Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Oct 16 2022 Valid 12Z Wed Oct 19 2022 - 12Z Sun Oct 23 2022 ...Overview... The medium range period begins Wednesday with an amplified/blocky upper level flow pattern consisting of potent troughing across the eastern half of the U.S. extending south from an anomalously strong upper low in eastern Canada and an upper ridge/high over much of the West. This pattern should persist through Thursday while strong shortwaves move through the eastern trough and a southern stream upper low slowly edges eastward across the Pacific toward southern California. Lake effect/enhanced precipitation (including some snow) will be common through midweek across the Great Lakes with lighter amounts later in the week. Through Thursday temperatures will be much below average in the eastern half of the country but warmer than average in the Northwest under the ridge. A pattern change appears likely late in the week, with overall deamplification of the upper flow Friday before troughing looks to come into the West over the weekend, which should focus the warmth over the Plains with a trend toward more normal temperatures elsewhere before eventual cooling in the West. ...Model and Predictability Assessment... The forecast during the first half of the period continues to show better than average agreement and continuity overall, representative of the typically good predictability of a highly amplified large scale pattern. There are still smaller scale differences involving embedded energies within the upper low/trough in the East and exact frontal positions, but without too many impacts on the sensible weather since frontal passages will mainly be dry by the medium range period. Meanwhile the models/means continue to develop some spread for the upper low offshore southern California. Over recent runs the GFS/GEFS and CMC have tended to be on the faster side with the ECMWF on the slower side, with the 00Z ECMWF mean between the two extremes. Newer 00Z runs are on the whole slower to bring the feature onshore compared to the 12/18Z guidance, so the details are yet to be figured out. Over the past day or so, guidance has come into better agreement on the large scale with respect to how quickly incoming Pacific flow breaks down the upper ridge over the West. This looks to begin on Thursday with energy over western Canada with low amplitude troughing perhaps getting established atop the Northern Rockies by Friday, while additional energy upstream comes in to amplify troughing over the Pacific Northwest nudging eastward for the weekend. The incoming 00Z guidance has tended to be on the same page with the timing and evolution of the trough development. Less certain is the potential for the timing of the phasing of this developing trough with the southern stream, which will take additional time to resolve. Another possible issue for the latter part of the period is with energy separating from the eastern trough into the southern stream while the overall northern stream eastern trough retreats late week. This relatively smaller scale feature remains less predictable as ECMWF runs have persistently shown a slow-moving and energetic feature but GFS and CMC runs have waffled, and most ensemble members that are aggressive with the feature are ECMWF members. The WPC forecast for fronts/pressures was based on a blend of the deterministic 12/18Z models to resolve lingering detail questions during the first part of the period, and then incorporated GEFS and EC ensemble means by day 5 and beyond as operational models diverged with some details. This approach seemed to provide good continuity with the previous forecast though with a nudge toward recent trends. ...Sensible Weather and Potential Hazards... The contiguous U.S. should overall be rather dry on Wednesday-Thursday, with the exception of lake effect rain/snow due to upper level energy and the westerly flow across the relatively warm Great Lakes compared to the cold air temperatures. Precipitation amounts are likely to be higher through Wednesday downwind of the Lower Great Lakes in particular but with some lingering showers possible into Thursday. South Florida may see a few rain showers as well, along with southern California into the Southwest with the southern stream low/trough approaching. By late in the week, the upper trough arrival/amplification across the West and an associated surface front pushing across the region is forecast to spread notable precipitation chances into and expanding across the West, with typical terrain enhancement in the Pacific Northwest and Rockies with higher elevation rain and lower elevation snow. Meanwhile some rain may be possible along the East Coast this weekend, but with low confidence on how much rain could fall onshore. The amplified western ridge in place for the early part of the period will cause warmer than average temperatures across the Northwest in particular, with highs commonly 10-20F above average, potentially leading to some daily record highs through Thursday. Conversely, the anomalous upper low/trough in the East should produce rather cold temperatures more common to November and December from the Plains eastward. Below normal temperatures by 10-25F will be widespread and translate into lows in the 30s spreading south possibly all the way to the Gulf Coast (Mobile, Alabama for example) for Wednesday-Thursday. The Southeast could also see highs in the 50s and 60s, and record low maximum and minimum temperatures are likely to be set in the east-central U.S. As the upper level pattern relaxes around Friday and then transitions to a western trough/eastern ridge pattern, above normal temperatures should get pushed into the central U.S., with the north-central Plains seeing highs of 10-20F above average. Tate/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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml