Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Fri Nov 15 2019 Valid 12Z Mon Nov 18 2019 - 12Z Fri Nov 22 2019 ...Pattern Overview and Guidance/Predictability Assessment... The large scale pattern will feature moderately progressive northern stream flow with a tendency for upper troughing in the East, while some energy nearing the West Coast will drop southward into a slow-moving closed low. One upper low already west of Baja California at the start of the period early Mon will promote a northward flow of moisture associated with Tropical Storm Raymond. Some energy within a trough nearing the West Coast by early Tue should brush the northern tier while the rest drops into an upper low forecast to close off over or near California, and in the process eject the upper low initially to the south. The California upper low should drift slowly eastward late next week. Northern tier/Canada trough energy should reach the East by Fri, replacing the amplified trough over the East during Mon-Wed. Ahead of the initial eastern trough aloft, guidance continues to show two defined systems but with poor clustering/continuity. The first one, tracking close to New England Mon-Mon night, has had a wide track spread (due to smaller scale detail differences aloft) with significant influence on the degree of sensible weather impacts over the Northeast. Majority model/ensemble clustering and continuity leaned toward latest ECMWF/GFS runs and their means so the updated forecast put greater emphasis on that scenario. However UKMET/CMC runs including the 12Z cycle and a number of 00Z CMC ensembles have advertised a farther west track and could have some validity. For updated model preferences based on 12Z data please refer to the Model Diagnostics Discussion PMDHMD. Models and ensembles still show a second system developing off the East Coast from about late Tue onward but with 24-hour trends showing a farther offshore track and less impact on the East Coast. Details differ but there is fair agreement that some leading trough energy coming into the West by Tue will cross the northwestern U.S. while the rest drops into a closed low over or near California. Latest CMC runs have been showing the farthest west track with the 00Z ECMWF a little eastward, while remaining solutions are along the coast or inland. By late in the week the GFS and ECMWF have varied with their timing of ejection. The old 12Z/14 ECMWF was faster than most other recent runs while the 00Z GFS was closer to the 00Z ECMWF than the 06Z run. The combination of ridging to the north/northwest of the upper low and distant/weak nature of upstream Pacific energy seem to favor timing in the middle to slower half of the spread--leaving the 00Z ECMWF/00Z GFS (CMC not out of the question but perhaps a tad extreme) as the closest operational models fitting this idea. Toward the end of the week the operational models and ECMWF/CMC means provide a good signal for what could be a fairly vigorous system reaching the Great Lakes-New England and/or southeast Canada in association with the shortwave energy reaching western North America earlier in the week. GEFS means have been much less defined with the system. Based on above considerations the forecast blend during the first half of the period emphasized the 00Z ECMWF and a little less 00Z GFS, with modest CMC/UKMET input. By days 6-7 Thu-Fri the forecast blend transitioned to a model/mean blend incorporating the 00Z ECMWF/GFS/CMC in order of highest to lowest weight along with more ECMWF mean than GEFS mean. ...Weather Highlights/Threats... The most prominent signal for significant focused precipitation exists over the Southwest and vicinity, near and ahead of the mid-late week upper low. The best model/ensemble agreement for highest totals remains over the favored terrain of central Arizona. Depending on the exact path of the upper low there may also be a period of significant precip along the coastal ranges of southern California and possibly over other locations between southern California and southern Nevada/Utah/Colorado. Moisture from T.S. Raymond could enhance precipitation totals. Expect mostly rain over southern areas with some higher elevation snow possible over the Great Basin/central Rockies. Highest elevations to the south could see a little snow if the upper low is sufficiently deep. Northern parts of the West will see rain and high elevation snow during the first half of the week with snow levels decreasing as precip trends lighter. Some of this moisture should extend eastward across the northern tier in the form of snow around midweek. Developing low pressure later in the week may produce higher amounts--snow north and rain south--over parts of the Great Lakes/Northeast but with sensitivity to exact surface low track. Rainfall of varying intensity should accompany the trailing cold front over the central/eastern states. The leading system tracking near/offshore New England should bring some precipitation into the Northeast early in the week, with some snow possible over higher elevation/inland areas and rain closer to the coast. The tight pressure gradient around the northern/western side of the system may produce a period of strong winds. Poor agreement with exact track keeps confidence lower than desired for the coverage and intensity of effects over the Northeast. There is lingering uncertainty for the second offshore system around midweek but at the moment there is better agreement that most effects should bypass New England. 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