Quantitative Precipitation Forecast
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Issued by NWS
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289 FOUS30 KWBC 240751 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 351 AM EDT Fri May 24 2024 Day 1 Valid 12Z Fri May 24 2024 - 12Z Sat May 25 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX TO THE ARKLAMISS... ...ArkLaTex into the Mid-South... A broad marginal risk area continues from continuity, with a new Slight Risk added near the LA/AR border. Precipitable water values across these regions will remain above average, with additional shortwaves in from the west supporting scattered convection and locally heavy rainfall amounts. The ingredients in place support hourly rain totals to 2.5" with local amounts to 6". However, the 00z NAM was far more bullish, though somewhat to the north, with a maximum of 11"+. The concern is greatest in and near the Slight Risk area, which saw heavy rainfall Wednesday and Thursday. Considering the NAM signal, localized Moderate Risk impacts cannot be ruled out should a long enough period of training evolve. ...Upper Mississippi Valley into the Upper Great Lakes... The next strong area of height falls moving into the northern High Plains will continue to push eastward through the Northern Plains and into the Upper Mississippi Valley. The associated front on the leading edge of the upper trough will have strong frontal convergence which causes an axis of PW values 1 to 1.5 standard deviations above the mean. This will support potential for moderate to locally heavy rainfall totals. This front does look progressive, which may limit very heavy amounts. The marginal risk was drawn to where there may be overlap of locally heavy rains Friday through Friday night with heavy rains that have fallen as of late. The area saw tweaks from continuity, but nothing major. Hourly rain totals to 2" and local amounts to 4" are expected where cells merge or manage to train. ...Portions of the East... Near and ahead of a front and lee trough, precipitable water values of 1.5-1.75" pool within a region with weak to modest low-level inflow and MU CAPE up to 2000 J/kg. Effective bulk shear of ~25 kts is expected, which could lead to some convective organization, particularly during the period of daytime heating. Interestingly, the mesoscale guidance was a bit more bullish in this area than the global guidance. But, given the ingredients, hourly rain totals to 2" with local amounts to 4" are possible where cell training or cell mergers occur. Flash flood guidance values are rather high partially due to soil type in the area, but also because the otherwise frequent rainfall lately hasn`t been heavy --- 7 day precipitation has been fairly low, roughly 25% of average and 14 day rainfall is more or less average. Synoptically, this set up tends to lead to isolated flash flooding, particularly in urban areas, so added a Marginal Risk area from the coastal Carolinas up into the Eastern Shore on this update. Roth Day 2 Valid 12Z Sat May 25 2024 - 12Z Sun May 26 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR THE KANSAS/MISSOURI BORDER... The Marginal Risk remains similar to continuity, with some changes made to the Slight Risk area. A cyclone emerging into the central Plains will bring precipitable water values of 1.5"+ across the region on the heels of sufficiently strong low level inflow/effective bulk shear. The region lies within a general instability gradient, with CAPE expected to rise to 4000+ J/kg within an increasingly capped atmosphere across the southern Plains. Pockets of moderate to heavy rainfall are forecast within this region. There is increasing overlap in the QPF pattern near the KS/MO border within the 00z guidance where three hourly flash flood guidance is modest, so went ahead with an upgrade to a Slight Risk for this update. However, given the ingredients, hourly rain totals to 2.5" with local amounts to 5" are possible where mesocyclones form and/or cells manage to train. ...Portions of the East... Near western PA and WV, showers and thunderstorms along a progressive front could drop locally heavy rainfall within an area of modest flash flood guidance. Hourly rain totals to 1.5" with local amounts to 3" are possible where short bouts of cell training or cell mergers manage to occur. Since the flash flood threat appears to be isolated, added a Marginal Risk. Down south in the Carolinas, higher moisture pools near the coast as a front slowly moves through. With MU CAPE as high as 3000 J/kg and effective bulk shear expected to be sufficient to organize convection to some degree, added a Marginal Risk within a regime that can lead to isolated flash flooding during the warm season. Roth Day 3 Valid 12Z Sun May 26 2024 - 12Z Mon May 27 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE MIDWEST AND THE MISSISSIPPI & OHIO VALLEYS... A cyclone moving across the Midwest and Great Lakes is expected to draw ample moisture (precipitable water values of 1.50-1.75") into the region. The area that is expected to see the greatest residency time with higher moisture and instability is across portions of TN and KY. However, warm temperatures at 700 hPa appear to be a limiting factor the farther down its front you go. The best warm air advection ahead of the low is slated to occur from portions of IL across OH, where the guidance is more robust in its QPF signal, with local amounts in the 3-5" range. Considering the progression of the warm air advection pattern and best moisture across the region, that degree of heavy rainfall would need to fall quickly; hourly rainfall potential to 2.5" is possible within the organized thunderstorms, with embedded mesocyclones, could get them there given the low- level inflow and effective bulk shear expected. Went ahead and added a Slight Risk across the region, where flash flood guidance values are modest. Roth 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