Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Fri Jan 26 2024 Valid 12Z Mon Jan 29 2024 - 12Z Fri Feb 02 2024 ...Overview... Most guidance agrees on a large scale upper pattern featuring an amplified Pacific trough arriving into the West late in the week, a downstream ridge drifting from the Intermountain West into the Plains/Mississippi Valley, and an East Coast mean trough that should trend more shallow by the end of the week. Strong and moist southerly flow ahead of the Pacific trough will support an atmospheric river event whose focus should progress southward along the West Coast over the course of the week, with some moisture likely extending farther inland over the West after Wednesday. The forecast pattern will produce drier conditions farther east, with the primary areas of precipitation being the potential for lingering snow over southern New England on Monday with a departing storm system and mostly light rain/snow with a trailing system crossing the East early-mid week. ...Guidance/Predictability Assessment... Over the West, the pattern seems to exhibit better than average predictability as an overwhelming majority of dynamical models/ensemble means and 12Z ECMWF-initialized machine learning models bring the amplified Pacific upper trough to or just inland from the West Coast as of early next Friday. GFS runs have exhibited some variability at that Friday valid time, with the 12Z run straying a bit fast but the 18Z and new 00Z runs slower than consensus. As for other embedded details, the GFS eventually strays northward with the fairly deep low pressure offshore the Pacific Northwest, with the majority of guidance holding onto a defined surface low (albeit weakening) that drifts toward the coast late in the week. There are also differences in exactly how quickly leading height falls reach the West Coast, with a guidance average providing a reasonable starting point in light of no clear clustering at this time. Overall there is a lot more spread and variability with details over the East. Recent GFS runs have been on the slow side of the spread for the amplifying shortwave crossing the region. This is still the case when adding in the new 00Z runs but slower trends in the UKMET/ECMWF make the GFS look a little less extreme and thus the full guidance average is now a little slower with the associated surface reflection than what was supported by the non-GFS majority among the 12Z/18Z dynamical models/ensemble means and machine learning models. Eastern Canada into New England flow by mid-late week has also been an issue, with the 12Z ECMWF showing a particularly deep and slow upper low in contrast to remaining solutions that depict either a more modest trough or faster progression (as reflected in the new 00Z ECMWF). Another shortwave may drop into the East Coast mean trough by Friday, with latest GFS runs being on the slow side. The GEFS/ECens means provide the best starting point late in the period given the spread and run-to-run inconsistency in the guidance. For the updated forecast based on 12Z/18Z guidance, an operational model composite provided a reasonable starting point early in the period. Decreasingly favorable comparisons for the GFS over the East from midweek onward and to some degree over the West by Friday, as well as low confidence in the 12Z ECMWF over the East after midweek, led to phasing out the GFS and quickly increasing total 18Z GEFS/12Z ECens weight to 70 percent by days 6-7 Thursday-Friday. Minority lingering input from 12Z CMC/ECMWF provided a little operational detail over the West without adversely impacting the pattern over the East. ...Weather/Hazards Highlights... The upper trough approaching the West Coast Monday-Wednesday and then arriving into the West by late week will promote a strong southerly flow of enhanced moisture and likely bring a period of significant rainfall and high elevation snow to the West Coast states, eventually reaching farther inland with time. During the period covered by the Days 4-5 Excessive Rainfall Outlooks (early Monday through Tuesday night), the best focus should initially be over southward facing terrain in western Washington near the Olympics with some moisture possibly reaching southern Oregon and northern California by the Day 5 time frame. Both days depict a Marginal Risk area over western Washington in light of wet conditions leading into the outlook period, with some lingering uncertainty over the precise magnitude and persistence of anomalous moisture. Meanwhile a separate Marginal Risk area along coastal areas of southern Oregon/northern California on Day 5 reflects an intermediate timing of heavier rainfall arrival (closest to the 12Z ECMWF/ECens and latest NBM runs) versus the faster/southward CMC and slower GFS. Again with some uncertainty on exact timing, heaviest totals currently appear to be most likely over northern-central California on Wednesday and then shifting into central-southern California by Thursday. Also by Thursday expect organized precipitation to extend into the Intermountain West/Southwest. The southern Rockies/Plains could see some precipitation by Friday. The system tracking away from the East Coast on Monday may produce some lingering snow over southern New England along with brisk to strong winds over the western Atlantic. A system passing through the East early-mid week may spread an area of mostly light precipitation (snow north, rain south) across the region. The northern-central Plains will see anomalously warm temperatures through the period, with northern areas tending to see the best chance for multiple days of min/max temperatures at least 20F above normal. Some morning lows could be up to 25-30F above normal and a few daily records will be possible. The West will be warm as well early in the week with somewhat less extreme anomalies (generally up to plus 10-20F), but that may still be enough to produce some daily records. The upper trough moving in should bring a cooling trend for the latter half of the week with highs over California and an expanding portion of the southwestern U.S. trending below normal Thursday-Friday. Below normal readings over the East will tend to be confined to Florida early in the week. The remainder of the East will likely see morning lows that are warmer than average and daytime highs near to slightly above normal, with a couple frontal passages providing some day-to-day variability. 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