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FOUS30 KWBC 070810 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 AM EDT Tue May 7 2024 Day 1 Valid 12Z Tue May 07 2024 - 12Z Wed May 08 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY... ...Ohio Valley... The strong spring system will evolve into a large closed upper low over the Northern High Plains while a piece of embedded shortwave energy quickly lifts through the Midwest and lower Great Lakes. Initial decaying line of convection will move into the region later this morning which will likely be fast-moving and have diminishing intensity in rain rates. This will likely set up an outflow boundary across parts of Indiana and Ohio that will then be the focus for additional thunderstorm development in the afternoon/evening hours once convective temperatures and maximum destabilization occurs. Meanwhile, a zone of higher PWs is forecast to develop, characterized by PWs of 1.5+", along the Ohio River. As storms develop later today, they will be moving generally along/parallel to the mean flow with some south/southeast component and the latest hi-res guidance shows some support for training/repeating rounds and the latest HREF neighborhood probabilities indicate hourly totals of 1-2" appear possible. Some of this rainfall may fall over more saturated areas of southern Ohio and northern Kentucky. Given the environmental ingredients, possible 1-2 rounds of convection, and some greater sensitivity in ground conditions, a Slight Risk was introduced across parts of Indiana, Ohio, and Kentucky. ...Upper Midwest and Great Lakes... The impressive line of thunderstorms currently moving eastward across the Plains and Midwest will continue to lift north/northeast this morning into the Upper Midwest, generally working along the leading edge of the warm frontal passage and best isentropic lift. Moisture in the low levels will continue to rise through the period, with an axis of PWs > 1.25" expected from Minnesota southward into Illinois which will support isolated/embedded cores producing heavier rain rates. The latest hi-res guidance does suggest potential for some repeating/redeveloping convection as the flow becomes parallel to the storm motions over Wisconsin, which could produce isolated/streaks of higher rain totals. Altogether, the risk for flash flooding lies in the lower end of the range and the Marginal Risk looks good for this forecast update. ...Northern Plains/Rockies... The closing mid/upper level low over the Northern High Plains will linger through the period, with embedded vort energy pivoting/retrograding westward across the region. This should continue the forcing/lift across the region and with the anomalous moisture in place and orographic enhancement, an additional 0.5-1" is expected and this could cause additional flooding concerns. Taylor Day 2 Valid 12Z Wed May 08 2024 - 12Z Thu May 09 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MID-MS VALLEY, TENNESSEE VALLEY, AND OHIO VALLEY... ...Mid-MS Valley to Ohio Valley... The strong closed low from today/tonight is expected to open up into a large positive tilted trough axis stretching from the Rockies to the Great Lakes. Embedded within that, pieces of shortwave energy will ripple through the Plains to Ohio Valley and be the primary focus for forcing in the mid/upper levels. At the surface, a low pressure will deepen/organize in response to the large scale forcing and track through the Mid-MS Valley to Ohio Valley, pulling a warm front northward into the Ohio Valley by the afternoon. In the warm sector, the airmass will be characterized by dewpoints well into the 60s, perhaps into the low 70s, across the region and along with PWs rising well above 1.5", this should be more than sufficient moisture to work with. And with the increasing height falls aloft and steepening lapse rates, a large area of favorable instability is expected, reaching 1500-2000+ J/kg MUCAPE. With daytime heating and convective temperatures reached, rounds of strong to severe thunderstorms are expected from the Ohio Valley initially that then build/grow along the advancing frontal boundary southwest across the Mid-MS Valley. Compared to earlier issuance, the northern edges of the ERO outlines have been trimmed back some as the latest guidance suggests the warm front may not lift as far north, stalling along the Ohio River. This may keep the greatest threat for training/intense thunderstorms south into KY and into TN tied to the better instability. This is where there is the greatest overlap of forcing, moisture, and instability and the setup for training convection that pushes the ERO risk toward the higher-end of the Slight Risk range. This also lines up also where the antecedent ground conditions are wet with recent heavy rainfall events giving lower/reduced FFGs and more vulnerability to flash flooding. ...Upper Midwest... Compact area of deeper convection is expected to develop along an inverted trough underneath the core of the upper low in the Midwest that will be capable of producing localized heavy rainfall and flash flooding. A narrow axis of higher moisture is expected, perhaps above 1-1.25" from far southeast South Dakota through Iowa and southern Minnesota. Given this greater moisture, broad forcing, and steepening lapse rates, pockets of slow-moving and pivoting showers and thunderstorms are likely. Rain rates may approach/exceed 1-2"/hr at times and while isolated, there is at least some flash flood risk so the Marginal Risk covering the area looks good. Taylor Day 3 Valid 12Z Thu May 09 2024 - 12Z Fri May 10 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST U.S.... ...Southeast... The strong spring system over the Central U.S. will begin to elongate/stretch in a lengthy positive tilted shortwave trough by the start of the forecast period and sink southward toward the region. Earlier convection from the previous day is likely to push outflow boundaries and a cold front through the region while another embedded shortwave trough is expected to move through in the southern stream. Altogether, this should put the region in a favorable region of forcing for ascent. Deeper moisture is expected to pool along/ahead of the surface boundaries and will be characterized by PWs greater than 1.5", which is around 2 std above the climatological normal for this time of year. With sufficient instability developing that will overlap with the better forcing and moisture, rounds of thunderstorms are expected to develop along the boundary before slipping south/southeast toward the Gulf Coast. By this point in the storm system evolution, the flow is likely to be more parallel to the overall storm motions and this could favor some repeating rounds or training convection particularly over portions of MS/AL. The environmental ingredients will support intense rain rates over 1-2"/hr at times and this could bring scattered instances of flash flooding and for this update, the Slight Risk inherited looks good was largely unchanged from the previous forecast. ...Central Appalachians to the Northeast... Low pressure is expected to track from the lower Ohio Valley to the eastern Great Lakes over the forecast period as shortwave trough energy aloft and height falls over the region. A strong southwesterly flow in the low levels will bring higher moisture, characterized by dewpoints in the upper 50s to lower 60s and PWs above 1", especially from the Central Appalachians and southern areas of the NOrtheast. With the uncapped and warm/moist environment, there should be scattered to numerous showers and thunderstorms that develop that will be capable of producing isolated/localized rain rates in excess of 1-1.5"/hr at times. This could cause isolated flash flooding, particularly for some of the areas that have seen recent heavy rainfall and more saturated ground conditions as well as urban locations. Taylor 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