Extended Forecast Discussion NWS Weather Prediction Center College Park MD 500 PM EDT Wed Sep 08 2021 Valid 12Z Sat Sep 11 2021 - 12Z Wed Sep 15 2021 ...Overview... Quasi-zonal flow over the northern two-thirds of the CONUS with high pressure over the southern third will persist this weekend well into next week. This makes for a mostly dry pattern outside of a frontal system tracking across the northern Plains and Great Lakes and a plume of tropical moisture that threatens heavy rain for the TX and LA coasts. Temperatures will generally be near to below normal across the northern tier north of the front and near the Gulf, but warmer than normal in between. ...Guidance Evaluation and Preferences... Notable model divergence in the flow pattern over the CONUS begins Saturday night with recent runs of the GFS (including the 12Z today) featuring a weaker and much faster trough crossing from the Pacific Northwest by Sunday night. These quicker GFS solutions then allow ridging to return across The West ahead of the next wave in the middle of next week. Meanwhile, the 00Z ECMWF, 00Z/12Z UKMET, and 00Z/12Z CMC all feature a more amplified, positively tilted, and slower trough that lingers over the Pacific Northwest through Monday with troughing lingering over The West until the next wave arrives in the middle of next week. With a focus on consensus, the forecast that is based on a deterministic blend of global models on Day 3 quickly becomes non-GFS and favoring the 00Z ECMWF by Day 4 with increasing weight to the ECENS, CMCE, and some GEFS through Day 7. Overall, removing the GFS from the model blend makes for a slower progression of fronts/precip across the CONUS than the previous forecast. Near the Gulf of Mexico, there remains uncertainty with the broad tropical plume shifting northwest to the Gulf Coast Sunday into the middle of next week. All guidance was considered given the general lack of trackable forcing mechanisms in the moisture plume. 06Z and 12Z runs of the GFS are farther north up the coast with their focus of QPF, highlighting the middle and upper TX coast where prior runs focused on the lower coast/lower Rio Grande Valley. The 00Z ECMWF also shifted farther east, featuring the central Gulf coast (centered on LA) while the CMC (GDPS) continued to generally keep heavy QPF offshore in the Gulf. Given this uncertainty, the Days 4-7 QPF along the Gulf coast features a compromise between the GFS and ECMWF, highlighting the middle TX coast to the southern LA coast with an apex near Galveston Bay/Houston metro. However, given the broad spread in guidance (including the ensemble means), there remains a general heavy rain threat next week from the lower Rio Grande Valley through the central Gulf Coast and any particular areas of focus are likely to continue shifting. ...Weather/Hazard Highlights... The frontal boundary over the northern tier will likely only support light rainfall for the northern Plains/Rockies before a modest injection of moisture from the south should promote more scattered showers across the Upper Miss Valley/Great Lakes Tuesday into Wednesday. This should slide into the Northeast Wednesday per current timing. As noted above there is a multi-day potential for heavy rain across the western and central Gulf Coasts later Sunday into or through the middle of next week with confidence on location pending any organization/focused area development. Temperatures will be above normal under the ridge and south of the surface boundary this weekend from the southern Rockies and eastward. High temperatures look to be 10 to 15 degrees above normal over the southern Rockies/Plains Saturday when several high temperature records will be challenged. This area of warmth will temper a bit as anomalies of generally 5 to 10 degrees above normal cross the Middle Mississippi, Ohio Valleys, and then Mid-Atlantic next week. Temperatures along the Canadian border will linger near to below normal through the period. Areas around the Gulf will see typical mid-September temperatures in the upper 80s to around 90F with plenty of humidity. Jackson Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Mon-Wed, Sep 13-Sep 15. - Heavy rain across portions of the Northern Plains and the Northern Rockies, Sat, Sep 11. - Much above normal temperatures across portions of the Central Rockies, the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Sat-Mon, Sep 11-Sep 13. 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml