Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Jan 28 2024 Valid 12Z Wed Jan 31 2024 - 12Z Sun Feb 04 2024 ...Large scale system likely to spread precipitation across much of the West this week with highest totals expected over California... ...Overview... Guidance continues to show the large scale pattern becoming more blocky by later this week and next weekend. Expect upper ridging to settle over the central U.S. while extending north into Canada, as a Pacific upper trough reaching the West becomes negatively tilted by Friday. The southern part of the trough will likely continue elongating eastward and break off an upper low that undercuts the central U.S. ridge next weekend. Some lingering upper troughing/possible embedded low should lingering near the northern-central West Coast through the weekend. Meanwhile East Coast mean troughing will comprise one shortwave passing through around midweek and then reinforcing energy arriving farther north toward the end of the week and likely persisting through the weekend. Strong and moist southerly flow ahead of the Pacific trough will support an atmospheric river event whose focus should be most pronounced over northern California on Wednesday and farther south by Thursday. A broad area of rain/snow will likely extend farther inland over the West after Wednesday and rain should develop over parts of the Plains by Friday or Saturday. This moisture should then stream into the Southeast. An area of mostly light precipitation should accompany a modest midweek East Coast system while the Northeast may see some precipitation mostly in the form of snow late in the week and next weekend as reinforcing energy aloft settles into the mean trough position. ...Guidance/Predictability Assessment... As has been the case recently, most aspects of the western U.S. pattern (at least through Friday) are more stable and agreeable than for details over the East. Leading height falls with the upper trough arriving into the West still become a little faster in the CMC but otherwise a consensus approach looks good through Friday. Then guidance fairly rapidly diverges regarding the path of a potential upper low expected to break off from the southern part of the western trough. Latest CMC runs represent the northern extreme over the central Plains while the new 00Z GFS has become a southern extreme just offshore the western Gulf Coast as of early next Sunday, and the 12Z ECMWF strays to the fast side near the Gulf Coast (adjusted back to slower timing in the new 00Z run). A composite of the 12Z/18Z GFS, 00Z/27 ECMWF, and 18Z GEFS/12Z ECens means seem to provide the best intermediate starting point at this time. Worth monitoring is a potential southward trend as the 12Z ECMWF-initialized machine learning models leaned along or just north of the Gulf Coast for the upper low track. Over the East, the leading system on Wednesday is gradually coming into better agreement with most solutions depicting rapid progression into the Atlantic after early Wednesday. CMC runs are a notable slow extreme compared to this emerging majority. Behind this system, individual model runs continue to be quite diverse and erratic regarding how upstream southern Canada energy may amplify into the northeastern U.S. With the ECMWF-initialized machine learning models becoming more emphatic about an upper low reaching over or near New England by next weekend, latest GFS runs and 00Z/27 ECMWF match that idea the best overall within a favorably comparable GEFS/ECens mean trough. The 12Z/27 ECMWF was a southwest extreme with the upper low while the CMC/CMCens have been shifting the overall trough axis farther east. The new 00Z ECMWF has different details than some other solutions but does ultimately achieve a New England upper low by next Sunday. Based on guidance comparisons over the course of the period, day 3 Wednesday started with a model composite including a split of older 00Z/12Z ECMWF runs and 12Z/18Z GFS runs with some 12Z UKMET and minimal 12Z CMC inclusion. Thereafter, the blend first excluded the CMC and then eventually phased out 12Z ECMWF input while incorporating only 30 percent weight of 18Z GEFS/12Z ECens means as the 12Z ECMWF and two GFS runs together already compared fairly well to the means. ...Weather/Hazards Highlights... The upper trough and leading frontal system approaching the West Coast around midweek will support a strong southerly flow of enhanced moisture and likely bring a period of significant rainfall and high elevation snow to the West Coast states. During the period covered by the Days 4-5 Excessive Rainfall Outlooks (early Wednesday through Thursday night), guidance has been consistent in bringing the heaviest rainfall to northern California around Wednesday when a Slight Risk area remains as carried over from the prior Day 5 outlook. This outlook still corresponds to the best overlap of guidance signals within persistent timing spread, though now with a little closer agreement among the GFS/ECMWF/UKMET and the GEFS/ECens versus the CMC/CMCens that are still a little faster. The surrounding Marginal Risk area reflects an intermediate timing of significant but less extreme rainfall potential. There is also a Marginal Risk area over the Olympic Peninsula in western Washington in the Day 4 outlook with guidance still showing some relative QPF maxima with wet from prior days' rainfall. By Day 5/Thursday, expect the best precipitation focus to shift farther south over California as the upper trough axis nears the coast. A majority of heavy QPF over the Sierra Nevada should be in the form of snow, leaving areas near the southern California coast as the region of interest for excessive rainfall potential. Current plan is to introduce a Marginal Risk area over and south of the Transverse Ranges, with some acceleration of the moisture band relative to Day 4 helping to temper amounts somewhat. However the GFS/ECMWF suggest there may be some instability with this activity, so there could be potential for fairly intense rain rates over favored southward-facing terrain. A narrowing of existing timing spread and better resolution in the short-term models over the coming days could eventually help to define any potential embedded Slight Risk area. Farther inland, expect a broad area of moisture to spread across the West from Thursday into the weekend and produce rain/higher elevation snow. Southward-facing terrain should see the highest totals over the West, while portions of the central and/or southern Rockies into High Plains could eventually see easterly enhancement as elongating upper trough energy forms a closed low over or just beyond the Four Corners region. Then a fairly strong system most likely continuing eastward across the southern tier should extend rainfall from the central-southern Plains through the Southeast, with some of this rain potentially heavy. The system crossing the East into Wednesday will produce an area of mostly light precipitation which may be snow over the central Appalachians/northern Mid-Atlantic and rain farther south. There is still some uncertainty over the exact moisture coverage. Then upper troughing and potential surface development may produce some precipitation over the Northeast from late week through the weekend. Most of this precipitation should be snow, but some rain could be possible depending on details which remain fairly uncertain. The northern-central Plains into the Midwest and Great Lakes should see the most consistently above normal temperatures during the period, with northern areas seeing the best potential for multiple days of morning lows up to 20-30F above normal and highs up to 15-25F above normal. Some daily records will be possible, primarily for warm lows. The West will be warm into midweek with less extreme anomalies, generally up to plus 10-20F and especially for morning lows which could hold above some daily records. The upper trough moving in will bring a cooling trend for the latter half of the week through next weekend with highs over portions of California and Southwest U.S. up to 10-15F below normal Friday-Sunday. The above normal min temperatures will take an extra day or so to trend toward normal. Over the East, the southern part of the Florida Peninsula may see temperatures up to a few degrees below normal mid-late week but otherwise expect near to above normal temperatures. There is continued uncertainty for highs over the Northeast by next weekend, depending on the depth and location of upper troughing and a possible embedded closed low. There is better confidence in above normal morning lows over this region. 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