Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 02 2023 19:41:20 FOUS30 KWBC 021941 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 340 PM EDT Fri Jun 02 2023 Day 1 Valid 1939Z Fri Jun 02 2023 - 12Z Sat Jun 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR NORTHWEST TEXAS AND PORTIONS OF EASTERN MONTANA... ....Southern Plains... Another active day expected for the Southern Plains as a shortwave ejecting out of New Mexico overlaps with the LFQ of a subtropical jet streak and an advancing dry line to drive impressive ascent. This lift will occur within an extremely favorable thermodynamic environment characterized by PWs of +1 to +2 standard deviations above the climo mean according to the NAEFS ensemble, and a plume of 2000-3000 J/kg MUCAPE being drawn northward from the RGV. The result of this evolution is likely to be rapidly increasing convection which should intensify and expand across the High Plains of NM and organize into an MCS over the TX Panhandle. As convection expands, Corfidi vectors are progged to collapse to just around 5 kts, indicating the potential for storms to backbuilding into the greater instability and then train to the E/NE. Eventually, the complex is progged by the high-res simulated reflectivity to surge eastward this evening, but until that time rain rates of 2+"/hr training across areas could result in locally more than 3" of rain as shown by a good consensus among the HREF/ECENS/GEFS/SREF ensembles, with locally more than 5" possible. This rain will occur atop primed soils from recent heavy rains which have been 300-600% of normal the last 14 days leading to already high USGS streamflows. This has compromised FFG to as low as 0.25-1"/3hrs, for which the HREF indicates has a better than 60% chance of being exceeded. Although the ensemble probabilities have shifted just a bit east, the inherited MDT risk was adjusted only slightly/cosmetically as the antecedent conditions over the Panhandle are more susceptible than areas farther east. This did necessitate some expansion of the surrounding SLGT risk, but the area of greatest concern for more numerous flash floods remains over the TX Panhandle through this evening. ....Northern/Central Rockies and Northern High Plains... A fairly blocked mid to upper level pattern will persist today from the Northern Rockies, eastward through the Northern Plains into the Upper Mississippi Valley and Great Lakes. Very slow moving mid to upper level troughing will persist on the western side of the blocking upper high forecast to remain in place from the Upper Mississippi Valley into the Great Lakes. Within this trough, a wave of low pressure drifting along a nearly stationary boundary across central/eastern MT today will allow increased moisture to rotate into the area. 850mb inflow with a source region of the Gulf of Mexico will increase to 20-30 kts over the Dakotas and then turn eastward into MT, lifting isentropically atop the front, converging into weaker flow to the west, and also resulting in enhanced ascent through upslope. Additionally, an axis of deformation is still progged to pivot over the central part of the state, resulting in prolonged ascent to wring out moisture which will be nearing +3 standard deviations according to the NAEFS ensemble tables. AS MUCAPE rises to more than 1000 J/kg to the east, some of this will resupply into central MT, resulting in thermodynamics that are favorable for rainfall rates that have a greater than 50% chance for exceeding 1"/hr according to the HREF probabilities. The inherited rare (for this part of the country) MDT risk still appears warranted as it overlaps where all the global ensembles indicate at least a low-end risk for more than 3 inches of rain today, with the HREF also indicating a 10-20% chance for more than 5 inches. This will fall atop saturated soils from 14-day rainfall that is generally 150-300% of normal, including pockets of 2-3" of rainfall yesterday according to MRMS. The MDT risk was pulled east just a bit from inherited however to better align the overlap of heavy rates and antecedent soils, but there will likely be numerous instances of flash flooding today. ....Central and Southern Florida... Although Tropical Depression Two will remain well west of Florida, the associated mid-level trough and tropical moisture plume (PWs to 1.75 inches) will continue to plague the peninsula today. A weak surface trough will be a focus for low-level convergence, along which scattered to widespread showers and thunderstorms are once again expected. With favorable thermodynamics in place to support efficient 2"+/hr rain rates, storm motions that are expected to be just around 5 kts based on the 0-6km mean wind will support local rainfall amounts of 2-4 inches, locally higher where any mergers or collisions can occur. This could result in isolated instances of flash flooding, especially if any cells drift across the more urban southeast or southwest coasts. ....Northern New York State and New England... The backdoor cold front that will drop across New England and Upstate New York today