Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 PM EDT Wed Apr 17 2019 Valid 12Z Sat Apr 20 2019 - 12Z Wed Apr 24 2019 ...Overview and Guidance Preferences... Guidance agrees that mean flow across the Lower 48 will become less amplified in the wake of the deep system lifting northeastward over the East during the weekend. As illustrated by ensemble spaghetti plots, shortwave details will be rather uncertain for most of the period--starting with energy coming into the West on day 3 Sat and then for details of Pacific flow reaching western North America. This favors maintaining a blend approach for the forecast, mostly using operational guidance early and then trending toward more ensemble mean input late. For the departing eastern system the primary consideration is to downplay the 00Z GFS which becomes faster than consensus after late Sat. The past day of ECMWF runs has been fairly stable while GFS runs have been less consistent. Regarding the weekend western U.S. energy and trailing Pacific shortwave(s) whose combination becomes one of the primary forecast problems through the rest of the period, GFS/GEFS runs have been persistent in showing a deeper and slower overall trough than most other guidance. However the past couple ECMWF runs have trended toward digging somewhat more energy into the southwestern U.S./Rockies and northern Mexico versus previous runs. Teleconnections relative to negative height anomaly centers well north of Hudson Bay and the Gulf of Alaska (or farther west-northwest depending on the model/ensemble mean) all generally support moderately above normal heights and possibly modest mean ridging over the west-central U.S. This relationship would suggest that whatever troughing reaches the West should be a little weaker and/or faster than depicted in the GFS/GEFS. ECMWF trends and a GFS-like solution in the 00Z UKMET favor an intermediate blend approach but tilted just a bit in the ECMWF/ECMWF mean direction in light of teleconnections. During the latter half of the period the 00Z CMC/CMC mean are most aggressive to bring height falls across the northwestern U.S. Aforementioned teleconnections recommend staying closer to the GFS/ECMWF and their ensemble means. The fast mean flow and sensitivity to smaller-scale shortwave details lead to low confidence for surface front/wave specifics as each cycle of guidance has shown meaningful changes for eastern Pacific fronts that should tend to die out upon entering western North America. Farther east, guidance consensus over the past couple days has been trending stronger/eastward with eastern Canada high pressure that will likely help to push a backdoor front southward along the East Coast next Tue-Wed. ...Weather Highlights/Threats... The strong system affecting the East to start the weekend may produce a band of enhanced cool-sector rainfall over parts of the Ohio Valley/east-central Great Lakes and a leading band of locally moderate/heavy rainfall focused over parts of New England. Winds may be fairly strong at some locations on Sat as well. The overall shortwave trough aloft affecting the western/central U.S. and associated surface features will spread rain and high elevation snow over central latitudes of the West and then rainfall of varying intensity over central/southern parts of the Plains and possibly the Mississippi Valley. Confidence in specifics of this precipitation area is lower than desired due to the guidance spread for details aloft. There is a signal for low level upslope flow to provide some precipitation enhancement along the eastern slopes of the central Rockies around Sun. Thereafter ensemble guidance shows relatively higher probabilities for locally heavy rainfall over the southern Plains which should see a multi-day feed of moisture from the Gulf. After a relatively dry weekend the Pacific Northwest should see periods of precipitation from Mon onward as multiple fronts approach/cross the region. Most of this activity should be in the light-moderate range. The strong eastern system will bring a brief period of chilly temperatures to the Mississippi Valley/East from Sat into Sun morning. Highs may be at least 10-15F below normal on Sat from parts of the Ohio Valley into the Southeast. Aside from a pocket of below normal highs dropping southward along the High Plains, temperatures should otherwise be near to above normal given the flatter trend of flow aloft. Expect the highest warm anomalies over the West and into the northern High Plains with one or more days of max and/or min readings 10-20F above normal. Rausch 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 Hazards: - Heavy rain across portions of the Southern Plains, Tue-Wed, Apr 23-Apr 24. - Flooding possible across portions of the Mid-Atlantic, the Lower Mississippi Valley, the Central/Southern Appalachians, the Northern Plains, the Tennessee Valley, the Great Lakes, the Northeast, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley. - Flooding occurring or imminent across portions of the Northeast, the Northern Plains, the Mississippi Valley, and the Ohio Valley. - Flooding likely across portions of the Southeast, the Upper Mississippi Valley, the Mid-Atlantic, the Northern Plains, and the Northern Great Basin. - High winds across portions of the Central Plains and the Southern Plains, Sun-Mon, Apr 21-Apr 22. 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