Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Nov 21 2023 Valid 12Z Fri Nov 24 2023 - 12Z Tue Nov 28 2023 ...Pattern Overview... The models and ensemble means generally suggest a trend toward a broader and more amplified mean trough extending from Canada into the lower 48 going from late this week into early next week, supported by a persistent upper ridge over the eastern Pacific and northwestern North America. As this evolution develops, an initial western U.S. upper trough with embedded low as of Friday will open up by the weekend, with some of the energy potentially phasing with northern stream flow while weaker residual troughing extends through the Southwest. Northern stream shortwaves and the ejecting western energy will support one or more waves/frontal systems east of the Rockies while southern tier energy could also contribute to waviness from the Gulf of Mexico into western Atlantic with uncertain effects along the East Coast. Snow and cold temperatures over the central Rockies/High Plains will be a prominent weather story late this week, less confidence in specifics of precipitation coverage and amounts on a day-to-day basis farther eastward. ...Guidance Evaluation/Predictability Assessment... As has been the case recently, guidance continues to show various important detail differences within the more agreeable large scale evolution. After a lot of variability/trending in the past day or so, there is an improving consensus that whatever Gulf/Atlantic waviness exists late week into early Sunday due to southern tier shortwave energy should be fairly weak and a ways offshore (minus the 12Z ECMWF that was close to North Carolina). However right after that time solutions at the surface begin to diverge more as the ejecting western energy and northern stream troughing approach (potentially supporting a separate wave/frontal system). The 00Z GFS has reverted to a suppressed and well offshore track, in contrast to 12Z/18Z runs that had a more north-northeast track, with the 12Z ECMWF and new 00Z CMC just far enough offshore likely to have modest influence on the East Coast. Ensemble members continue to exhibit enough spread to keep the means very ill-defined. Meanwhile guidance continues to show one or more waves/frontal systems dropping south from Canada during the period but with typical differences and variability. The most common cluster shows a front reaching the northern tier by Sunday, followed by another potential wave around Tuesday (ECMWF/CMC runs and their means; GFS runs showing higher pressures at that time with the GEFS mean ill-defined). Finally, model/ensemble spaghetti plots get quite messy across the eastern Pacific into western U.S. early next week as solutions increasingly differ on the distribution of energy within a trough between Alaska and Hawai'i and downstream effects on the ridge nearing the West Coast plus the downstream details. A blended solution closest to the means looks best there, between the 12Z ECMWF's upper low over northern Baja California by early Tuesday and some GFS runs/new 00Z CMC that bring more significant height rises into the West. The updated forecast started with a blend of the 18Z GFS/12Z ECMWF and lesser weight of the 12Z CMC/UKMET early in the period and then gradually increased 18Z GEFS/12Z ECens mean weight mid-late period to yield a half model/half mean mix by day 7 Tuesday. By that time, more pronounced differences in operational runs over the West offset to keep the forecast closer to the means. ...Weather/Hazards Highlights... Snow over the central Rockies and High Plains (including lower elevations) should continue at least into Friday with some locally enhanced totals possible. Expect the snow to trend lighter and more scattered as the initial upper low over the West opens up and progresses eastward. Some precipitation (snow north, rain south) may continue into the central/eastern U.S. along a front but most should be fairly light. One or more periods of lake effect snow will be possible as well, under broadening chilly cyclonic flow aloft. Recent forecasts have been quite variable for how much precipitation may occur across parts of the southern/eastern U.S., with latest trends tending to keep coverage fairly low. However there are some mixed signals about a potential brief period of relatively heavier rainfall across parts of Florida late this week, with minor possibilities of other parts of the Gulf Coast/East Coast getting brushed with meaningful precipitation at some point during the period. Currently there no sufficiently coherent signals for organized heavy rainfall to warrant any excessive rainfall areas on the Day 4 and Day 5 Excessive Rainfall Outlooks. Most of the country will likely see near to below normal high temperatures from late this week into early next week. Coldest readings versus climatology should be over the central High Plains and vicinity Friday-Saturday with highs generally 15-25F below normal. Low temperatures will be less extreme but parts of Texas may experience some frost/freeze conditions for the first time this season and the combination of morning lows and breezy conditions may bring noticeably cold wind chills to valley locations such as Las Vegas. A decent portion of the Plains may see a sufficient warmup to bring highs within a few degrees on either side of normal by next Tuesday. The immediate central-northern West Coast should be an exception to the chilly theme, with East Pacific upper ridging close enough to keep highs a little above normal. 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw