Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Oct 14 2022 Valid 12Z Mon Oct 17 2022 - 12Z Fri Oct 21 2022 ...Overview... The medium range period will continue to be characterized by persistent amplified/blocky upper level flow, consisting of troughing in the eastern U.S. held in place by strong ridges over the Northwest U.S./western Canada and the Atlantic. The Northwest ridge will be part of a long-term Rex Block configuration, with an initial upper low crossing the Southwest and then energy within the southern part of a splitting eastern Pacific trough forming a new upper low offshore California by midweek. This pattern will support well below normal temperatures over the eastern half of the country as well as the southern Rockies/High Plains, along with very warm temperatures over the Northwest. The first upper low over the Southwest should help produce notable rainfall for portions of New Mexico and Texas Monday and perhaps lingering in southern Texas Tuesday. Meanwhile the energy aloft and westerly flow over the Great Lakes will lead to multiple days of lake effect precipitation (including some snow) while a developing low pressure system may bring meaningful precipitation to the Northeast Monday-Tuesday. ...Model and Predictability Assessment... Model consensus at the start of the medium range period Monday/day 3 remains quite good for troughing across the eastern half of the country and ridging in much of the West, with the exception of lingering energy in the Southwest. Models indicate Canadian energy digging southward into the Great Lakes should close off an upper low within the eastern trough at some point Monday, and the closed low should last through at least Thursday as it slowly drifts northeastward. Somewhat more questionable is the placement of surface lows and their associated fronts (and thus areas of organized precipitation) that will depend on exact axes of the energy at any given time. But with the last few runs of the GFS trending slower to eject the upper low, guidance was in good enough agreement for a multi-model deterministic blend. By Wednesday-Thursday preferred the 18Z GFS over the 12Z as the 00Z upper low center remained farther south of consensus and the ensemble means. By day 7/Friday there is much more uncertainty depending on how upstream energy enters to evolve the trough. Models agree that initial energy in the Southwest should weaken Monday-Wednesday as it tracks eastward, with reasonable agreement in the QPF as well. Then some Pacific energy is likely to split off into a southern stream low offshore California with some spread in its position. In terms of the 12/18Z model cycle, the GFS runs and the CMC were fairly well clustered with this upper low, while the UKMET was on the faster side with the ECMWF slower. The incoming 00Z guidance shows better clustering as the EC was farther east slightly while by around day 6/Thursday the GFS/CMC remained farther west, so hopefully the position and the eventual possible shift into the Southwest and the associated potential for QPF may become more agreeable. Meanwhile, predictability remains quite low for the northern energy that ultimately rounds the western ridge and drops into southern Canada and northern CONUS Wednesday-Friday. ECMWF runs have been persistent in suppressing the western North America ridge somewhat, especially up in Canada, with this energy, while GFS runs have wavered somewhat and the 12Z stayed particularly ridgy. The ensemble members of each model suite followed their deterministic models for the most part, so there is still some considerable uncertainty with the upper flow as well as frontal placement at the surface. The WPC forecast thus was based on a blend of deterministic 12/18Z guidance for the early part of the period with increasing amounts of the GEFS and EC ensemble means to about half by days 6-7 as detail uncertainties increase. ...Sensible Weather and Potential Hazards... Heavy rainfall is likely to remain across New Mexico and much of Texas on Monday, aided by a potent but gradually weakening closed low crossing the Southwest and a surface front pushing through the southern Rockies/Plains. Isolated flash floods are possible with the highest rainfall amounts in southern Texas, but the drier soil moisture may preclude widespread issues, plus the placement of the highest QPF varying in guidance led to not enough confidence for any Slight Risk area in the Day 4 experimental Excessive Rainfall Outlook. The initial clouds and rain and the cooler airmass behind the front will keep high temperatures 10-20F below normal through the first half of the week for the Southern Rockies/Southern Plains, gradually warming to near normal. Within the persistent eastern upper trough, Canadian energy digging into a Great Lakes upper low will develop a potent surface wave and frontal system that could produce areas of moderate to heavy rainfall over the Great Lakes and Northeast on Monday and Tuesday. Important timing differences in the guidance are becoming less pronounced but are still enough to maintain some uncertainty in coverage/amounts of enhanced rain. Tuesday there may actually be slightly more confidence compared to Monday with the placement of the rainfall axis across Maine with considerable moisture transport ahead of the cold front. Well below average temperatures (minus 10-20F anomalies, locally greater for Tuesday highs in the central Appalachians) are forecast to spread through the eastern half of the country behind this system, and records for low maximum temperatures may be most common, though a handful of record low minimum temperatures may be set as well. Areas of lake effect rain/snow will be widespread Monday-Tuesday but should gradually decrease in coverage/intensity after midweek. High pressure building over the Midwest and into the Southeast in the wake of a cold front will support a broad area of dry weather. The persistent Northwest upper ridge will keep the region's temperatures well above normal through the period, with each day featuring some plus 10-20F anomalies. Scattered record highs are possible. The West is also forecast to remain mainly dry through the period until some showers may reach California into Arizona later in the week. Tate/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, experimental excessive rainfall outlook, 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/#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