Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 04 2023 00:58:27 FOUS30 KWBC 040058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Sun Sep 03 2023 Day 1 Valid 01Z Mon Sep 04 2023 - 12Z Mon Sep 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE GREAT BASIN AND INTERMOUNTAIN WEST... ....16Z Update... Mostly minor edits were made to expand the inherited Slight risk west to include far southeastern OR with this update. This is in line with both current and recent radar trends that show thunderstorms having moved over the area, and the expectation of more widespread shower and thunderstorm development over this area later this afternoon. A northward moving jet streak immediately ahead of a strong trough will help increase upper level divergence and lift in the Slight Risk area. Training thunderstorms have developed across much of northern NV and will continue to grow upscale through the afternoon and evening into ID. Meanwhile a secondary development of training showers and thunderstorms is expected to develop across central and northern UT today, as a jet streak rounding the base of the aforementioned trough greatly increases upper level diffluence ahead of it. As the trough moves overhead late tonight, there will be a period where central UT will be in the highly favorable left exit region of the U-shaped jet streak, which maximizes both divergence and subsequent lift. Thus, expect the showers and storms that develop this afternoon with the help of daytime heating/instability to persist, likely with a gradually weakening trend due to loss of daytime heating, right through the night in this area. Burn scars in this region will locally increase the likelihood of flash flooding. The Slight Risk area was also expanded southward to include more of central UT as a result, in coordination with the SLC/Salt Lake City, UT forecast office. No changes were made to the Marginal Risk area along the central Gulf Coast. As expected, thus-far most of the significant thunderstorm activity as stayed just off the coast. With daytime heating, it's possible an isolated storm or two may move onshore and produce isolated flash flooding, so the Marginal Risk remains in effect. Wegman ....Previous Discussion... ....Great Basin to Intermountain West... An active and unsettled day is shaping up across much of the Great Basin and Intermountain West as the deep closed mid-level low currently centered over California begins to move eastward toward Nevada later today and tonight. Ahead of that feature, impressive strong forcing for ascent will be provided by a coupled jet streak over the region. Moisture anomalies remain very high, characterized by PWs 1-1.5", which is nearly 3 standard deviations above the climatological mean. The best overlap of the moisture anomalies and forcing lies across portions of central/northern Nevada northeast through southwest Montana including much of southern Idaho, northern Utah, and western Wyoming. 00Z model guidance shows favorable instability is expected to develop this afternoon, highest across portions of NV (1000+ J/kg MUCAPE) while 500-1000 J/kg is expected across southern Idaho through Utah. The Slight Risk area was largely unchanged from the previous cycle and captures the greatest HREF probabilities for 1" hourly totals (peaks at 20-25 percent this afternoon) and the highest risk of seeing 1-2" totals. Some training and repeating rounds of thunderstorms will be possible, particularly across portions of central Nevada through southeast Idaho resulting in isolated/scattered instances of flooding especially for the vulnerable areas including recent burn scar areas with a secondary area of concern possibly over southern Utah late in the period (after 06Z Monday) though confidence in higher rain rates there is lower. ....Central Gulf Coast... A stationary boundary in the vicinity interacting with a very moist airmass (PWs 2-2.25"+) may lead to additional heavy rainfall along the Gulf Coast again today, prompting localized flash flooding for the more vulnerable urban and low-lying locations. The deep moisture axis will begin to shift west with time today, pinning the best setup for localized heavy rainfall across southeast Louisiana through the far western Florida Panhandle where isolated 2" hourly totals (isolated 2-4" 24-hr totals) will be possible with any convection that develops. Taylor Day 2 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 INTO THE PLAINS... ....20Z Update... A small expansion of the inherited Slight Risk area towards the east to include the counties in and around the MT/ND/SD junction area was included with the update this afternoon. Anomalous moisture to 3 sigma above the climatological mean, with PWATs as high as 1.5 inches will advance eastward ahead of a weakening upper level trough, which will be largely absorbed into the broader jet just north of the border by Monday night. The influx of moisture up the Plains ahead of the trough will lead to higher rainfall totals into the MT/ND/SD junction point area, where totals locally exceeding 2 inches are probable. Despite recent dry weather, it's likely those totals will be seen from thunderstorms that form over that area in the evening hours, then as a surface low develops, is followed by wraparound rainfall and gusty winds overnight Monday night as a surge of colder air races south out of Canada. Elsewhere, the Slight Risk area was largely unchanged following the MT/WY border. Rain will be ongoing in this area Monday morning from Sunday's rainfall, but will translate eastward with time through the day. Lingering rainfall Monday morning should have an easier time producing flash flooding due to the rainfall expected to have fallen from the overnight tonight. Meanwhile further east, the development of strong afternoon thunderstorms will be the driving factor potentially leading to flash flooding. 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 though the bulk of the rainfall is expected to be more beneficial due to the dry antecedent conditions. Taylor Day 3 Valid 12Z Tue Sep 05 2023 - 12Z Wed Sep 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS AND UPPER MIDWEST... ....20Z Update... Very few changes needed for this update. The inherited Marginal Risk was extended southwestward to include northwestern SD, as lingering rain from Monday night will continue into Tuesday. Any resulting flash flooding may be worsened by the continued rainfall into Tuesday morning into this area, though amounts will largely stay under an inch, so only isolated instances of flash flooding are possible. Further east into MN, showers and thunderstorms developing ahead of the main low will be the driving factor for any flash flooding, though much of eastern ND into northern MN have been very dry, so flash flooding will be very isolated and limited to any flood-sensitive, urban (such as Duluth) and poor drainage areas. Wegman ....Previous Discussion... ....Northern Plains and Upper Midwest... Locally heavy rainfall will be possible across portions of the Dakotas through northern Minnesota Tuesday into Tuesday night as the main shortwave trough energy moves out of the Rockies into the northern Plains. Meanwhile, a 130+ kt jet over southern Canada Tuesday will provide further forcing for ascent across the region. Ahead of the cold front, anomalously high moisture will wrap into the system characterized by PWs > 1". A frontogenetical band of precipitation is expected northwest of the low track, generally across north-central SD through north-central MN where localized 1-1.5" totals will be possible. Instability will be limited, particularly across ND, and this may keep rain rates modest. Across portions of MN through WI ahead of the cold front, upwards of 1000 J/kg MUCAPE may develop and lead to a few stronger thunderstorms capable of producing locally 1"+ hourly totals and a few flooding concerns may develop over the more susceptible/vulnerable locations. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!56647aEA6Am-IcQ6yMNFJDWX7d4sm61ecO-7awOB9cW2= U5eHm_Ud0bV2bcUb4Udl5AUy3rwYbgRvpM1bPFs3e3ma9JM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!56647aEA6Am-IcQ6yMNFJDWX7d4sm61ecO-7awOB9cW2= U5eHm_Ud0bV2bcUb4Udl5AUy3rwYbgRvpM1bPFs3_l0BwxI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!56647aEA6Am-IcQ6yMNFJDWX7d4sm61ecO-7awOB9cW2= U5eHm_Ud0bV2bcUb4Udl5AUy3rwYbgRvpM1bPFs3SF4b0jY$=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) .