Extended Forecast Discussion NWS Weather Prediction Center College Park MD 233 AM EDT Thu Oct 28 2021 Valid 12Z Sun Oct 31 2021 - 12Z Thu Nov 04 2021 ...Overview and Guidance/Predictability Assessment... The start of the medium range period on Sunday will feature a deep upper low and associated surface low tracking into the Northeast, though the bulk of the weather hazards associated with this system should be into the Canadian Maritimes by then. Model guidance has maintained reasonably good agreement and continuity over the past few days and once again a consensus model blend provides a good starting point. Behind this, a shortwave should drag across the northern tier of the U.S. as multiple shortwave perturbations rotate southward providing reinforcement for mean troughing across the central into parts of the Eastern U.S. through the middle of next week. Models show good agreement with the initial shortwave through the Great Lakes Sunday-Monday, but timing and detail differences quickly begin to arise regarding energy rotating through the base of the mean trough Tues-Thur next week. Out West, the guidance continues to suggest a compact upper closed low may slide into the Pacific Northwest, but diverge after day 3/Sunday regarding what happens to the remnant energy as it drops southward through the Great Basin. The GFS has consistently been south and faster with this system, while the ECMWF/UKMET/CMC maintain better continuity with the ensemble means. There remains enough run to run variability in this though across all the models that confidence is low in the specifics. The next shortwave moves into the Western U.S. by Monday of next week as another amplified trough digs upstream becoming established off the West Coast by Thursday. The WPC progs for this cycle used a blend of the deterministic solutions days 3-5 (with slightly more weighting towards the ECMWF and CMC). After that, increased contributions from the ensemble means in order to help mitigate the timing and detail differences which arise mid to late period for several systems. This approach also maintained reasonable continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Heavy rains associated with the departing Eastern U.S. cyclone should lift into the Canadian Maritimes by the start of the period on Sunday with only some lingering light showers across Maine and interior New England. Elsewhere, light rain/mountain snows will be possible Sunday across parts of the central Rockies and High Plains. Some snow may be possible during the colder overnight hours even at the lower elevations of the High Plains. Confidence in the precise coverage and intensity of precipitation across these regions remains low, based on the uncertainty of important details of flow aloft. Locations from the southern Plains into the Midwest/Ohio Valley and eventually the interior northeast should see expanding coverage of rainfall along the potentially wavy front early next week. Most of the rain should be on the light to moderate side but there is a developing signal for some heavier totals over or near the southern Plains/Lower Mississippi Valley by next Tuesday or Wednesday. Pacific shortwave energy and moisture may bring one or more episodes of precipitation to central/northern parts of the West Coast Monday-Wednesday, still with fairly modest confidence in the details at this time. The only notable temperature hazard is below to much below normal temperatures expanding across much of the Central U.S. through much of the period. Areas from the northern High Plains through the south-central Plains and east into the Middle Mississippi Valley/Ohio Valley should see one or more days with highs 10-20F below climatological values for this time of year. Anomalies for min temperatures should be less extreme but could still be noteworthy for producing the first freeze in some areas from the central Plains into the Midwest around next Tuesday. Santorelli/Rausch 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