Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Mar 04 2024 Valid 12Z Thu Mar 07 2024 - 12Z Mon Mar 11 2024 ...Wet pattern for parts of the Eastern U.S., especially across the South... ...Multi-day unsettled period along the northern-central West Coast starting late this week... ...Overview... With a leading wave tracking over the western Atlantic early in the period, the primary focus of the forecast will be on an upper trough tracking across the lower 48 and supporting a vigorous surface system that will spread a broad area of moisture across the eastern half of the country from late this week through the weekend, after producing some snow/low elevation rain over the southern half of the Rockies. The best potential for snow with this system will be over New England by the weekend. There will be a broadening drier trend with time behind this system, while a mean trough aloft setting up near the West Coast will bring a series of frontal systems and persistent rain/higher elevation snow to the northern-central West Coast late this week into next week. ...Guidance/Predictability Assessment... The forecast continues to show relatively good agreement in the big-picture themes and operational models/ensembles plus 12Z ECMWF-initialized machine learning models as a whole do not seem to show pronounced clustering toward one side or another of the spread where disagreements emerge. Early in the period, there was more prominent clustering away from the 12Z UKMET/CMC which tracked leading western Atlantic low pressure closer to New England than other guidance. 00Z model trends have generally shifted farther offshore. Within the large scale upper trough crossing the lower 48, lower-predictability embedded shortwave details continue to make it difficult to have much confidence in associated surface low specifics from the Plains into the East. Thus that part of the deterministic forecast will likely continue to shuffle around some over coming cycles. By Sunday-Monday there is a general signal in the guidance that northern stream energy could try to consolidate an upper low over or near the Northeast, with dynamical and machine learning models not really clustering in a pronounced fashion in terms of location/timing. Regardless of the Plains through East evolution, there is relatively better agreement toward low pressure emphasis shifting to near New England or the Canadian Maritimes by next Monday. Farther west, a leading shortwave should come ashore into the West on Saturday (weakening as it progresses toward the central U.S. mean ridge) and followed by another shortwave/frontal system by Monday. Some guidance starts to trend out of phase over the eastern Pacific/western U.S. toward the end of the period. For the updated forecast based on 12Z/18Z guidance, an operational model composite emphasizing the GFS/ECMWF more than the UKMET/CMC looked best early in the period. Then a transition to a model/ensemble mean mix helped to keep the forecast fairly steady later in the period as greater detail uncertainties arose. ...Weather/Hazards Highlights... The forecast continues to show an active weather pattern over eastern half of the country from late this week onward. Precipitation along the East Coast in association with a leading wave tracking over the western Atlantic should be confined mostly to parts of New England and northern Mid-Atlantic by the start of the period early Thursday. There are some lingering signals for at least isolated significant totals but this is within a broader trend for less coverage/lower amounts overall. This scenario continues to favor no risk area in what will now be the Day 4/Thursday Excessive Rainfall Outlook. Some precipitation may be in the form of snow where moisture reaches far enough north in northern New England. Upper troughing that emerges from the West will support a fairly vigorous surface system tracking from the central U.S. into the East/Northeast from late this week through the weekend. With the combination of guidance scatter for details and fairly low soil moisture percentiles over the region, the Day 4/Thursday-Thursday night ERO maintains a only a Marginal Risk area from parts of the southern half of the Plains into the south-central Mississippi Valley as convection develops in association with low pressure and attached fronts. As the Plains system tracks east/northeast by Friday-Friday night and lifts a leading warm front northward (with interacting moisture/instability), guidance is persistent with its heavy rainfall signal over parts of the South/Southeast. Therefore the Day 5 ERO has introduced a Slight Risk area covering locations from Mississippi east/northeast into the far western Carolinas. The northwestern part of the surrounding Marginal Risk area accounts for a signal in some of the guidance for a secondary axis of enhanced rainfall from the south-central Plains northeastward, near the surface low track. Meaningful rainfall could extend farther north/northeast of the current Marginal Risk area but in the absence of instability in the guidance thus far. Precipitation of varying intensity will continue to overspread the East going into the weekend. Some pockets of snow will be possible on the northwest periphery of the moisture shield, from the central High Plains into the Great Lakes, but with probabilities not reaching much more than 10 percent for 0.25 inch liquid in the form of snow. New England will see greater snow potential, with some areas already sufficiently cold as the system reaches the Great Lakes/Mid-Atlantic and then the pattern becoming increasingly favorable as low pressure emphasis shifts more offshore by around late Sunday-Monday. This system may produce some brisk winds as well. The upper trough moving through the West late this week will produce snow and lower elevation rain over portions of the Southwest and southern half of the Rockies. Then the mean trough aloft setting up near the West Coast will bring a series of frontal systems and rain/mountain snow to the Pacific Northwest and northern California from Friday into next week. Most of this activity should be in the light to moderate range Friday-Saturday, with potential for locally heavier totals by Sunday-Monday. Some of this moisture should eventually extend into the northern Rockies. System progression across the lower 48 late this week into the weekend will bring an area of moderately below normal temperatures (pockets of highs up to 10-15F below average) across the West into the High Plains Thursday-Saturday, while warmth over the eastern half of the country on Thursday will get confined more toward the East Coast by the weekend. Near to slightly below normal highs should prevail over the East by next Sunday-Monday. Also during that time, the northern-central Plains and vicinity will trend steadily warmer (highs reaching 10-20F above normal by Monday) under the influence of mean ridging aloft while the mean trough setting up near the West Coast will promote slightly below normal daytime highs and gradually warming lows. Rausch 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