Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1044 AM EST Fri Nov 16 2018 Valid 12Z Mon Nov 19 2018 - 12Z Fri Nov 23 2018 ...16 UTC Update... Model agreement is relatively good for the first portion of the extended period and then goes quickly downhill by the later portion. Similar to the overnight issuance, the forecast during days 3-5 (Mon-Wed) was based on a multi-model deterministic blend (00Z ECMWF/FV3 and 06Z GFS). Models showed similar solutions for a couple shortwaves crossing the Great Lakes at the base of the Hudson Bay mean trough during this period, and with an elongated area of shortwave energy crossing the south central/southeastern U.S. The main forecast difficulty arises by days 6-7 (Thu-Fri) with respect to the weakening of western U.S. ridging and the arrival of height falls from the Pacific. The deterministic ECMWF continues to be among the faster solutions (joined by the CMC) to bring a leading shortwave into the West by Wed/Thu, quickly eroding the ridge. The GFS and FV3, while previously showing a rather consistent slower solution, have now become more variable. In general, a trend away from the slower solutions seems to have been observed over the last couple model cycles, although this is by no means universal. This is perhaps best demonstrated by the persistence of the CMC ensemble mean with a strong ridge and slower trough arrival despite the operational CMC more resembling the ECMWF. Nonetheless, arrival of height falls and a cold front are likely by the middle of next week along the West Coast. If a faster ECMWF/CMC-like solution were to verify, the development of a low pressure system across the central U.S. by day 6-7 would become more likely in response to a fast/amplified leading shortwave. Needless to say, forecast confidence is low by day 6-7, and weighting of ensemble means (ECENS and GEFS) was increased by that time. More weight was placed toward the GEFS mean in order to show some deference to continuity given the significant observed fluctuations. Ryan Previous Discussion (issued at 0554 UTC) ...Overview... Western ridging and eastern troughing will slowly translate eastward next week in a mostly dry pattern for much of the lower 48. Much needed rain may finally work into California by later in the week next week. ...Guidance/Uncertainty Assessment... Forecast for Mon-Tue was formed from a multi-model deterministic blend of the recent 12Z/18Z guidance. Weak clipper will dip through the Great Lakes/Northeast as southern front sinks into the Florida Straits. A weakening southern upper low will open up off Southern California and push inland. For Wed-Fri, ensembles still diverge on how quickly to lower heights into the west. The ECMWF and its ensembles (with the 12Z Canadian) remain quicker than the GEFS/Canadian ensembles while the deterministic GFS runs were slowest. The FV3 and UKMET were somewhat in between, near the 18Z GEFS mean. Trend over the past several cycles has been slower in the ECMWF ensembles and just a bit quicker in the GEFS members, which suggests leaning away from the quicker members (12Z ECMWF included). It has been seen at least a couple times in as many months that when the ECMWF ensembles were quicker than the GEFS ensembles verification favored the GEFS (though perhaps not completely). Opted to continue with the previous shift preference with a slower breakdown of the ridge along the line of the 18Z GEFS mean and somewhat the 12Z FV3 GFS. Indeed the 00Z GFS was considerably quicker than the previous runs, further exhibiting the uncertainty. Either way, front should eventually make its way to the coast, perhaps as soon as Wednesday but more likely early Thursday. In the east, heights should slowly rise but there remains differences across Canada with how much northern stream troughing may linger in the Northeast. Leftover southern stream reflection of the weakening upper low through the Southwest early in the week may attempt to carry into the Gulf of Mexico later in the week. ...Weather Highlights/Threats... Rather quiet period early in the week dominated by NW flow out of central Canada. Exception may be in the western Gulf along an inverted trough to the north of a stationary boundary. Potential for heaviest rain would be along the coast where instability is highest. That will continue through the week with perhaps some enhancement by the incoming southern stream shortwave. Entire west coast will see in increase in the chance of rain and elevation snow starting as soon as late Wednesday. Focus appears to be around northern California but this may extend into SW Oregon and/or central California given the ensemble spread. Amounts may be in the 1-3 inch range Thu-Fri over favored areas and even about a half inch in the Central Valley. Temperatures will be below average in the Great Lakes/Northeast for much of the week as troughing will be slow to lift out. Daytime highs in the 20s away from the coastal plain may be near record cold values Tue/Wed with record lows overnight possible as well especially Wednesday morning in New England with widespread teens and single digits possible. Temperatures in the west will be above average (by about 5-15F) along/inland from the central West Coast through at least the first part of next week. Cooler temperatures will accompany upper troughing and the frontal approach, nudging the warmer than average temperatures into the Plains by the end of the week. Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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