Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 05 2023 08:28:48 FOUS30 KWBC 050828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Sat Aug 05 2023 Day 1 Valid 12Z Sat Aug 05 2023 - 12Z Sun Aug 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST/NORTHERN PLAINS... ....Northern Plains... A Slight Risk remains in place across parts of the Upper Midwest/Northern Plains as a vigorous vertically-stacked low makes its way eastward across much of the Northern Plains as a compact but vigorous vertically stacked low makes its way across the Dakotas and into Iowa and southern Minnesota by late this evening. An axis of 1.75 inch precipitable moisture plume will continue to wrap into the system...helping provide a sounding profile favorable for heavy rainfall and rainfall rates. Given operational model deterministic amounts...3 to 5 inch amounts somewhere in the Slight Risk area seems plausible and the HREF continues to be bullish. So some places may meet the criteria for Moderate risk area...but soils in the eastern Dakotas have been dry, which may allow for more efficient absorption of the heavy rain and instances of which are expected to be widely scattered. Saw little reason to alter the Slight Risk area that was extended as far south as Kansas City. ....Northern IL and northwest IN... A separate compact low that that has been moving across Missouri into far western Illinois during the overnight hours will continue to make its way eastward today...bringing the potential for several inches of rain along its path. Being small and compact...its slow forward motion is expected to continue with the risk of heavy to excessive rainfall as it taps deep moisture over portions of Illinois and Indiana.=20 ....Coastal S. Alabama and the western FL Panhandle... Maintained the Marginal Risk area introduced earlier...but downward trend noted in deterministic model QPF and the ensembles pointed to removal of the Slight Risk area. Convection is still expected to form along the sea breeze and an old frontal boundary...so this hardly telegraphs a dry forecast. But flooding or run off will be mainly a concern in urban areas and low lying areas with poor drainage.=20 Bann Day 2 Valid 12Z Sun Aug 06 2023 - 12Z Mon Aug 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MISSISSIPPI VALLEY... After a major shift in the guidance that necessitates big changes in the excessive rainfall outlook for Sunday...the 00Z suite of numerical guidance did a better job of maintaining run to run continuity with the day-shift runs. As a result...changes to the new Day 2 outlook were pretty small. There were enough changes to warrant a subtle westward shift in the 2- and 3-inch contours from the SREF and EC Ens to warrant a nudge in that direction and a nudge southward given the model QPF showing less QPF up in the Minnesota Arrowhead and a bit more extending southeastward into Illinois along a region of isentropic lift. As previously mentioned, the shortwave energy will remain rather slow moving, as it will be somewhat cutoff from the broader flow between the 2 jet streams, so the surface low is also expected to continue a slow-movement setting up the potential for heavy to excessive rainfall along its path by late Sunday night/early Monday morning. Bann Day 3 Valid 12Z Mon Aug 07 2023 - 12Z Tue Aug 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN GREAT LAKES MONDAY... The shortwave responsible for heavy to possible heavy rainfall on Sunday will continue to move slowly from Illinois or northwest Indiana at the start of the period skirting the southern shore of Lake Erie by later Monday evening. Precipitable water values around 1.75 inches...with isolated embedded maximum values of 2 inches...will be in place as the shortwave moves across the area.=20 A stripe of several inches of rainfall is expected along its path...which implies locallu higher amounts. Placed a Slight Risk area where confidence was greatest. The question became how much QPF reaches parts of the Northeast U.S. and the Mid-Atlantic.=20 Some ingredients may be in place by then...but the spread in the model guidance was large enough to preclude one at this time but the area was covered with a Marginal for the time being. ....Northwest... Maintained the presence of a Marginal risk area in the northern Rockies as a minor shortwave trough progresses inland on Monday.=20 WPC deterministic guidance was near the mid-range of the model QPF. Nearly an inch of liquid precipitation in the complex terrain may be problematic...and the main question will be the amount of instability and whether or not the rainfall rates will be sufficient for run-off problems.=20 Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4c8PVA-IyhsOgokAfxj2-78EZuai62xpjBuFjN6gRW8t= r_oqd6LyldStcow3b-O4xeEQdvWE_tybIyMn015FtD6aG04$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4c8PVA-IyhsOgokAfxj2-78EZuai62xpjBuFjN6gRW8t= r_oqd6LyldStcow3b-O4xeEQdvWE_tybIyMn015F2bzFZkM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4c8PVA-IyhsOgokAfxj2-78EZuai62xpjBuFjN6gRW8t= r_oqd6LyldStcow3b-O4xeEQdvWE_tybIyMn015FIbnSqRg$=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. 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