Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Oct 11 2023 15:55:33 FOUS30 KWBC 111555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Wed Oct 11 2023 Day 1 Valid 16Z Wed Oct 11 2023 - 12Z Thu Oct 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF FLORIDA AND THE MIDWEST... ....16Z Excessive Rainfall Outlook Discussion... No changes needed in either area. Even though there was a fairly broad shield of rain across the Gulf coast states...the better instability and higher rainfall rates remain off-shore. Maintained the Marginal Risk area in Florida given its proximity overlap of better moisture with a slow moving warm front located to the south. The 12Z HREF still highlights the area in its probability of 3 or more inches of rainfall through 12Z Thursday.=20 Overall...though...suppressed rainfall rates should result in this being a more beneficial rain than a flash flood threat. Across the Midwest....things are expected to remain fairly quiet in the immediate term but then the threat for heavy rainfall should increase by evening as moisture gets drawn northward and rides up/over a front by a strengthening closed low to the west.=20 The 12Z run of the HREF was comparable with the previous run and maintained the highest probability of higher rainfall amounts from western Iowa to northern Illinois. So only minor adjustments were needed, Bann ....0830Z Excessive Rainfall Outlook Discussion... ....Southeast... A surface wave developing over the western the Gulf of Mexico this morning is forecast to track quickly east-northeast across the northern Gulf to the Southeast Coast this period. Deep southwesterly flow ahead of the wave will feed tropical moisture into a boundary lifting north through the northern Gulf. Guidance shows the northern edge of 2+ inch PWs lifting across Florida and the central Gulf Coast into southern Georgia. While there remains some model disagreement regarding the timing and amplitude of the system, most guidance agrees that the heaviest amounts will remain offshore along with the greater instability. An axis of relatively heavier amounts is expected near the track of the low, which is forecast to move inland near Apalachee Bay and then track east-north across North Florida early Thursday. Convection developing along a trailing front may contribute to some additional heavy amounts late in the period. The Marginal Risk highlights the area where the HREF shows higher probabilities for accumulations of 3 inches or more. For areas near Tampa Bay and east, the HREF does show some low probabilities for meeting or exceeding 3 hour FFGs, but these appear to be largely influenced by the FV3 which seems to be both a heavier and more southerly outlier in comparison to the other HREF members. Therefore, opted to go with only a Marginal Risk at this point.=20 Farther to the north, and well north of the wave, most models show a stripe of strongly-forced stratiform rain developing from the central Gulf Coast northeastward into central Georgia and southern South Carolina. Expect the lack of instability and high FFGs to limit any impacts across that region. ....Eastern Nebraska to northern Indiana... The threat for heavy rainfall is expected to increase across this area as a strengthening low level jet ahead of a developing closed low in the West begins to support pooling moisture along a slow-moving boundary. As PWs climb to around 1.25 inches, persistent, strong inflow into an area of sustained lift may support redeveloping storms along boundary. The Marginal Risk highlights an area where the HREF shows high probabilities for accumulations of 2 inches or more, with the highest probabilities centered over west-central Iowa and east-central Iowa through northern Illinois.=20 Pereira Day 2 Valid 12Z Thu Oct 12 2023 - 12Z Fri Oct 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF THE MID MISSOURI VALLEY... ....Northern Plains/Mid Missouri Valley/Upper Midwest... A deep cyclone developing over the central Rockies and High Plains early Thursday is forecast to drift east across the Plains and into the mid and upper Mississippi Valley by early Friday. Strong southerly winds ahead of the system will continue to transport moisture into a region of strong ascent afforded in part by favorable upper jet forcing. For most areas the combination of ample moisture and strong forcing, but little instability, will produce an extended period of moderate rains, resulting in locally heavy accumulations but limited impacts. The placement of the Slight Risk area remains largely the same and centered near where greater instability and deeper moisture along the axis of strongest inflow may help to increase rainfall rates and the threat for short-term heavy accumulations and rapid runoff.=20 Training storms may also raise the threat for heavy amounts and excessive runoff across this region. ....Southeast... Low pressure moving from the northern Gulf to the Southeast coast Thursday morning is forecast to move quickly offshore. However, rich moisture and mid level energy embedded within deep southwesterly flow in its wake may continue to produce periods of heavy rainfall across the same portions northern and central Florida impacted on Day 1. Pereira Day 3 Valid 12Z Fri Oct 13 2023 - 12Z Sat Oct 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MISSISIPPI VALLEY/GREAT LAKES REGION... ....Upper Mississippi Valley to the western Great Lakes... The overall setup on Day 3 is largely similar to the one expected on Day 2, except farther east as a closed low tracks from the Plains into the Great Lakes. Similar to Day 2, ample moisture and strong forcing will support an extended period of moderate rains north of the low, but the lack of instability will limit the threat for heavy rainfall rates and flash flooding concerns. A relatively greater threat is expected to center near and ahead of the low, where deeper moisture and greater instability along the low level jet may help to elevate rainfall rates and short-term runoff concerns. Areas that may possibly be impacted with heavy rain on Day 1, including northern Illinois, may see additional heavy rains this period, increasing the risk for flash flooding for those areas. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8rjzMEB58sFkFRQx7fZCay3eIWPg6nX3V4Uvk2BQM_Zw= 8x4FW1efjjuLKkGL49wBW1JgwFDvjhzha2PgokSpX6BIcSU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8rjzMEB58sFkFRQx7fZCay3eIWPg6nX3V4Uvk2BQM_Zw= 8x4FW1efjjuLKkGL49wBW1JgwFDvjhzha2PgokSpdB_Q2iE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8rjzMEB58sFkFRQx7fZCay3eIWPg6nX3V4Uvk2BQM_Zw= 8x4FW1efjjuLKkGL49wBW1JgwFDvjhzha2PgokSprTuH9K8$=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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