Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Wed Apr 24 2019 Valid 12Z Sat Apr 27 2019 - 12Z Wed May 01 2019 ...16Z Update... Large-scale changes were not made to the medium range forecast suite during this issuance. Much like the previous issuance, products were based on a multi-model deterministic blend early in the period, with much more emphasis on the EC and GEFS means late. More specifically, on day 3/Sat, differences remained regarding the trough pulling away from the Northeast U.S., with the 00Z and 06Z GFS slowing up the trough with more vorticity present farther south. Decided that the 06Z parallel GFS (FV3) was a better fit with this feature than the operational runs; it clustered better with the 00Z EC, UKMET, and CMC. This blend kept lower precipitation in New England compared to the west GFS. Secondly, the 00Z ECMWF came in stronger with the shortwave crossing the Great Lakes/Ohio Valley (mentioned below), more in line with the other guidance, so consensus is growing there. The 00Z UKMET, while usable until day 4/Sun, is more energetic with its shortwave moving through the northwestern to north-central U.S. on Sun, which spins up into a strong upper low in the Northern Plains and develops a surface low there on Mon. Ensembles do not support this evolution, keeping the shortwave progressive instead, so excluded the 00Z UKMET from the forecast at day 5/Mon. Model run-to-run variability continues to be large regarding how a trough moving through the Northwest will merge with the Eastern Pacific/California originally cutoff low toward Tue/Wed, so stuck with the means for now. Please see the previous discussion for details on sensible weather. Tate ...Previous discussion... ...Overview and Guidance Preferences... Models suggest the potential for at least some degree of pattern change across the CONUS during the medium range. Initially separate streams with a progressive northern stream and less progressive southern stream appear set to consolidate by the middle of next week. This is expected to occur as the northern stream undergoes amplification in response to a persistent upstream Rex Block extending from Alaska to Hawaii. Amplification of an upper trough across the western U.S. by the middle of next week is expected along with an expansion of the subtropical ridge northward into the Southeast and southern Mid-Atlantic. A negatively tilted upper trough is expected to be in place on day 3 (Sat) across the Northeast, with a phased surface low pressure system expected to be deepening as it pulls away from New England. Farther west, the northern stream remains active with one compact but energetic shortwave crossing the Midwest/Great Lakes Sat-Sun and yet another diving into the Pacific Northwest/northern Rockies. Meanwhile, a cutoff upper-level low should approach California by Sun night/Mon before moving inland and interacting in some fashion with the broader upper trough amplification across the West by early-mid next week. The forecast during days 3-5 (Sat-Mon) was based on a multi-model deterministic blend including the 12Z ECMWF/UKMET and 18Z FV3-GFS. Model solutions differed on the evolution of the negatively tilted upper trough and deepening surface low as they pull away from the Northeast Sat-Sun, but a blended approach appeared best at this time. Preferred the GFS/FV3/UKMET idea of a more amplified shortwave crossing the Great Lakes/Ohio Valley Sat-Sun over the weaker/more suppressed ECMWF solution. Farther west, consensus has improved with respect to amplified shortwave energy reaching the Northwest/northern Rockies Sat/Sun, but some timing differences remain, and models also differ as to whether the wave quickly moves east into the Northern Plains by late Sun or shears out with more energy remaining across the interior Northwest. The 12Z UKMET suggested that a compromise between these ideas was feasible and thus this was the preference. Ensembles do provide a fair degree of support for the idea of keeping a significant portion of the shortwave energy back across the interior West into early/mid next week, with the arrival of additional energy helping to lower heights and amplify a broader upper-level trough. The evolution of the upper trough will also impact what happens farther south as the initially cutoff Pacific energy moves inland across the Four Corners region, with the eventual forward speed of this system as it begins to become absorbed tied to the degree of northern stream amplification. While models differ as to exactly how this occurs, ensemble means agree in principle, thus leaned heavily toward ECENS and GEFS means by days 6-7 (Tue-Wed). ...Weather Highlights/Threats... Widespread rain is expected across New England on Sat as the deepening low pressure system moves away from the region. Low pressure moving from the Central/Northern Plains to the Midwest/Great Lakes/Northeast Sat/Sun should be accompanied by showers and thunderstorms, with the potential for locally heavy rainfall. A few areas on the northern fringe of this precipitation area may see the potential for snow or a rain/snow mix, particularly portions of northern Wisconsin and Michigan on Sat. Through the reminder of the forecast period, into the middle of next week, the focus for scattered to numerous showers and thunderstorms will shift back to the central U.S., where a stationary surface front along with arrival of additional upper-level shortwaves will focus multiple rounds of showers and thunderstorms. Models currently show the greatest potential for heavy rainfall across areas from the Central Plains to the Upper Midwest Mon-Tue. The central/northern Rockies will also see an increase in precipitation (rain and mountain snow) by early to mid next week as the upper trough amplified across the West and additional southern stream Pacific energy arrives. Below average temperatures are expected through much of the forecast period from the northern Rockies to the Upper Midwest and Great Lakes, where high temperatures of 5 to 15 deg F below average may be common. Above average temperatures initially in place from California to the southern Rockies and adjacent Plains should shift east to the Southeast and southern Mid-Atlantic regions by early to mid next week as the western trough amplifies and ridging builds into the Southeast. 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