Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 PM EST Tue Feb 27 2024 Valid 12Z Fri Mar 01 2024 - 12Z Tue Mar 05 2024 ...Large system to spread a Heavy Rain/Snow/Wind threat across the West this weekend and the North-Central to Southern U.S. through early next week... ...Guidance/Predictability Assessment... A non-UKMET blend was utilized throughout the Medium Range period. A general model blend consisting of the 00z EC/CMC and 06z GFS were used on day 3. The 06z GEFS was introduced to the blend on day 4 to account for a weakening trend in the latest guidance regarding an East Pacific trough. By day 5, the blend is spread out equally between the Euro and GFS deterministic and ensemble means as well as the deterministic 00z CMC. The 00z EC has a longwave trough over the Northwest U.S., which it has been trending toward over its last several runs, while the GFS and Canadian consistently portray a more amplified system over the West. This CMC-GFS solution cluster well with their ensemble means. The Euro suite is considerably reduced in weighting on day 6 due to its continued signal for a long and ever expanding trough across southern Canada. The Canadian and GFS suites continue to cluster on the evolution of this deep upper feature so they were favored in this blend. The amount of dispersion in the guidance with respect to the Central U.S. trough lead to a general ensemble mean blend on day 7. The 00z Canadian and 06z GFS suites also signal a southern stream shortwave developing and propagating into the Southwest beginning on day 7 while the 00z EC trended away from that beginning with yesterday's 12z run. ...Overview and Weather/Hazards Highlights... A leading southern stream upper trough with origins over the East Pacific will eject from northern Mexico and work across The South Friday before exiting the East Saturday, and may prompt a modest/uncertain coastal low to lift focus up/off the East Coast. In this pattern a surface low/frontal system also lags back over the Gulf of Mexico. Moisture return the Gulf of Mexico and onshore fetch from the Atlantic will fuel an organized area with enhanced rain/convective chances. Locally heavy downpours may focus over the Central Gulf Coast late week, with potential for a secondary maxima over the Southeast/southern Appalachians. These areas will be monitored for runoff issues, but no WPC Excessive Rainfall Outlook (ERO) threat area has been issued at this time given relatively high Flash Flood Guidance values. Some rain/showers will linger over the Southeast into early next week with weaker impulses, lingering boundaries and inflow. Upstream, there continues to be a very strong guidance signal that an amplified upper trough digging into the eastern Pacific later this week will shift over the West this weekend. This will support chances for a moderate to heavier rain focus into the Pacific Northwest mid-later week that along with a moderate long fetch moisture plume/Atmospheric River will shift steadily down the coast through southern California later week into the weekend before easing. The WPC Day 4/Friday and Day 5/Saturday EROs shift Marginal Risk areas southward from SW Oregon through southern California. There is also a significant threat for high winds and very heavy snow to spread inland across the Intermountain West and Rockies, especially for favored terrain/mountains as highlighted in the WPC Winter Weather Outlook (WWO). Confidence continues to increase in the risk for moderate to extremely disruptive high snowfall rates/blowing snow, particularly for the Sierra Nevada. Additionally, valley snowfalls may develop more over time as snow levels decrease with upper system and wavy frontal push. This translation should also favor development of an emerging precipitation shield over the cooling Rockies/Plains by Sunday/Monday with Plains cyclo/fronto genesis. Widespread showers/thunderstorms look to develop across the Plains/Mississippi Valley then east-central U.S. with focus over the Gulf Coast States into Tuesday. This may include some severe weather potential, as well as the chance for heavy snows in quite windy flow on the northwest periphery of activity across the Northern Plains to Upper Midwest with low deepening and the digging of cold Canadian air southward in the wake of this main system, as per the WWO. Pacific upper trough amplification and inland progression this weekend will cool temperatures over much of the West, leading to the spreading of values as cold as 15 degrees below normal. Downstream, expect much above normal pre-frontal warmth later week over the north-central U.S. that will spread to much of the central and eastern U.S. this weekend/early next week downstream of the main upper trough/lead frontal system approaching from the West. Springlike minimum and maximum temperatures ranging upwards to 20-30+ degrees above normal over the Midwest will overspread the Northeast and offer widespread record values. Kebede/Schichtel 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