FOUS30 KWBC 270828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sat Jul 27 2024 Day 1 Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE TEXAS GULF COAST|... ...Texas Coast to Southeast US... Maintained the Slight Risk area with onshore southerly flow of deep tropical moisture off the Gulf set to bring another day of scattered convection capable of producing heavy rain over an area where flash flood guidance has been lowered by several days of moderate to heavy rainfall...with the expectation that the convective organization should be less today compared with previous days. Elsewhere, only modest adjustments were made to the expansive Marginal Risks surrounding the Slight Risk area and eastward to the mid-Mississippi Valley given persistent south to southeasterly flow on the east side of an upper level trough/closed low. The trough/upper low could lead to enough upper support to focus and sustain storms that produce localized downpours...but even convective initiation along local outflow boundaries will be forming in an environment supportive of downpours from the Mississippi Valley into parts of the Southeast U.S...so isolated instances of multi- inch rainfall totals are quite possible. ...Southwest United States... Signals from the 27/00Z suite of numerical guidance continues to show decreasing threat of excessive rainfall as mid level flow increases across the Intermountain region. The exception remains over parts of Arizona and New Mexico where enough moisture/instability linger in a region with meager southwesterly steering flow...so will be maintain at Marginal risk area. ...Upper Midwest... Maintained the Marginal Risk area mainly over northern Minnesota as given the presence of increasing southwesterly low level flow feeding an airmass with precipitable water values locally in excess of 1.75 inches ahead of a cold front making its way across the area. Given spread in forward speed of the front...expanded the Marginal Risk area but largely maintained continuity in terms of the area covered. Bann Day 2 Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST... ...Northern Minnesota... Rain associated with a cold front moving across the northern tier of the country should be ongoing across northern Minnesota. While there is likely to diminish in intensity for a period early in the period...there should be a renewed risk of excessive rainfall later in the day and evening. With precipitable water values increasing to 1.75 inches or greater continuing to be fed into the region by south to southwesterly 850 mb flow with divergence aloft being provided by the right entrance region of an upper level jet...the 27/00Z HREF neighborhood probability values of 10 to 15 percent for 2+ inch amounts in an hour seem pretty reasonable. Assuming that the GFS was too fast with the forward speed of the front during the period...he risk of training cells and repeat convection are additional concerns for excessive rainfall in addition to the intense rainfall rates. ...Tennessee Valley and Surrounding Areas... A stationary front turned warm front will track northeastward up the Tennessee Valley through late Sunday night/early Sunday morning. Increasing atmospheric moisture associated with the advance of an airmass originating off the Gulf will allow for potential training and backbuilding of strong storms capable of increasingly heavy rainfall. The surface warm front will likely slow as it approaches the southern Appalachians...supporting slower moving storms that with interactions with the terrain will be capable of flash flooding. With very efficient warm rain processes at work...multiple inch per hour rainfall rates . Urban areas are at a higher risk for localized flash flooding with any repeating storms. ...Southwest US... The risk of excessive rainfall continues to get less conducive with time and gets shunted eastward as mid-level westerly flow strengthens over much of the West outside of Arizona and New Mexico. Maintained a Marginal Risk here given the lingering moisture. Bann Day 3 Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY FOCUSED OVER THE TENNESSEE VALLEY... ...Southeast US... Focus for excessive rainfall shifts eastward and is mainly focused in the central and eastern portion of the Tennessee Valley as mid level height falls approach from the west and precipitable water values at or above 1.75 inches will already be in place. While the height falls are associated with a trough that was beginning to fill...the diffluent flow located above an instability axis should be enough to support downpours from slow moving storms in the afternoon and evening. Elsewhere in the Southeast US...the forcing remains weak and nearly impossible to time with any accuracy at this range but localized downpours from any convection that can develop and was covered by an expansive Marginal Risk area. ...Upper Midwest to far Western Great Lakes... The same cold front helping to focus heavy to potentially excessive rainfall on Sunday/Sunday night will continue to make its way eastward on Monday. The pattern becomes somewhat less favorable for heavy to excessive rainfall but the potential for some isolated enhanced amounts linger...especially from far northern Minnesota into the western Great Lakes where a Marginal Risk was placed. ...Southwest US... Steering flow in the mid and upper levels begin to weaken as heights begin to rise while precipitable water values linger at or above an inch (even 1.25 inches in places over the desert)...meaning a slight westward expansion in the Marginal risk area over portions of Arizona and New Mexico compared with the previous couple of days. Bann 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