Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 15 2023 15:56:58 FOUS30 KWBC 151556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sat Apr 15 2023 Day 1 Valid 16Z Sat Apr 15 2023 - 12Z Sun Apr 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ....16Z Update... No changes were made to the Slight Risk area or the immediate surrounding Marginal Risk area along the central Gulf Coast with this update. Showers and thunderstorms are tracking across southern LA and just offshore of the MS Delta as of the time of this writing. These storms should grow upscale as they encounter increasing instability, and thus, an increasingly favorable atmosphere along the Gulf Coast from eastern LA through AL. Further north, the Marginal Risk area in WI was expanded north to include the western Upper Peninsula of MI, as FFGs are also very low here, and rivers are running very high, with numerous river gauges reporting flooding largely due to snowmelt. The 1/2 to 1 inch of rain forecast through tonight will worsen already ongoing flooding in this area, such that even these low amounts of rain have the potential to cause localized flash flooding. ....Mid-Atlantic... HREF signals for flash flooding remain elevated in the Marginal Risk area, particularly over eastern PA and NJ this afternoon. Instability being drawn northward ahead of the upper low as well as through daytime heating will support strong thunderstorms capable of producing 1-1.5 inch/hour rainfall rates at times. While this area has been exceptionally dry recently, this amount of rain falling in areas with terrain have the potential to result in isolated flash flooding. This area was trimmed eastward to remove much of western PA and parts of the Southern Tier of western NY, as instability that far west will be limited. Wegman ....Previous Discussion... ....Central Gulf Coast... A strong shortwave trough currently streaming through the southern Texas early this morning is expected to fire up a round of convection that tracks generally along the Gulf Coast from southeast Texas through southern Mississippi and the northwest Florida Panhandle beginning later this morning. The environment is expected to be characterized by anomalous PWs, reaching 1.5-1.75" particularly from southeast Louisiana through southern Mississippi and the northwest Florida Panhandle. Combined with an axis of higher instability, intense rain rates approaching 1-2"/hr will be common as the line moves through. There is some potential for training or backbuilding with some components of the embedded line segments oriented more parallel to the mean flow. The 00Z HREF probabilities show potential for localized 2-3" totals, which may impact the urban and more vulnerable areas of New Orleans to Mobile. Some recent heavier rainfall events have primed the soils, with the 0-40 cm layer soil moisture percentiles running above 70-80 percent, contributing to the flash flood potential. ....Mid-MS Valley to Upper Midwest... Shortwave energy coming out of the central Rockies today will help deepen an area of low pressure over the Plains that then tracks toward northern Illinois by early Sunday morning. The air mass along/ahead of the advancing frontal boundary will be characterized by modest moisture (PWs > 1") and with peak heating today, a line of showers and thunderstorms is expected across portions of southern Iowa through Missouri into Arkansas. Here, isolated instances of flash flooding will be possible due to intense rain rates of 1" to locally 2" in an hour. Further north, as the low lifts toward northern Illinois, a steady/prolonged rain is expected from portions of eastern Iowa into Wisconsin. A few rounds of moderate to locally heavy rainfall will be possible. For areas further north into Wisconsin and the U.P. of Michigan, ongoing snowmelt flooding could induce additional flooding more easily. Taylor Day 2 Valid 12Z Sun Apr 16 2023 - 12Z Mon Apr 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST AND SOUTHEAST FLORIDA... ....Upper Midwest and Michigan... Low pressure will continue to drift north/northeastward through the region as the mid/upper level energy begins to close off by Sunday evening. Along/downstream of the 700 mb low, an axis of heavier precipitation is expected to set up across portions of Wisconsin, initially on the warm side of the system, before much colder air crashes eastward, leading to a changeover to rain/snow or all snow. The impressive dynamics/lift will help support a narrow band of intense precipitation, likely over areas that are somewhat more susceptible to flooding given the snowmelt concerns and previous day rainfall. The latest guidance suggests potential for another 1-2" areal averages across portions of central/eastern Wisconsin, the U.P. of Michigan, and northern L.P. of Michigan which may lead to isolated instances of flash flooding. ....South Florida... Deeper convection is expected to initiate along existing sea breeze convergence ahead of an approaching cold front slated to arrive later in the period. The latest guidance shows anomalously high moisture pooling in the region, upwards of 1.75", which when combined with the daytime heating/sea breeze interactions, several clusters of strong thunderstorms capable of producing intense rain rates will be possible. Some of the area is still susceptible/vulnerable due to recent rainfall so this additional threat of heavy rainfall may lead to flash flooding over the urbanized corridor of southeast Florida. Taylor Day 3 Valid 12Z Mon Apr 17 2023 - 12Z Tue Apr 18 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9_MKQbcJ1lOyn8loO3oyTQxMhZSYUT6PwBwveeWGnTsa= yDYzGClLw21J-b-IYcvAa-S0YeBZCqJWORSQTbeThGF1Els$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9_MKQbcJ1lOyn8loO3oyTQxMhZSYUT6PwBwveeWGnTsa= yDYzGClLw21J-b-IYcvAa-S0YeBZCqJWORSQTbeTm_zK4NA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9_MKQbcJ1lOyn8loO3oyTQxMhZSYUT6PwBwveeWGnTsa= yDYzGClLw21J-b-IYcvAa-S0YeBZCqJWORSQTbeTPN0pRPM$=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) .