is still progged to move through by this evening, with associated showers and thunderstorms developing along it. Overall the area has been quite dry based on 14-day rainfall departures, which has left FFG around 2-3"/3hrs. This suggests that despite a favorable overlap of instability and moisture to support rainfall rates of 1-2"/hr, these dry antecedent conditions will somewhat cap the excessive rain threat. This is also aided by storm motions that should be generally to the south at 15-20kts. However, in some places multiple rounds of heavy rain producing convection could occur, and there are also some more sensitive terrain features across this region. This could result in some isolated runoff or flash flooding instances, and the MRGL risk was broadened to account for Mesoscale Precipitation Discussion #410. See that discussion for more information. Roth/Weiss Day 2 Valid 12Z Sat Jun 03 2023 - 12Z Sun Jun 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS NORTHEAST NEW MEXICO INTO NORTHWEST TEXAS... ....Southern High Plains... The overall stagnant blocked mid to upper level pattern will persist into day 2 across much of the central to western U.S., comprised of a slowly retrograding closed upper high over the Upper Mississippi Valley into the Northern Plains and mean troffing from the Northern to Central Rockies into the Southern High Plains. Models are indicating another upper level difluence maxima sinking east southeastward late Saturday afternoon into evening from northeast New Mexico into northwest Texas. This area of shortwave energy/upper difluence will likely support another round of convection pushing east southeast in what will be a northward surge of instability across this region late Saturday afternoon. While model precip amounts are not as high as day 1, there will be potential for additional heavy rainfall amounts and potential overlap of day 1 amounts across a region that has seen much above average precip over the past few weeks. For these reasons, a small slight risk area was added for the day period over portions of northeast New Mexico into Northwest Texas. ....Southern to Central Plains into the Central to Northern Rockies... No significant changes made to the very broad marginal risk area across portions of the Southern to Central Plains, into the Central to Northern Rockies. An axis of above average PW values will continue across much of these regions between the mean troffing from the Rockies into the Southern Plains and the closed high over the Upper Mississippi Valley and Northern Plains. Model consensus for heavy amounts day 2 across much of the Southern and Central Plains is to the east of the day 1 axis. Given this and higher ffg values across the farther eastward locals in the Southern to Central Plains, the risk level was maintained as marginal. Across the Central to Northern Rockies, model consensus is for lesser qpf amounts day 2 than day 1. While there will be potential for locally heavy rainfall amounts, confidence is low with respect to placement, keeping the threat level across these areas at marginal. In addition, the CSU first guess fields continue to indicate only a broad marginal risk across these regions. Oravec Day 3 Valid 12Z Sun Jun 04 2023 - 12Z Mon Jun 05 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN TO CENTRAL PLAINS INTO THE NORTHERN ROCKIES AND ACROSS THE NORTHERN SIERRA... There is not expected to be any significant changes to the overall flow pattern across the lower 48 day 3. The blocking pattern comprised of the closed high across the Northern Plains/Upper Mississippi Valley and the mean trof extending from the Northern Rockies into the Central to Southern Plains is expected to continue. Model consensus is for a widespread region of scattered convection in what will remain an axis of above average PW values. Similar to the day 1-2 time period, there is not a lot of overlap with the day 2-3 precip areas to support more than a broad marginal. One area that may need to have a slight introduced in later outlooks would again be over portions of the Southern High Plains. Models on day 3, again show potential for another round of convection pressing east southeastward across eastern New Mexico into Northwest Texas where there is expected to be heavy rains days 1 and 2.=20=20=20 No changes made either to the marginal risk area over the northern Sierra. Models do show some enhanced upper difluence moving across this area Sunday afternoon. This will support potential for some isolated heavy totals that may produce isolated runoff issues where stream flows remain high. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_OEwFyLFBaWfceQNJkRR7EDYMiveqaPDJI7jgQZdZ27O= HxPeRjxJGQI8RHHKjkYpkkLSUKHoHfIiD9xbYAf-pUxYpKM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_OEwFyLFBaWfceQNJkRR7EDYMiveqaPDJI7jgQZdZ27O= HxPeRjxJGQI8RHHKjkYpkkLSUKHoHfIiD9xbYAf-SF8fV7Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_OEwFyLFBaWfceQNJkRR7EDYMiveqaPDJI7jgQZdZ27O= HxPeRjxJGQI8RHHKjkYpkkLSUKHoHfIiD9xbYAf-bIHLMzE$=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) .