Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Jan 26 2021 Valid 12Z Fri Jan 29 2021 - 12Z Tue Feb 02 2021 ...California heavy precipitation to taper off late week followed by another significant event likely to focus along the central West Coast from the weekend into early next week... ...Weekend into early next week system to spread a broad area of rain and wintry weather across the eastern half of the lower 48... ...Weather Pattern Overview and Guidance/Predictability Assessment... The latest guidance continues to show a pattern involving amplifying upper troughs over the eastern Pacific followed by progression across the lower 48. For the Fri-Tue period of interest, this evolution will lead to areas of enhanced precipitation along the West Coast with lesser amounts farther inland over the West along with a system that will spread a large area of moisture with various precipitation types over the eastern half of the country. A deep and compact upper low crossing the Northeast late this week and lingering cyclonic flow aloft will hold chilly air over the East into the weekend, supporting potential for fairly wide coverage of wintry weather over the northern half of the East. Models and ensembles have maintained reasonable continuity for features in general but with some typical spread and trends. For the system forecast to reach the central U.S. this weekend, solutions are split over the relative emphasis of leading shortwave energy crossing the West late this week and a trailing weaker shortwave entering the West by early Sat. GFS/UKMET runs have been emphasizing the leading feature while the ECMWF/CMC have been weakening the leading shortwave in favor of the trailing one (or some interaction of the two). The trailing impulse is rather small in scale and thus has low predictability. Partially due to these differences, once the system reaches the East the GFS/GEFS have tended to be somewhat faster/northward with the parent low as well as the secondary development off the Mid-Atlantic/New England coast. On the positive side, in spite of these detail differences there is good consensus on the overall upper trough and potential embedded closed low formation. The past couple ECMWF/ECMWF mean runs have extended prior southward trends for low pressure over the East in response to the New England upper low/lingering cyclonic flow. More recently the GFS/GEFS have displayed less of this trend. After the aforementioned question marks about energy reaching the West Sat, there is ongoing spread for the amplitude and timing of the next upper trough approaching the West. GFS runs have been on the slower/amplified side of the spread but with runs through the 18Z cycle hedging a bit toward the ECMWF cluster for the trough itself and upstream flow. However the new 00Z GFS reversed this trend a bit and the 00Z UKMET is fairly close to it by the end of its run late Sun. The GEFS mean is closer to GFS runs in principle. A composite of operational 12Z/18Z models reflected a reasonable intermediate solution for the first half of the period with CMC input sufficiently low to downplay some regional specifics that compared a little less favorably to consensus. For the rest of the period, trends through the 18Z cycle recommended a slight tilt toward the 12Z ECMWF/ECMWF mean relative to the 18Z GFS/GEFS mean while maintaining a small CMC component. The operational models were close enough for the general pattern/system evolution through the end of the period to allow for majority inclusion of their solutions and yield systems with somewhat better definition than the means. ...Weather/Threats Highlights... The ranges over far southwestern California may still see moderate to heavy precipitation late this week while lighter activity lingers over the rest of the state. Expect generally moderate amounts of rain and higher elevation snow to spread across the remainder of the West as the supporting shortwave aloft crosses the region. Then from the weekend into next week upstream energy aloft and a couple frontal systems will bring additional moisture to the West Coast. Much of the Pacific Northwest may see a period of moderate to heavy rain/mountain snow early in the event followed by a more concentrated emphasis of heavy precipitation on southwestern Oregon and northern California. Some of this moisture should extend into the northern Rockies. Specifics of the heaviest precipitation will depend on the exact amplitude and timing of the approaching upper trough but the general signal for southwestern Oregon/northern California has been fairly stable over recent days. The Northeast will see breezy to windy conditions late this week in the gradient between strong Atlantic low pressure and Great Lakes high pressure. The system forecast to emerge over the Plains to start the weekend should track eastward and ultimately lead to secondary low pressure off the Mid-Atlantic/New England coast by around late Sunday or early Monday. This system will spread a broad area of precipitation across the eastern half of the country with some wintry weather possible from the northern Plains/Midwest through the Great Lakes/Ohio Valley and Mid-Atlantic/Northeast. Snowfall totals within portions of this area may be significant depending on exact system evolution. Check the Winter Weather Outlook forecasts for a probabilistic depiction of locations expected to see the best potential for meaningful snow. Rain will prevail to the south with some locally moderate to heavy activity possible but system progression may be a limiting factor for rainfall totals. Areas near the East Coast will tend to see below normal temperatures in the late week/early weekend time frame. The upper low crossing the Northeast will bring at least one day of highs 10-20F below normal to locations near its path. As the system initially near the West Coast progresses eastward late this week into the weekend, temperatures over the Plains and then east-central U.S. will rise to above or well above normal values--especially for morning lows which will see broad coverage of 10-20F anomalies. Then closer to the East Coast clouds/precipitation will keep lows above normal but highs should be within a few degrees on either side of normal. Meanwhile some areas over the Plains may see readings 10F or more above normal into next week. The southwestern states will be moderately below normal through the period with mostly single-digit negative anomalies and perhaps a few pockets that are slightly cooler. Rausch 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