Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Sun Sep 06 2020 Valid 12Z Wed Sep 09 2020 - 12Z Sun Sep 13 2020 ...Record cold over parts of the central U.S. Wed-Thu followed by gradual moderation... ...Significant rainfall possible over parts of the Plains/Midwest while rainfall increases over the East... ...Overview and Guidance/Predictability Assessment... Most guidance shows similar ideas for the broad pattern evolution during the period--sharp and amplified Rockies-Plains troughing between strong West Coast/western Atlantic ridges early in the period followed by a gradual broadening of the central/east-central North American mean trough as the ridges weaken somewhat. The West Coast ridge should reach into the western U.S. by next weekend. Corresponding to the initial upper trough there is high confidence in the Rockies/Plains cold surge bringing temperatures more typical of late fall. However models and ensembles have had considerable difficulty with resolving the details of an embedded upper low that seems likely to form, leading to much lower confidence in determining areas most likely to see enhanced rainfall due to the upper low. Looking at the array of guidance from the 12Z/18Z cycles, ensemble members maintain unusually large spread for the Rockies/Plains trough relative to typical days 3-4 forecasts. The western side of the envelope keeps a Four Corners closed low in play, such as now depicted in the new 00Z CMC and ECMWF, while the eastern side is more open and progressive than any recent operational model runs. Overall guidance trends over the past couple days have been increasing the potential of a closed low but slowly trimming the probability of formation as far west as the Four Corners (before being countered by the aforementioned CMC/ECMWF). This led to the updated forecast following a compromise among the 12Z GFS/ECMWF/UKMET/CMC for the first half of the period. The 18Z GFS was less favored due to being a little faster than the 12Z run. The new 00Z GFS has nudged a bit slower and a compromise among the 00Z GFS-UKMET/12Z ECMWF is close to the resulting forecast. Beyond the uncertainty for this low to begin with, the degree to which flow rounding the western ridge will influence the continued progression of the low by next weekend becomes an added uncertainty. Guidance has been inconsistent with this upstream energy as well. The ensemble means provide useful input by this time frame with moderate timing/amplitude for the upstream flow, leading to earlier phasing than in the 12Z ECMWF but slower timing than the GFS. Thus the forecast blend quickly transitioned to 60-70 percent means by days 6-7 Sat-Sun. The upstream energy will support a front expected to push into the northern tier of the lower 48 next weekend, with fairly low confidence in precise southward extent. ...Weather/Hazard Highlights... During Wed-Thu many locations over the southern half or two-thirds of the Plains/Rockies should see record cold max/min temperatures, with readings up to 20-35F below normal. Record cold highs may extend into the Upper Midwest on Wed. Moderation after Thu will likely bring the most extreme anomalies into the minus 10-20F range during Fri-Sat and minus 5-12F for Sun. The initial upper trough and leading surface front may generate some locally moderate/heavy rainfall over southern parts of the Plains around midweek. Otherwise there is still a decent signal for some areas of moderate/heavy rainfall from the Plains into the Midwest and possibly northeastward but low confidence on the specifics due to guidance spread/run-to-run variability on the path of the upper low. An upper low reaching as far back as the Four Corners would also produce more precipitation over/near the southern half of the Rockies. How much rainfall occurs over the east-central U.S. will depend on the path/timing of the upper low, which will affect progression of the leading front. Meanwhile areas along the East Coast should see increasing rainfall, some locally heavy, as moist Atlantic flow pushes a warm front just inland. Above normal temperatures (plus 10-20F anomalies) concentrated over western Washington/Oregon as of midweek will gradually expand eastward/southeastward while greatest anomalies decrease toward the plus 10-12F or so range. This will correspond to the gradual weakening but eastward expansion/drift of the upper ridge initially along the West Coast. The eastern Great Lakes and Northeast will be at the center of warmest eastern U.S. temperatures versus normal mid-late week, with clouds and rainfall tending to favor more anomalous lows versus daytime highs. Readings are likely to be within a few degrees on either side of normal next weekend though above normal lows may persist over the Mid-Atlantic and vicinity. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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