513 FXUS21 KWNC 101932 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT September 10 2026 SYNOPSIS: A developing area of mid-level low pressure and an unseasonably cooler air mass east of the Rockies is expected to reduce the risk of persistent extreme heat across the lower Great Plains and Mississippi Valley, but also possibly bring first frost and/or freezes to the northern tier of the CONUS during the week-2 period. Another round of enhanced tropospheric moisture is favored to overspread the western and central CONUS, potentially bringing heavy precipitation and possible flooding over portions of the Four Corners. HAZARDS Slight risk of much below normal temperatures for parts of the Northern Plains, Upper Mississippi Valley, and eastern Great Lakes, Fri-Sun, Sep 18-20. Slight risk of much below normal temperatures for parts of northern New England, Sat-Tue, Sep 19-22. Slight risk of heavy precipitation from the Four Corners region, Fri-Sun, Sep 18-20. Flooding possible for portions of the Desert Southwest, Great Basin, and Rockies. DETAILED SUMMARY FOR SUNDAY SEPTEMBER 13 - THURSDAY SEPTEMBER 17: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR FRIDAY SEPTEMBER 18 - THURSDAY SEPTEMBER 24: Consistent with previous ensemble based guidance, both the GEFS and ECENS remain in good agreement in regards to the evolution of the 500-hPa pattern late next week and into the following weekend. As an amplified ridge shifts eastward from the Gulf of Alaska into western Canada, a developing longwave trough is favored downstream, where models favor a deeper solution of the trough axis over eastern North America compared to yesterday. With more cyclonic flow developing over the eastern CONUS early in week-2, much of the enhanced subtropical ridging that has been established for quite some time is expected to ease and become more shunted to the west, resulting in cooler and drier conditions for many areas east of the Rockies. Such a cooldown is anticipated to be very much welcomed, particularly for areas embattled by persistent heat conditions since August across the Great Plains and Mississippi Valley, though the cold air advection may possibly produce first frosts and/or freezes for portions of the northern tier of the CONUS during the period. Underneath the amplified mid-level ridge over the Gulf of Alaska, models continue to favor secondary mean troughing encroaching the West Coast early in week-2. Although weak, this feature combined with additional Tropical Cyclone (TC) activity in the eastern Pacific may bring another round of enhanced moisture advection into the western CONUS supportive of heavy precipitation and flood risks over the Four Corners. Relative to yesterday, the eastern extent of this moisture intrusion looks to become more concentrated over the Interior West due to aforementioned troughing and the associated drier, northwesterly flow overspreading the east. Tied to the deepening trough favored over the eastern CONUS, a slight risk of much below normal temperatures remains posted over the Upper Midwest for Sep 18-20, with its coverage shifted eastward in the updated outlook to include (exclude) more of the Great Lakes (Northern High Plains). A separate slight risk area of much below normal temperatures remains designated for northern New England valid for Sep 19-22 to capture the evolution of the cold air mass. Within the highlighted areas, there continues to be agreement in uncalibrated temperature tools showing increased chances for temperatures falling below 40 deg F, with the ECENS Probabilistic Extremes Tool (PET) favoring 20-30% chances of minimum temperatures falling below the 15th climatological percentile. While near freezing temperatures are not markedly unusual for this time of year (climatological first freezes occur by late September for the highlighted areas), any frost and/or freezes associated with the favored cold air mass may be impactful to some vegetation and unsuspecting populations. While cooler temperatures are favored further south associated with an advancing frontal boundary, temperatures may be slow to moderate across portions of the Southern Plains and Lower Mississippi Valley. PETs maintain increased chances for temperatures remaining above the 85th percentile over this part of the country and along parts of the Southeast, however chances of extreme heat are well diminished based on lower heat index values indicated by the tools by the start of week-2, thus no corresponding hazard areas are posted. Continued above-normal temperatures may exacerbate dry ground conditions and sustain an increased wildfire risk, but given the relatively cooler temperatures favored and decreasing sun angles with the start of autumn soon approaching, the prospect of additional impacts associated with the persistent warmth is lower overall. In addition, the latest U.S. Drought Monitor indicates drought conditions now being registered over many parts of the south-central CONUS, and with increased rainfall amounts favored over the next week or so, this supports the removal of the Rapid Onset Drought (ROD) risk in the updated outlook. With troughing and onshore flow favored near the West Coast, with the potential for added tropical moisture from the eastern Pacific, a slight risk of heavy precipitation remains posted (Sep 18-20) over the Four Corners where uncalibrated tools maintain elevated chances for 3-day precipitation amounts exceeding a half an inch, and in some locations more than one inch. The slight risk coverage is now omitted over the Great Plains, Mississippi and Ohio Valleys, and lower Great Lakes in the updated outlook. This adjustment is based on a reduction of signals in both the GEFS and ECMWF PETs, and an area of mean surface high pressure descending from the north-central CONUS which is anticipated to limit the eastern extent of enhanced moisture by the start of week-2. In addition, given the continued indications of potentially heavy precipitation during the next week or so, a possible flooding hazard remains posted for portions of the Four Corners. This region is prone to flash flooding and caution should be exercised near canyons and creek beds, and additional rainfall may hamper recovery efforts from recent flooding in the Grand Canyon. Across the eastern Pacific, the National Hurricane Center (NHC) is currently monitoring newly formed TC Norbert as of 11am PDT, as well as a pair of watch areas located near 150W and 110W with 30% and 70% chances of development, respectively, during the next week. While the latter higher likelihood area looks to be more of a factor in regards to the enhancing moisture advection into the CONUS, a continued westward track from Norbert beyond the official forecast track from NHC, as well as the lower likelihood watch area in proximity to Hawaii may pose an additional heavy precipitation threat to the state during the next few weeks. Any additional TC related precipitation may worsen already saturated grounds across parts of Hawaii, as there has not been much time for recovery following the impacts associated with TCs Lowell, Moke, and Lala this season. FORECASTER: Nick Novella $$