisture, instability across the Southern Plains has been weak given cloud cover during the day on Monday and relatively poor mid-level lapse rates of 5-6 C/km. Through 12Z Tuesday, a couple of weak mid-level impulses from western Texas into northern Mexico embedded in southwesterly flow will aid in generating lift across west-central Texas into western Oklahoma. Short term forecasts from the RAP support an increase in the 850 mb flow into the 40-50 kt range overnight and at the leading edge of that LLJ, a southwest to northeast oriented axis of low level convergence extending from southern portions of the Permian Basin into southwestern Oklahoma. Areas of training will be possible along the low level axis of convergence given similarly oriented steering flow. While instability is expected to remain weak (~750 J/kg or less) and highest across southern portions of the outlook area near the Rio Grande, the potential for 1-2 in/hr rainfall rates will exist where cell training occurs, with the highest probabilities depicted within the Slight Risk area. Additional rainfall totals through 12Z of 3 to 5 inches will be possible on a localized basis across west-central Texas, with dry antecedent conditions being overcome where occurs training of high rainfall rates. Adjustments to the previous outlook were minor and included a contraction of the Slight and Marginal Risk areas across western Texas, in line with the latest hi-res model consensus. Otto 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!_lcGJ_IU7KnfjJmhaItpTyN9_ZspVPLOaQh7Xx9B7Oy2= jWkT7ySA_6q99JY2c1t9ku2ibcchV2WWJIS08K-Ucp2UtPk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_lcGJ_IU7KnfjJmhaItpTyN9_ZspVPLOaQh7Xx9B7Oy2= jWkT7ySA_6q99JY2c1t9ku2ibcchV2WWJIS08K-Uso2083w$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_lcGJ_IU7KnfjJmhaItpTyN9_ZspVPLOaQh7Xx9B7Oy2= jWkT7ySA_6q99JY2c1t9ku2ibcchV2WWJIS08K-UQa76MT0$=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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