Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Tue Apr 23 2019 Valid 12Z Fri Apr 26 2019 - 12Z Tue Apr 30 2019 ...Overview and Guidance Preferences... Upper-level flow across the CONUS is not expected to change significantly during the extended forecast period. A progressive northern stream and less progressive southern stream, similar to the pattern seen in recent weeks, is expected to persist. Models/ensembles do hint at potential northern stream flow amplification from the North Pacific into Canada by next weekend, however. Development of a broad Rex block extending from Hawaii to Alaska is expected, which will reinforce upper troughing centered west of Hudson Bay. Conceptually, this setup seems to favor potential southward transport of polar air into central Canada and the north central U.S., and this bears out when teleconnections associated with the Pacific Rex block are examined. Began the forecast during days 3-5 (Fri-Sun) with a multi-model deterministic blend (12Z ECMWF/UKMET/CMC/18Z GFS). Models have trended slightly more amplified with northern stream shortwave energy crossing the Great Lakes on Fri, resulting in a quicker progression of energy initially across the Southeast, and more phasing between the two streams. The resulting phased low pressure system should deepen as it quickly lifts into the Canadian Maritimes by Sat. Consensus has improved around an energetic shortwave expected to dive southeastward into the Pacific Northwest and northern Rockies Sat-Sun, as another cutoff southern stream upper low approaches California. Beyond Sun, there remains a significant question as to how much heights fall from the northern Rockies to the Northern High Plains. Upstream ridge amplification across the north Pacific should favor the tendency for cyclonic upper-level flow and potential trough amplification across the Northwest, and this is the direction models seem to be trending at the moment. Current model trends suggest that the amplifying trough across the Northwest by early next week should begin to absorb the initially cutoff southern stream feature as it moves into the Four Corners region, more rapidly kicking it eastward into the Southern/Central Plains by Tue. Ensemble spaghetti plots reveal a fair amount of spread involving the trough amplification as well as the evolution of the flow across Canada (with implications for whether colder polar air is able to spread south into the north central U.S. Thus, confidence is below average by early next week, and a trend toward heavier weight of ensemble means was favored in the forecast. Regardless of the specifics of the western trough, ensembles show broad agreement that subtropical ridging should gradually build into the Southeast and southern Mid-Atlantic regions by early next week. ...Weather Highlights/Threats... The shortwave and surface low pressure system traversing the Southeast Fri-Sat should be accompanied by scattered to numerous showers and thunderstorms, with the potential for at least locally heavy rainfall. As the phased low pressure system develops across New England late Fri into Sat, widespread rain is expected, perhaps even mixing with snow across interior portions of far northern New England. Farther west, a low pressure system moving from the Northern/Central Plains to the Great Lakes Sat-Sun should bring rain to much of the Midwest, with the potential for some areas of snowfall on the northern edge of the system, from northern Minnesota east into northern Wisconsin and Michigan. A stalled frontal boundary across the central U.S. in the wake of this system will continue to generate areas of showers and thunderstorms into early next week from the Central Plains to the Midwest, with further enhancement possible by Tue as the next shortwave approaches. Rain and mountain snow will become more common across much of the interior western U.S. by the weekend into early next week as well, as upper troughing amplified overhead. Above average temperatures are expected Fr-Sun from California and the Great Basin east to the High Plains. High temperatures of 10 to 15 deg F above average are expected to be common. Early next week, warm temperatures are expected to spread eastward to the Southeast/southern Mid-Atlantic as the subtropical ridge builds. At the same time, falling heights across the northern Tier should bring below average temperatures from the northern Rockies to the Upper Midwest from Sun onward. Ryan 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