Quantitative Precipitation Forecast
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Issued by NWS
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598 FOUS30 KWBC 262006 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 PM EDT Sun May 26 2024 Day 1 Valid 16Z Sun May 26 2024 - 12Z Mon May 27 2024 ...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS MUCH OF SOUTHERN KENTUCKY TODAY... 16Z Update... Extended the Moderate Risk area westward across much of southern Kentucky as several waves of heavy rainfall work their way eastward through late morning and early afternoon...followed later today by the potential for a second round of convection. The HREF probabilities for 24 hr amounts in excess of 3 inches are in excess of 45 pct across the outlook region with spotty coverage of probability of 24 hour rainfall exceeding 5 inches embedded within that area. These amounts are certainly reachable given some observed values and MRMS depicted amounts that have exceeded 2.3 inches in under an hour. After a lull later today...high resolution CAM guidance tends to be in pretty good agreement that a new round of convection will develop over southeast Missouri or western Kentucky that tracks eastwards...with a pretty good chance of traversing at least some of the heavy rainfall footprint left behind by the storms this morning (even though the placement of storms in round 2 differs a bit from high res model to high res model). This same area depicted by the expanded Moderate risk area was highlighted by the National Water Model rapid onset graphic. Elsewhere...changes tended to be made based on trends seen in mid- to late-morning satellite and radar imagery. Bann ...0830Z Excessive Rainfall Discussion... A cyclone moving across the Midwest and Great Lakes is expected to draw ample moisture (precipitable water values of 1.50-1.75"), low-level inflow, effective bulk shear, and instability into portions of the MS, TN, and OH valleys. A convective complex trying to congeal near the KS/MO border at the present time is expected to continue moving east to east- southeast across KY and portions of TN this morning into this afternoon, near and to the north of an old/weak segment of the polar front. Making matters worse, a second convective round is expected Sunday evening moving from the confluence of the OH/MS rivers east to east- southeast across the same region as a cold front moves through. Rainfall over the past week across portions of the TN/KY border is 300-600%, so soils should be saturated. With the specter of hourly rain totals to 2.5", and local amounts of 6", an upgrade to a Moderate Risk was necessary in this area, despite the 00z guidance spread. This evolution is supported by the 00z HREF which was on the southern portion of the guidance spread and since 700 hPa temperatures are low enough to support a relatively uncapped atmosphere, a more southern solution as advertised by the HREF makes sense conceptually. A broad Slight Risk surrounds this region where similar amounts are possible and soils are less saturated. Mesocyclones, cell training, and cell mergers are the expected reasons for excessive rainfall here. While lower rainfall amounts generally are expected in the East and the Upper Midwest, the expected low-level inflow, effective bulk shear, and precipitable water values indicates that an isolated excessive rainfall threat exists, with hourly rain totals to 2" and local amounts to 4". The difference between the two regions are the degree of soil saturation/recent rains and the available instability. Much of WI has experienced 300-600% of their average seven day rainfall so soils should be sensitive. However, instability should be minimal so low- to mid-level frontogenesis appears to be doing the heavy lifting, heavy rainfall production- wise. Went ahead and raised a Slight Risk in and near portions of WI. Roth 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 haavy 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