Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Mon Oct 03 2022 Valid 12Z Thu Oct 06 2022 - 12Z Mon Oct 10 2022 ...Overview... A stubborn compact upper low/shortwave stuck over the Mid-Atlantic region during the short range period will finally be departing off the Northeast coast as the medium range forecast starts on Thursday. In its wake, larger scale mean troughing should prevail over eastern North America (led by a shortwave diving into the Upper Great Lakes from Canada, possibly followed by a weaker shortwave by Sunday-Monday) while a persistent upper ridge just inland from the West Coast gradually retrogrades offshore. Despite the somewhat amplified pattern, expect relatively quiet weather conditions across most of the CONUS into the start of next week. Exceptions will include some Southwest rainfall in response to monsoonal moisture and a weak upper low settling just south of the region, and a potent central/eastern U.S. cold front producing some rain especially in the Great Lakes region along with considerably cooler temperatures after its passage. The western U.S./eastern Pacific upper ridge will support very warm temperatures through the period. ...Guidance Evaluation/Predictability Assessment... Latest models and ensembles maintain agreement for the general pattern consisting of the western U.S. ridge retrograding into the eastern Pacific as initial weak energy settles over parts of the West, the mean trough prevailing over the eastern two-thirds of the lower 48, and a weak Arizona/New Mexico upper low settling over northern Gulf of California for a time before opening up by Sunday or Monday. Embedded details are more uncertain though. Models develop 12-24 hours or so of timing spread for the system affecting the northeastern U.S. and southeastern Canada late this week into the weekend. The 00Z ECMWF was fastest while the 00Z/06Z GFS were slowest. Latest UKMET runs lean much closer to the slow side while the 12Z CMC has trended somewhat faster than its 00Z run (now closer to the ECMWF) and 12Z GFS accelerated to a compromise. Thus it will take additional time to determine any conclusive trends. Behind this system, 00Z through 12Z runs are showing an increasing signal for another shortwave (possibly closed per the 12Z UKMET/CMC) reaching the far northern tier by early day 6 Sunday. This feature originates from around southern Alaska/northwestern Canada on Sunday. There is enough agreement on its existence but favor a conservative open wave depiction at this time since models are inconsistent in its evolution. A blended approach looks good for the weak energy dropping into the West (00Z CMC was extreme with forming a closed low). Finally, current spread/run-to-run variability suggest an intermediate solution for how late-period Gulf of Alaska into Canada energy may break down the northern part of the mean ridge. Based on guidance through 06Z, the updated forecast started with an operational model composite for the first half of the period and then trended to a half models/half ensemble means blend by day 7 Monday. The primary continuity change was introduction of the Sunday-Monday northern tier wave/frontal system. QPF adjusted the 13Z National Blend of Models to reflect a wider footprint for light amounts (mostly in the Great Lakes/Northeast and southwestern U.S.) given the typically low bias for light precipitation in the NBM. ...Weather/Hazards Highlights... The combination of persistent monsoonal moisture and an upper low settling over the northern Gulf of California will produce diurnal showers and thunderstorms over parts of the Southwest/southern Rockies, with locally heavy rainfall possible in some spots. Intensity and coverage may vary from day to day, with some eastward expansion of rainfall possible by early next week as the upper low opens up. To the north, a cold front shifting across the Midwest and Great Lakes may bring some light rain mainly around Thursday, and then lake enhanced showers in the westerly flow behind the front Friday and perhaps Saturday. Temperatures out West will be warm into early next week underneath a blocky upper ridge, with daytime highs over the Northwest 10 to 15 degrees or so above normal. Despite the anomalies, the resulting temperatures peaking in the 70s and 80s (90s for parts of California) are unlikely to create any heat related hazards. Conversely, Canadian high pressure spilling into the north-central U.S. and expanding eastward behind the cold front will result in much below normal temperatures shifting from the northern Plains into the Midwest and into much of the East Thursday-Monday. Daytime highs in some locations could be 15 to 20 degrees below average (mainly Thursday-Saturday) while overnight temperatures are forecast to drop into the 30s for many locations from the northern half of the Plains into central Appalachians, and even 20s for North Dakota and northern Minnesota particularly Friday morning. Rausch/Santorelli 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