Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Nov 13 2021 Valid 12Z Tue Nov 16 2021 - 12Z Sat Nov 20 2021 ...Overview... Latest guidance continues to show the upper pattern settling into broadly cyclonic mean flow, with a hint of a strengthening eastern Pacific ridge by next weekend. The relatively low amplitude and progressive nature of this regime should decrease predictability for forecast details behind a vigorous shortwave that will move eastward from western North America Tuesday onward and support a strong southern Canada surface low and trailing cold front crossing most of the lower 48. This system will produce a period of strong/gusty winds over northern tier areas of the U.S. and then some rain over the east-central U.S. Well above normal temperatures over the Plains and southern half of the West at the start of the period on Tuesday will moderate and move eastward ahead of the advancing cold front. Somewhat below normal temperatures behind the front will spread into the central/eastern U.S. mid-late week. During the latter half of the week the precipitation and surface pattern details farther west become increasingly uncertain. ...Guidance/Predictability Assessment... Models have continued to show some difficulty in resolving the details of the strong upper trough (likely with an embedded closed low tracking just north of the U.S.-Canadian border around Wednesday) and thus with the forecast of the cold front crossing the lower 48 and anchoring low pressure in Canada. 12Z/18Z GFS runs had been on the slower/sharper side of the envelope with the upper trough and thus slow with the cold front. The new 00Z GFS made a considerable adjustment toward the 12Z ECMWF--as did the 00Z UKMET from a previously more suppressed surface evolution. Meanwhile latest CMC runs and the 12Z CMC mean are on the fast side. Finally, latest 00Z runs thus far appear to confirm 12Z guidance favoring a moderately flat/progressive handling of the upper trough once it reaches the East, in contrast to the slow/amplified depiction in yesterday's 00Z ECMWF. An initial blend of 12Z/18Z guidance available at the time of forecast preparation ultimately led to an evolution closest to the 12Z ECMWF. Upstream shortwave details still appear to be a ways from being resolved. There is a leading feature around 145-150W longitude as of early Tuesday and even latest 00Z runs range between a diffuse shearing out (most models in some fashion) and a stronger one (CMC), leading to dramatic differences in western U.S. precipitation coverage/intensity. Whether left over from the initial shortwave or originating farther upstream, additional Pacific energy comes into the West while northern stream flow from the Northeast Pacific and vicinity may also reach Canada and the northern lower 48 late in the period. Thus far the GFS/CMC runs have been leaning toward the progressive side for the general area of shortwave energy expected to reach the West by Thursday and continue along thereafter while the ECMWF/ECMWF mean have been slower. Latest GEFS means have been moderately progressive too but with less of a surface reflection. The past 12 hours of GFS runs have been trending less amplified with the overall trough, thus leading to a more suppressed surface system over/near the Southeast by late in the period--a nod to the ensemble means that have generally tended to keep fairly strong high pressure over much of the East. The progressive nature of the pattern could suggest the 12Z ECMWF/ECMWF mean (with a wave back over the Plains early Saturday) could be somewhat slow but at least they have been more consistent thus far. The latter half of the manual forecast trended toward a majority weight of the 18Z GEFS/12Z ECMWF means due to the uncertainty, with the rest composed of a little more 12Z ECMWF weight versus the 18Z GFS. ...Weather/Hazards Highlights... Expect mostly light to moderate rain and higher elevation snow over the northern Rockies through Tuesday in association with the upper trough/leading cold front pushing through the region. The potent surface low tracking across southern Canada Tuesday onward will bring a period of strong/gusty winds to northern tier areas, especially the Northern Plains on Tuesday into Wednesday. Moisture should increase ahead of the low's trailing cold front from midweek onward, leading to mostly light/moderate rain over portions of the Mississippi, Ohio, and Tennessee Valleys along with some light precipitation in the Great Lakes region. The Florida Peninsula may see increasing rainfall for a time mid-late week as moisture interacts with a stalled front to the south, with fairly low confidence in coverage/intensity details at this time. Likewise confidence is below average for any precipitation that could reach the West around mid-late week, with the full range of possibilities ranging between only light/scattered activity and an area of heavier/more organized precipitation. The significant uncertainty extends into the central and eastern U.S. by Friday-Saturday with minimal confidence in any specific solution for precipitation coverage/intensity. Well above normal temperatures will continue into Tuesday over the Plains and southern half of the West. The warmest readings versus normal should be over the Plains where plus 15-25F anomalies will be common and some daily records for highs/warm lows will be possible. Currently the best potential for highs to approach/reach record values is over the western half of Texas. As the cold front emerging from the Northeast continues onward, warmth will extend into the South and East Wednesday-Thursday but in modified form with highs/lows generally only up to 10-15F above normal. Cooler air behind the front will spread mostly 5-10F below normal temperatures across the central and eastern U.S. from Wednesday through the rest of the week. Rausch/Tate 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