Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Mar 23 2023 Valid 12Z Sun Mar 26 2023 - 12Z Thu Mar 30 2023 ...Overview and Guidance/Predictability Assessment... With a leading system departing from the Northeast Sunday, there will be two dominant features of interest during the Sunday-Thursday period. One will be an upper low dropping southward over the eastern Pacific, with some potential complexity over how it may incorporate one or more separate features approaching from the west and lead to a rather strong and concentrated offshore storm early next week. This system should produce some areas of significant rain and mountain snow, especially over California early next week. Consensus shows the upper system moving inland by around Wednesday-Thursday. Meanwhile initial western U.S. troughing with one or two embedded lows as of Sunday will progress eastward, likely reaching the East by around Tuesday. There is a lot of uncertainty over the character of the shortwave by that time and what kind of surface system it may produce near the East Coast by Tuesday-Wednesday--with a lot of spread for sensible weather, which could range between significant over some areas to minimal on the other side of the spectrum. The large scale pattern will support persistently below normal temperatures over the West, while flow around the southern periphery of a central Canada upper low should keep the northern Plains cold as well. Eastern U.S. temperatures will fluctuate around either side of normal with system progression. Comparisons among 12Z/18Z guidance led to starting the updated forecast with a blend of the 12Z/18Z GFS and 12Z ECMWF early in the period and then starting to incorporate some 18Z GEFS/12Z ECMWF means by day 5 Tuesday on the way to 60 percent total ensemble weight by the end of the period next Thursday. The 12Z UKMET and CMC both strayed from the other models/means over the eastern Pacific, with the UKMET straying farther east with the upper low and the CMC southward. The new 00Z UKMET looks a lot more like the GFS/ECMWF while the 00Z CMC is somewhat weak/eastward. Recent GFS/ECMWF runs, and now the UKMET, show a complex evolution that incorporates a surface system originally tracking north of Hawai'i as well as a separate shortwave farther north. As the upper system opens up and begins to move inland, the 00Z GFS strays a bit fast but overall clustering is better than average relative to forecasts 6-7 days out in time. Meanwhile guidance has been having a lot of difficulty in resolving the specifics of energy that emerges from the West and supports some degree of surface low pressure/frontal evolution over the eastern half of the country and into the western Atlantic/Canadian Maritimes. The preferred blend provided a reasonable degree of continuity in terms of the eastern U.S. surface evolution Tuesday and then a fairly climatological track off New England thereafter, less extreme than the literal 12Z ECMWF/GFS but farther north than the 18Z/00Z GFS. Ensemble spread is making it difficult for the GEFS/CMCens means to develop much definition offshore, while the 12Z ECMWF mean seemed just a bit fast with its coastal development compared to other guidance. ...Weather/Hazards Highlights... The system departing from New England on Sunday will leave behind a wavy front over the Gulf Coast/Southeast for a day or so before the system emerging from the Plains approaches. There should be sufficient moisture and instability for areas of heavy rain/thunderstorms, with the setup allowing for training over some areas. Therefore the proposed day 4 (12Z Sunday-12Z Monday) Excessive Rainfall Outlook introduces a Slight Risk area from the southeast corner of Mississippi through central Georgia. The day 5 (12Z Monday-12Z Tuesday) outlook will show a Marginal Risk area for a lingering heavy rainfall threat, with less consensus for specifics than in the day 4 period. Other aspects of the forecast across the central/eastern U.S. have lower confidence overall. The upper dynamics/surface reflection crossing the Plains into east-central U.S. may produce some precipitation of varying intensity, with some snow possible in the northern part of the moisture shield. Then the details become even more uncertain closer to the East Coast, as possible solutions range between strong low pressure tracking close to New England (with a wind and snow threat) to a weaker and/or farther south low track that would lead to a more suppressed area of precipitation and farther south rain-snow line. Continue to monitor forecasts for the latest details. Parts of the Interior West and Rockies/High Plains may see some areas of snow on Sunday. Then latest guidance trends for the developing storm off the West Coast by the start of next week suggest an increasing potential for strong winds along and offshore the coast, plus gradual convergence toward the idea that leading moisture flow could bring some heavy rainfall/mountain snow to locations from the central West Coast into the Sierra Nevada. The day 5 Excessive Rainfall Outlook will introduce a Marginal Risk area corresponding to the arrival of this Pacific moisture. Expect much of the West and portions of the northern Plains to see below normal temperatures during the period. Coldest readings over the West will likely be through Sunday-Monday when parts of the Great Basin into central Rockies may see highs at least 20-25F below normal. Northern Plains locations should see highs consistently 10-20F below normal with some even colder readings possible early-mid week. Some of the cold air may spill into the central Plains on Sunday-Monday. A few locations may approach or set daily records for cold highs and/or lows. Temperatures over the Northwest should be less extreme but still be mostly below normal. Areas along the East Coast, and especially the Southeast, should see above normal temperatures Sunday into Monday. Otherwise the eastern U.S. should see variable temperatures on with system progression, including slightly below normal readings around Tuesday-Wednesday. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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