Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 AM EDT Sat Oct 15 2022 Valid 12Z Tue Oct 18 2022 - 12Z Sat Oct 22 2022 ...Overview... The medium range period begins Tuesday with an amplified/blocky upper level flow pattern consisting of an anomalously strong upper low centered near the Lower Great Lakes with troughing extending through the eastern half of the U.S. and an upper ridge/high over much of the West. This pattern should stay pretty persistent through Wednesday-Thursday while strong shortwaves move through the eastern trough and a southern stream upper low slowly edges eastward across the Pacific toward California. Lake effect/enhanced precipitation (including some snow) will be common through midweek across the Great Lakes with lighter amounts later in the week. Temperatures are forecast to be much below average in the eastern half of the country but warmer than average in the Northwest under the ridge. Then by Friday-Saturday, the pattern becomes more uncertain but with a likely trend toward overall deamplification in the upper levels. ...Model and Predictability Assessment... Model guidance continues to show good clustering for the upper low over the Great Lakes Tuesday and then lifting northward over western Quebec, and even reasonable agreement for shortwaves embedded within the trough such as over the Upper Great Lakes by day 5/Thursday in the 12/18Z guidance compared to previous runs. Minor differences could be resolved through the blending process of the 18Z GFS and 12Z ECMWF/CMC/UKMET through the early part of the period. Models also agree well with ridging across the Intermountain West through around Thursday while an upper low in the eastern Pacific tracks eastward. The exact position of this southern stream low has been variable from cycle to cycle. The 12/18Z deterministic models were fairly well clustered with a western position but with the ensemble means centered a bit east, and the 00Z models generally came in a bit eastward as well. This is within fairly typical spread for the medium range timeframe but its timing will cause some sensible weather differences with clouds/showers coming into California and the Southwest dependent on when the energy/low trough moves ashore likely later in the week. The most uncertain part of the period arises by Thursday-Saturday as the pattern evolves dependent particularly on rounds of northern stream energy across western Canada. The ECMWF has been persistent in showing the ridge should start getting suppressed by western Canada energy around Thursday, with other guidance including the GFS and some GEFS ensemble members playing catch up to this idea, though the 12Z CMC remained on the amplified ridge side, but overall models seem to be converging somewhat at that point. By Friday the 12/18Z GFS and 12Z ECMWF and their 00Z runs along with the 00Z CMC indicate some retreat of the main upper low but with energy remaining within the trough farther south. The 12Z ECMWF seemed particularly aggressive with closing off a separate low in the east-central U.S. with a slower movement, while the GFS is more progressive. There are also some differences with models for the western U.S. and how/when the southern stream trough may merge, along with how additional northern stream energy may suppress the ridge further. The WPC forecast leaned toward a middle ground for all this by favoring the GFS/ECMWF/EC mean/GEFS mean for late week, but the forecast leans toward the lower predictability side by later in the period so significant changes to the forecast are possible with time. ...Sensible Weather and Potential Hazards... By Tuesday, a frontal system will have passed across much of the East with a drying high pressure system behind it. However, the front could continue to cause showers and thunderstorms across Maine, the Florida Peninsula, and southern Texas ahead of the frontal passage in those areas. Recent model trends have been for a slightly faster shortwave and front in New England, which would push a swath of moderate to potentially heavy rainfall due to considerable moisture transport east into Canada rather than over Maine, but the exact positioning may not be resolved until the short range period. Meanwhile upper level energy and the westerly flow across the relatively warm Great Lakes compared to the cold air temperatures will produce considerable lake effect precipitation downwind of the lakes. Support is highest for notable amounts of rain and/or snow on Tuesday with a surface low pressure system nearby, but at least the flow aloft will be favorable for additional lake effect precipitation through much of next week. Other than those areas, much of the contiguous U.S. is forecast to be dry during the medium range period, until a few showers could come into southern California and the Southwest with the southern stream upper low/trough during the latter half of the week, and then precipitation chances could increase across the Northwest by Friday/Saturday with upper level energy and a surface frontal system. The amplified western ridge will cause warmer than average temperatures across the Northwest in particular, with highs commonly 10-20F above average through much of next week, potentially leading to some daily record highs. 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 teens in portions of the north-central U.S. Tuesday and 30s spreading south possibly all the way to the Gulf Coast (Mobile, Alabama for example) by 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 towards the end of the week, temperature anomalies should lessen as well, with current forecasts showing the most anomalous temperatures to be 10-15F above average across the central U.S. on Saturday. 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, 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