827 FOUS30 KWBC 172013 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 413 PM EDT Wed Jun 17 2026 Day 1 Valid 01Z Wed Jun 17 2026 - 12Z Thu Jun 18 2026 ...THERE ARE MODERATE RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF ILLINOIS/INDIANA AS WELL AS THE MISSISSIPPI DELTA... ...1600 UTC Update... The Moderate Risk in the Midwest has been shifted south based on current observational trends and the latest 12Z hi-res guidance with regards to an ongoing MCS over northern Illinois. This will likely limit the northward progression of the warm front/effective outflow into the afternoon hours bringing the greatest threat for heavy rainfall/flash flooding along a west-northwest to east- southeast axis through central Illinois into central Indiana. This includes both the more intense storms on the southern/southwest end of the ongoing MCS able to tap into greater moisture/instability along the warm front as well as for additional storm development that will focus along the warm front/effective outflow. Repeated development/potential training of robust, heavy rainfall producing storms (rainfall rates ~ 2 inches/hour) leading to totals of 3-5", locally as high as 6-7", will pose a threat for scattered to widespread incidents of flash flooding. Further south, the Moderate/Slight Risk areas have also been minimally adjusted based on the latest 12Z guidance along and east of the track of what is now Tropical Storm Arthur. Plentiful tropical moisture will support additional rainfall totals of 3-5" with localized amounts as high as 6-10". The highest totals look to focus along a corridor within southeastern Louisiana generally between Baton Rouge and New Orleans where the greatest threat for more widespread instances of flash flooding will exist. Putnam Southwest United States... Another round of isolated late day and evening thunderstorms capable of producing locally heavy rainfall is possible. Given low level moisture being capped by broad but fairly weak subsidence associated with an upper high...the expectation is for any convection to develop over or near the complex terrain which would act as localized heat sources. As such...loss of daytime heating should lead to fairly quick dissipation following sunset. Bann ...Previous Discussion... ...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 produce locally heavy 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... ...2030 UTC Update... Only a few minor areal adjustments were made to the inherited risk areas with the prior forecast reasoning remaining on track for the Southeast, Ohio/Tennessee Valleys, and areas further west into Arkansas/Oklahoma. The most notable was to expand the Moderate Risk back northeast a bit to include metro Atlanta. While rain totals in the hi-res guidance tend to be lower overall further north than the noted shift south overnight, the expected track of the remnants of what is now Tropical Storm Arthur will pass very close to the region. This will bring intense low-level flow (50 kts+) and high PWATs (2-3") northward which could lead to some more intense rainfall and heavier totals than currently depicted in the guidance, and at least some of the hi-res runs showed the potential for totals of 3-4". The urban area will also remain more sensitive compared to elsewhere even if totals do not match some of the higher end amounts anticipated further south. Otherwise, the rest of the region was left mostly untouched. The latest 12Z hi-res guidance depicted the potential for significant rainfall totals particularly across southern portions of the Moderate Risk area (southern Alabama into the western FLorida Panhandle), with areal average totals in the 4-5" range and locally higher totals as much as 6-10", more than sufficient for scattered to widespread flash flooding. The HREF and RRFS both showed moderate probabilities (40-60%) for totals of greater than 8 inches. One other adjustment was to extend the Marginal Risk further westward across the southern Plains into portions of the Texas Panhandle and eastern New Mexico where afternoon/evening storms ahead of a sagging cold front could bring a couple heavier downpours and possibly some isolated flooding concerns within cities. Putnam ...Previous Discussion... ...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 Day 3 Valid 12Z Fri Jun 19 2026 - 12Z Sat Jun 20 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN ALABAMA... ...2030 UTC Update... Only very minimal areal changes were made to the inherited risk areas as the prior forecast is consistent with the latest 00/12Z guidance for both the Southeast as well as central/western Texas. Rainfall totals across the Moderate/Slight Risk areas in the Southeast will likely be down overall compared to Thursday as the remnants of what is now Tropical Storm Arthur depart the region. However, the region will remain much more susceptible to additional rainfall given the amounts expected Thursday, particularly for those in the Moderate Risk area across southern Alabama into the Florida Panhandle which overlaps Thursday's risk area. It likely will not take much to lead to additional flash flooding as well as exacerbate any ongoing flooding. Putnam ...Previous Discussion... ...Southeast... Any lull in heavy rainfall across southern Alabama from Thursday night behind the precipitation shield associated with the remnants of Potential Tropical Cyclone 1 will end in a hurry Friday morning as the cold front over the Tennessee Valley continues to press southward and runs into the ongoing southerly flow and low-level jet of deep tropical moisture from off the Gulf. The clash of these air masses looks to maximize over southern Alabama, where storms are likely to continue in some form or fashion for much of the day Friday. Assuming the multiple inches of rain from Thursday occurs in these same areas, the slightly lower storm total rain on Friday will make little difference for the flooding risk, as multiple inches of rain fall over the same areas very hard hit from Thursday. This very good overlap in the storm placement from Thursday to Friday necessitated a risk upgrade across southern Alabama for the Day 3/Friday period. Should this overlap continue to hold and the widespread multiple inches of rain with localized totals to double digits from Thursday occurs as forecast, it's entirely feasible that the hardest hit areas may need further upgrades to the flooding risk. Unfortunately the track record with the CAMs ensembles on maintaining continuity day to day on where the storms will occur is not very good, so any movement breaking the overlap will greatly improve the rather ominous flooding outlook for southern Alabama on Friday. Convective coverage over Alabama should diminish greatly Friday night as heavier rains to the west and the departing system to the east should put the state in a relative ridging pattern, allowing for a much needed break from the rainfall. ...Texas Hill Country... Southeasterly flow of deep moisture from off the Gulf will continue across much of Texas through the Day 3 period. Northwestward moving convection on the leading edge of the deep moisture will collide with storms that form along the dry line and the Mexican mountains Friday evening, resulting in MCS development that turns the storms back towards the best instability to the southeast. This should result in some areas seeing multiple rounds of heavy rain producing storms over the historically flash flood prone region of the Texas Hill Country. Thus, a Slight Risk area remains in effect for the region. Lesser amounts of rain are expected further east into north-central Texas and the Metroplex, so the Slight Risk inherited with downgraded in that area. Wegman Day 4 and Day 5 Valid 12Z Sat Jun 20 2026 - 12Z Mon Jun 22 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OF CENTRAL PLAINS THROUGH THE MID MISSISSIPPI INTO THE LOWER OHIO VALLEY... ...2030Z Update... The daytime runs of the deterministic models and the ensemble guidance continued to focus on the Central Plains on Day 4 and in/near the Mid-Mississippi Valley on Day 5 as having the highest probability for excessive rainfall. Even though there were some run to run adjustments...the placement of the outlook areas were consistent and well covered by the on-going outlook. Few changes were made this cycle. Bann Day 4... A well-defined 500mb shortwave trough will move from the Great Basin into the northern Rockies midday Saturday, reaching the northern High Plains by Saturday night. Ahead of a southward advancing Canadian cold front, a northward-lifting warm front across the central Plains will lead an anomalous moisture plume into an increasing unstable environment. While exact convective placement varies, the deterministic models remain in generally good agreement -- depicting a deepening surface low over the central Plains that will help to focus moisture along the leading edge of a strengthening low level jet. A deepening pool of moisture (PWs increasing to 1.5-2 inches) over central Nebraska/Kansas eastward into southwestern Iowa and western to central Missouri will support efficient rainfall rates. This ample moisture along with large- scale ascent afforded by mid-level shortwave and right- entrance region jet forcing will further encourage heavy rainfall and the potential for flash flooding across the region. Therefore, a Slight Risk was maintained across the region. Farther south, a weakening frontal boundary and residual tropical moisture will trigger additional thunderstorms from South Texas across the Gulf Coast. Localized flooding will likely remain a threat given the increasing sensitive soils. Day 5... Forecast confidence lowers notably heading into the latter half of the weekend as guidance begins to diverge on the track of the leading wave forming over the central Plains on Day 4. Notably, the ECMWF offers one of the more northerly solutions compared to other guidance. Beyond the primary wave's track from the Midwest into the East, considerable model spread exists regarding the timing of the trailing front and subsequent wave and storm development. With the continuation of ample moisture, instability, and strong forcing areas of heavy rainfall and flash flooding appear likely. Given the model spread, the outlook area, including a Slight Risk extending from the lower Missouri to the lower Ohio Valley, relied most heavily on a multi-model ensemble mean. Pereira Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$