Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Jan 30 2024 Valid 12Z Fri Feb 02 2024 - 12Z Tue Feb 06 2024 ...Late week into weekend system to spread significant precipitation across much of the West and then the Plains and Gulf Coast/Southeast... ...Another strong weather system increasingly likely to impact California late this weekend into early next week... ...Overview... Guidance continues to show an evolving omega block pattern aloft, with varying degrees of mean troughing along or offshore the West coast and over the Canadian Maritimes while an upper ridge prevails over the Upper Midwest/Great Lakes. There will be the added wrinkle of late week western U.S. upper trough energy breaking off an upper low that tracks across the southern tier during the weekend into early next week. The large scale pattern will promote episodes of significant precipitation and potentially other active weather across the West, with highest totals likely to be over California with a late weekend into early next week system and the southern tier with the system crossing the region during the weekend. Meanwhile, the forecast pattern would support persistence of much above normal temperatures over the Upper Midwest and vicinity in contrast to cool daytime highs over the Southwest/California. ...Guidance/Predictability Assessment... As has been the case over recent days, most guidance shares general themes for the large scale pattern but with some embedded spread and run-to-run adjustments. One notable trend over the past day has been for guidance to shift eastward somewhat for the Upper Midwest/Great Lakes ridge and downstream trough/potential embedded upper low, with the latter now more likely settling over or near the Canadian Maritimes instead of New England. The ECMWF-initialized machine learning models that had initially been most aggressive with the Northeast troughing/low have also shifted eastward some, but their 12Z cycle still offers some potential for troughing to linger in more amplified form than most dynamical guidance by early next week. Meanwhile the guidance has been waffling for the precise latitude and speed of the upper low and associated surface system crossing the southern tier. Consensus of 12Z/18Z models and ensembles has returned back to where it was two days ago after drifting a little south for a time. The new 00Z GFS/UKMET stray on the faster and southern side of the spread. What becomes of this system upon reaching the western Atlantic early next week is still somewhat uncertain given the spread and inconsistency for troughing to the north. Along the West Coast, recent days of model runs have varied for evolution of low pressure off the Pacific Northwest. Currently the GFS/ECMWF/UKMET and ensemble means cluster toward a southward drift Friday into the weekend with the CMC trending weaker/eastward. Then there is a fairly good consensus toward positively tilted upper troughing composed of lingering Pacific Northwest coast troughing and an incoming southern stream Pacific feature. Details are still very murky however, so confidence remains quite low regarding the strength and track of associated low pressure that could be quite deep/north (GFS/ECMWF) or slower (per CMC runs that have been showing less phasing aloft). ECMWF-initialized machine learning models have been favoring the faster side of the spread for the primary surface system. At this point preferences favor an intermediate solution until better clustering emerges. 12Z/18Z guidance comparisons led to a starting blend of more 12Z/18Z GFS/12Z ECMWF relative to the UKMET/CMC early in the period and then transitioning to some 18Z GEFS/12Z ECens mean input while phasing out what minority component of the CMC there was earlier on. ...Weather/Hazards Highlights... The leading upper trough moving into the West will produce a broad area of rain and higher elevation snow, with significant accumulations possible Friday-Saturday over much of the northern and central Rockies. There will also be some lingering precipitation closer to the West Coast during that time. The upper low breaking off from the southern part of the trough along with the associated surface system may lead to a period of easterly precipitation enhancement along the central Rockies, and should begin to generate rain and thunderstorms over the southern Plains by Friday or Friday night with lighter totals extending into the central Plains. Eastward progression of the system over the weekend will spread some heavy rainfall across the Gulf Coast and Southeast. The new Day 4 Excessive Rainfall Outlook covering Friday-Friday night maintains the prior Day 5 Marginal Risk area covering most of the eastern half of Texas into parts of Oklahoma, with guidance continuing to signal potential for locally heavy (though not exceptional) rainfall. Then the Day 5 ERO depicts a Slight Risk area along the central Gulf Coast where the best instability and ensemble probabilities of highest rainfall totals overlap, with operational model runs showing heavy rainfall potential as well. The surrounding Marginal Risk area extends north-northwest into southwestern Missouri and southeastern Kansas, for less extreme rainfall totals but with ground conditions starting out on the damp side. Heavy rainfall potential should continue eastward into the Southeast/Carolinas by early next week. The details remain very uncertain but the overall forecast pattern continues to show the potential for a significant atmospheric river event to affect California from around Sunday into early next week. There are also some signals that the associated surface system could be fairly strong, which would produce a period of strong winds. This event still appears likely to produce greater low elevation rain/mountain snow totals than the preceding event due to a longer duration of southwesterly flow aloft and a more direct connection with lower latitude moisture. Continue to monitor forecasts for this time frame as specifics gradually become better resolved. The persistent upper ridge over and near the Upper Midwest will support a core of persistently well above normal temperatures over that part of the country. Recent guidance adjustments for specifics of the upper pattern have increased potential temperature anomalies, with morning lows possibly as warm as 25-35F above normal exceeding daily records for multiple days. Expect daytime highs to be less extreme, up to 15-25F above normal. Some of this warmth, especially for morning lows, will extend across a much broader area between the Rockies and Appalachians. In contrast the Southwest U.S./California should see cool daytime highs through the period with some locations up to 10-15F below normal. Clouds/precipitation over the West should keep morning lows near to above normal. 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