Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 14 2024 00:55:45 FOUS30 KWBC 140055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Wed Mar 13 2024 Day 1 Valid 01Z Thu Mar 14 2024 - 12Z Thu Mar 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSOURI AND MID MISSISSIPPI VALLEYS... ....01Z Update... Bottom line up front: Have expanded the Slight=20 Risk area a little farther east (into western IL, just west of=20 Peoria), while also making some adjustments to the Marginal Risk=20 area based on the latest observational and guidance trends.=20 Latest satellite IR and WV imagery, along with last few loops of the visible prior to the setting sun, show convective clusters getting better organized across portions of north-central and northeast KS eastward along the I-70 corridor to south-central IL. Convection is firing along/near the surface warm front, and is expected to grow upscale this evening as the leading edge of the southern stream upper level jet streak moves toward the MO/KS/AR/OK 4-state area (placing the outlook area in the favorable left exit region). Meanwhile, increased upper level confluence to the north will lead to the development of another jet streak across the=20 Upper Midwest. The result would put the outlook areas within the left exit/right entrance region of the southern/northern upper level jet streaks, thereby allowing for robust divergence aloft and deep-layer ascent overnight.=20 850 mb flow will increase to 40-50kts overnight as the LLJ veers from south to southwesterly. Latest HRRR and RAP continue to show the veering LLJ becoming more aligned with the mean 850-300 mb flow toward 05-07Z, which would allow for more upwind propagation and areas of training convection downwind of the surface triple point. PWs approaching 1.25" in an expansive convective environment,=20 along with elevated mixed-layer CAPEs averaging 1000-1500 J/Kg,=20 will allow for 1-2 inch/hr rainfall rates and totals of 3-4+ inches where cell training is most pronounced. Hurley Previous discussion... ....16Z Outlook Update... Morning model guidance (particularly CAMs) depict a scenario for developing (and initially slow-moving) convection to form along a warm front along and north of the I-70 corridor in northeastern Kansas/northwestern Missouri this afternoon. Models also generally grow the developing convection upscale, promoting cell mergers and eventual transition into one or two forward-propagating linear segments through the night. This evolution should prolong rain rates in certain spots - allowing for local 1-2 inch/hr rates and 2-3 inch rainfall totals to materialize where training is most pronounced. Given the scenario, a targeted Slight Risk area (15% or greater flash flood probabilities) was introduced where the most likely axis of heavier rainfall is expected to develop primarily between 22-06Z. A broader Marginal Risk area was maintained for this outlook, but was expanded northeastward into more of northern Illinois/Indiana where recent guidance depicts potential for localized training of deep convection to materialize tonight. See the previous discussion below for more information. Cook Day 2 Valid 12Z Thu Mar 14 2024 - 12Z Fri Mar 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS AREAS FROM NORTHEAST TEXAS INTO THE MID-SOUTH AND MISSOURI OZARKS... ....21Z Outlook Update... Westward and northward expansions of the ongoing Slight Risk area were made for this update amid substantial model spread and uncertainty. Early in the D2/Thu period (around 12-15Z), models depict potential for training convection to focus along a warm frontal zone across northern Illinois/Indiana. That activity will be heavily modulated by the eventual eastward movement of convective complexes late in the D1 forecast period. That activity should maintain isolated flash flood potential through the morning, although a weakening trend in convection is expected after 15Z. Later in the D2 period, a southward-moving cold front is expected to provide a focus for scattered to numerous thunderstorm development from the Red River in southern Oklahoma into southwestern Missouri. Models vary substantially with respect to development and upscale growth of this activity, and rising geopotential heights throughout the forecast period over the front and warm sector raise concern that convective coverage may be sparse in some parts of the Slight Risk area. However, storms are expected to develop and foster local cell mergers/training to support flash flood potential. The highest potential for flash flooding should reside from southeastern Oklahoma into southern Missouri (including much of the Ozarks) where areas of 2 inch/hr rain rates are expected to materialize. Some uncertainly also exists with respect to upscale growth of convection and downstream rainfall potential across portions of the ArkLaTex, central Arkansas, and the Mid-South. The Slight risk area continues to depict potential for at least localized flash flood potential through the overnight hours depending on the eventual evolution of upstream convective complexes that form in the 19Z-00Z timeframe. Portions of Mississippi and northeastern Louisiana were trimmed from the inherited Slight as it appears less likely that deep convection will materialize in these areas during the 00Z-12Z period (per 12Z models). The Marginal risk area was expanded southwestward across central Texas with the expectation that