Extended Forecast Discussion NWS Weather Prediction Center College Park MD 419 PM EDT Mon Nov 01 2021 Valid 12Z Thu Nov 04 2021 - 12Z Mon Nov 08 2021 17Z Update: The latest models and ensembles are in above average agreement on the synoptic scale through Thursday across the continental U.S., before more timing differences become apparent starting on Friday with eastern Pacific shortwave impulses. The 12Z GFS becomes more progressive and stronger with the shortwave approaching the Pacific Northwest by the end of the week, and also faster with the one approaching over the weekend. Additionally, there has been more run-to-run variability with the past several GFS runs regarding the magnitude of the eastern U.S. trough for later in the forecast period, and there slightly more weighting was applied to the ECMWF and its ensemble mean for the Saturday to Monday portion of the forecast period. The UKMET was slightly out of phase off the West Coast by Friday and Saturday with a flatter trough axis compared to the ECMWF/GFS, and a stronger upper ridge. Otherwise, the existing forecast is mainly on track with no major changes necessary for this forecast update. The previous discussion is appended below for reference. /Hamrick ...Overview and Guidance/Predictability Assessment... The medium range period will begin Thursday with a ridge axis across the Southwest to northern High Plains separating troughing over the eastern Pacific and in eastern North America. Model guidance agrees on this much, before diverging by the end of the week with a couple of key features. First, shortwave energy moving through the trough in the East creates differences in the trough's evolution Friday into the weekend, namely the depth that the trough digs, whether or not a separate/possibly closed low develops in the southern stream with this energy, and the speed the trough tracks eastward. These differences also impact the possibility of a surface low forming and its track likely in the western Atlantic, along with precipitation chances for the East Coast. There have been ample run-to-run and model-to-model variations with these features over the past couple of days, with generally the GFS suite taking the weaker/faster side compared to other guidance. For the 12/18Z cycle, somewhat better model agreement was finally seen as the 12Z ECMWF (and now the new 00Z ECMWF) no longer spin up a deep closed low with the energy on Friday and lasting into the weekend, but take a more middle ground approach with weaker but still potent energy producing a southern stream open trough. Additionally, the 18Z GFS, while overall still fast compared to consensus, keeps troughing persistent across the East into Saturday, though is now on the aggressive side with a small closed upper low off the East Coast by Sunday. The 12Z CMC shows a brief closed low forming Saturday but certainly within reason. Thus for these features in the East, a blend mainly based on the 18Z GFS, the 12Z ECMWF, and the 12Z CMC was utilized through the weekend, weighting the GEFS and EC ensemble means (which were better clustered tonight) more heavily as the period progressed. With this blend, surface cyclogenesis occurs off the East Coast, and most guidance tonight shows a trend toward most precipitation remaining offshore with the low, but with the possibility for lighter amounts over the East Coast Friday and Saturday. Farther west, there is uncertainty with track and timing of shortwave energies entering the West Coast late this week. Most guidance (though the 00Z CMC is on the weaker side) takes a shortwave across the Northwest and southwestern Canada Thursday into Friday, which pushes upper ridging somewhat eastward into the central U.S. Just behind this shortwave there is potential for additional shortwave energy to come into the West Friday into Saturday. This is where model guidance begins to diverge more notably. The 12Z UKMET is quite strong with a closed mid-upper low forming in the central U.S. on Saturday, the 12Z ECMWF splits energy to take some into central Canada while some tracks quickly southward over the Mississippi Valley and reinforces the East Coast troughing by Sunday but keeps ridging over the central U.S., and the 12Z CMC takes most energy eastward along the U.S./Canadian border and creates zonal flow over the western and central CONUS. The 12Z and 18Z GFS differ notably, as the 12Z GFS aggressively combines strong energy to create a central U.S. closed low Sunday, somewhat like the UKMET but a day later, while the 18Z shears the shortwave energy north of the west-central U.S. ridging. The new 00Z GFS is more like the 18Z GFS, though does close off a 500mb low in the Upper Midwest more quickly than the 18Z run. For this feature leaned toward the 18Z GFS and GEFS mean for the forecast, which seemed to be a good middle ground solution among guidance, as it maintains some ridging in the central U.S. but accounts for the shortwave energy as well. Overall, variability in model and ensemble guidance leads to below average confidence in the forecast evolution, though may be improving somewhat compared to previous cycles. ...Weather/Hazards Highlights... Light to moderate rain is expected across the Gulf Coast and the southeastern quadrant of the country on Thursday as a weak shortwave moves through the base of the trough and sufficient moisture streams north of a Gulf Coast front. The heaviest precipitation will likely be in the Southern Appalachians, and the highest peaks there could see snow. Then some rain may spread into the Mid-Atlantic and Northeast Friday into Saturday, but how far west precipitation reaches will depend on the surface low track in the western Atlantic. If moisture does reach the higher elevations of the Central Appalachians and interior Northeast, snow is possible there as well. Abundant moisture ahead of frontal systems and upper-level shortwaves edging in from the Pacific will result in periods of enhanced precipitation during the latter half of the week into the weekend near the West Coast. Through the period, the Pacific Northwest/far northern California can expect the heaviest precipitation, with normal terrain enhancements and higher elevation snow. Precipitation is likely to spread into the Intermountain West and Rockies as well, with highest amounts Thursday into early Friday. Behind the aforementioned front across the Gulf Coast, a cold high pressure system will maintain well below normal temperatures across the central and eastern CONUS. This will especially be the case for highs, which should be 10-20F below average from southern parts of the Plains and spreading across the Mississippi Valley toward the eastern U.S. Chilly lows are also forecast, with temperatures about 5-10F below normal across the Great Lakes/Ohio Valley and stretching toward the south-central U.S. by Friday, and into the Southeast over the weekend. Expect above normal temperatures under the western/central U.S. ridge aloft, with highs about 10-15F above average for the north-central U.S. southward into the central High Plains, with the Desert Southwest a few degrees above normal as well. Meanwhile, periods of unsettled weather will keep highs near to somewhat below normal over the West Coast on most days. Tate Hazards: - Heavy precipitation across portions of the Pacific Northwest and along the Cascades, Thu-Fri, Nov 4-Nov 5. - Heavy precipitation across portions of the Pacific Northwest and along the northern Cascades, Thu-Sun, Nov 4-Nov 7. - Heavy precipitation across portions of northern California into Oregon, Mon, Nov 8. - Flooding occurring or imminent across portions of central New York. - Flooding likely across portions of central Illinois. 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