Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Fri Oct 22 2021 Valid 12Z Mon Oct 25 2021 - 12Z Fri Oct 29 2021 ...Multi-Day Heavy Rainfall and Mountain Snow Threat over the West... ...Overview... An 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 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 steady over the past few model cycles, but embedded system details regarding timing, track, and depth remain very uncertain, especially at longer lead times. With the leading system/west coast trough, the models (with the exception of the UKMET) show good agreement with depth of the initial deep/powerful cyclone off the British Columbia coast, but timing and evolution of this trough as it tracks through the central U.S. become problematic after Wed/day 5. Theres indications in the deterministic guidance that a deep closed upper low may form over the Great Lakes/Northeast by late period, but this is a newer theme in the last run or two so confidence is low. The GFS also is quite a big faster than the ECMWF/CMC with the associated cold front into the East late week. The ensemble means seem to show better consistency and agreement. In the East, problems start right off the bat on day 3-4 with the GFS much faster with a shortwave/closed low through the Ohio Valley and eventual possible coastal low development off the Northeast Coast. The GFS has been consistenty farther north and faster with this feature. The ECMWF is the farthest south, and the CMC in the middle, along with the ensemble means. There are also timing issues with the next trough/system into the West Coast later next week. The WPC blend used a majority deterministic guidance (more EC/CMC than GFS) for days 3-4, with increasing ensemble mean contributions thereafter to help mitigate the detail differences which could take a few days to resolve. This maintains fairly good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... A lead and wavy front across portions of the Midwest/Mid-Atlantic then western Atlantic 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 across the eastern Mid-Atlantic/New England Monday-Tuesday 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 into Monday as a deep and long fetch moisture plume (Atmospheric River) shifts southward from the Pacific Northwest into California. Several inches of rain are possible over the multi-day period (which begins during the short range/this weekend), particularly across favored windward terrain which may result in some instances of runoff issues and/or flooding especially over already vulnerable burn scar areas. 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. 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 out West, albeit farther north this time, 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 early in the week 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. Santorelli/Fracasso 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