Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1142 AM EDT Sat Oct 26 2019 Valid 12Z Tue Oct 29 2019 - 12Z Sat Nov 02 2019 ...Well below average temperatures expected over much of the West/Plains next week... ...Potential storm may affect the eastern half of the country during the latter half of the week... ...16Z update... Fortunately, model guidance has come into considerably better agreement in the medium range period. As shown in the previous discussion below, older runs of the GFS/GEFS were much faster with the main trough/energy crossing the U.S. compared to non-NCEP guidance. Each run of the GFS suite has trended slower beginning with yesterday's 18Z run, and today's 06Z run is much better aligned with the 00Z ECMWF and ECENS in particular, and also the 00Z UKMET and CMC through day 5. This has increased confidence in a much slower track for the upper low/trough. Since this slower solution was where our forecasts leaned heavily over the past couple of days, major changes were not needed for the medium range forecast, and some 06Z GFS and GEFS were able to be incorporated. This pattern would still lead to the potential for a strengthening surface low to track through the Tennessee and Ohio Valleys into the Great Lakes region on Thu/Fri. Please see the previous discussion for more details on sensible weather. Tate ...Previous Discussion... ...Overview and Guidance/Uncertainty Assessment... Models and ensembles indicate that the northern periphery of the strong eastern Pacific ridge, extending through western Canada as of Tue, will quickly flatten. This should change the shape of downstream troughing over the course of the period--aligned from near Hudson Bay into the western U.S. during the short range time frame into midweek followed by a migration into the central U.S. Within this transition the dominant feature of interest and forecast problem will involve the energy digging into the Interior West/Rockies on Tue and then ejecting eastward/northeastward with time. Guidance thus far has varied considerably with the details but there seems to be an improving signal for the general idea that this energy will support a strengthening storm system that may support areas of heavy rainfall, meaningful snow in parts of the cold sector, and possibly strong winds. Over multiple model runs the GFS/GEFS have consistently been on the progressive extreme with the energy initially digging into the West, while teleconnections/recent verification in this pattern/historical biases have favored some variation of a slower solution such as represented by recent ECMWF runs. Starting with the 18Z cycle the GFS/GEFS have made a significant adjustment toward the slower scenario but remain conspicuously fast/east, especially with the new 00Z UKMET somewhat slower than its previous run. Meanwhile the 00Z CMC has trended a bit faster than its past couple of runs that were on the slow edge of the spread--and is probably a good adjustment in light of approaching upstream flow and recent tendency for initially slow solutions in this pattern to adjust a little faster. 12-hourly ECMWF runs have been oscillating with 12Z runs slower than 00Z runs while the ECMWF mean has been somewhat more stable. As a result the updated forecast emphasized an average of the past two ECMWF runs and the 12Z ECMWF mean. For days 3-4 Tue-Wed the blend included the 12Z UKMET as well as the 12Z CMC before it become noticeably slow. This kept the forecast well-centered given the ongoing detail uncertainty. Model runs are providing a steadily improving signal that the surface low may deepen significantly as it tracks into the Great Lakes and southern Canada so as confidence improves the forecast will likely reflect a deeper system. Behind this system model differences have less significant impact on sensible weather. However preferences still lean away from the GFS/GEFS mean as those two solutions (especially the GEFS mean) have been on the weaker side of the spread for the eastern Pacific ridge aloft. ...Weather Highlights/Threats... The forecast remains on track for a large portion of the western and central U.S. to see much below normal temperatures, with a broad area seeing one or more days with highs and/or lows at least 20F below normal. Expect the most extreme anomalies over the northern and central Rockies/High Plains on Tue-Wed with some readings 30-40F below normal. Such temperatures will likely challenge daily records for lows and/or cold highs at some locations. There will be some moderation after midweek but it will be gradual. Still expect to see strong winds to the south of the strong surface high building southward over the northern into central Rockies. The signal from the guidance is steadily improving for the idea that ejecting western dynamics will support a strengthening storm system affecting the eastern half of the country with an array of significant weather after midweek, with a surface low most likely tracking through the Ohio Valley/Great Lakes in the Thu-Fri time frame and then continuing into Canada. Best potential for meaningful snow will extend from the central Rockies across the central Plains and then northeastward into the Upper Midwest/Great Lakes. Areas of heavy rainfall and potentially strong convection will be possible in the warm sector as low level flow brings Gulf moisture northward. Some locations over the southern Plains/Lower Mississippi Valley east/northeast into the Appalachians should see the highest totals from this event but significant rainfall should extend well northeastward. Potential strength of the system may support some strong winds, most likely over the Great Lakes and Northeast. After this system departs the pattern should be quite dry over much of the lower 48 with perhaps some scattered light snow over the extreme northern tier with a cold front dropping southeast from Canada and a little lingering rainfall over Florida. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml