Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Feb 08 2023 01:01:26 FOUS30 KWBC 080101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 800 PM EST Tue Feb 07 2023 Day 1 Valid 01Z Wed Feb 08 2023 - 12Z Wed Feb 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF WESTERN ARKANSAS, SOUTHEASTERN OKLAHOMA, AND NORTHEAST TEXAS... The Slight risk looks on track across portions of northeast TX into southeast OK and western AR. Persistent forcing over the area will support a prolonged period of showers and embedded heavier thunderstorms through tonight. Strong low level frontogenesis and moisture convergence, combined with an approaching closed mid level low and an upper jet to the north, will provide this persistent upstream forcing. Meanwhile moisture will be plentiful, with PWs already as high as 1.5" per GPS, which is around the climatological 99th percentile. The environment is also pretty favorable for repeat activity...with the lower level convergence not moving all that much, deep layer winds generally aligned with this convergence axis, and the mid/upper forcing persistent/increasing with time tonight. The main limiting factor for excessive rainfall is probably instability. We do have 500-1000 j/kg east of the low level front...however most of the convection is expected right along or on the cool side of the front. This activity will still have some elevated instability to work with...but probably more on the order of 500 j/kg or less. This should be enough to get hourly rainfall locally up towards 1"-1.25", but probably not much above that. Recent HRRR runs and the 18z HREF are pretty well aligned in favoring far northeast TX into southeast OK and western AR for heaviest rainfall through 12z. Neighborhood probabilities of exceeding 3" through 12z are 30-60% across this area, and recent HRRR runs also support localized 2.5-3.5" totals. We were able to trim back some on the west and northwest side of the Marginal and Slight risk with this update as confidence increases on the corridor most favorable for heavy rainfall. Rainfall rates high enough to potentially cause flash flooding should stay confined near and just north of the front where enough instability will persist. Soil moisture and streamflows are generally neutral across this area. Generally near normal values suggest these ground conditions won't aid or hamper flood potential compared to a typical Feb day. Soil/stream conditions are a bit more sensitive as you get closer to the ArkLaTex region...but through 12z the heaviest rainfall is expected to generally stay northwest of these more sensitive areas. So overall the hourly rates seem unlikely to exceed FFG across the area. But the persistence of the rain mentioned above should begin to result in 3hr and 6 hr FFG being approached and locally exceeded as we go through the overnight hours. The greatest threat is across the Slight risk area, but some localized risk also exists further south over south central TX where activity may be a bit less organized, but higher instability may support some locally more intense rainfall rates. Chenard Day 2 Valid 12Z Wed Feb 08 2023 - 12Z Thu Feb 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF ARKANSAS AND SOUTHERN MISSOURI... ....2030Z Update... Few changes were made for this afternoon's update. The guidance remains in excellent agreement on an area of training convection that will be ongoing at the start of the period across northwestern AR, and extending northeastward into southern MO. Expect multiple rounds of convection through much of the day across this region. While there's great agreement where the heavy rain will occur, there are definite discrepancies on the intensity. The 12Z HREF guidance shows a swath of probabilities over 40% of exceeding 2-year ARIs extending from central into northeastern AR. Soil moisture percentiles from NASA Sport shows soils generally 80-90 percent saturated over the southern half of AR, with quite a bit drier soils in the northwest corner of AR into southern MO. With those dry soils, the biggest changed from the inherited forecast was to trim the northern part of the inherited Slight and especially Marginal Risk areas north of the AR/MO border, as these areas should be able to handle more rainfall before any flooding would occur as compared with areas further south into AR. Soil moisture continues to decline the further north from the AR/MO border one goes. One final cold front pushes all the rainfall to the MS River and points east around 06Z, so the flooding threat from excessive rain will end by then. Wegman ....Previous Discussion... A Slight Risk area has been maintained for the new Day 2 outlook, primarily covering northern Arkansas and southern Missouri (with a small portion in adjoining states). Models continue to show a relatively slow forward speed of the main upper system as it lifts northeastward and takes on a negative-tilt, leading to increased overlap of QPF on Days 1 and 2. The overall setup and forcing remains similar to Day 1, with deep layer southerly flow ahead of the trough increasing tropospheric moisture levels across the MS/OH Valleys, resulting in precipitable water values reaching as high as 1.5-1.8 inches. This increased moisture, along with strong lift and sufficient instability (MUCAPE of 500-1000 J/kg), will result in the potential for heavy rainfall, with areal average rainfall amounts expected to range from 1-3" across the Slight Risk area (and locally up to 3-4 inches in the best overlap areas on Day 1). NASA SPoRT data shows elevated 0-40 cm relative soil moisture values across portions of the Lower MS Valley (as high as the 80-90th percentile across southern AR), while soil moisture values in the Middle MS and OH Valleys are well below normal (as low as the 10th percentile across portions of southern MO/IL/IN and western KY). Churchill/Bann Day 3 Valid 12Z Thu Feb 09 2023 - 12Z Fri Feb 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE WESTERN FLORIDA PANHANDLE, SOUTHERN ALABAMA, AND WESTERN GEORGIA... ....2030Z Update... The inherited Marginal Risk area was adjusted primarily on the eastern side to remove much of the eastern FL Panhandle. A squall line of convection is expected to stall out over the area, with the guidance still struggling, but generally agreeing on a more western (slower) stopping point as compared with inherited. Thus, the Marginal Risk area was also expanded a bit towards the northeast to include the Atlanta metro area on the northern end as training convection is likely to develop starting at the Gulf and riding the stalled front northeast into the Atlanta area. Where the heaviest rain falls and whether Atlanta itself is directly impacted remains to be seen as there is some disagreement to that level. Convection also looks to be significantly weaker compared with previous days, so at this point no upgrades beyond the Marginal Risk are expected. Wegman ....Previous Discussion... A Marginal Risk of excessive rainfall has been introduced for the Florida Panhandle and surrounding portions of AL and GA for the new Day 3 outlook, mainly due to a stalling cold front moving into the region. High tropospheric moisture levels are expected ahead of the front, as indicated by GFS progged precipitable water values of 1.5"+. The presence of sufficient instability (MUCAPE of 500-1000 J/kg) will increase the potential for heavy rainfall, with rates possibly exceeding 2"/hr at times. The possibility of repeating rounds of heavy rainfall is also expected, increasing the threat of excessive runoff potential (with ECENS probabilities for 3" exceedance as high as 5% locally). However, antecedent soil conditions in the region are quite dry, with NASA SPoRT 0-40 cm soil moisture values indicated to be as low as the 10-20th percentile. This should significantly limit the areal extent of any flash flooding, with poor drainage/urban areas the most at risk of realized isolated flooding impacts. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4cPnLnjixzfc2EC6R_M3gqm5S1mu1I6_YGZz9xMHcu5o= N-C0asxf_5Ibw4uytNmU2P8F8of81tqiI97rv7C99AJN8zA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4cPnLnjixzfc2EC6R_M3gqm5S1mu1I6_YGZz9xMHcu5o= N-C0asxf_5Ibw4uytNmU2P8F8of81tqiI97rv7C9bB20eRQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4cPnLnjixzfc2EC6R_M3gqm5S1mu1I6_YGZz9xMHcu5o= N-C0asxf_5Ibw4uytNmU2P8F8of81tqiI97rv7C9us3BTrU$=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) .