Extended Forecast Discussion NWS Weather Prediction Center College Park MD 441 PM EDT Thu Sep 08 2022 Valid 12Z Sun Sep 11 2022 - 12Z Thu Sep 15 2022 ...General Overview... A pattern change will be underway across much of the Rockies and the central/northern Plains into the Midwest as a strong cold front heralds a change to much cooler temperatures for the weekend, compared to the anomalous warm weather presently. The upper trough that is accompanying this front is forecast to evolve into a slow moving closed upper low across the Ohio Valley and Great Lakes region by early Monday, with a ridge axis near the East Coast slowing its eastward progression, and moisture ahead of this feature may bring locally heavy rainfall to the portions of the Southeast and Mid-Atlantic late this weekend into early next week. Moisture associated with what will eventually be the remnants of Hurricane Kay is expected to advect inland across California and the Great Basin, albeit with lighter rainfall by this time. There will likely be another trough approaching the Pacific Northwest by the middle of the week. ...Guidance Evaluation/Predictability Assessment... Overall, the model guidance was in reasonably good agreement and showed decent consistency and continuity from the previous forecast cycle. The two areas of most model concern were the developing closed mid/upper low over the Ohio Valley and it's longitudinal position. The CMC remains a slower/west solution compared to the consensus while the GFS may be slightly ahead of the consensus but well within tolerable range. There are more sensible weather differences/potential impacts though, as a surge of moisture lifting out ahead of it may result in a heavy rainfall event somewhere over the Southeast to Mid-Atlantic (and potentially Northeast) later in the period. The other area of concern was the developing longwave trough over the western U.S. that advances later this weekend into next week, helping to break down the strong/anomalous upper ridge that has been in place over the West. The typical model biases were seen here, so overall using a multi-model consensus/blend was favored and this led to good continuity from overnight and adjusts well to the latest model trends. Overall, the blend incorporated the 13Z NBM with inclusion of the 00Z/06Z GFS, 00Z ECMWF, 00Z CMC, and 00Z UKMET for pressures/fronts and mainly a combination of the 13Z NBM and a multi-model ensemble bias corrected blend for QPF days 4-7. ...Weather/Hazards Highlights... The first strong cold front of September is forecast to bring quite a change to the Plains and then the Midwest and Great Lakes region for the weekend. Highs are expected to be only in the 60s across most of the Upper Midwest on Sunday, which is roughly 5-15 degrees below normal for this time of year. Significant abatement of the heat is also expected across California and Desert Southwest, with much of this cool down attributable to clouds and rainfall associated with a weakening Tropical Storm Kay by that time. There will be warmer conditions for the Pacific Northwest for Sunday and Monday, but not to the same magnitude as this past week. Across the eastern U.S., readings should generally be within 5 degrees of climatological averages for highs, while overnight lows near the East Coast should be about 5-10 degrees above normal most nights, owing to higher humidity and more cloud cover. Moisture from the future remnants of Hurricane Kay will likely be in place across the Intermountain West on Sunday, although the most impactful rain should be over by that time. Farther to the east, a cold front that will be in the process of becoming occluded is expected to produce moderate to locally heavy rain from Iowa to southern Wisconsin and northwestern Illinois, and a Slight Risk of excessive rainfall is now in effect for these areas on the Day 4 experimental outlook. Across portions of the Southeast into the Mid-Atlantic, there is an increasing signal for a heavy rainfall event. Favorable forcing for ascent provided by ample upper divergence ahead of the mid/upper level low, plenty of moisture being lifted northward out of the Gulf, and the expectation for sufficient instability in place could produce moderate to heavy rainfall. As a result, a Slight Risk was introduced in the Day 4 Experimental ERO (12Z Sun-12Z Mon) for portions of the Carolinas into the Mid-Atlantic region. Taylor Hazards: - Heavy rain across portions of the Mid-Atlantic into the central Appalachians and along the Carolina coast, as well as the upper Midwest into lower Michigan, Sun-Mon, Sep 11-Sep 12. - Heavy rain across portions of central Nevada, Tue, Sep 13. - Heavy rain across portions of the northern Mid-Atlantic into northern New England, Tue-Wed, Sep 13-Sep 14. - Heavy rain across portions of northern Utah to southwestern Wyoming, Wed-Thu, Sep 14-Sep 15. - Flooding possible near the southern border of the Big Bend National Park in Texas. - Flooding occurring or imminent across portions of the lower Mississippi Valley. Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml