Extended Forecast Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Mon Sep 06 2021 Valid 12Z Thu Sep 09 2021 - 12Z Mon Sep 13 2021 ...Overview... The medium range period will begin Thursday with an upper high over the Four Corners region and mid-upper troughing across the eastern third of the country. The upper high is forecast to weaken and shift/expand eastward toward the Southern Plains/Lower Mississippi Valley through the period and allow for quasi-zonal flow to take hold across much of the CONUS, suggesting lower confidence in system timing/details. The pattern remains rather dry with generally near to above normal temperatures. ...Guidance Evaluation and Preferences... Model guidance aligned pretty well for the early part of the medium range period with the pattern described above. However, by late week as the pattern deamplifies fairly quickly, several features will contribute to forecast uncertainty and cause timing and amplitude differences particularly with shortwaves moving through the quasi-zonal flow. These include variations with flow out of the Pacific, differences in positioning and strength of troughing across Canada, and the manner in which the broad-scale flow interacts with what is currently Hurricane Larry as it transitions to an extratropical cyclone. Run-to-run continuity is also poor, leading to added uncertainty. However, the overall pattern appears to be handled reasonably well by the GEFS and EC ensemble means, though confidence in the smaller-scale details is low. Through Friday, a multi-model deterministic blend of the 00Z models was able to be used (that guidance happened to be fairly agreeable on a shortwave moving through southwestern Canada into the north-central U.S., though the 06Z GFS had different timing), but the forecast shifted to a blend mainly of the ensemble means by early next week to reduce the effects of the variations in the deterministic models. In terms of possible tropical development, a tropical system is likely to form in the East Pacific per the National Hurricane Center but is forecast to track offshore/west of Baja California. In the Gulf, tropical development remains uncertain but moisture transport northward/northeastward continues to look likely into the northern/northeastern Gulf later this week, so maintained a modest QPF signal across the FL Panhandle northeastward, but ultimately need to wait and see if any organization will occur. Please see the NHC outlooks for more information. ...Weather/Hazard Highlights... Rainfall will focus along and ahead of a cold front along much of I-95 Thursday morning, with its tail-end lingering over the Southeast for a few days before dissipating. Amounts should generally be light except over the northeastern Gulf and western Atlantic coasts (FL/GA/Carolinas), where tropical moisture may support modest to perhaps locally heavier amounts. Daily showers/storms are likely for much of the FL peninsula to the south of the boundary. Some moisture will work back through the Southwest around the upper high as it drifts southeastward, focused over inland CA (Sierra) into the Great Basin and parts of the Desert Southwest. This may lift northward into the northern Rockies by later in the week into the weekend, ahead of another Pacific front. These amounts should be generally light as well. With shortwaves and frontal systems traversing across the north-central to northeastern U.S., periods of rain showers can be expected there. By around Monday, another possible low pressure system with tropical moisture inflow could spread rain toward western parts of the Gulf Coast, but uncertainty remains high. Temperatures will be near to above normal over most areas, especially over the Rockies and High Plains ahead of the incoming Pacific front/trough Thu-Fri. Temperatures 5-15 degrees above normal are possible as the area of warmth moves into the southern/central Plains later in the week into the weekend and toward the Middle Mississippi Valley next week. Temperatures in the Great Lakes/Northeast will be near to a bit below normal via northwest flow behind the cold front later this week, moderating a bit over the weekend as the surface high settles over the region. Tate/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 Hazards: - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Mon, Sep 13. - Heavy rain across portions of the Northeast, Thu, Sep 9. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, and the Mid-Atlantic. - Much above normal temperatures across portions of the Central Rockies, the Central Plains, the Northern Plains, and the Northern Rockies, Thu-Fri, Sep 9-Sep 10. - Heavy rain across portions of mainland Alaska, Thu, Sep 9 and Sun-Mon, Sep 12-Sep 13. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Fri, Sep 10 and Sun-Mon, Sep 12-Sep 13. 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