835 FXUS21 KWNC 201804 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT September 20 2026 SYNOPSIS: A stalled frontal boundary and prolonged easterly flow are expected to set up across the Florida Peninsula and the Gulf, accompanied by an elevated risk for periods of heavy rain all week-2. No other hazards are posted under a deamplifying zonal mid-level pattern across the Contiguous U.S. (CONUS), but a few situations need to be monitored. Unseasonably hot weather is possible early in part of the south-central U.S. while enhanced rainfall expected across the Plains could prove more beneficial than hazardous. Farther west, low pressure in the Gulf of Alaska has the potential to bring increased precipitation and winds to parts of southern Alaska at times, and the continuing active tropical season in the East Pacific increases the chances that at least one disturbance impacts Hawaii. HAZARDS Slight risk of heavy precipitation for the Florida Peninsula and the Gulf Coast region, Mon-Sun, Sep 28-Oct 4. DETAILED SUMMARY FOR WEDNESDAY SEPTEMBER 23 - SUNDAY SEPTEMBER 27: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR MONDAY SEPTEMBER 28 - SUNDAY OCTOBER 04: Dynamical models generally agree that week-2 starts with a deamplifying mid-level ridge extending from the central or interior western CONUS northeastward, with the greatest 500-hPa anomalies over eastern Canada. Meanwhile, a mid-level low pressure along or near the Atlantic Seaboard will meander near or into the eastern CONUS while dissipating by the middle of the period. This results in generally zonal flow across the CONUS with modest positive 500-hPa height anomalies covering most of the CONUS during the last half of week-2. A flagging ridge axis is expected to waver around the central to interior western CONUS at this time, extending northward into somewhat more impressive positive 500-hPa anomalies across much of Canada. Mid-level heights are relatively lower to the south and east, establishing a weak de facto mid-level trough near or off the East Coast despite expected near- to slightly above-normal 500-hPa heights there. Meanwhile, the European ensemble (ECENS), GEFS, and GEFS-Artificial Intelligence (GEFSA-AI) means all start week-2 depicting a fairly strong mid-level trough extending from Alaska into the North Pacific, with negative 500-hPa height anomalies reaching -12 dm well west of British Columbia. The Canadian ensemble (CMCE) mean has a weaker trough centered farther north, but this appears to be an outlier, so a stronger solution consistent with model consensus is favored. With time, this trough remains approximately in place and weakens, but 500-hPa heights remain at least 6 dm below normal through all of week-2 over part of the central North Pacific and adjacent Gulf of Alaska. Marginally anticyclonic mid-level flow is expected to prevail across the eastern half of the CONUS between the weak ridge axis to the west and a couple of iterations of mid-level troughing to the east. This induces weak but persistent surface high pressure across the central and eastern CONUS for most of week-2, with lower pressures expected to the south and east, resulting in a weak frontal boundary around the eastern and southern periphery of the surface high pressure. Initially this feature is draped southward from the mid-level trough centered near the Atlantic Seaboard, then southwestward across the Florida Peninsula south of the surface high pressure. With time, surface pressures are expected to rise in the western Atlantic as the mid-level trough becomes more dissolute, and the front takes on an east-west orientation south of surface high pressure, extending from the northwestern Tropical Atlantic across Florida and the Gulf and potentially into adjacent Texas or Louisiana. The weak, wavering frontal boundary is expected to serve as a focus for convective development, with a protracted fetch of easterly winds keeping moist, tropical air flowing across the region. Models consistently highlight southern Florida as the area at greatest risk of hazardous rainfall totals, but most now show considerable amounts along much of the Gulf Coast and significantly lower amounts inland, although the details of timing and intensity are uncertain. In addition, the Probabilistic Extremes Tools (PETs) from the ECENS and GEFS show marginally enhanced chances for 3-day precipitation to exceed both the 85th climatological percentile and one inch total across varying parts of the Florida Peninsula and Gulf Coast through at least the middle of the period. Furthermore, any tropical waves or low pressure systems that interact with the frontal boundary would further enhance rainfall over at least part of the region, further supporting a risk of hazardous precipitation amounts. The boundary is expected to drift around Florida and the Gulf Coast region all week, so a slight hazard for heavy precipitation is posted there for the entirety of week-2. Despite most models showing south Florida as the area expected to receive the most rain, there is too much uncertainty with this set-up to justify any moderate risk areas. Some areas of heavy rain could be slow-moving, potentially triggering isolated flash flooding, although no particular area appears vulnerable to widespread and persistent flood conditions. Despite the anticipated deamplification of the mid-level ridge over the central and south-central CONUS, 500-hPa heights are expected to remain broadly above normal almost nationwide, allowing unseasonably high temperatures to linger from the southern Great Plains through the Lower Mississippi Valley early in the period. Temperatures or heat indices there have a 30 to 50 percent chance of exceeding the 90th climatological percentile and meeting hazards thresholds according to internal heat forecast tools based on the ECENS and GEFS, but declining 500-hPa heights and the time of year both argue against posting any heat hazards. Early in week-2, significant rainfall is expected to reach eastward out of the southwestern CONUS across the Plains and Mississippi Valley. This rain is more likely to be beneficial than hazardous in most areas as antecedent conditions have been dry, especially across the southern half of this region. And while the ECENS PET shows nominally enhanced chances for 3-day rainfall amounts to exceed the 85th climatological percentile and one inch total, the GEFS and CMCE PETs show little chance for amounts that high. Overall, there isn't enough support for heavy rainfall among the tools to support posting a hazard at this time, although models overall are slightly wetter today. A mid-level trough is expected to be centered over the Gulf of Alaska at the start of week-2 and remain approximately in place through the period while slowly weakening. This is expected to induce and maintain low surface pressures south of Alaska, which has the potential to enhance precipitation totals and winds across parts of southern Alaska at times. Most of the tools, however, show somewhat lower precipitation totals and less of a chance for hazardous winds today, so while above-normal precipitation and some gusty winds are possible, tools are not sufficiently robust to support posting any hazards at this time. As the active tropical season in the East Pacific, the increase in tropical cyclone development enhances the odds that at least one system may influence conditions across Hawaii. Grounds are saturated in many areas due to periods of heavy rain over the past few months, and any additional episodes of heavy rain may renew impacts caused by the wetness, such as flooding and possibly mudslides. At this time, several areas of potential tropical development are being monitored in the East Pacific, but no specific system is threatening the island chain at this time. Refer to products from the National Hurricane Center for more detailed information, including any tropical cyclone development. FORECASTER: Rich Tinker $$