Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Thu Sep 09 2021 Valid 12Z Sun Sep 12 2021 - 12Z Thu Sep 16 2021 ...Overview... Quasi-zonal flow over the northern two-thirds of the CONUS with ridging along 30N will persist through the period. This makes for a mostly dry pattern outside of a frontal system tracking across the northern tier and a plume of tropical moisture that may suppoer a heavy rain threat for parts of 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 from the Southwest to the Mid-Atlantic. ...Guidance Evaluation and Preferences... The guidance continues to have problems resolving the eastern Pacific flow into the West that translates downstream next week, showing important differences even before the medium range period. The GFS and many GEFS members generally detached a portion of an eastern Pacific trough much more than the other models/ensembles, allowing its northern part to move swiftly eastward next week, well ahead of the larger consensus that includes the ECMWF, Canadian, and UKMET. Once that timing difference starts, it expands through the period rendering an all-encompassing blend ineffectual. Preferred the non-GFS consensus that was closer to continuity but realize that the GFS does present a plausible solution. However, opted to rely on the older 00Z/08 ECMWF rather than its even slower 12Z/08 run. Its 12Z/08 ensemble mean was weighed most heavily after next Tuesday as it was preferred over the quicker GEFS mean. Around the Gulf of Mexico, there remains uncertainty with the broad tropical plume shifting northwestward to the northwest 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. 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 with each model run. Potential exists for several inches over the multi-day period. ...Weather/Hazard Highlights... The frontal boundary over the northern tier will likely only support light rainfall for the Great Lakes Sunday until a modest injection of moisture from the south should promote more scattered showers across the Upper MS Valley/Great Lakes Tuesday into Wednesday. This should slide into the Northeast Wednesday per current timing. Over the middle MS Valley, the tail-end of the frontal boundary will act as another focus for showers on Tuesday into Wednesday as the front eventually lifts northward. Temperatures will be about 5-15 deg F above normal under the ridge and south of the surface boundary from the Four Corners/southern Rockies and points eastward. 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 into the 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 yielding heat indices near 100F. Fracasso 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