Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Nov 01 2022 Valid 12Z Fri Nov 04 2022 - 12Z Tue Nov 08 2022 ...System emerging from the Rockies to bring a potential for heavy rain to portions of the Plains... ...Late week atmospheric river event likely to bring an episode of heavy precipitation to parts of the Northwest... ...Overview... Guidance continues to show an amplified pattern late this week into the early weekend as an upper trough and embedded low moves from the Rockies into the Plains and a strong ridge builds near the East Coast. Strong North Pacific flow pushing into western North America will help to accelerate the Plains system northeastward, deflected by the East Coast ridge that will become centered over the western Atlantic. In response to a strong upper ridge that builds over and near Alaska, by Sunday-Tuesday the mean pattern over the eastern Pacific through western two-thirds of the lower 48 should become broadly cyclonic. The leading Plains system will support a heavy rain threat over parts of the central U.S. late this week into the early weekend. Meanwhile the Northwest atmospheric river event should be most pronounced around Friday-Friday night, with the pattern potentially supporting multiple days of less extreme but still meaningful rain/mountain snow over the West thereafter. ...Guidance and Predictability Assessment... The models continue to show a fair amount of spread and trending for the specifics of the upper trough/embedded low ejecting through the Plains and beyond during the weekend. In the 12Z guidance there was a new wrinkle compared to previous cycles as the ECMWF/UKMET depicted a more elongated and farther north low for a time. GFS runs through 18Z were still on the slower side of the spread, joined by the 12Z CMC, but the latter became a pronounced southern extreme with the upper low from Sunday onward. New 00Z runs generally add to the faster trend and the 00Z CMC evolution is now much closer to other guidance. Based on the 12Z/18Z guidance and ongoing uncertainty, an operational model composite provided a reasonable faster adjustment from previous forecast but the next issuance reflecting newest solutions will likely be faster yet. As upper ridging builds toward and into Alaska, latest consensus shows an upper low/trough dropping down from the Gulf of Alaska toward Vancouver Island and Pacific Northwest during the weekend--supporting a leading surface system that pushes into the Northwest. Latest GFS/ECMWF/CMC runs all suggest that the upper trough energy should continue eastward and bring low pressure into the Plains/Upper Midwest toward the end of the period next Tuesday. This system is a relatively new feature in the guidance and the GEFS mean thus far barely hints at it. The 12Z ECens/CMCens means had a weak but defined wave as of early Tuesday. Enough signal exists to depict the system, but for now in fairly conservative form by way of a combination of the 18Z GFS/12Z ECMWF and their means. In spite of the relative agreement in principle at the moment, it should be noted that the full ensemble spread for the eastern Pacific/lower 48 pattern becomes quite broad by next Monday-Tuesday. This seems to reflect the significantly lower predictability of the eastern Pacific/lower 48 pattern that can result from typical uncertainty over the details of any upper ridge building over Alaska during the cool season. ...Weather/Hazard Highlights... The leading system emerging from the Rockies into the Plains will bring an episode of heavy rain potential to parts of the central U.S. late this week and into the weekend. Some strong convection is possible over parts of the Plains as well, per the latest outlooks from the Storm Prediction Center. The experimental Day 4 Excessive Rainfall Outlook reflects the best potential for heaviest rainfall during Friday-Friday night, over and just east of portions of the south-central Plains. A band of moderate to heavy rainfall may develop along a front extending northward into the Midwest as well. Heavy rainfall should become less widespread over the course of the weekend as the supporting dynamics lift northeastward. The portion of the surface front stalling over the Lower Mississippi Valley may provide the best focus for lingering areas of significant rainfall. This event may feature a combination of flash flood threats (in mainly urbanized/metro regions) where the heaviest rain occurs but also beneficial rainfall given the long-term drought over the region. Monitor Storm Prediction Center outlooks for latest information on severe potential. The other significant precipitation focus will be over the Northwest late this week into the weekend, due to an atmospheric river event associated with strong westerly flow reaching into the region. A period of heavy rain/mountain snow appears likely with highest totals tending to be during Friday-Friday night. Some significant precipitation should continue into the weekend but the southward progression of the Pacific jet (extending the precipitation shield gradually farther south) should be sufficient to allow for a somewhat lighter trend. A period of strong winds will be possible in this event as well. Somewhat less extreme but still significant precipitation will reach into the northern Rockies. The pattern may support continuation of some precipitation over the northern half of the West through the first half of next week. Snow levels over far northern areas may trend notably lower depending on the southward extent of cold air pushing down from western Canada. Confidence is not very high thus far, but guidance suggests that a system reaching the Northwest late in the weekend may lead to Plains/Upper Midwest low pressure by next Tuesday. This system could produce some snow over the norther tier. Near the East Coast, by very late in the period some moisture could approach from the Atlantic as an upper low near the Bahamas retrogrades toward Florida. The southern three-fourths of the West will see a broad area of daytime highs 10-25F below normal continue through Friday. Double-digit anomalies should become more scattered during the weekend as temperatures moderate but most areas should remain below normal for highs. The evolving upper pattern should support a push of cold air reaching the northern High Plains/eastern slopes of the Rockies by next Monday-Tuesday, possibly pushing highs down to 20-35F below normal over Montana. Persistence of mean troughing aloft to some degree should keep the West below normal early next week with minus 5-15F anomalies for highs. While it will feel more like winter over the West, the East will see temperatures more typical for earlier in the fall. Expect a broad area of plus 10-25F anomalies that should produce some daily records, with morning lows tending to be slightly more extreme than the daytime highs. 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