Quantitative Precipitation Forecast
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Issued by NWS
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169 FOUS30 KWBC 270023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 823 PM EDT Sun May 26 2024 Day 1 Valid 01Z Mon May 27 2024 - 12Z Mon May 27 2024 ...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF KENTUCKY, NORTHERN TENNESSEE, SOUTHERN ILLINOIS/INDIANA, AND SOUTHEAST MISSOURI... The Moderate risk still looks in pretty good shape across portions of the OH Valley where numerous instances of flash flooding are expected into tonight. Convection continues to grow upscale over portions of southeast MO and southern IL this evening, and is expected to develop into a squall line that will push southeast across KY and TN tonight. The squall line will likely become progressive in nature, however intense rainfall rates will still allow for a quick 1-2" of rainfall. Recent rainfall has saturated soil conditions and lowered FFG across the the MDT risk area...thus this additional rainfall is likely to result in some flash flood impacts. Given the strong low level jet and increasing forcing along/ahead of the developing squall line, expect we will continue to see some discrete supercells develop ahead of the convective line. Where these cells merge into the squall line there will be localized areas where the duration of heavy rainfall is extended. If this occurs over any more sensitive basin or urban area then areas of significant flash flooding could evolve. The MDT risk was expanded a bit north to account for model/observational trends. Chenard Day 2 Valid 12Z Mon May 27 2024 - 12Z Tue May 28 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE NORTHERN MID-ATLANTIC STATES... ...2030Z Update... A deep cyclone for the month of May will continue to lift into southeast Canada. Circulation around the low will continue to draw moisture northward over portions of the eastern U.S.. The 12Z run of the models were not showing quite as much instability and somewhat lower precipitable water values...but still mesoscale forcing to keep the Slight Risk with just a couple minor adjustments. Elsewhere...the placement of the Marginal still adequately covers the uncertainty of where heavy rainfall may occur given the instability and heard-to-time shortwave energy. Bann ...0830Z Excessive Rainfall Discussion... ...Great Lakes/East... A cyclone on the deep/strong side of May climatology lifts into southeast Canada. A broad area of 1.5"+ precipitable water values advects in ahead of the system and focuses along the system`s frontal boundaries and within area terrain. The Slight Risk for portions of PA and NY is similar to continuity. There is a 6-12 hour period where Atlantic moisture advects in while the 850 hPa flow remains quasi-stationary and fairly strong, which could lead to a decent period of moderate to heavy rainfall. In this area, hourly rain totals to 2.5" and local amounts to 5" are considered possible where cells merge, train, or where mesocyclones form. As 700 hPa temperatures are modest, heavy rainfall potential is there along its cold front as it sweeps through the Cumberland Plateau, the Appalachians, and the Southeast. There is some concern that MCVs from previous activity in the Midwest and Mid-South could sail into the area to increase divergence aloft, effective bulk shear, and thunderstorm organization across the Southeast, so broadened the Marginal Risk in this area. There`s even some convective potential within the system`s comma ahead across portions of MI; the guidance shows a narrower range of possible outcomes compared to this time yesterday, which appears to be due to timing differences. Within the Marginal Risk area, hourly rain totals to 2" and local amounts to 4" are possible where cells merge, train, or mesocyclones form. Roth Day 3 Valid 12Z Tue May 28 2024 - 12Z Wed May 29 2024 ...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF TEXAS AND SOUTHERN OKLAHOMA... ...2030Z Update... The 12Z suite of guidance continues to favor portions of Texas and southern Oklahoma for another round of convection capable of producing heavy to potentially excessive rainfall. Return flow draws moisture back into the region...setting the stage for convection to develop late Tuesday afternoon and Tuesday evening as a mid-level shortwave trough crosses Texas from west to east. Model agreement was still fairly good allowing the Slight Risk to stay in place without too many adjustments. Bann ...0830Z Excessive Rainfall Discussion... Weak shortwaves rippling through and dampening mid-level ridging across TX are expected to force a return flow of moisture and instability back into the western Gulf Coast. Precipitable water values of 1.5-1.75" are forecast, and low-level inflow and effective bulk shear should be just sufficient for convective organization near a front. The 00z NAM advertises MU CAPE of 5000+ J/kg, so there`s instability to spare. While 700 hPa temperatures are plenty warm, implying a decent mid-level cap, the front should be an effective focus and allow the cap to break. Most of the guidance shows reasonable agreement, with the exception of the 00z GFS that streaks heavy rainfall eastward. Since it can be prone to convective feedback which can lead to such an evolution, decided to account for its solution in the Marginal Risk area by showing some eastward expansion. The inherited Slight Risk resembles continuity. 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