Extended Forecast Discussion NWS Weather Prediction Center College Park MD 221 AM EDT Tue Oct 04 2022 Valid 12Z Fri Oct 07 2022 - 12Z Tue Oct 11 2022 ...Overview... A fairly amplified upper pattern will be in place when the medium range period starts Friday with large scale troughing across the East, and a blocky ridge over the West Coast and East Pacific. This trough over the East will push a cold front off the East Coast with expansive high pressure in its wake spreading from the north-central U.S. on Friday into the Midwest/East this coming weekend and bringing a period of considerably cooler temperatures. Behind this Eastern trough, another shortwave/possible closed low will skirt through the northern tier states likely associated with some additional showers. A week upper level low over the Southwest U.S. will keep monsoonal moisture plenty during the period across the Four Corners region as well. By next Monday-Tuesday, models show another amplified feature dropping into the northern High Plains from western Canada. ...Guidance Evaluation/Predictability Assessment... The latest models and ensembles maintain fairly good agreement on the overall pattern, particularly the first half of the period. Embedded details remain somewhat uncertain though, as there are some timing differences with the Eastern U.S. trough as it shifts through the Great Lakes/Northeast. There is also good agreement out West regarding the ridge which should slowly retrograde into the eastern Pacific as a week upper low near Arizona/New Mexico settles into the northern Gulf of California before eventually opening up by Sunday or Monday. For the first half of the period, a general model blend between the GFS, ECMWF, and CMC worked well. The last few cycles of guidance have trended stronger with a compact shortwave (with some runs suggesting an embedded closed low) dropping into the far northern states around day 5/Sunday. The ECMWF has consistently been strongest with this, maintaining a small closed low feature all the way into Tuesday just to the north of Lake Superior. The GFS is weaker and a bit quicker, but still fairly comparable to the ECMWF. The CMC on the other hand wants to drop the low due south and with it lingering over the northern High Plains region into the middle of next week. This likely is a result of a much stronger/blockier ridge over the Western U.S. which keeps the next batch of energy from the Gulf of Alaska late period much weaker and staying well north into Canada. Both the GFS and ECMWF have support from the ensembles that this Alaskan troughing will drop into southwest Canada and eventually the northern Rockies/High Plains, which would act to push the first shortwave out to the northeast much quicker. Based on this assessment, the blend for the latter half of the period trended more towards the ensemble means to help mitigate some of the detail uncertainties, but still kept a modest amount of the deterministic GFS and ECMWF. Given the uncertainty in timing and amplitude of both systems of note, the pattern depicted by the ensemble means gets understandably flat, so it seemed necessary to include deterministic solutions to try and maintain some level of weak amplitude with features. For the QPF, the National Blend of Models once again seemed much too light and not broad enough with its QPF footprint, especially across the Southwest. QPF from the NBM was essentially non-existent so some considerable amounts of the GFS and ECMWF were needed to add some QPF across the Northern Plains/Upper Midwest associated with the compact shortwave through the region late weekend and early next week. ...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 east and west expansion of rainfall possible by early next week as the upper low opens up. To the north, the first 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. Behind this front, much below normal temperatures will shift from the northern Plains into the Midwest and East. 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 the central Appalachians, and even 20s for North Dakota and northern Minnesota particularly Friday morning. The next cold front into the northern tier this weekend likely brings some showers with it, but still considerable uncertainty in coverage and amounts. Out West, temperatures 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. Western U.S. temperatures may trend back closer to normal by Tuesday next week as the ridge retrogrades into the Eastern Pacific. 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