Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Feb 20 2024 Valid 12Z Fri Feb 23 2024 - 12Z Tue Feb 27 2024 ...Overview... An upper ridge traversing the country late this week into early next week will spread much above normal temperatures particularly to the Plains and Midwest. Troughing will be in place across the East late this week ahead of the ridge, with some precipitation across the Eastern Seaboard Friday and cooler temperatures by Saturday behind a cold front, before transitioning to more zonal flow. Meanwhile an eastern Pacific upper low should spread some increasing precipitation chances to the West Coast once again, possibly by late week but more confidently by early next week as northern and southern stream troughing phase and track into the West. ...Guidance/Predictability Assessment... Model guidance is rather agreeable on the synoptic scale through the weekend, including east-central U.S. troughing on Friday shifting into the East and then the Atlantic by Sunday, the upper low in the eastern Pacific, and ridging in between. A multi-model deterministic blend of the 12/18Z GFS and 12Z ECMWF, CMC, and UKMET worked well to blend out smaller differences for a compromise solution. The most uncertain aspect of the forecast is how quickly the upper low in the eastern Pacific tracks east into the western U.S. early next week. Model guidance shows northern stream troughing digging southeastward at the same time, and their phasing likely plays a role in the timing differences. With the 12/18Z model cycle, the GFS/GEFS mean were quicker to bring the southern stream trough into the Southwest by early Monday compared to the slower ECMWF and EC mean, with the CMC in between. Interestingly, most of the 12Z ECMWF-based machine learning/AI models were even slower than the operational run. This led us to weight the EC/EC mean somewhat more in the model blend to produce a slower solution that was more of a compromise between the faster GFS suite and the slower machine learning models, so slower than continuity. The incoming 00Z operational guidance all tend to be slower than their previous runs so this seems like a good choice for now. The 00Z ECMWF may be too far west though. By Tuesday there is a little bit better agreement with the phased trough axis. However the 12Z and now 00Z CMC have been bringing upstream troughing/energy toward British Columbia where GFS/ECMWF runs show ridging, and this affects the downstream flow somewhat. So by the late forecast period, phased out the CMC while maintaining the ECMWF and a bit of the GFS in the blend while increasing the proportion of ensemble means to over half by Day 7. ...Weather/Hazards Highlights... Light to moderate precipitation is forecast for the Eastern Seaboard on Friday ahead of a cold front. Higher elevations of New York and New England have relatively higher probabilities of seeing notable snow with this activity but the details of the exact low track and temperatures will determine placement and amounts of snow. Brisk winds are also possible for New England particularly late Friday into Saturday as the surface low strengthens. Behind this system, parts of the Great Lakes may see mostly light snow from a combination of lake effect and lake enhanced activity caused by nearby fronts this weekend into early next week. An upper low and its surface reflection look to meander in the eastern Pacific late this week, perhaps reaching close enough to California to provide some shower activity. But by Sunday model guidance is more agreeable that precipitation should spread into the Pacific Northwest and California with some moisture spreading farther inland into the Intermountain West. Snow and rain should continue to increase in interior parts of the West into Monday, while another low pressure/frontal system provides forcing to increase precipitation (mainly in the form of rain given the anomalous warmth) in the Midwest. Warmer than average temperatures by 10-20 degrees in the central U.S. on Friday look to only increase in coverage and magnitude by the weekend and early next week. Temperatures of 20-30F above average will stretch from the northern Plains/Upper Midwest Saturday southward into the Plains and Mississippi Valley early next week. This puts actual temperatures into the 80s and even 90s in Texas Monday-Tuesday while 50s and possibly even 60s reach the Dakotas. A gradual eastward spread/shift should warm the Southeast in to the 70s next week. The East looks to be above average Friday but near to below average Saturday behind a cold front, gradually warming next week. Meanwhile the West is forecast to see near to slightly above normal temperatures late week into the weekend, but cooling to below normal particularly in terms of highs by Monday-Tuesday as upper troughing comes in. 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 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