scattered thunderstorms could focus along a cold front in that area late Thursday and train/repeat, boosting local rain rates. The Marginal risk area was also expanded northeastward across the Ohio Valley with the expectation that isolated to scattered, fast-moving storms could train and promote isolated areas of 1+ inch/hr rain rates from afternoon through the overnight hours. A low-end risk of similar activity extends as far northeast as southern New York State after 00Z Fri, although confidence in this scenario was too low to add Marginal probabilities at this time. See the previous discussion below for more information. Cook ....Previous discussion... The low level southerly flow is expected to once again strengthen significantly late Thursday into early Friday across the eastern portions of the Southern Plains into the Lower Arkansas and Lower Mississippi Valleys downstream of the strengthening closed low over the Southern Great Basin. 850-700 mb moisture flux anomalies and PW anomalies rise to 2 to 2.5+ standard deviations above the mean along and ahead of the associated cold front pushing southeast through the Southern Plains into the Lower Arkansas Valley. There is a strong model signal for heavy precipitation from northeast Texas, southeast Oklahoma, northern Louisiana, much of Arkansas, northwest Mississippi, far western Tennessee, far southeast Missouri, far southern Illinois and far western Kentucky in a region of favorable large scale conditions, strong frontal convergence, well defined upper difluence, late Thursday into the early hours of Friday. Similar to the past few model cycles,there continues to be north-south model spread with respect to where the max convective qpf axis will set up. The NAM continues to be the northern outlier, the EC and EC mean, the farthest south and the GFS, GEFS mean and UKMET in between. The slight risk continues to be drawn to encompass the current model spread, with only some minor changes from the previous outlook for this period. The slight risk is across areas that have had below average precip over the past few weeks and subsequently relative high FFG values. The heaviest precip totals over the past few weeks and the most saturated soils area well to the southeast of the slight risk area across portions of the South and Southeast.=20 Given this and the continued spread in the max qpf axis, the risk was kept at slight for the time being.=20 Oravec Day 3 Valid 12Z Fri Mar 15 2024 - 12Z Sat Mar 16 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EAST TEXAS, ACROSS THE LOWER MISSISSIPPI VALLEY AND INTO PORTIONS OF THE SOUTH... ....21Z Outlook Update... An uncertain convective/flash flood scenario is expected to continue into the D3/Fri forecast period. Ongoing convection (potentially extending from northeast Texas through the Mid-South) is expected to migrate/propagate southeastward into and through the Slight Risk area primarily during the morning hours. Varying model solutions suggest that whatever remaining low-level boundaries/remnant convection should reach portions of the central Gulf Coast and southeast Texas by midday despite substantial ridging aloft over the area. The low-level boundary in central Texas has potential to aid in renewed convective development later in the afternoon, and could result in a few instances of training/cell mergers and increased rainfall rates. Strong consideration for removing the Slight was given, although enough of a convective signal remains in the model guidance to keep existing Marginal/Slight areas along with some geographical restriction especially across the southern Appalachians and southeastern Tennessee. Cook ....Previous Discussion... The surface frontal boundary pressing southeastward day 2 into the Southern Plains, Lower Arkansas Valley and Mid Mississippi Valley will begin to accelerate to the southeast day 3 through the Tennessee Valley and Lower Mississippi Valley, moving slower to the south into the Southern Plains. An axis of anomalous PW values, 1.5 to 2.5+ standard deviations above the mean will accompany this southeastward moving front day 3. This and the continued overall favorable large scale conditions ahead of this front will support another day of potentially widespread heavy precip along and ahead of the front. There is the typical amount of spread with respect to the max qpf axis, but overall model consensus for potentially heavy rains. The day 3 qpf axis does move back over areas of Mississippi, Alabama and Georgia that have had above average precip over the past week or two, resulting in higher stream flows and greater soil saturation. There were not a lot of changes made to the previous broad slight risk area, extending it slightly farther to the west across northeast Texas to cover the model spread. The northern portion of the marginal risk area over the Northeast was cutback to the Central Appalachians to better fit the latest model qpf. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!44A6dmToNhpdEz3_v64SsYq3KnXw7njVgSy3zZ7tC-ER= 0jxoXBWNL1AAwR2QzxLD8SRjNqXxxK31ijXZFOoQTLQxP4M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!44A6dmToNhpdEz3_v64SsYq3KnXw7njVgSy3zZ7tC-ER= 0jxoXBWNL1AAwR2QzxLD8SRjNqXxxK31ijXZFOoQMtdJxIA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!44A6dmToNhpdEz3_v64SsYq3KnXw7njVgSy3zZ7tC-ER= 0jxoXBWNL1AAwR2QzxLD8SRjNqXxxK31ijXZFOoQsQHwhqU$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .