Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Oct 19 2023 Valid 12Z Sun Oct 22 2023 - 12Z Thu Oct 26 2023 ...Overview... Models and ensembles have maintained a fairly agreeable pattern evolution over the East from this weekend into the first half of next week, with strong Canadian Maritimes low pressure producing gusty winds and some lingering precipitation over the Northeast on Sunday followed by a cool and drier pattern as the supporting upper trough departs in favor of ridging at the surface and aloft. In contrast, recent guidance runs have been showing increasing uncertainty for the eastern Pacific into North America pattern from about Tuesday onward and the new 00Z model runs have raised the uncertainty to another level. The most likely scenario would have a mean trough digging into the West to bring a wetter and cooler period to the region, with expanding coverage of northern Rockies/Plains snow by midweek, while moisture from Hurricane Norma in the East Pacific may contribute moisture to a general pattern favorable for some areas of heavy rainfall over the central U.S. by Tuesday. Alternative possibilities include a much warmer and drier pattern over the West and less precipitation coverage over the central U.S. ...Guidance Evaluation/Predictability Assessment... The primary source of the recent widening spread in the guidance from the far eastern Pacific into North America is the evolution of upper ridging that builds into Alaska and the pattern just to the south. During the first half of the week, the new 00Z GFS/CMC and UKMET (through the end of its run late Tuesday) have shifted toward varying degrees of elongating shortwave energy underneath the Alaska ridge. This ultimately leads to an upper ridge building into the West by days 6-7 Wednesday-Thursday with broad cyclonic flow downstream across southern Canada and northern tier U.S. On the other hand, recent consensus has been for the higher latitude shortwave energy to drop into the western U.S. to form a larger scale trough with possible embedded upper low (albeit with a fair amount of spread for specifics, including some earlier CMC runs being quite aggressive with the trough/upper low). The new 00Z ECMWF still goes along with the western trough, though in slightly less amplified form than the 12Z run. Thus far there has been a sufficient majority of ensemble members favoring the western trough solution for the means to depict that idea. This includes the new 00Z GEFS mean that ends up very close to the 12Z ECMWF mean by day 7. That said, 12Z GEFS/ECens/CMCens clustering actually showed a total of about one-third of members grouped into a variation of the western ridge pattern. Meanwhile teleconnections relative to the strong core of positive height anomalies that most models/means have been showing over the southern coast of Alaska in the D+8 multi-day means support western U.S. troughing. Based on guidance through the 12Z/18Z cycles and teleconnection relations, preference sided with the western trough solution that represents continuity. Ahead of this mid-late period western pattern issue, specifics will play a role in how long an initial trough/upper low moving into the West lingers near the western U.S.-Mexico border. In general more amplified upstream troughing leads to a faster ejection of the leading feature, though with the 12Z GFS possibly being too fast. Plus there is a wide dispersion for the track of Norma in the eastern Pacific, with latest GFS runs a considerably fast extreme. For the system affecting the Northeast, latest models have gravitated to a moderately progressive solution over the Canadian Maritimes as of the start of the period early Sunday--closest to what the ensemble means had been showing in recent runs. The updated forecast, based on information from the 12Z/18Z runs, started with a model composite early in the period (more GFS/ECMWF weight relative to the UKMET/CMC) and then transitioned toward half models and half ensemble means (18Z GEFS/12Z ECens) by the end of the period to downplay lower confidence specifics from individual model runs. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks that cover Sunday through Monday night depict no risk areas at this time. On Sunday guidance shows signals of some lingering Northeast precipitation that could produce localized enhanced totals, but lack of instability and fairly rapid departure of anomalous moisture should temper short-term rainfall rates. On Monday the GFS is the only model to depict the onset of locally heavy rainfall over the southern Plains due to its fast progression of Norma, with other models much drier. Otherwise moisture will spread across the West with the central West Coast and northern Rockies likely seeing relatively higher but not exceptionally heavy totals. Probabilities for meaningful snow at high elevations of the Rockies should start increasing early in the week. The western and central U.S. forecast during Tuesday-Thursday will depend in part on the increasingly uncertain pattern aloft. Based on the current majority scenario of a deepening western U.S. upper trough, moderately cool highs over the West early in the week would trend significantly colder over the Intermountain West into northern Plains. Northern High Plains locations could see highs at least 20F below normal. This trend would lead to increasing snow coverage including at some lower elevations, with some higher elevation snow in the Cascades as well. A smaller minority scenario would lead to a warmer and drier pattern over the West, so continue to monitor upcoming forecasts. Meanwhile the pattern should become favorable for increasing coverage and intensity of rainfall over the central U.S. by Tuesday or so, with moisture from Hurricane Norma likely contributing to this activity. The current most likely pattern would produce a fairly broad area of moderate to heavy rainfall, while the minority western upper ridge scenario would probably result in a more confined area of rainfall ahead of the leading shortwave/upper low along the Mexico border. Expect the Northeast to see gusty winds on Sunday behind the departing Canadian Maritimes system. Cool temperatures over the East Sunday-Monday should moderate to near or above normal by midweek. The early week cool spell may bring the first frost or freeze of the season to some locations from the Great Lakes into the Appalachians. Warm temperatures over the Rockies and then Plains early in the week, with some highs at least 10-15F above average, should moderate thereafter but morning lows will remain quite warm (10-20F above normal) across the Plains into the Great Lakes through Thursday in the moist flow around the western side of East Coast/Atlantic high pressure. 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