Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Tue Mar 05 2024 Valid 12Z Fri Mar 08 2024 - 12Z Tue Mar 12 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... One prominent focus of the forecast will be on an upper trough tracking out of the Rockies on Friday, 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 eastern U.S. system will be over New England by the weekend. There will be a broadening drier trend with time behind this system. Meanwhile a mean trough aloft setting up near the West Coast will bring a series of shortwaves and surface fronts into the region, leading to persistent rain/higher elevation snow along the northern-central West Coast late this week into next week. Some of the moisture from these systems will likely reach the northern Rockies by early next week. ...Guidance/Predictability Assessment... The most consequential area of uncertainty in the medium range is at the beginning of the period over the Southeast. Small differences in the 500mb pattern over the Plains/Mississippi Valley is reflected in the various models' QPF axes over the Southeast on days 3/4. The 00z UKMET and CMC both have shortwave energy amplifying notably more than the other guidance over the Mississippi Valley on day 4. Thus a general model blend favoring the 00z Euro and 06z GFS with less weighting for the 00z CMC and UKMET was utilized for days 3 and 4. The 00z Euro and 06z GFS deterministic and ensemble means were favored with some 00z Canadian sprinkled in on day 5 since they all clustered fairly well around a phasing solution for the northern and southern streams. The evolution of an East Pac trough propagating through the Gulf of Alaska this weekend is still very much in flux with plenty of spread identified by the ensemble guidance. In fact the ECE seems to be the only ensemble to depict a trough while the GEFS and CMCE have a zonal pattern present. A blend favoring the 06 GFS/GEFS is used with some lesser weighted 00z EC/ECE/CMC/CMCE used on day 6 due the continued uncertainty around this East Pac evolution. There's reasonable agreement with respect to the deep upper low over the Northeast on day 6. Things tend to amplify enough in the GEFS and CMCE means to suggest a trough will be present over the West Coast by day 7, although there remains plenty of spread among the Canadian ensemble members. ...Weather/Hazards Highlights... While some details are yet to be resolved, confidence remains high in the general theme of an active weather pattern over eastern half of the country from late this week into the weekend, and likely continuing into next Monday over the Northeast. The combination of upper troughing that emerges from the West and separate northern tier energy may support one or two vigorous surface waves tracking from the central U.S. into the East/Northeast from late this week through the weekend. Surface low pressure should consolidate into a storm system near New England by Monday and then depart to the east. As the Plains system (whether the primary low or the southern of two waves) 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. The new Day 4 Excessive Rainfall Outlook has a higher-end Slight Risk area covering locations from around the central Gulf Coast region into far western South Carolina, with the only adjustment from the prior Day 5 ERO being to expand the northwestern part of the area a bit per guidance signals. The northwestern part of the surrounding Marginal Risk area continues to account 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. During the Saturday-Saturday night period covered by the Day 5 ERO, guidance shows one area of enhanced rainfall potential over the Southeast, reflecting eastward continuation of convection from the previous 24 hours. Thus far there has been some spread and variability for the timing and details of convection (with ECMWF/ECens runs being slowest) so initially introduced a Marginal Risk area with the expectation that an embedded Slight Risk may arise once guidance clustering improves. There is another area of enhanced rainfall potential across parts of the Ohio Valley/Lower Great Lakes/northern Mid-Atlantic, but with less instability suggested. A Marginal Risk area over this region accounts for a combination of model QPF signals, and over the Mid-Atlantic wet ground conditions from shorter-term rainfall. A few pockets of snow will be possible on the northern periphery of the moisture shield but with New England easily seeing the best potential for meaningful snow. Some locations in New England may be sufficiently cold for some snow as the system reaches the Great Lakes/Mid-Atlantic, and then the the pattern would become increasingly favorable for snow as low pressure emphasis shifts more along/offshore the coast by around late Sunday-Monday. The Winter Weather Outlook probabilities for at least 0.25 inch liquid in the form of snow currently exceed 50 percent across portions of interior New England for the Sunday-Sunday night period. This system may produce some brisk winds as well. The upper trough moving through the Four Corners region late this week will produce snow and lower elevation rain over the southern half of the Rockies, followed by a drier trend as the trough departs. Then the mean trough aloft and multiple embedded shortwaves/surface fronts affecting the West Coast will produce persistent rain and mountain snow from the Pacific Northwest into northern California from Friday into next week. Early in this period the precipitation totals should be in the light to moderate range with highest totals over the Olympics. Currently there is not a strong enough signal for any risk area in the Olympics through the Day 5 period of the Excessive Rainfall Outlook (Saturday-Saturday night). However guidance suggests potential for somewhat heavier (though not exceptional) amounts along the northern-central West Coast by Sunday-Monday. By early next week confidence in specifics decreases as guidance diverges for shortwave/frontal details. Expect moisture from these systems to start reaching into the northern Rockies by early next week with meaningful snowfall possible. System progression across the lower 48 late this week into the weekend will bring an area of moderately below normal temperatures (some highs up to 10-15F below average) across the West into the central-southern High Plains Friday-Saturday, while warmth over the eastern half of the country as of Friday 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 before starting to rebound. Meanwhile the Plains/Mississippi Valley will trend steadily warmer by Sunday-Tuesday under the influence of mean ridging aloft, with some northern locations possibly seeing highs up to 20-25F or so above normal early next week. The series of frontal systems reaching the West Coast from the weekend into early next week will promote slightly below normal daytime highs over that region and moderately above normal lows over most of the West. Kebede/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