Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EDT Thu Apr 25 2019 Valid 12Z Sun Apr 28 2019 - 12Z Thu May 02 2019 ...Overview and Guidance Preferences... An expansive Rex Block across the north/central Pacific, extending from Hawaii to Alaska, will support a downstream mean upper trough centered across the northern Rockies through the extended forecast period. A persistent upper low centered west of Hudson Bay, and slowly retrograding through the period, will support southward transport of chilly polar air into the north central U.S. Farther east, the subtropical ridge is expected to gradually exert its influence northward into the Southeast and southern Mid-Atlantic early next week. Preferred a multi-model blend approach initially during days 3-5 (Sun-Tue), incorporating the 12Z ECMWF/CMC and 18Z GFS (with heaviest weight place toward the ECMWF). This blend leaned toward the slower solutions (which were well supported by ensemble member clustering) with the shortwave/surface low crossing the Ohio Valley/Northeast on Sun. Additional northern stream shortwaves, one crossing the north central U.S., and another diving into the Pacific Northwest Sun-Mon were handled similarly by these preferred solutions. Farther south, a cutoff southern stream upper low approaching southern California late Sun-Mon continued to show some timing differences, and the slower ECMWF was preferred over the somewhat faster GFS timing (causing the GFS solution be further deemphasized by Mon). During days 6-7 (Wed), timing differences with the southern stream feature continue to grow as the feature moves into the central U.S. by Wed, with the GFS at least 18 hours faster than the ECMWF/CMC. Farther north, model consensus has improved a bit that the arrival of additional shortwave energy into the Pacific Northwest early-mid next week should serve to further amplify a broad upper trough axis centered over the northern Great Basin/northern Rockies, with a consolidated area of shortwave energy perhaps lifting out into the Northern Plains by Thu. Ensemble means showed general agreement on this idea despite increasing spread. Thus, the forecast by Wed-Thu placed increasing weight on ECENS/GEFS ensemble means, with some continued use of the deterministic ECMWF through the end of the period. ...Weather Highlights/Threats... Showers and perhaps a few thunderstorms are expected to accompany the low pressure system moving from the Ohio Valley/lower Great Lakes to the Northeast on Sun. After that, the focus shifts to the central U.S., where a persistent stalled frontal boundary along with a few passing upper shortwaves will focus multiple rounds of showers and thunderstorms, with the potential for areas of heavy rainfall. The strongest signal in the guidance for heavy rain was seen from portions of the Southern/Central Plains to the Mid-Mississippi Valley from Mon through the middle of next week, with QPF guidance suggesting the potential for multi-inch rainfall totals over the course of a few days. On the northern fringe of this area of precipitation in the central U.S. sufficient cold air may be in place to support some snow on Sun across portions of the Northern Plains. A second round of potential snow or rain/snow mix will be possible across portions of the Northern High plains Tue-Wed. Farther west, lowering heights and the passage of multiple shortwaves will generate rain and mountain snow for portions of the Great Basin and central/northern Rockies through the period. Below average temperatures are expected from Sun into next week, from the northern Rockies to the Great Lakes. The greatest anomalies are expected Sun-Wed from the northern Rockies to the Northern High Plains, where high temperatures may be 10-20 deg F below average. Warming temperatures are expected across the Southeast and southern Mid-Atlantic by early next week as the subtropical ridge builds. Highs across these areas may reach 10 to 15 deg above average Tue-Thu. 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