Extended Forecast Discussion NWS Weather Prediction Center College Park MD 205 PM EDT Sun Oct 17 2021 Valid 12Z Wed Oct 20 2021 - 12Z Sun Oct 24 2021 ...Overview... A fairly typical late October weather pattern is expected to be in place across the majority of the nation during the medium range forecast period. A surface low and trailing cold front will track from the Midwest to the Northeast, with the front exiting the East Coast by the end of the week. A rather active pattern will be in place across the eastern Pacific with a persistent upper level trough off the West Coast that will support several shortwaves rotating around it, along with a couple of cold frontal passages and rounds of precipitation. Meanwhile, a ridge axis is expected to briefly build over the Intermountain West and then be positioned over the Plains by next weekend. ...Guidance Evaluation/Preferences... 00Z/06Z model guidance remains in good agreement with the overall synoptic scale pattern through about Thursday, as positioning of the closed low over the central U.S. Wednesday appears well handled by deterministic and ensemble guidance, even as it tracks eastward into the Great Lakes region and starts getting absorbed into northern stream flow on Thursday. Guidance is also becoming more agreeable with a northern stream closed upper low centered over James Bay Thursday, but uncertainties remain with this flow over the southeastern quadrant of Canada by Friday. While the 00Z GFS appears in line with the consensus of 00Z guidance with a relatively slow track of the upper low eastward, the 06Z GFS lifts a trough more quickly northeastward, and the new 12Z GFS also followed this faster track. The 06/12Z GFS runs do appear to be somewhat fast outliers at this point, while the CMC is slowest, and the 00Z ECMWF provides a good middle ground solution that aligns with the 00Z EC and GEFS ensemble means. Overall consensus does suggest a slightly faster trend with this feature compared to earlier runs by Friday/Saturday, but likely not as fast as the recent GFS runs. Troughing is expected across the eastern U.S. by next weekend, but its amplitude will depend on these northern stream factors. While all model guidance shows the overall troughing pattern in the eastern Pacific, shortwave timing and magnitude differences persist. The 00Z/06Z GFS runs appear to be a more amplified and slower outlier with troughing approaching California on Friday, and even the GEFS ensemble members are generally deeper than members from the CMC and EC ensemble suites. Once again the ECMWF seems to show a middle ground solution compared to the deep GFS runs and the weak UKMET. Differences in deterministic guidance persist through the weekend with shortwave tracks and if/how they amplify the broader flow, but fortunately the ensemble mean guidance seems generally agreeable. Thus for this forecast, a multi-model blend of the deterministic ECMWF/GFS/UKMET/CMC was able to be used at the beginning of the period, generally weighting the ECMWF slightly heavier through the period. Began leaning away from the 06Z GFS and 00Z CMC by Friday, while increasing contributions from the ensemble means by next weekend to reduce individual model issues. ...Weather/Hazards Highlights... For the West Coast, surges of moisture along and ahead of cold fronts approaching the region will lead to multiple rounds of moderate to possibly heavy precipitation for northern California into the Pacific Northwest through next weekend. Currently, the heaviest round of precipitation looks to occur next weekend as a more west-east oriented atmospheric river takes aim at the West Coast. Some precipitation is likely to spill into the Intermountain West and northern Rockies around Friday and through the weekend. In general, lower elevation rain and higher elevation snow are expected, with snow levels possibly lowering slightly next weekend behind the cold frontal passage. The quantitative precipitation forecast for the upcoming week (including the short range period) shows 3-10 inches of rain/melted snow amounts over favored windward terrain, particularly the Coastal Ranges and to a lesser extent the Cascades and northern Sierra Nevada. Gusty winds are also possible near the coast. Meanwhile for the eastern half of the U.S., showers and possibly a few thunderstorms are expected along the track of the surface low and its associated cold front late next week, but rain amounts should remain generally light given not much moisture to work with. As moisture increases by next weekend, rain chances are also forecast to increase around the Middle Mississippi Valley. In terms of temperatures, it will feel more like mid November across much of the Northern Plains in the wake of the departing storm system on Wednesday, with high temperatures running about 10 to 15 degrees below average. There should be a gradual moderation in temperatures going through the end of the workweek and into the weekend as an upper-level ridge begins to build in, and then readings up to 10 degrees above average for the High Plains by next Sunday. Generally cooler than normal temperatures are forecast for the West Coast, influenced by the upper trough and mainly overcast skies. Ahead of the cold front, it will likely feel more like late September from the Great Lakes and Ohio Valley to the East Coast with highs on the order of 5-10 degrees above average for the middle of the week, although humidity levels should remain in check. Slightly below normal highs are then expected behind the cold front. Tate/Hamrick 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