Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Thu Nov 19 2020 Valid 12Z Sun Nov 22 2020 - 12Z Thu Nov 26 2020 ...Overview and Guidance Evaluation... The medium range forecast pattern continues to feature multiple embedded shortwaves within progressive and broadly cyclonic mean flow. Each shortwave or cluster of impulses has various uncertainties involving amplitude/timing or more complicated issues involving stream interaction or potential for temporary separation of some energy. As a result the guidance over recent days has exhibited a fair number of continuity changes/trends, whether abruptly between runs or more gradually over a number of days. On the gradual side of the spectrum, the wave development expected to be in progress at the start of the period on Sun over the Middle Mississippi Valley has been trending southward over recent days. The consensus track still takes this wave into southeastern Canada by Mon though with more interaction from an upstream wave/front crossing southern Canada. The shortwave expected to reach the West Coast on Sun ultimately produces the greatest forecast difficulty during the rest of the period. In previous days guidance had been fairly diffuse with the embedded energy, leading to weak surface reflections depending on where within the overall trough embedded energy decided to concentrate. Starting with the 00Z ECMWF from 24 hours ago and more recently joined by GFS/CMC runs, recent trend has been toward a more vigorous upper trough crossing the West and then a closed low possibly forming for a time over the central/eastern U.S. This evolution leads to a more pronounced western U.S. cold frontal passage and stronger surface development downstream. The GEFS is the one ensemble mean through the 18Z run that has a central/eastern surface evolution somewhat resembling the operational runs, albeit with an open trough aloft. The 00Z GEFS maintains this idea. Among 00Z operational runs the GFS/CMC have adjusted slower than their prior runs (especially the CMC). At the moment confidence in the slower side of the spread is not especially great given the progressive mean pattern. The ECMWF has toned down its development from its prior two runs but still fits into the prior cluster in principle. On the other hand the UKMET splits this energy over the West to yield a completely different evolution--a lower probability scenario given consensus/continuity thus far. Overall spread and variability are not as dramatic for the next upper trough arriving into the Northwest by Tue-Wed. GFS/GEFS runs have tended to be on the flatter/broader side of the spectrum though latest runs seem to be hedging back toward a more intermediate solution. With the mid-late period development to the east of the Rockies becoming sufficiently common among operational guidance not to ignore, the updated forecast kept at least 50 percent total operational model weight through the end of the period after reflecting an operational consensus blend into early day 5 Tue. The 12Z CMC strayed south of the GFS/ECMWF after day 6 Wed so it dropped out of the blend by day 7 Thu. Ensemble input used somewhat more GEFS than ECMWF mean to account for the eastern evolution but an overall model/mean average was reasonable for the system entering the West during the latter half of the period. ...Sensible Weather and Hazards... From Sun into Mon expect the wave tracking northeastward from the Mid-Mississippi Valley into southeast Canada to bring an area of organized precipitation from the south-central Plains into southeastern Great Lakes and Northeast. Most of the precipitation should fall as rain but parts of the moisture shield's northern periphery may contain some snow. The gradual southward trend for the precip axis southwest of the Great Lakes has been continuing. Confidence in specifics remains low but there is an increasing signal for a more defined Tue-Thu system that could produce another episode of meaningful precipitation from the Plains into the East, with some snow possible in northern latitudes. Over the Northwest, progressive mean flow aloft will promote a wet period as a series of fronts approach/reach the region. Some of the fine details are fluctuating as would be expected for the pattern but there is good continuity for persistence of rain and higher elevation snow with highest totals over Coastal and Cascade Ranges of the Pacific Northwest and a secondary maximum over northern parts of the Rockies. Lighter precipitation may extend as far south as northern California and into the central Rockies. The progressive pattern will keep temperatures variable over a majority of the lower 48. Most areas should remain within 10 degrees on either side of normal for highs but there are enough uncertainties with embedded system details that there may end up being some pockets of more anomalous highs. Larger areas of morning lows 10F or more above normal are possible ahead of the front pushing into the South/East Sun into Mon and another front reaching the Plains by Tue. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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