Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Sep 03 2023 09:07:00 FOUS30 KWBC 030815 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 AM EDT Sun Sep 03 2023 Day 1 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... ....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 Sun Sep 03 2023 - 12Z Mon Sep 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN ROCKIES... ....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 Mon Sep 04 2023 - 12Z Tue Sep 05 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS AND UPPER MIDWEST... ....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. 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