Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Dec 22 2022 Valid 12Z Sun Dec 25 2022 - 12Z Thu Dec 29 2022 ***Unsettled conditions for the West Coast with heavy rain and heavy mountain snow with an atmospheric river event early next week*** ...Synoptic Overview... The overall weather pattern improves going into the weekend across the eastern half of the nation as the core of the intense low lifts farther northward across central Canada and weakens with time. However, there will still be some lingering lake effect snow downwind of Lakes Erie and Ontario as well as a continuation of much below normal temperatures at the start of the period. The impressively cold airmass that has plagued the eastern 2/3rds of the country for the prior days will begin modifying as the surface high settles across the southern tier states, with colder temperatures lingering longest for the Midwest. Looking ahead to early next week, multiple shortwave passages near the Pacific Northwest will keep things unsettled from northern California to Washington state with an atmospheric river pattern setting up leading to multiple days of heavy precipitation for the region. The eastern half of the country will see much closer to normal temperatures and generally dry weather. ...Guidance/Predictability Assessment... Both the deterministic and ensemble mean model guidance were very similar at the start of the period (Sunday) with the main upper-level features consisting of the closed low associated with this week's powerful storm system shifting northeastward towards Hudson Bay and mean troughing over the eastern U.S., a couple shortwaves in the mean flow sliding southeastward across the Southern Plains and Northern Rockies, and troughing upstream over the northeastern Pacific. The WPC forecast began with an initial even blend of the deterministic guidance (00Z ECMWF/UKMET/CMC and 06Z GFS) to accentuate the smaller scale details of the more subtle shortwaves compared to the ensemble means. More noticeable differences between the guidance and individually run-to-run were rapidly apparent by 12Z Monday in terms of the timing of the shortwaves rotating through the mean troughing in the eastern U.S. as well as the amplification of the pattern upstream. The influence of the 00Z CMC was decreased in the forecast as it was more of an outlier compared to the other guidance in terms of the phasing of the shortwaves, which had specific ramifications on the development of low pressure at the surface over the Plains/Midwest, and the amplification of the pattern upstream. Further into the period, the guidance continued to differ in the phasing of shortwaves in the southern and northern stream over the eastern U.S., specifically the energy of a shortwave over the South, and a shortwave moving east through the Rockies as a closed low quickly replaces this energy upstream over the northeastern Pacific. The 00Z CMC was again an outlier with a lower degree of mean troughing over the South. There was at least an encouraging convergence towards more similar solutions run-to-run between the GFS and ECMWF to keep the same blend with decreased influence of the CMC. The deterministic guidance continued to be emphasized despite the differences because the ensemble means were notably less amplified. The trend compared to the prior WPC forecast was for deeper mean troughing over the eastern U.S. and a closed low upstream over the northeastern Pacific. Late in the period, divergence amongst the model solutions and run-to-run differences increases with the phasing of the shortwave energy over the eastern U.S. between the northern and southern stream and how the closed low over the Pacific evolves downstream over the West. The influence of the 00Z ECens and GEFS means was increased as the means still captured shortwaves over the southeastern and western U.S. with the differences between the deterministic guidance becoming more difficult to resolve. This pattern lasted through the end of the period on Thursday with a difference between the phasing of shortwave energy moving off the East Coast and the degree of energy with the shortwave to the west digging southward into the Southern Rockies and Plains. A blend of the deterministic 00Z ECMWF and 06Z GFS and the 00Z ECens/GEFS means was able to capture the overall pattern relatively well towards the West with the deterministic guidance helping to capture the shortwave energy off the East Coast. ...Weather/Hazard Highlights... Saturday will feature widespread temperature anomalies 20-30 degrees below average for morning lows and daytime highs across much of the central and eastern U.S. Sunday will still be quite cold over the eastern half of the country with a broad area of readings 15-25 degrees below normal. The theme for next week will be a steady warming trend as the upper pattern becomes less and less amplified. Moderately above normal temperatures will become established over the West with mean ridging aloft (lows tending to be a little more above normal than daytime highs) and then spread with downsloping winds into the High Plains by next Tuesday-Wednesday with some areas reaching 10-20F above normal. These winds may be strong and gusty at times. The eastern half of the country will continue to moderate, leading to mostly single-digit negative anomalies by next Wednesday. A series of northeastern Pacific systems should bring a multi-day period of atmospheric river events to the West Coast, leading to significant totals of rain and higher elevation snow. Some of this moisture will also extend eastward into the Great Basin and the Rockies, though there is less certainty in terms of total amounts in these locations. The primary focus during the weekend is forecast to be over the Pacific Northwest. Then the precipitation shield will likely push southeastward during the first half of the week as a more amplified upper trough and leading surface front move in. As this occurs, the heaviest rain/mountain snow should focus along the central West Coast into the Sierra Nevada while some moisture extends farther south over California and east into the Great Basin/Rockies, and noteworthy precipitation persists over the Northwest. There is still some uncertainty over the precise intensity and southward extent of precipitation at any particular time, though at the moment the best signal for highest amounts is over the Olympics and Washington Cascades. These systems may also bring periods of high winds to some portions of the West Coast and Great Basin. Elsewhere, a weak Alberta Clipper may bring an area of light snow from the northern Plains into the Midwest during Sunday-Monday. Gulf Coast shortwave energy and associated stationary front should spread some moisture across the Gulf of Mexico and western Atlantic early-mid week, with Florida possibly seeing some rainfall during that time. There may be an increase in rain chances for the Gulf Coast states as well, but this looks to be much more likely just beyond the current forecast period. Putnam/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, 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