Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Sun Apr 04 2021 Valid 12Z Wed Apr 07 2021 - 12Z Sun Apr 11 2021 ...Overview and Guidance Evaluation/Predictability Assessment... Latest guidance continues to have different ideas for some specifics within the increasingly blocky regime expected to develop over parts of eastern North America as well as for individual shortwaves within a mean trough over the northwestern part of the continent and extending down to portions of the West Coast. Early in the period there is decent clustering for the upper low forecast to track over the Plains/Midwest. However by days 4-5 Thursday-Friday GFS/UKMET runs with some hint in GEFS members have been bringing leading western trough energy quickly toward the Midwest upper low, leading to varied interactions or forming of a new upper low near the Great Lakes. The 12Z CMC was somewhat closer to the consistently slower ECMWF/ECMWF mean for the western trough (which would let the Midwest upper low drift along on its own for a longer time) but the new run has trended more to the GFS cluster. Regardless of the scenario, by late week there has been a recent trend toward greater downstream ridging aloft over the East Coast/western Atlantic and eastern Canada, thus slowing progression of the system. By late in the period recent D+8 multi-day means have agreed on establishment of a strong core of positive height anomalies to the east-northeast of Hudson Bay. Teleconnections relative to this feature suggest a zone favorable for one or more upper lows from the Great Lakes into southeastern Canada and the Canadian Maritimes. Guidance is signaling this idea but with no real clustering or continuity for specifics over this area. Returning to the mean trough to the west, recent GFS/GEFS trends have been eastward toward other guidance for upstream energy that digs into the feature during the late week/weekend time frame. The ECMWF/ECMWF mean have also trended a bit eastward over the past couple days. Persistence of the GFS cluster for energy emerging from the western U.S./Canada and heading into the Midwest upper low suggested a closer to even balance for which scenario may occur based on guidance through the 18Z cycle. Given the ongoing uncertainty, preference to stay closer to continuity in principle led to trending the initial operational model blend more toward the past couple ECMWF runs, 12Z CMC (only into day 5), and 18Z GEFS/12Z ECMWF means--with the understanding that meaningful future change in some details may be possible. This blend maintained the idea of slow progression of the parent system over the Midwest while an East Coast wave develops by next weekend. Specifics of one or more fronts over the West will depend on the uncertain shortwave details aloft. The new 00Z ECMWF has jumped to the GFS cluster for leading western trough energy, significantly raising the probability of that scenario, but at least the general ideas of an upper low drifting into the Great Lakes and a wave developing near the East Coast next weekend remain intact. ...Sensible Weather and Potential Hazards... Some locally heavy rain may fall over parts of the Midwest around midweek, near the upper low and associated surface system affecting the region. Other areas of enhanced rainfall will be possible along the trailing front--most likely from near the south-central Mississippi Valley into the southern Appalachians. The lingering front may provide a focus for additional showers and thunderstorms over parts of the South through at least Friday or Saturday. Many other portions of the eastern U.S. will also see one or more days of rain with varying intensity. The Northwest should see periods of rain and mountain snow, with highest totals over favored terrain in the Olympics and Washington Cascades. Specifics of coverage, timing, and amounts are still uncertain with significant shortwave differences. Locations from the Midwest into Ohio Valley and Great Lakes will see well above normal temperatures during mid-late week. Morning lows should be particularly anomalous with some areas seeing a day or two of 20-25F above normal readings. Warmest highs should be 10-20F above normal. Clouds and precipitation should keep min temperatures above normal over the East into the weekend. Expect Texas to be very warm to hot during the period with highs 10-20F above normal most days. Above normal temperatures initially over a majority of the West will likely moderate with time while below normal highs become more common from the Northwest into northern Rockies. 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, 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