Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sun Mar 17 2019 Valid 12Z Wed Mar 20 2019 - 12Z Sun Mar 24 2019 ...Overview and Guidance Assessment/Preferences... Latest guidance continues to anticipate a split flow regime over western North America, with a persistent western Canada mean ridge deflecting periodic bundles of shortwave energy into the western Lower 48 and the rest into or around mainland Alaska. Aside from some specifics there has been decent agreement that the leading feature coming into the West will form an upper low near the California coast around midweek and then gradually weaken as it reaches the central U.S. Another shortwave should reach the West by early next weekend but there is greater uncertainty for specifics of this feature and upstream flow. Mean troughing will prevail over eastern North America mid-late week and then gradually shift into the western Atlantic. Within this trough there is still a lot of guidance spread/run-to-run variability with individual shortwaves and thus low confidence in forecast specifics. Compared to the past couple days guidance seems to be better clustered for the leading system moving into/through the West and eventually reaching the Plains. A consensus approach seems reasonable given the low predictability of embedded small scale impulses that will play a role in determining exactly where the upper low center will be at a particular point in time. Upstream details rapidly diverge during the latter half of the period. There are essentially two clusters of guidance, the GFS/GEFS which stray faster with North Pacific flow from earlier in the period versus most other solutions (ECMWF/ECMWF mean/CMC/UKMET/NAVGEM and to some degree the CMC mean) that are slower. As a result by late in the period the GFS cluster already has another strong trough nearing the West Coast by day 7 Sun and rapidly weakens the shortwave reaching the coast the previous day. On the other hand other solutions are slow enough upstream to allow more of the day 6 Sat energy to move into the southern half of the West on Sun. Prefer to lean 3/4 toward the ECMWF cluster given the more pronounced guidance signal toward that scenario and the fact that it would represent somewhat better persistence of the mean pattern that has become established. Confidence remains well below average for details over the central/eastern U.S., including possible surface low development off the East Coast. The closed upper high forecast near or north of the northern Montana border as of early Wed is continuing to cause havoc with the forecast of shortwave energy over the Plains/Mississippi Valley at that time. The 00Z ECMWF/NAVGEM are in the minority today, holding more energy over the north-central Plains on Wed versus established consensus. Further complicating the forecast, there are various possibilities depicted for amplification/timing of Canadian energy that may drop into the eastern U.S. with some potential for interaction/phasing to occur. The 06Z FV3 GFS is on the extreme side of guidance regarding associated surface development and effects along parts of the East Coast but there is in general a stronger signal for surface development just close enough to the coast to merit close monitoring. Note that the 12Z/16 ECMWF contributes to this signal, with an initial Plains/MS Valley shortwave more similar to the current majority. Additional shortwaves may feed into the mean trough aloft behind the possible late week Atlantic development but should reflect merely as cold fronts at the surface. Due to lower confidence in the 00Z ECMWF east of the Rockies early-mid period, the primarily operational model blend weights that solution somewhat lower than would normally be the case. Then by next weekend the forecast phases out the less probable GFS over the Pacific/western U.S. while emphasizing the 00Z ECMWF/ECMWF mean/CMC and modest weight of the 06Z GEFS mean. ...Weather Highlights and Hazards... The southern stream system initially heading into the West will bring rain and mountain snow to most locations from California eastward into the central/southern Rockies mid-late week. Most of the precipitation may stay just north of the Desert Southwest/southern Arizona though. Typically favored terrain should see the highest totals with this system. Expect the central/southern Plains to see greater coverage and intensity of showers/thunderstorms during the late week into weekend time frame as moisture increases ahead of the western system. Some areas of heavy rainfall are possible. The next system reaching the West Coast late Fri-Sat will likely focus rain and high elevation snow over central/northern areas with lighter activity extending farther south. Areas near the central West Coast may see a brief period of moderate-heavy precipitation. Confidence decreases in coverage/areas of greater focus over the West by next Sun. To the east of the Rockies, some rainfall may accompany a weakening wave/frontal system over the Plains/east-central U.S. early in the period. Frontal passages may bring one or more episodes of mostly light rain/snow to the Great Lakes/Northeast. Specifics of potential surface low development off the East Coast continue to be a major uncertainty around Thu-Fri. The most probable scenario is for any system to track far enough offshore to keep most precipitation away from East Coast locations. However there is still an ongoing lesser threat for more meaningful effects that require monitoring. The overall pattern should keep daytime highs somewhat below normal over the central/southern latitudes of the West, though clouds/precipitation will keep lows on the warmer side of normal (by 10-15F in some cases). Aside from periodic cool pushes into the Northeast, there should be expanding coverage of above normal temperatures to the east of the Rockies during Fri-Sun, with some plus 10-20F anomalies possible over parts of the Plains/Mississippi Valley. 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