Subj : HVYRAIN: Excess Rain D1-2 To : All From : Mike Powell Date : Wed Jun 17 2026 10:42:34 FOUS30 KWBC 170822 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 422 AM EDT Wed Jun 17 2026 Day 1 Valid 12Z Wed Jun 17 2026 - 12Z Thu Jun 18 2026 ....THERE ARE MODERATE RISKS FOR EXCESSIVE RAINFALL FOR THE CHIACAGOLAND REGION AS WELL AS FOR PORTIONS OF THE MISSISSIPPI DELTA... ....Chicagoland... A potent low racing out of the Plains and across the Great Lakes today will run into an increasingly moist air mass in place ahead of the low over the Midwest. As the low approaches the Chicagoland region late morning, convection will break out along the low's warm front extending east of the low center. The initial hit of rain with the warm front is unlikely to feature too much heavy rain, though embedded elevated convection could make for brief heavier downpours. Immediately behind the warm front, however, a surge of moisture and instability will fuel backbuilding thunderstorms during the afternoon ahead of a strong cold front trailing behind the low. These storms are progged by many of the CAMs to generate into several broken lines of storms as they cross northern Illinois and into northern Indiana and southern Michigan through the late afternoon/early evening. These storms will have ample forcing allowing them to organize into severe clusters of storms. They are likely to locally produce rainfall rates of up to 2 inches per hour where they backbuild. However, since the series of line segments will be sufficiently separated by much lighter or no rain, it seems any flash flooding that results from these storms will be more localized and less widespread. Further, as mentioned, the storms will be racing very quickly, likely moving at 40+ mph. This will greatly reduce the time any one area sees heavy rain, but since the lines are expected to align parallel to the windflow, those areas may see a bit longer time of heavy rainfall. Hydrologically, the Chicagoland area has seen several bouts of heavy rain in recent weeks that have left the soils above normal for moisture content as well as prone to flash flooding with sufficiently heavy rain. Of course, the large urban area of Chicago will also help to worsen any flash flooding that develops, so in those local areas where heavy rain overlaps with urban areas, locally considerable flash flooding is possible. Further east into Indiana, eastern Michigan, and eventually northwestern Ohio, the lines of storms this evening should align with the cold front and largely orthogonal to the flow. This should lower the flash flooding risk in these areas, despite similarly favorable hydrology. ....Mississippi Delta... The moisture plume of Potential Tropical Cyclone 1 (PTC1) will continue to slowly advect northeast across the northwestern Gulf and into southern Louisiana today into tonight. For much of the day, the heaviest rains will remain offshore, and except for occasional showers and an isolated storm or 2, much of the area should not contend with much heavy rain. That changes tonight as the center of circulation of what may be Tropical Storm Arthur moves ashore near the TX/LA border. All of the deep moisture will be east of the center of circulation, so that will direct the fire-hose of heavy rain into south-central Louisiana, from the Abbeville/New Iberia area northeast near Baton Rouge and into southern Mississippi after midnight. By that point, the diurnal strengthening of the nocturnal jet will function as the the primary driver of heavy rain as northeastward moving storms backbuild into the jet. The area from around Baton Rouge northeast to the north of Lake Pontchartrain and into far southern Mississippi is likely to be the corridor with the heaviest rain. There is considerable uncertainty first of the eastern extent of the axis of heaviest rain, particularly in relation to the New Orleans Metro. For now most of the guidance makes a very tight gradient of moisture, which most of the city remains on the drier side of. However, this axis of heaviest rain has been shifting east from model run to model run over the last couple days, which any further eastward drift would bring New Orleans right into the corridor of multiple-inch per hour rainfall for multiple hours. Regardless, even if New Orleans misses out on the heaviest rainfall, feeder bands of showers and storms could still cause localized flash flooding in the city. Thus, the Moderate Risk remains largely unchanged and includes New Orleans. More than a couple CAMs do bring strong cells over the city, which would very quickly lead to flooding problems. Wegman Day 2 Valid 12Z Thu Jun 18 2026 - 12Z Fri Jun 19 2026 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... ....Southeast... Very heavy rain associated with what will be the remnants of PTC1 will be ongoing at the start of the period over most of southern Mississippi and likely extending into southwestern Alabama. These storms will be fueled by a strong low level jet pumping deep tropical moisture into a corridor that covers much of the Moderate Risk area. By around midday, the rapidly increasing instability and the typical weakening of the low level jet will allow the widespread convection to transition into training lines of storms that track east with time, as the line gradually drifts south towards the greatest instability and moisture source that is the Gulf. Thus, the greatest focus of heaviest rain now appears to set up from southeastern Mississippi east across the southern tier of Alabama and far western Georgia. 3+ inch/hour rainfall rates from individual storms will combine with multiple hours of rain as the storms train along the east-west oriented lines. Storm total rainfall for the day could easily exceed 8 inches in spots. Mobile, AL will be the biggest city in the crosshairs of the heaviest rainfall. With this rather dramatic southward shift in the guidance of where the heaviest rainfall totals are expected, the Moderate Risk inherited was trimmed significantly on the north side, while expanded across southeast Alabama, the western Florida Panhandle, and far southwestern Georgia. The Moderate Risk was downgraded out of both the Birmingham and Atlanta metros as those areas see leftover heavy rains from the much more potent and heavy rain producing storms much closer to the Gulf Coast. Nonetheless, periodic convection and a long duration of light to moderate rain is still expected over those metros, so the higher-end Slight remains in effect. Convective coverage greatly decreases Thursday night over all of the Southeast, but isolated to widely scattered storms could persist through the night in some areas. ....Ohio/Tennessee Valleys... Further north from the Gulf Coast along the Ohio and Tennessee Valleys, a cold front will start the day following roughly parallel to the Ohio River. The front will slowly sink southward over Kentucky through the day. Meanwhile, daytime heating and unidirectional southwesterly flow ahead of the front is expected to generate a long line of storms. The storms will be training and backbuilding along the KY/TN border and extending into West Virginia. Guidance has shifted a bit further south, especially with the western end of the line, so the higher-end Slight now follows the entire length of the KY/TN border, and now includes Nashville, while the Ohio River has largely been removed from the risk area. With nightfall and the loss of daytime heating, the flash flooding risk from storms will also diminish. ....Arkansas/Oklahoma... On Thursday night, the westernmost extension of the low-level jet over Texas will run into the same stalled out front that extends east into Kentucky. This clash of air masses is likely to result in the development of an MCS over eastern Oklahoma, which will then propagate southeastward into western Arkansas. Here too the soils are wetter than normal from multiple bouts of heavy rain in recent weeks, so a Slight Risk remains in place over the area with few changes. Wegman $$ --- SBBSecho 3.28-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/107) .