Extended Forecast Discussion NWS Weather Prediction Center College Park MD 412 PM EDT Mon Oct 18 2021 Valid 12Z Thu Oct 21 2021 - 12Z Mon Oct 25 2021 ...Heavy rainfall potential next weekend from central California to western Washington... ...Overview... Guidance shows a fairly amplified mean pattern that will be slow to evolve over time. A mean trough aloft over the eastern Pacific/West Coast will support a period of active weather in its vicinity from late week onward, with a series of strong northeastern Pacific storms bringing at least a couple fronts into and through the West with heavy precipitation and gusty winds. Downstream from an upper ridge drifting across central Canada, an upper low expected to be over Ontario/Quebec late this week and weekend should anchor a trough that extends over the eastern half of the country. This trough will likely show some progression/flattening by early next week. Weak mean ridging aloft will drift across the central U.S. This ridge may connect with the Canadian ridge at times but one or more shortwaves ejecting from the West could filter through it as well. ...Guidance Evaluation/Preferences... For about the first half of the period the best model clustering consisted of the 00Z GFS/ECMWF, latest ensemble means, and 00Z UKMET. The starting blend used the 00Z GFS/ECMWF and a little CMC as the 00Z UKMET was not available locally. The 06Z GFS strayed to the west of consensus with the upper low between the Great Lakes and Hudson Bay, ultimately leading to premature flattening of the eastern U.S. upper trough. The preferred cluster showed an earlier establishment/definition of a front that sets up over the Plains and Ohio/Tennessee Valleys. Meanwhile continuity and agreement are good with an initially very deep Pacific storm that brings a leading front into the West and then another front that approaches the West Coast by early Saturday. By the latter half of the period, guidance generally shows a steady deepening of the upper trough whose axis should reach the West Coast by early next week. This suggests an evolution toward heavier/farther south precipitation but individual model runs/ensemble members show quite a bit of spread for the details of upper dynamics and the track/strength/timing of any potentially vigorous storm systems. At this time prefer to lean toward a model/mean blend that provides a conservative depiction of surface waviness while awaiting better agreement for specifics of a better defined system. Also this blend leans away from 00Z/06Z GFS runs that are on the aggressive side of guidance with bringing height falls through the West. The 12Z GFS compares better to the model/ensemble mean consensus. Farther east, the combined details of how quickly the eastern trough begins to lift out and low-predictability details of energy passing through the central U.S. mean ridge will affect the position of the Plains into east-central U.S. front and embedded waves. GFS/ECMWF runs have not been very consistent thus far. ...Weather/Hazards Highlights... Across the West Coast region, expect multiple periods of rain/showers from northern California to western Washington, and some of the rain will likely be moderate to heavy at times with moist onshore flow ahead of Pacific storm systems/fronts. Highest totals of at least several inches for the five-day period will be possible over favored coastal and windward terrain. Snow is likely for the highest mountains of the Cascades and Sierra Nevada, mainly above pass level. By the weekend guidance continues to show the signal for an atmospheric river event reaching into the coastal ranges of northern California and southwest Oregon. Strong winds are also possible near the coast. By Sunday and into Monday some of this moisture should reach the Intermountain West and the central/northern Rockies while gradual deepening of the upper trough may push meaningful precipitation southward into central California. At this time confidence is below average for important details of any potentially strong eastern Pacific storms that could enhance winds and precipitation focus. Great Lakes into eastern Canada low pressure and a trailing wavy front crossing the East late this week will produce mainly light to moderate rainfall due to system progression. Meanwhile an area of locally moderate to heavy rain will be possible over the Midwest/Lower Ohio Valley during the weekend, to the north of a front that sets up over parts of the south-central U.S. In terms of temperatures, the West Coast states will see a slowly expanding area of below normal highs during the period due to the upper trough and clouds/precipitation. The core of highs 10-15F or so below normal should be centered over the northern half of California and far southwest Oregon. Below normal highs may reach into the Great Basin by next Monday. In contrast the southern Plains will see increasing coverage of above normal readings with plus 10-15F anomalies becoming more common Saturday-Monday. It will feel more like mid November across much of the Northern Plains in the wake of the departing storm system on Thursday, with high temperatures running about 5-10F and locally slightly more below average. Expect a gradual moderation in northern tier temperatures going through the end of the workweek and into the weekend as an upper-level ridge begins to build in. The northern High Plains may reach up to 10F above average by next Monday. On Thursday it will feel more like late September over the Mid-Atlantic/Northeast with highs on the order of 5-10F or so above average ahead of an advancing cold front. Frontal passage will bring highs down to near or slightly below normal. Rausch/Hamrick Hazards: - Heavy rain across portions of California, the Central Great Basin, and the Pacific Northwest, Thu-Fri, Oct 21-Oct 22. - Heavy rain across portions of California and the Pacific Northwest, Sun-Mon, Oct 24-Oct 25. - Heavy rain across portions of the Central Plains and the Middle Mississippi Valley, Sun, Oct 24. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Sun-Mon, Oct 24-Oct 25. - Flooding occurring or imminent across portions of the Southern Plains. - Flooding likely across portions of the Southeast. - High winds across portions of the Aleutians, Thu, Oct 21. 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml