Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 02 2023 20:00:01 FOUS30 KWBC 021959 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 359 PM EDT Sat Sep 02 2023 Day 1 Valid 1954Z Sat Sep 02 2023 - 12Z Sun Sep 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST... ....20Z Special Update... The Slight Risk area along the Gulf Coast was removed with this update in coordination with MOB/Mobile, AL and TAE/Tallahassee, FL forecast offices. Any weak convection this afternoon has remained well offshore, with any convection over land expected to remain west of the previous Slight Risk area into MS/LA. Wegman ....Previous Discussion... ....Arizona to southeast Nevada and northward into Utah... Another round of locally strong convection is expected this afternoon/evening across portions of the Southwest into the Intermountain West in response to the mid-level closed low over California helping to channel anomalously high moisture northward through the region. Compared to yesterday, the moisture axis isn't as deep though instability is expected to top out around 1000-1500 J/kg MLCAPE especially across western AZ, southwest UT, and portions of southern Nevada. This should support intense rain rates greater than 0.5-0.75"/hr at times with the strongest cores with a few localized/isolated 1"+ hourly totals possible. Based on the 00Z guidance, localized 1-2" totals will be possible and combined with the heavy rainfall footprint from Friday, some greater sensitivity is likely to be present so the Slight Risk still looks reasonable and covers where 2-day totals may exceed 3-4" in spots over some more vulnerable locations. ....West Coast... Closed mid-level low is forecast to remain nearly stationary over the course of the period, generally across northern California and southern Oregon. The lower heights contributing to steeper lapse rates combined with at least some higher than normal moisture and potential for marginal instability should support some stronger showers and perhaps thunderstorms capable of producing locally intense rain rates greater than 0.5"/hr. This may lead to a few localized flooding concerns particularly for the terrain areas of northern California and around recent burn scars. ....Gulf Coast... The axis of deep moisture, characterized by precipitable water values in excess of 2-2.25", will remain along the Gulf coast, aided in part by onshore flow provided by mid level low positioned over portions of eastern Texas and western Louisiana. The strong onshore flow will overrun a nearly stationary boundary draped west to east onshore and with a steady surge of low level moisture transport, expect steady/repeating showers and thunderstorms through much of the day across the central Gulf Coast. The high PWs and warm cloud depths will support efficient rain producing thunderstorms with hourly totals likely to be 1-2" at times. The best instability will remain generally confined to the immediate coast where model guidance and 00Z HREF neighborhood probabilities continue to suggest 2-3" totals (isolated 3-5+") will be common. Given the favorable setup for training/repeating rounds and very efficient/intense rain rates, a Slight Risk was introduced along the immediate coastal areas lining up with the greatest HREF probabilities for 3"+. The intense rainfall and rain rates may lead to isolated/localized instances of flash flooding for mainly urban and low-lying locations. Taylor 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!-LZJwTvd3jXG6kQvLnMTVWLVnpjeHFtzz1WrDIUYtwxN= wnVazmsxed6hgzzI0tAUKcRnE5PYVnxozP4koyMJ2qew9DM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-LZJwTvd3jXG6kQvLnMTVWLVnpjeHFtzz1WrDIUYtwxN= wnVazmsxed6hgzzI0tAUKcRnE5PYVnxozP4koyMJ9Buf5uw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-LZJwTvd3jXG6kQvLnMTVWLVnpjeHFtzz1WrDIUYtwxN= wnVazmsxed6hgzzI0tAUKcRnE5PYVnxozP4koyMJ0Eha-l0$=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) .