Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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961 FXUS21 KWNC 231831 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT September 23 2024 SYNOPSIS: At the beginning of week-2, there are considerable model differences regarding the remnants of Potential Tropical Cyclone Nine which is forecast to impact the southeastern contiguous U.S. (CONUS) during week-1. Possibilities range from dissipation to a more consolidated system lingering near the Eastern Seaboard. Above-average temperatures and a more tranquil pattern are forecast elsewhere across the CONUS. Alaska is forecast to be in between significant weather systems at the beginning of week-2, with chances for hazardous precipitation and winds potentially increasing across the southern part of the state by the second half of the period. HAZARDS Slight risk of heavy precipitation across portions of the Southeast, Tennessee Valley, Central and Southern Appalachians, and Mid-Atlantic, Tue-Thu, Oct 1-3. Flooding possible across portions of the Southeast. Risk of Rapid Onset Drought across portions of eastern Oklahoma and northeastern Texas. DETAILED SUMMARY FOR THURSDAY SEPTEMBER 26 - MONDAY SEPTEMBER 30: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR TUESDAY OCTOBER 01 - MONDAY OCTOBER 07: As of 2pm EDT today, the National Hurricane Center (NHC) forecasts Potential Tropical Cyclone Nine (PTC-9) to intensify into a hurricane before striking the Gulf Coast later this week. While direct coastal impacts are expected to end prior to the start of the week-2 forecast period, the remnant circulation is forecast to meander inland across the Southeast. Dynamical models depict differing scenarios, with the 0z GEFS indicating a faster dissipation of the system with minimal impacts during week-2 and the 0z ECENS maintaining a more coherent circulation near the Eastern Seaboard. This is also reflected in the ECENS Probabilistic Extremes Tool (PET) which depicts a 20 percent chance of 3-day precipitation totals exceeding the 85th climatological percentile and 1-inch early in week-2 over parts of the East. Both models also indicate shortwave troughing moving into the eastern CONUS which may also lead to increased cyclonic flow across the region. Given the synoptic pattern, more robust precipitation forecast in the ECENS, and continuity from last week, a slight risk for heavy precipitation is posted across portions of the Southeast, Central and Southern Appalachians, and the Mid-Atlantic, Oct 1-3. There is more uncertainty than usual, especially given any changes to the track forecast of PTC-9 could have implications for the forecast evolution later in week-1 into week-2. Flooding is possible across portions of Alabama, Georgia, and Florida in the wake of PTC-9 and consistent with high week-1 projected rainfall totals from the Weather Prediction Center (greater than 4-inches over some areas). Heavy rainfall is also forecast to expand westward toward the Middle Mississippi and Ozarks, which should mitigate the drought concerns in that region, but may also need to be monitored for flooding as well. Antecedent heavy rainfall across western and central portions of Oklahoma also favor decreased chances of drought development in that area. Therefore, only portions of eastern Oklahoma and northeastern Texas remain at elevated risk for rapid onset drought development where precipitation deficits of around 3-inches remain and below-normal precipitation is forecast during week-2. A frontal system is forecast to move across the Northern Plains and Upper Midwest toward the end of week-1, bringing in a period of relatively cooler temperatures. The GEFS and ECENS PETs indicate at least a 40 percent chance of minimum temperatures falling below 40 deg F over some areas, and frost cannot be ruled out over susceptible vegetation. However, since the prevailing temperatures are still forecast to be above-normal, no related below-normal temperature hazards are posted. The ECENS PET also depicts at least a 20 percent chance wind speeds exceed the 85th climatological percentile and 20-mph early in week-2. However, the best synoptic set-up is on day-7 tied to the frontal passage, and no related wind hazard is highlighted over the region. Surface low pressure is forecast across the Aleutians and Gulf of Alaska late in week-1 bringing the potential for high winds and heavy rain across the southern coast of the state. This system is forecast to weaken at the outset of week-2, with another area of surface low pressure taking a similar track across the region toward the middle of the period. Given that Alaska is likely to be in-between these two weather systems at the start of week-2, no related hazards are indicated. However, wind and precipitation hazards may need to be considered related to the second system, but holding off for now given the weaker depiction of the surface low in the GEFS relative to the ECENS and CMCE. FORECASTER: Thomas Collow $$