Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM EST Fri Jan 26 2024 Valid 12Z Mon Jan 29 2024 - 12Z Fri Feb 2 2024 19Z Update: The 12Z model guidance has come into better agreement with the amplifying trough from the Great Lakes to the East Coast for the middle of next week, although the GFS remains a slightly slower solution. It appears that ensuing coastal low cyclogenesis should be far enough away from the coast to keep precipitation amounts light as the forecast stands right now. There is better than average model agreement with the large scale trough setting up along the West Coast by mid-late week, with the greatest model differences residing across southeast Canada with timing differences in northern stream flow. By Friday, the GFS is stronger with a shortwave/upper low over the Great Lakes region, but it is compatible with the model consensus elsewhere. A multi-deterministic model blend sufficed as a starting point in the forecast process for the Monday-Wednesday time period, and then gradually increasing percentage of the ensemble means for later in the week to iron out the differences across the eastern U.S. and southeast Canada. The previous forecast discussion is appended below for reference. /Hamrick -------------------- ...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 Marginal Risk area that was over the Olympic Peninsula in western Washington on Day 4 has been dropped, with the latest overall QPF model signal of less than 1 inch and hourly rainfall rates likely to be under a quarter inch, but the Marginal Risk remains here for the Day 5 period as moisture flux increases. Meanwhile, the separate Marginal Risk area along coastal areas of southern Oregon/northern California on Day 5 remains in place, and this reflects the consensus timing for the arrival of moderate to locally heavy rainfall with the atmospheric event expected on Wednesday. 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