Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 07 2023 20:28:23 FOUS30 KWBC 072028 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 327 PM EST Tue Mar 07 2023 Day 1 Valid 16Z Tue Mar 07 2023 - 12Z Wed Mar 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND EASTERN OKLAHOMA... ....16Z Update... No big changes were made to the inherited Slight and Marginal Risk areas mainly across OK. The meteorological discussion below remains largely in effect. The main changes with this morning's update follow the trend in the CAMs guidance, that the line of storms over OK should have a southwest to northeast focus. The line is generally expected to develop this evening on the northwestern side of the SLGT risk area, then slowly progress south and east through the overnight hours tonight. Early on, the line is not expected to move much, which will optimize the potential for training storms. As the night progresses, the storms will grow upscale as is typical, which will broaden the width of the line of storms. This will be offset by a bit faster movement of the storms by the time they reach southeastern OK and central AR. There remains uncertainty as to where the line of storms will develop, how fast it will move, what the southern extent of the line into TX will be, and how strong the storms will be. Thus, the SLGT and MRGL risk areas remain fairly large. As the storms move northeast into MO, instability will rapidly decrease, so the storms are expected to stop strengthening, and thus have their rainfall potential limited. Western areas of the SLGT and especially MRGL risk areas also have the benefit of very dry soils. Thus, expect some stronger storms to develop over these areas of far northern TX along the Red River, but the flood potential is lower than over OK and AR where the antecedent conditions show the soils are much closer to saturated, and thus less able to handle heavy rain. Otherwise, the highly anomalous atmospheric moisture as described below remains the same. Wegman ....Previous Discussion... The forecast remains on track for the new Day 1 period as the threat of heavy rain increases across portions of the Southern Plains after 00z tonight. An upper-level pattern featuring a longwave trough over the West and a ridge axis spanning from the central Gulf Coast to the Ohio Valley combined with a sharp frontal boundary and increasing moisture will lead to the threat of flash flooding. Shortwave energy rotating through the base of the western trough will amplify the downstream mid-level flow as backing flow aloft over the southern Plains and lower MS Valley will encourage the return of low-level southerly flow. Moisture will become increasingly focused north along a well-defined front draped across the Red River Valley of the South and stretching east-southeast across the lower MS Valley. Initially, the forcing along the front will be rather modest, but by the evening and overnight hours, the arrival of low-amplitude shortwave energy in the deep layer southwest flow aloft along with some uptick in the low-level jet should encourage the development of showers and thunderstorms along and north of the front for areas of central/eastern OK and north-central TX, before gradually shifting into adjacent areas of northwest AR and southwest MO in vicinity of the Ozark Plateau. The initial convection extending to the north of the front is also likely to be co-located along an inverted trough, which will support convergence and stationary/training cells as the low-level jet continues to pump moisture northward. Aside from increasing isentropic ascent, there will also be the nose of at least a modest axis of elevated instability with MUCAPE values of 500+ J/Kg advancing up across south-central OK that will facilitate convective development, with 1000+ J/kg not that far away in northern TX. The PW anomalies increase with time and reach upwards of 3 standard deviations above normal in the 00Z-12Z/Wednesday time frame, with actual PW values close to 1.50" and near the daily max moving average for FWD and OKC according to SPC's sounding climatology page. This will be reflective of not only the low-level Gulf of Mexico moisture transport, but also the infusion of mid and high-level subtropical moisture from the eastern Pacific Ocean which is showing up in a well-defined manner in the global models. The convection that does develop will likely be capable of producing rainfall rates as high as 1 to 1.5 inches/hour given the relatively efficient moisture transport regime and available instability, and the guidance supports the potential for multiple rounds of convection that may repeat/train over the same area. CAMs suggest the potential for maximum rainfall amounts upwards of 3-4" during this timeframe, with the greatest chances for these higher amounts across parts of central and eastern OK. 00z HREF has 30-40% neighborhood probabilities of exceeding 3" here. The only minor trend with recent guidance is to initiate convection farther to the southwest over parts of northern TX and limit the heavy rainfall potential over southwest MO. Additionally, these intense rainfall rates may fall over some relatively sensitive soil conditions as a result of the most