Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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... Guidance forecast spread has recently decreased through medium range time scales in active and amplifying upper flow to bolster forecast confidence. Favored a 18 UTC GFS and 12 UTC ECMWF/Canadian/UKMET composite for Friday into Sunday. Predictability decreases into early next week and a composite of best compatible 18 UTC GFS/GEFS mean and 12 UTC ECMWF/ECMWF ensemble means seems to provide a good forecast basis along with the National Blend of Models through these longer time frames. WPC product continuity is well maintained with this plan. However, it is a bit concerning that while latest machine learning models and the bulk of latest 00 UTC guidance remains in line, the 00 UTC ECMWF has trended toward less amplified flow into/out of the West. ...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. 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