Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Thu Oct 24 2019 Valid 12Z Sun Oct 27 2019 - 12Z Thu Oct 31 2019 ...Well below average temperatures expected for much of the West next week... ...Overview and Guidance/Uncertainty Assessment... Strongly amplified pattern out of the eastern Pacific across North America will dominate next week driven by a +3.5 sigma height anomaly over southeastern Alaska/Yukon territory of Canada. In addition, persistent subtropical ridging over the Bahamas (+2 sigma) will favor troughing over the western and central CONUS and broad southwesterly flow through the East. The models/ensembles have struggled with the amplitude/depth of the troughing, most notably the GFS/GEFS. Given the lead-in pattern, recent verification, and long-standing known model biases, will continue to favor the slower/deeper/more amplified solutions that include the 00Z ECMWF/Canadian and ECMWF ensemble mean. This pairs well with WPC continuity. 00Z UKMET also fell into the slower camp but was among the slowest 5% of ensembles early next week. Multi-day trends have shown that the deterministic ECMWF can be too deep/slow with strong troughing and closed off systems, and the ECMWF ensemble mean is often most correct at the expense of some lost detail. By next Wed/Thu, additional height falls out of western Canada may slow/drag the mean trough axis via surface wave development along the strong cold front (as shown by the past two ECMWF solutions) but the Canadian was much more progressive. Trended toward the ECMWF ensemble mean by the end of the period which does show modest wave development out of the lower MS Valley into the Great Lakes as most ECMWF ensemble members held the front along or just west of the Appalachians by 12Z next Thu. ...Weather Highlights/Threats... Deep troughing in the West will carry a lead cold front into the Desert Southwest and southern Plains Sun/Mon with temperatures 10-30F below average in its wake, centered from Montana southward to the Four Corners region and eastward to the western Plains. On Tue/Wed, another cold front will bring even colder air to the same region, taking temperatures down to 20-40F below average over much of Wyoming and eastern Colorado. This may challenge some daily record lows or low maxes as temperatures stay in the 10s or 20s. Modest cooling is likely even into the Great Basin and southern California to the southern Rockies along with increased winds (~1036mb high pressure over Wyoming next Wed). The lead front out of the Plains on Sunday will likely get hung up across the Great Lakes/Midwest and wait for the second cold front to merge/overtake it with low pressure likely lifting northeastward. This could lead to a swath of snow in the colder air to the northwest of the low out of the central Rockies Sun/Mon and across the central Plains to the Upper Midwest/western Great Lakes Tue/Wed. Please consult the latest WPC day 4-7 winter weather probabilities for highlighted areas. As the frontal boundary pushes eastward into the northwestern Gulf and Lower MS Valley Wednesday, increased moisture to its east will help expand the rainfall areas into the TN Valley and northeastward to the Ohio Valley and Appalachians by Halloween. Confidence is lower regarding system timing but a slower front would likely favor a heavier rainfall potential. Fracasso 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