Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 15 2024 20:08:16 FOUS30 KWBC 152008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Mon Apr 15 2024 Day 1 Valid 16Z Mon Apr 15 2024 - 12Z Tue Apr 16 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST NEBRASKA INTO SOUTHEAST SOUTH DAKOTA... 16z Update: Went ahead with a targeted Slight risk upgrade across portions of northeast NE into southeast SD for tonight into Tuesday morning. Convection is expected to periodically train/backbuild near and just north of the stationary front across this region. Enough instability is forecast to support hourly rain up to about 1.5"/hr, with totals through 12z Tuesday locally exceeding 3" within/near the Slight risk area. This should be enough to result in an isolated to scattered flash flood threat. Given the dry antecedent conditions and low streamflows this is likely a lower end Slight risk...but excess runoff and flash flooding is possible in urban areas. Even outside urban areas the dry ground and lack of green vegetation does suggest that rainfall rates may overcome the infiltration rate in areas...resulting in at least isolated flooding concerns. Chenard ....Previous Discussion... A deepening low and amplifying jet will pivot into the Upper Midwest creating broad areas of ascent while increasing low-level southerly flow from the Gulf of Mexico. Precipitable water values will surge to 1.25/1.5 over the Plains with moisture anomalies peaking around 2.5 standard deviations. The environment will be conducive for convection to develop ahead of the front that will be capable of multiple hazards like very large hail, destructive winds(possible tornadoes) and heavy rainfall. The nature of these storms will be progressive which may somewhat limit the threat for flooding however with the support of mixed-layer CAPEs of at least 1000-2000 J/Kg over the southern portion of the Marginal Risk area, there will be a higher probability of more intense 1-3hr rainfall rates compared to areas farther north. Higher FFGs (lower soil moisture percentiles lower) over the southern portion of the outlook area will maintain a Marginal or localized flash flood risk. Meanwhile, lower FFGs/wetter antecedent soils over the northern portion of the outlook area, along with a better chance for more prolonged activity along/north of the surface warm front, will be offset by decided lower deep-layer instability (even elevated). So even here, again the flash flood risk remains marginal. Campbell/Hurley Day 2 Valid 12Z Tue Apr 16 2024 - 12Z Wed Apr 17 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EASTERN PORTIONS OF THE NORTHERN PLAINS SOUTHWARD THROUGH MUCH OF THE MISSISSIPPI VALLEY... 20z Update: We considered a Slight risk upgrade across portions of northeast IA into southern MN and southwest WI. It is here where the heaviest rainfall rates and totals are expected on Tuesday. Enough instability is forecast to support 1-2"/hr rainfall over this area, which will likely exceed 1hr FFG in spots. One limiting factor for flash flooding will be the expected quick movement of cells, limiting the duration of heavy rates. Some periodic training/backbuilding is possible near the warm front, which supports some areas exceeding 3" of total rainfall. However HREF 5" exceedance probabilities drop to near 0, likely indicative of the overall progressiveness of the system. Antecedent conditions and streamflows are also generally below average across most of the region. Thus despite the likelihood of some 1hr and 3hr FFG exceedance, the coverage and magnitude of impacts are expected to remain below Slight risk levels. The main threat will be to urban areas, where the intense rates may lead to localized flash flooding. Outside of urban areas the dry ground and limited green vegetation may allow for some excess runoff, however these impacts are expected to remain isolated as well. After coordination with=20 local WFOs, the consensus was to stick with just a Marginal risk=20 for now. There may be an ongoing flash flood threat over southeast SD to begin the period, but will let the day 1 Slight risk over that=20 area handle this threat for now. Pending observations later=20 tonight we may need to extend the Slight into this day 2 period. Otherwise, did expand the Marginal risk into WV, where some slow moving convection near a warm front may result in localized heavy rains over an area that already has elevated soil saturation and=20 streamflows. Chenard ....Previous Discussion... The closed low and trailing cold front will advance from the High Plains into the Upper Mississippi Valley while an west-east orientated warm front extends across the Upper Midwest to the Mid- Atlantic region. An axis of anomalous precipitable water values up to 2 standard deviations above the mean, will persist along and ahead of the well defined cold front while pooling along the warm front. Organized convection will likely fire up in proximity to this anomalous moisture axis. Like the previous day, the convection is expected to be fairly progressive that continues to limit the potential for heavy rainfall and associated areas of localized flooding. Locations along and north of the warm front have the greater potential for organized precipitation to span from the Northern Plains to the Upper Mississippi Valley/Upper Midwest. Heavy rain will likely spread across locations north of the warm front and maintain for longer durations as the moisture surging northward from the Gulf of Mexico over the frontal boundary persists. Consensus suggests broad areal averages of 1 to 2 inches with locally higher amounts certainly possible. This region has been relatively dry over the past few weeks, resulting in dry soils and below average stream flows. Campbell Day 3 Valid 12Z Wed Apr 17 2024 - 12Z Thu Apr 18 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT LAKES, OHIO VALLEY AND WESTERN PARTS OF THE NORTHEAST... 20z Update: Only minor changes to the inherited Marginal risk. Convection moving across the OH Valley will be strong and capable of producing high rainfall rates, but should generally be progressive in nature. There is less instability with eastward extent, which should result in lower rainfall rates. But given the above average soil saturation and streamflows over portions of PA and NY the Marginal looks good. Chenard ....Previous Discussion... Showers and thunderstorms will shift east as the closed low and fronts advance through the Ohio Valley toward the Mid-Atlantic. Much of the guidance has 1 to 2 inches for this period for parts of the Great Lakes, eastern Ohio Valley and portions of the Northeast. This part of the country has much lower FFG guidance from antecedent rain thus are more sensitive to additional rainfall. A Marginal Risk spans from Michigan/Wisconsin south to Kentucky and east into western New York, Pennsylvania and Maryland. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_IaKb8_FVdTWFGFqGxU51fBk0lLHbkvu1GxqOcD6jsSY= wUVxFO9GA7xS21pHf7jOQKIX7D_GECLwGj8yg4sv22nyd-g$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_IaKb8_FVdTWFGFqGxU51fBk0lLHbkvu1GxqOcD6jsSY= wUVxFO9GA7xS21pHf7jOQKIX7D_GECLwGj8yg4svBZ7c1GI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_IaKb8_FVdTWFGFqGxU51fBk0lLHbkvu1GxqOcD6jsSY= wUVxFO9GA7xS21pHf7jOQKIX7D_GECLwGj8yg4sv3zYeH1A$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .