Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 PM EST Mon Feb 26 2024 Valid 12Z Thu Feb 29 2024 - 12Z Mon Mar 04 2024 ...Heavy Precipitation Pattern Emerging Again for the West... ...Guidance/Predictability Assessment... Model guidance remains fairly well clustered after initially struggling to find a consensus with a split-stream flow pattern over the CONUS earlier this week. Following the departure of a weakening southern stream shortwave moving northeast from northern Mexico across the Southeast and off the coast of the Mid-Atlantic, attention then turns to to the West as energy upstream over the northeastern Pacific begins to slowly dig southward along the West Coast. This will allow for a multi-day influx of Pacific Moisture over the West and yet another Atmospheric River event. The pattern over the CONUS will become more amplified as this deep longwave trough shifts eastward over the West and towards the central U.S. this weekend into early next week, with stronger ridging/higher heights over the East. There remains the typical increasing uncertainty with time on some of the details of individual shortwave energies/frontal systems within the flow, specifically with respect to a system tracking along the Canadian border across the north-central U.S. this weekend, and the intensity and northward trend in location with a potential embedded closed upper-level low/deepening surface cyclone pushing northeastward across the Plains/Midwest next week as the longwave trough shifts eastward. However, the overall pattern evolution looks to remain fairly consistent with little run-to-run variation through yesterday's 00Z and today's updated 12Z forecasts. Given the guidance is well clustered, a multi-model blend is used for the updated WPC forecast for the early to mid-forecast period, with a contribution from the ECens/GEFS means added for the later period to account for some increasing uncertainty with the smaller-scale details. The consistency in the guidance along with a similar guidance blend to the prior WPC forecast led to minimal changes overall with this update. ...Overview and Weather/Hazards Highlights... A leading/main southern stream upper trough with origins over the East Pacific will eject eastward from northern Mexico and traverse The South/Southeast Thursday and Friday as a surface low/frontal system takes position over the Gulf of Mexico. Moisture return from the southern Gulf and easterly fetch from the Atlantic are expected to fuel an area with rain/convective chances. Locally heavy downpours may focus over the Central Gulf Coast, with additional but more uncertain potential for a secondary maxima inland over the Southeast/southern Appalachians. The area will be monitored for any runoff issues, but no ERO threat area has been issued at this time given relatively high Flash Flood Guidance values. Rain/showers will linger over the Southeast this weekend with more, but weaker impulses in a similar pattern. 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 increasingly support chances for a moderate to heavier rain focus into the Pacific Northwest mid-later week that along with an associated/moderate long fetch moisture plume/Atmospheric River will shift steadily down the coast through southern California later week into the weekend to monitor. The WPC Day 4/Thursday and Day 5/Friday Excessive Rainfall Outlooks (EROs) offer Marginal Risk areas for coastal Oregon through Northern California. There is also a significant threat for high winds and very heavy snow to spread inland across the West, especially for favored terrain/mountains as highlighted in the WPC Winter Weather Outlook. Confidence continues to increase in the risk for moderate to extremely disruptive high snowfall rates/blowing snow, particularly for the Sierra Nevada. Additionally, some valley snowfalls may develop more over time as snow levels decrease with upper system and wavy frontal translation. This translation should also favor development of an emerging precipitation shield over the cooling Rockies/Plains by Sunday/next Monday. Widespread showers/thunderstorms look to range across portions of the Plains/Mississippi Valley which may include some severe weather potential, as well as the chance for some accumulating snows on the northwest periphery of activity across the Northern Plains to Upper Midwest. Upper trough amplification and inland march by the weekend will cool temperatures over much of the West, leading to maximum values of as much as 10-15 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 working over the West. Spring maximum temperatures ranging upwards to 20-30 degrees above normal will spread from the Midwest toward the Northeast to offer potential for some record values. Putnam/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