Extended Forecast Discussion NWS Weather Prediction Center College Park MD 512 PM EDT Mon Oct 17 2022 Valid 12Z Thu Oct 20 2022 - 12Z Mon Oct 24 2022 ...Overview... A major shift in the upper-level pattern is expected to occur across the U.S. during the medium range period. A much amplified upper low/trough anchored over the eastern U.S. is forecast to retreat by late this week as an anomalous ridge over the West is forecast to be replaced by major troughing arriving from the northeastern Pacific by the weekend. This will likely result in a rather amplified trough to exit the Rockies into the Plains by early next week. Meanwhile, the presence of an upper low west of Baja California will introduce uncertainty into the forecast for the Southwest early next week. A smaller scale upper low and its interaction with an offshore front will also introduce uncertainty to the forecast along the East Coast from the weekend to early next week. ...Model and Predictability Assessment... Deterministic model solutions are in good agreement as the medium-range period begins on Thursday. Timing agreement remains good through late week regarding the amplified upper low/trough lifting out of the eastern U.S. By the weekend, models have generally trended toward a more amplified trough to dig faster across the northeastern Pacific toward the Pacific Northwest. Meanwhile, models continue to show poor agreement on the evolution of an upper low situated well off Baja California and the degree of interaction with the incoming major troughing across the western U.S. The 00Z CMC is fastest in terms of the western U.S. trough lifting the Baja upper low into northwestern Mexico by late in the weekend. The ECMWF and GFS both keep the Baja upper low farther offshore. By early next week, increasing southerly flow is forecast for the Plains, with GFS and CMC introducing heavier rains across southern Texas. On the other hand, the ECMWF keeps the southern Plains dry. The ECMWF is also most enthusiastic in developing an upper low near coastal southeastern U.S. this weekend into early next week, while the GFS is faster and weaker bringing this feature up and just off the East Coast. The CMC is somewhere in between these two extremes. The WPC forecast for fronts/pressures was based on a blend of the 00Z EC/EC mean, 00Z CMC/CMC mean, and the 06Z GFS/GEFS, transitioning to mostly a blend of the ensemble means by Day 7. This approach maintained good continuity with the previous forecast aside from typical nudges reflecting latest trends. ...Sensible Weather and Potential Hazards... The contiguous U.S. should overall be rather dry on Thursday, with the exception of lingering light lake effect rain/snow due to an anomalously cold upper low situated over the relatively warm Great Lakes waters. South Florida may see a few rain showers ahead of a strong cold front approaching from the north. By late in the week through next weekend, the upper trough arrival/amplification across the West and an associated surface front pushing across the region should produce a notable increase in precipitation coverage and intensity. The best focus will be over favored windward terrain over the Pacific Northwest on Friday and then extending southeastward into the northern/central Rockies and Wasatch over the weekend, with higher elevation snow and lower elevation rain. Confidence in a meaningful precipitation event is becoming fairly good, but exact amounts of rain/snow and timing will still need to be refined with time. The ECMWF has the fastest arrival time of the precipitation over the Pacific Northwest on Friday. Precipitation may also be possible across the Southwest Friday and especially Saturday depending on the timing and track of the southern stream low and its eventual possible phasing. By early next week, rain chances should generally increase across the Plains and into the upper Midwest ahead of the upper trough and a developing low pressure system with areas of enhanced rainfall possible but the specifics are not yet clear at this time. Meanwhile, some rain may be possible along or near the East Coast this weekend into Monday, 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 extend warm spell across the Northwest into Thursday with high temperatures challenging some daily record records. Conversely, the anomalous upper low/trough in the East should produce rather cold temperatures more common to November and December from the Plains eastward for Thursday. Below normal temperatures by 10-20F will be widespread and translate into lows in the 30s spreading south possibly all the way to the Gulf Coast (Mobile, Alabama for example), which could set low temperature records across the Southeast. As the amplifying trough barrels into the Pacific Northwest and replace the stagnant upper ridge, the warm spell across the West will abruptly ends this weekend followed by widespread below normal temperatures by early next week. Temperatures are also expected to gradually warm from the Plains to the Mississippi Valley and then expand into the Northeast early next week with expansive height rises ahead of the upper trough. Kong/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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Fri, Oct 21 and Sun, Oct23. - Flooding occurring or imminent across portions of the Southeast. 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