Extended Forecast Discussion NWS Weather Prediction Center College Park MD 343 PM EDT Fri Apr 19 2019 Valid 12Z Mon Apr 22 2019 - 12Z Fri Apr 26 2019 ...Overview and Guidance Preferences... Expect two pronounced streams aloft to prevail across North America next week. A fairly slow-moving southern stream will feature a trough/upper low that should promote a heavy rainfall threat over the southern Plains/Mississippi Valley, while a progressive northern stream across southern Canada and northern U.S. will contain shortwaves whose scale and speed will lead to below average predictability. Questions of whether any stream interaction may occur at some point during the forecast period produce further uncertainty in forecast details. Over the past day the southern stream upper trough (with embedded closed low at times) drifting from the western U.S. into the Plains/Mississippi Valley has shown the best relative stability over the past day. That said, there has still been a recent tendency toward greater southwestward digging of energy within the trough during the early part of the week before the system continues eastward. There is still considerable uncertainty over specifics of individual shortwaves so overall confidence in the exact forecast remains fairly low. At the very least the dominant cluster of guidance suggests the 06Z GFS is too quick to shear the upper trough later in the week. Model/ensemble spaghetti plots become chaotic across the North Pacific early in the period and this spread continues quickly eastward with time. Guidance continuity has also been poor for individual shortwaves across southern Canada and northern U.S., with significant corresponding variability in progression of surface waves/fronts across the northern and eastern U.S. A slower/stronger shortwave in latest guidance has resulted in a better defined wave tracking from the Midwest into New England during Mon-Wed. For a time this adjustment slows down frontal progression into the East but with the wave's passage the front pushes back down toward the previous cycle's consensus--but well southward of what yesterday's 00-06Z guidance was advertising. The other major change in latest guidance is faster progression of North Pacific flow across North America. As a result the overwhelming majority of current 00-06Z guidance brings a front much farther southeast into the northern states versus previous cycle. By the end of the period models/ensembles show completely opposite solutions over the northeastern Pacific, the ECMWF/ECMWF mean with troughing and the CMC/CMC mean plus the GFS to some extent with ridging. The upper low over northern Canada teleconnects to cyclonic flow over the northeastern Pacific so it seems reasonable to lean away from the ridge solutions. The upper low departing from the Northeast continues to display significant timing differences. The 00Z GFS continues to be on the fast side while the 06Z version actually becomes slower than the ECMWF after early Tue. An average among the 06Z GFS and 00Z ECMWF/CMC looks reasonable at this time. Based on the current array of guidance the updated forecast started with a blend of the 06Z GFS/00Z ECMWF/00Z CMC for the early week forecast, followed by a gradual increase of the 06Z GEFS/00Z ECMWF means to 70 percent total weight by next Fri. ...Weather Highlights/Threats... The most prominent weather threat next week should be with heavy rainfall from widespread showers/thunderstorms across the southern Plains and eventually into the Lower Mississippi Valley from Mon night/Tue onward. There will be multiple ingredients to support this event: the upper system tracking out of the Southwest, a leading slow-moving wavy front, and a multi-day feed of low level moisture from the Gulf of Mexico. Ensemble guidance shows best potential of highest rainfall totals during the period over Texas and Oklahoma. Early in the week expect precipitation over the eastern Great Basin and central-southern Rockies. Highest elevations in Colorado may see some snow. Meanwhile a wave tracking from the Midwest to New England may focus meaningful rainfall near its track. Any snow should be confined to areas north of the Canadian border except perhaps for northern Maine. A front moving into/across the Northwest and onward across the northern tier states will be accompanied by mostly light rain and possibly a little very high elevation snow. The western U.S. will see a broad area of plus 10-20F temperature anomalies with some of this warmth extending into the northern and eventually central Plains. A cold front moving into the Northwest will bring a cooling trend to that part of the West during the latter half of the week. Daily records appear most likely for warm lows over California and the Interior West Wed-Thu but a few record highs may be possible as well. Parts of the East will see a period of above normal temperatures in the warm sector of the Midwest to New England wave Mon-Wed. On the other side of the spectrum the central-southern Plains will be the primary focus for below normal highs during the period. 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 Southeast, the Lower Mississippi Valley, and the Southern Plains, Tue-Fri, Apr 23-Apr 26. - Flooding possible across portions of the Mid-Atlantic, the Mississippi Valley, the Central/Southern Appalachians, the Northern Plains, the Tennessee Valley, the Northeast, the Southeast, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of the Mississippi Valley, the Northern Plains, and the Ohio Valley. - Flooding likely across portions of the Northeast, the Northern Plains, and the Northern Great Basin. 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