Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 03 2023 00:58:08 FOUS30 KWBC 030058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Sat Sep 02 2023 Day 1 Valid 01Z Sun Sep 03 2023 - 12Z Sun Sep 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST AND GREAT BASIN... 0100 UTC Update -- Outside of minor tweaks to the ongoing Slight and Marginal Risk areas, have also hoisted a Slight Risk across central and north-central portions of NV. Increasingly favorable upper difluence ahead of the anomalous upper low off the CA coast, along with the equally anomalous thermodynamical profile (MUCAPEs 1000-2000 J/Kg, PWs ~1.0") is favoring an elongated band of convection with cell training becoming more likely. Hurley Day 2 Valid 12Z Sun Sep 03 2023 - 12Z Mon Sep 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE GREAT BASIN AND INTERMOUNTAIN WEST... ....20Z Update... ....Intermountain West... The inherited Slight Risk area was expanded well to the north and west with this update. An upper level trough will become negatively tilted as the low at the base of the trough moves northeastward on its way to rejoining the main jet stream over Canada. Atmospheric moisture anomalies are considerable, as much as 3 sigma above normal, with PWAT values approaching 1 inch. Finally, instability will range between 500 and 1,000 J/kg, with locally higher amounts. The combination of these ingredients will result in widespread development of showers and thunderstorms, capable of producing rainfall amounts to 1.5 inches per hour in those with the stronger cores. With the HiRes guidance fully in through the period, most of the guidance shows training showers and thunderstorms developing over northeastern NV, perhaps as far west as the eastern foothills of the Sierras, then training along that corridor, highlighted by the southwestern-most extension of the Slight Risk area, through south-central ID and into the mountains of southwest MT. Additional storms will track across the southeastern-most corner of ID and into the Tetons and Yellowstone of northwestern WY. Overnight Sunday night, the upper level low will open up into a trough as shortwave energy at the base of the trough shears northeastward, along with a renewed area of moisture moving into the ID/MT border region. This in turn will keep lighter, but still steady rain ongoing across central and southwest ID and southwestern MT, with embedded convective elements still possible. Widespread 1 to 1.5 inch rainfall totals with locally higher amounts could result in widely scattered instances of flash flooding overnight. ....Central Gulf Coast... A weak but not insignificant signal for locally heavy rainfall persists along the immediate Gulf Coast from as far east as Pensacola over to central LA. The greatest threat for localized flash flooding, most likely over any urban areas will shift westward with time, following the slowly retrograding cutoff low to the west. Much of LA will see the greatest chance of isolated flash flooding with any storms hold off until the overnight hours Sunday night. A small Marginal Risk was introduced with this update. Wegman Day 3 Valid 12Z Mon Sep 04 2023 - 12Z Tue Sep 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN ROCKIES... ....20Z Update... No major changes were made to the inherited Slight Risk area with this update. The shortwave trough from Sunday will continue tracking northeastward, which should enhance precipitation along the WY/MT border through the day, with the rain continuing into the overnight into the Dakotas, especially ND. The 1 to 2 inches of rain, locally higher, centered over southwestern ND should be largely beneficial rainfall for that area, due to very dry antecedent conditions, so little in the way of runoff is expected in this area. Thus, the Marginal was maintained for any locally heavy/training storms that may occur overnight Monday night. The Slight was expanded a bit towards the west in the Day 3 ERO to overlap more with the Day 2 area, as the rainfall from Sunday night persists in the ID/MT/WY area well into Monday. Wegman ....Previous Discussion... ....Northern Rockies and Northern Plains... The approach of the mid-level shortwave energy through the Intermountain West and Northern Rockies will provide the large scale forcing for ascent across the region Monday into Monday night. Meanwhile, anomalous precipitable water values lingering from the surge of deep monsoonal moisture will be found across much of northern Wyoming through southern/southeast Montana, characterized by PWs near 1" to locally 1.25", which is nearly 2.5-3 standard deviations above the climatological mean. During the peak heating, 00Z guidance does show a signal for an axis of 500+ J/kg of MLCAPE to develop to help promote stronger cores and more intense rain rates. Overall, a widespread moderate rainfall event is likely with embedded stronger cores across far northern Wyoming and southern Montana where localized 1-2" totals for the 24-hr period will be possible and this may lead to localized/isolated instances of flash flooding. Further east, low pressure ejecting out into the Plains Monday night into early Tuesday will promote thunderstorm development across the northern Plains (northern South Dakota through central North Dakota). A few stronger thunderstorms repeating over the same area will be possible and may lead to isolated flooding impacts. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8kjMgS8hTv6dLdZRRpuj0ppa_O-2d1vbaKpm1Rxv-kje= 19uKtvLf-XiahFwNSVBg6fymdgfPY0rvrdij1L3ZZK77ikU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8kjMgS8hTv6dLdZRRpuj0ppa_O-2d1vbaKpm1Rxv-kje= 19uKtvLf-XiahFwNSVBg6fymdgfPY0rvrdij1L3Zfdlstos$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8kjMgS8hTv6dLdZRRpuj0ppa_O-2d1vbaKpm1Rxv-kje= 19uKtvLf-XiahFwNSVBg6fymdgfPY0rvrdij1L3Z1pIDVxA$=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) .