Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sat Sep 25 2021 Valid 12Z Tue Sep 28 2021 - 12Z Sat Oct 02 2021 ...Overview... Guidance agrees upon an amplified upper pattern during the period. Expect a strong mean ridge to become established over the east-central U.S. and north/northeast into Canada while a downstream trough settles over or near the East Coast. Another mean trough over the West will consist of multiple pieces of energy and generally become more diffuse with time as a deeper trough reaches the Northeast Pacific. The most prominent forecast issues involve details of a potential southwestern U.S. upper low whose interaction with a Plains front could produce heavy rainfall over the southern Rockies/High Plains toward the end of the week, as well as specifics of an upper low likely to be embedded within the East Coast trough. ...Guidance Evaluation/Preferences... Confidence remains in the lower half of the spectrum for details of an upper low which most operational models suggest will be within the East Coast trough. Through the 00Z run, ECMWF runs over the past couple days have trended dramatically northward/northeastward for the upper low center while the GFS has adjusted northward from prior runs but from a less extreme starting point. The earlier ECMWF runs that had an upper low reaching the Southeast led to some 500mb height anomalies reaching 3-4 standard deviations below normal. Latest CMC runs are showing the low over New England, which is where the ECMWF mean has finally started to close off a low. GEFS means are lagging a bit as it took until the 06Z/12Z runs to provide at least a hint at a closed low and with a position north/northeast of the ECMWF mean and operational models. The new 12Z UKMET is now farthest south (close to Cape Hatteras by early Friday) which reflects the ideas from some earlier runs of other models. The new 12Z ECMWF actually ejects the Thursday New England upper low and then digs trailing trough energy into another farther south upper low whose center also reaches near Cape Hatteras by Saturday. There is general agreement that a shortwave moving into the West through midweek will separate, with an upper low closing off near the western U.S.-Mexico border and remaining energy continuing into the Northern Plains and southern Canada. Model/ensemble mean clustering for the upper low is decent, with typical spread for exact strength/track but models vary considerably for the details of the northern energy. The relatively small scale of the latter keeps predictability low. Another shortwave will likely reach the Northwest by Friday but it should be fairly weak to start with and then weaken further. The updated forecast started with a 00Z/06Z model composite during the first half of the period. This blend reflected the improving signal for an upper low to reach at least into New England, along with refinement of the western trough separation that should occur by early Thursday. With detail uncertainties rapidly increasing after early day 5 Thursday the forecast quickly trended to a nearly half model (06Z GFS/00Z ECMWF/00Z CMC) / half mean (06Z GEFS/00Z ECens) blend for days 6-7 Friday-Saturday. Ensemble input tilted more toward the ECens due to its better depiction/persistence of the New England upper low versus the GEFS. ...Weather and Potential Hazards... Through at least midweek the central U.S. will see an expanding area of showers and thunderstorms as shortwave energy ejects northward from the southern Rockies/High Plains and low level flow brings in Gulf moisture that will interact with a front moving from the West into the Plains. Best potential for some locally moderate to heavy rainfall will be over parts of the central and southern Plains. Then expect rainfall to become heavier and more focused over and near the southern High Plains by Friday-Saturday with an upper low currently forecast to track over the Southwest/Four Corners region and low level flow continuing to bring moisture into a stationary front. There will be periods of lighter precipitation early-mid period over parts of the Southwest, eastern Great Basin, and southern Rockies. Over the Northwest expect light to moderate precipitation on Tuesday with snow possible over high elevations followed by a brief episode of rain over the Pacific Northwest around Thursday. Highest totals with the latter event should be confined to the Olympics. Much of the East will tend to be dry most of the period but a front dropping south through the region early in the period could produce some areas of light rain and the upper low/trough near the East Coast could produce some scattered rain, most likely over the Northeast. The upper ridge settling over east-central North America will bring the most persistent warmth to the Northern Plains and Upper/Mid Mississippi Valley through the period. The Northern Plains in particular may see some plus 20-25F or so anomalies for daytime highs on Tuesday and morning lows on Wednesday. Otherwise temperatures will generally be 5-15F above normal. Meanwhile upper troughing should bring below normal temperatures across much of the West, with greatest coverage of minus 10-20F anomalies for highs likely to be from the Northwest and parts of California into the Great Basin and north-central Rockies Tuesday-Wednesday. By late week into the weekend the West Coast states and Great Basin will see a moderating trend while below normal highs focus over and just west of the southern Rockies. Central/southern parts of the East will see above normal temperatures Tuesday-Wednesday ahead of an advancing cold front. Cooler air behind the front will bring highs down to near normal, and perhaps slightly below normal along parts of the East Coast. Rausch/Tate 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