Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 PM EST Tue Nov 21 2023 Valid 12Z Fri Nov 24 2023 - 12Z Tue Nov 28 2023 ...Pattern Overview... The medium range period (Friday November 24 to Tuesday November 28) will feature a broad, amplified mean trough extending from Canada into the lower 48 while a persistent upper ridge remains in place over the eastern Pacific and northwestern North America. An initial western U.S. upper trough with an embedded closed low 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 may contribute to waviness from the Gulf of Mexico into the western Atlantic that could cause affect parts of the East Coast next week. Snow and cold temperatures over the central Rockies/High Plains will be a prominent weather story late this week, but there is less confidence in specifics of precipitation coverage and amounts on a day-to-day basis farther eastward. ...Guidance Evaluation/Predictability Assessment... Model guidance is in good agreement on the overall pattern; however, there are differences in some of the smaller scale details that could affect sensible weather. Models continue to show the system developing in the Gulf/Atlantic over the weekend remaining fairly weak and offshore of the East Coast through early next week then tracking into the Canadian Maritimes. Latest guidance suggests the low pressure center may track slightly closer to Cape Hatteras than the previous forecast, which would allow for increased precipitation chances in coastal North Carolina. Guidance also continues to show several upper waves and surface frontal systems dropping south from Canada with slight differences in position and timing. The ECMWF is faster and stronger with the wave late in the period, bringing a frontal system into the Upper Midwest on Tuesday, while the GFS and CMC are slower and weaker with this feature. The afternoon forecast started with a blend of the 06Z GFS/00Z ECMWF and lesser weight of the 00Z CMC/UKMET early in the period and then gradually increased 06Z GEFS/00Z ECens mean weight mid-late period to come to a solution close to the model 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 late this weekend into early next week 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. With model guidance shifting slightly east with the offshore low, precipitation chances have increased slightly for coastal North Carolina over the weekend. This coastal system is then forecast to track towards the Canadian Maritimes and could bring precipitation to the Northeast. Currently there is no sufficiently coherent signals for organized heavy rainfall to warrant any risk 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 cold morning lows and breezy winds may bring noticeably cold wind chills to valley locations such as Las Vegas. A decent portion of the Plains may see a sufficient warm up 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/Dolan 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