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Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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097 FXUS21 KWNC 211821 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT June 21 2024 SYNOPSIS: Mid-level low pressure across the northeastern contiguous U.S. (CONUS) favors a brief cool down in the East late in week-1, but excessive heat risk is forecast to increase during week-2 as mid-level ridging expands across the CONUS. Another mid-level low over central Canada may limit the northward extension of the heat across the Upper Mississippi Valley and Great Lakes. However, frequent periods of thunderstorms with locally heavy rainfall and an increased risk of flash flooding are expected to persist over these areas. The Southwest monsoon is forecast to become more active, leading to increasing chances of locally heavy rainfall and flooding across parts of the southwestern U.S. HAZARDS Moderate risk for excessive heat for portions of the Ohio and Tennessee Valleys, Sun-Mon, Jun 30-Jul 1. Moderate risk for excessive heat for portions of the Mid-Atlantic, Sun-Mon, Jun 30-Jul 1. Slight risk of excessive heat for portions of the eastern and central U.S., Sat-Fri, Jun 29-Jul 5. Slight risk of excessive heat for portions of the Great Basin, Rockies, Southwest, and California Central Valley, Mon-Fri, Jul 1-5. Slight risk of heavy precipitation for portions of the Northern Plains, Upper Mississippi Valley, and Great Lakes, Sat-Fri, Jun 29-Jul 5. Slight risk of heavy precipitation for parts of the Southwest and Southern Rockies, Sat-Fri, Jun 29-Jul 5. Flooding possible for portions of the Northern Plains, Upper Mississippi Valley. Flooding possible for portions of southern Arizona and New Mexico. Rapid onset drought risk extending from New Jersey and Pennsylvania southward through northern Georgia. Rapid onset drought risk across the eastern Corn Belt. Rapid onset drought risk across northern Oklahoma and adjacent areas. DETAILED SUMMARY FOR MONDAY JUNE 24 - FRIDAY JUNE 28: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR SATURDAY JUNE 29 - FRIDAY JULY 05: Late in week-1 mid-level troughing is forecast to move across the Great Lakes and Northeast ushering in a reprieve from the ongoing excessive heat. However, this break is predicted to be short-lived as the 0z GEFS, ECENS, and CMCE quickly rebuild ridging across much of the central and eastern CONUS early in week-2. The ECENS and GEFS Probabilistic Extremes Tools (PETs) depict some parts of the Ohio Valley and Mid-Atlantic having at least a 40 percent chance for maximum temperatures to exceed the 85th climatological percentile over portions of the Ohio Valley and Mid-Atlantic. Temperatures in the uncalibrated guidance have ticked upwards compared to yesterday, with the 0z ECENS continuing to average a few degrees warmer than the 0z GEFS. On day-8 (Jun 29), the warmer ECENS depicts high temperatures in the mid-90s deg F across portions of the Ohio and Tennessee Valleys (upper-80s to low-90s deg F in the GEFS). While the Mid-Atlantic may remain under the influence of troughing initially, both models build higher temperatures into the region by day-9 (Jun 30), with maximum temperatures in the GEFS and ECENS ensembles above 90 deg F, and perhaps further increasing on day-10 (Jul 1). Dewpoints are forecast to be relatively lower on day-8 (Jun 29) in the wake of troughing, but increasing moisture thereafter favors heat indices near or above 100 deg F across the Ohio Valley and 105 deg F across the Mid-Atlantic, with the calibrated GEFS/ECENS skill-weighted heat tool depicting at least a 40 percent chance heat indices exceed the 90th climatological percentile across these regions. Given these signals, moderate risks for excessive heat are posted across portions of the Ohio and Tennessee Valleys and Mid-Atlantic Jun 30-Jul 1. A slight risk for excessive heat is posted across portions of the eastern and central CONUS for all of week-2. Early in the period, troughing is forecast to lift out of the western CONUS and into central Canada. This favors an expansion of heat across the West, but may also help limit the northward extent of the heat across the Upper Mississippi Valley and Great Lakes. The GEFS and ECENS PETs depict expanding coverage of probabilities of at least 20 percent for maximum temperatures exceeding the 85th climatological percentiles across portions of the Interior West and California Central Valley beginning around day-10 (Jul 1) resulting in a slight risk for excessive heat Jul 1-5. Maximum temperatures range from the 90s deg F across the northern Great Basin and Rockies to triple digits over the California Central Valley and Southwest. Increasing monsoon moisture may help limit temperatures across parts of the Southwest and Four Corners. The 0z GEFS and ECMWF ensembles depict multiple shortwave 500-hPa troughs progressing east from the western to the north-central CONUS overtop the expanding mid-level ridge. This longwave pattern along with uncalibrated 24-hour precipitation amounts from the GEFS and ECENS support a slight risk of heavy precipitation across the Northern Plains, Upper Mississippi Valley, and Great Lakes through the end of week-2. Thunderstorms could trigger flash flooding. In addition, minor to moderate river flooding is expected to persist into the beginning of July, especially across southern Minnesota. The westward-expanding ridge favors the start of the Southwest monsoon, with dewpoints increasing into the 60s deg F across the region in several recent runs of the deterministic GFS. This is also aided by increasing southeasterly moist flow related to Tropical Storm Alberto and potentially another tropical disturbance later in week-1. Both the ECENS and GEFS PETs depict at least a 20 percent chance that 3-day rainfall totals exceed a half-inch across parts of the Southwest and southern Rockies, with the ECENS being the most robust, bringing these increased chances through much of Arizona later in the period, and also having at least a 20 percent chance 3-day rainfall exceeds 1-inch across extreme southern Arizona. This is also supported by the uncalibrated tools which depict expanding coverage of week-2 rainfall totals of greater than a half-inch into parts of Arizona and New Mexico. Given this support, a slight risk for heavy precipitation is highlighted across portions of the Southwest and Southern Rockies for all of week-2, with a flooding possible area covering southern parts of Arizona and New Mexico where the stronger precipitation signals are depicted. Precipitation is expected to be isolated and not all locations will experience heavy rainfall. However, flash flooding is possible both where precipitation occurs and in downstream valleys, particularly in and around burn scars where debris flows can be triggered by heavy rainfall. Rapid Onset Drought (ROD) risk extends from a region including New Jersey and Pennsylvania southward through northern Georgia, as well as portions of the eastern Corn Belt and northern Oklahoma. These areas are most vulnerable to the rapid onset of drought due to: 30-day precipitation deficits of 2 to 4 inches, low soil moisture, and above-normal temperatures with high evapotranspiration rates forecast during the next two weeks. FORECASTER: Thomas Collow $$