Extended Forecast Discussion NWS Weather Prediction Center College Park MD 539 PM EDT Thu Oct 21 2021 Valid 12Z Sun Oct 24 2021 - 12Z Thu Oct 28 2021 ...Multi-Day Heavy Rainfall and Mountain Snow Threat over the West... ...Heavy Rainfall Threat over the Midwest... ...Overview... A quite active storm track over the eastern Pacific in association with an anomalous upper level trough will result in multiple rounds of impactful weather for the West Coast and extending well inland across the Intermountain West and Rockies late this weekend into early next week. Systems will eject eastward across the Plains and then the Midwest states and Northeast/Mid-Atlantic, continuing a rainfall threat in tandem. This will result in an increasingly wetter eastern half of the Lower 48 next week, but amid relatively mild temperatures for the end of October. ...Guidance Evaluation/Preferences... The larger scale longwave pattern evolution remains rather steady over the past few cycles, but embedded system propagation/track/depth has not. Lead system into the West will start as a powerful/deep cyclone well offshore that will move into southwestern British Columbia. Multi-day trend has been a bit farther north with the upper trough and surface low in the ECMWF guidance (ensembles) while the GFS and GEFS were farther south. Consensus including the UKMET and Canadian lied toward the southern side but waves of low pressure along the front could disrupt the movement of the parent low. Regardless, strong cold front will move into the West and sink southeastward as another system moves into the Pac NW. Upper pattern will then transition to a bit more amplitude and possibly form a transient Rex Block over the Northeast and western Atlantic as a coastal system forms. Uncertainty increases quite a bit east of 90W midweek next week and relied mostly on the ensemble means to round out the period next Wed-Thu. Regardless, consensus shows more ridging into the West and the trough axis into the Mississippi Valley. ...Weather/Hazards Highlights... A lead and wavy lead front across portions of the Midwest/Mid-Atlantic then western Atlantic this weekend and into next week is expected to produce scattered to numerous showers with the potential for a few convective complexes that offer potential for locally heavy downpours/runoff issues, especially over the Midwest Sunday to the Mid-Atlantic/New England Monday with possible coastal low development. Proximity to the coast will influence how much rainfall will occur over the Northeast/Mid-Atlantic as temperatures remain mild. Meanwhile across the West Coast region, multiple rounds of enhanced rains are expected from the Pacific Northwest through especially California that will likely be heavy at times with breezy conditions as well. This will especially hold true for Sunday and into Monday as a deep and long fetch moisture plume (Atmospheric River) shifts southward from the Pacific Northwest across California. Several inches of rain are possible over the multi-day period, particularly across favored windward terrain which may result in some instances of runoff issues and/or flooding. Burn scars will also be especially vulnerable. Heavy snows are likely for the higher elevations of the Cascades and especially Sierra, with multiple feet possible at the highest elevations. Snow levels are expected to lower Sunday into Monday as the core of the upper trough moves inland along with the surface cold front. Strong winds are also possible for the coast and inland along mountain ridges. Some of this moisture is expected to reach the Intermountain West and the central/northern Rockies with significant snowfall for some of these areas as shown in the WPC Winter Weather Outlook linked below. The main associated cold front reaching the Plains by Tuesday and onward across the east-central U.S. Wednesday/Thursday is expected to present an emerging and expanding lead rainfall/convective focus with an influx of Gulf moisture. An additional upstream Pacific system is forecast to maintain the active pattern, albeit farther north, with periods with enhanced precipitation chances going into the Pacific Northwest well into next week. In terms of temperature highlights, it will feel more like late September across much of the central and southern Plains as the upper level ridge remains in place across this region. High temperatures are forecast to be 10-15 degrees above normal ahead of the cold front and possibly near record highs over parts of Texas. Cooler than normal temperatures are forecast for much of the West, influenced by the upper trough and mainly overcast skies. High temperatures may be 10-20+ degrees below normal but will moderate by the middle of next week. These may challenge daily record cool max values over parts of California early next week with highs only in the 60s. Fracasso/Schichtel 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 California, the Pacific Northwest, and the Northern Great Basin, Mon-Thu, Oct 25-Oct 28. - Heavy precipitation across portions of California, the Great Basin, the Pacific Northwest, and the Southwest, Sun-Mon, Oct 24-Oct 25. - Heavy precipitation across portions of the Northern/Central Rockies, and the Great Basin, Mon-Tue, Oct 25-Oct 26. - Heavy rain across portions of the Central Plains, the Upper/Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Sun, Oct 24. - Heavy rain across portions of the Middle/Lower Mississippi Valley, the Tennessee Valley, the Southern Appalachians, the Southeast, and the Ohio Valley, Wed, Oct 27. - Severe weather across portions of the Central/Southern Plains, and the Lower/Middle Mississippi Valley, Sun, Oct 24. - Flooding occurring or imminent across portions of the Southern Plains. - High significant wave heights for coastal portions of California and the Pacific Northwest, Mon, Oct 25. 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