Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Thu May 14 2020 Valid 12Z Sun May 17 2020 - 12Z Thu May 21 2020 ...Significant precipitation possible from northern California into the northern Rockies/Montana... ...Some heavy rainfall possible over parts of the East... ...Overview and Weather/Hazard Highlights... Guidance shows an evolution toward a very amplified regime aloft. A deep trough with embedded low will progress from the eastern Pacific into the western U.S. while farther east a sharpening upper ridge over the Plains should encourage a deepening trough and closed low to settle over the East. The system coming into the West will likely bring highest rainfall totals, and possibly some high elevation snow, from northern California into the northern Rockies/High Plains. Low level upslope flow north of a front that drops into Montana by around midweek could serve to enhance amounts over/near that state. In contrast to the recently good continuity over the West, guidance trends have been dramatic for the amplifying eastern system. In fact a couple days ago most solutions were showing an upper ridge moving into the eastern half of the lower 48. The eastern U.S. upper trough/low and associated low pressure, plus the increasing likelihood for a period of slow movement, may produce some areas of significant rainfall. The most common signal for highest amounts in the 00Z/06Z cycle of guidance extended from the Great Lakes into the Mid-Atlantic. This is subject to change given the behavior of guidance over recent runs. There may also be a period of strong onshore flow along parts of the East Coast, between the surface low/front and New England/eastern Canada high pressure. Meanwhile the National Hurricane Center is monitoring the near-future potential development of low pressure along a weakening frontal boundary over the Bahamas. The track of this low should remain offshore the East Coast but the possibility that some moisture from this system could interact with the amplifying flow over the interior eastern U.S. will have to be monitored. Early in the period a compact upper feature may produce locally heavy rainfall near the western Gulf Coast/Lower Mississippi Valley regions. The systems forecast to affect the western and eastern states will each support a period of cool highs with some locations at least 10-15F below normal for one or more days. In-between expect an area of warmth to drift from the Rockies into the Plains. Greatest anomalies for daytime highs should be plus 15-20F or so over the northern-central High Plains during Mon-Tue. ...Guidance/Predictability Assessment... Continuity and clustering over the West were better than average through day 7 Thu, with the core of the upper trough (likely to be a closed low for a decent part of the period) tracking from California into the Northern Rockies/Montana from early Tue into early Thu. The main model consideration was to remove the 00Z CMC from the blend after early Wed as it strayed from other guidance for upstream flow approaching the Northwest. Over the East, all guidance has trended significantly toward a slow/closed solution aloft over the past couple days but the GFS/GEFS mean have generally led the ECMWF/ECMWF mean in the slower/deeper trend. The 00Z ECMWF did adjust to the majority cluster though. It is hard to argue with this trend given the sharpening/amplifying nature of upstream flow--plus being a favored time of year for such features to close off. The 06Z GEFS mean actually ran a bit counter to the trend so GEFS input favored the 00Z cycle. 00Z ECMWF ensembles were diverse, eventually leading to a questionably weak/elongated upper trough. Thus as the operational model blend early in the period transitioned to a more even model/mean mix, the mean component used a higher weight of the GEFS mean relative to the ECMWF mean. New 12Z GFS/ECMWF runs continue to further the amplifying trend over the East with a farther southwest/south low track at the surface and aloft. 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