recent rainfall event. The latest NASA SPoRT soil moisture anomalies in the 0-40 cm layer are running high (locally in the 80 to 95 percentile range). USGS data also shows streamflows running high along the OK/AR border and over into the Ozark Plateau. Thus, given the sensitive antecedent conditions, and heavier rainfall potential for the latter part of this period, a Slight Risk has been maintained with only minor adjustments to the Marginal Risk area. Scattered areas of flash flooding can be expected within the highlighted area. Snell Day 2 Valid 12Z Wed Mar 08 2023 - 12Z Thu Mar 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF ARKANSAS, SOUTHEASTERN OKLAHOMA, AND PORTIONS OF NORTH TEXAS... ....2030Z Update... Some fairly significant changes were made with this afternoon's update in coordination with the TSA/Tulsa, OK and LZK/Little Rock, AR forecast offices. The model trends continue to decrease the forecast of storm total rainfall across the Slight Risk area. While there remains some overlap from the Day 1 Slight Risk area, most of the expected rainfall is largely offset to the southeast from the Day 1 Slight Risk area. Despite anomalous atmospheric moisture expected over the area, the CAMs and longer range models for this time frame remain in disagreement with each other as regards the potential for training convection and the tracks of the convective activity. Regardless, the continued southward and drier trend, with widespread rainfall totals closer to 1 inch and lowering chances for localized amounts much above that, the flooding potential has decreased enough that a Moderate Risk for flash flooding was no longer needed. Despite the highly saturated soils, the addition of only about an inch of rain, that may be separated into multiple rounds of fractions of an inch of rain should allow those soils to handle this little bit of extra rainfall. FFGs are over 2"/1 hr and close to 3"/3-hrs in the Slight Risk area, which is now considered unlikely to occur outside of an isolated instance from a stronger thunderstorm. Wegman ....Previous Discussion... The heavy rainfall threat from day 1 extends into day 2 and potentially overlaps with regions that contain sensitive antecedent soil conditions, which may exacerbate the possibility of numerous instances of flash flooding. Relatively organized convection is expected to be ongoing across AR and southeast OK early Wednesday morning as the shortwave energy responsible advances to the east across the lower/mid Mississippi Valley. This convection should weaken and become less widespread by midday as instability lacks downstream. This should yield a lull in activity until the low-level jet reloads by Wednesday night. Anomalous PWs (above the 90th climatological percentile) will be readily available as convection develops in response to sufficient 850 mb inflow and MUCAPE values upwards of 1000 J/kg. A lingering stationary front will also remain in the area and drape from the lower Mississippi Valley to northern TX, likely sinking southward some during the day. However, a stronger shortwave crossing the Four Corners region should eject into the High Plains by Thursday and aid in increasing ascent over the Southern Plains, which will support convective development to the north of the front and spanning from southeast OK to central AR. The Moderate Risk area incorporates a region where guidance highlights both the greatest chances for additional rainfall amounts in the 1-3" range and the potential for 1-1.5" per hour rainfall rates, while also overlapping with saturated ground conditions. This rainfall, combined with the previous day convection, could yield areal averages of 3"+ in the 48-hr period. Given the antecedent conditions (AHPS 14 day precip departures are 200-300 percent of normal and NASA SPoRT soil moisture percentiles in the 10-40 cm layer are above 80 percent in areas) and likely overlap of heavy rainfall areas, the spatial coverage of flash flooding impacts should be enough to support the maintained Moderate Risk. High-end impacts may be highly dependent upon the convection overlapping with the susceptible soils and CAM trends (as well as the outcome of Tuesday nights rainfall) will be closely monitored to determine if the outlook area remains viable. The broader Slight and Marginal Risk areas remain in decent shape and incorporate locations where more isolated-to-scattered flash floods are likely along the modest forcing stretching just to the north of the frontal boundary. Eastern regions remain most susceptible due to wet antecedent conditions, while the potential for higher rainfall rates may be closer tied to the better instability over northern TX and near the Red River Valley of the South. Snell Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!51mKGa8M_7PJKIuOU638cERJhjVeo2uJiY2qnRDcEE-0= S8UXz5EU9rTW4J7uNHBcaoghLhvQm18G6knXHq9rzTRwc_w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!51mKGa8M_7PJKIuOU638cERJhjVeo2uJiY2qnRDcEE-0= S8UXz5EU9rTW4J7uNHBcaoghLhvQm18G6knXHq9rkvG0bfQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!51mKGa8M_7PJKIuOU638cERJhjVeo2uJiY2qnRDcEE-0= S8UXz5EU9rTW4J7uNHBcaoghLhvQm18G6knXHq9rz32AaVo$=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) .