Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Tue Apr 06 2021 Valid 12Z Fri Apr 09 2021 - 12Z Tue Apr 13 2021 ...Overview and Guidance Evaluation/Predictability Assessment... Most guidance continues to suggest that strong positive height anomalies east of Hudson Bay and associated upper ridging will support a mean trough aloft centered near the Upper Midwest. As a result low pressure systems associated with shortwaves entering western North America will tend to gravitate toward that region and the Great Lakes. Models and ensembles are still showing some spread for details within the blocky pattern, with predictability for some specifics typically not very high given the likelihood of various closed features between the northeastern quadrant of the lower 48 and eastern Canada/North Atlantic. Between Friday and Sunday the models are gradually refining the details of how shortwave energy initially over the northern High Plains will close off an upper low and displace a leading Midwest system. Consensus for the associated surface evolution is for the leading low to lift northward while a developing southern Plains/Mississippi Valley wave tracks into the Great Lakes. A leading wavy front extending through the Mid-Atlantic has trended somewhat northward from recent continuity, with 18Z/00Z GFS runs on the northern side of the spread. GEFS means are a little less extreme in that regard, recommending a more moderate continuity adjustment. Further adjustments can be made if these trends continue. Farther upstream the guidance is well clustered for another vigorous shortwave reaching western North America by early Saturday. Most solutions say this feature will close off another upper low which should track over or near the northern tier states. 18Z/00Z GFS and GEFS mean runs have trended a bit eastward of established consensus for this feature late in the period. For now would favor leaning somewhat more toward the 12Z run for GFS/GEFS input. By next Monday-Tuesday a number of recent GFS runs have wanted to dig a fairly deep trough/upper low over the West. Teleconnections relative to the eastern Canada ridging favor some modest troughing over the southwestern U.S./far northwestern Mexico but not so much troughing over the Northwest, while some ridging should extend into western Canada. GEFS/ECMWF/CMC means all compare more favorably to teleconnection-favored flow over the West. Based on the above considerations, the forecast blend started with a 12Z model composite early in the period followed by increasing 12Z GEFS/ECMWF/CMC mean input with the rest consisting of the only the last two ECMWF runs by day 7 Tuesday. ...Sensible Weather and Potential Hazards... The forecast pattern will likely produce a fairly broad area of precipitation over the eastern half of the lower 48 for portions of the period. The best signal for heaviest rainfall currently exists from near the Gulf Coast into the southern-central Appalachians, and possibly west into the Tennessee/Ohio Valleys, late this week into the weekend. During this time frame there will be a decent flow of Gulf moisture ahead of southern Plains into Mississippi Valley/Great Lakes surface development that will be supported by High Plains into Midwest dynamics aloft. Separate southern stream energy will also likely to play a role in southern tier rain/thunderstorms. The Pacific Northwest into the northern Rockies will see a period of rain and mountain snow from late this week into the weekend. The forecast has been consistent in showing highest liquid totals over the Olympics and Washington Cascades. By late weekend into early next week the associated development at the surface and aloft may produce areas of enhanced precipitation over the northern Plains and vicinity. Some of this precipitation could fall in the form of snow. Other areas of rain may develop farther south over the central U.S. The majority of guidance indicates a drying trend to the west of the Plains early next week but there is still a modest potential for some precipitation to linger over the north-central Rockies. The most extreme temperature anomalies will be over southern Texas on Friday with highs up to 10-20F above normal and over portions of the East from late week into the weekend, especially for morning lows that will be 10-20F above normal due to warm moist flow ahead of an initial system and trailing development. Portions of the Great Lakes could see morning lows near record warm values on Friday. Best potential for highs 10-15F above normal will be over the Ohio Valley/Great Lakes area Friday-Saturday. From the weekend into early next week expect highs of 5-15F below normal to progress across the Northwest and northern Rockies behind a cold front pushing through the Northwest and into the Plains. Anomalies may moderate a bit by the time the airmass reaches the Plains. The southern half of the West will tend to see above normal temperatures with most anomalies in the plus 5-12F range. Warmest days should be Friday-Saturday with a slight rebound again over the West Coast states next Tuesday. 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