Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sun Apr 21 2019 Valid 12Z Wed Apr 24 2019 - 12Z Sun Apr 28 2019 ...Overview and Guidance Preferences... Models and ensembles continue to show a lot of spread for possible solutions within the progressive northern stream and slower southern stream, and degree of interaction between the two streams. This type of scenario lends itself to low predictability at extended time frames. In addition larger scale pattern differences that develop over parts of the northern/central Pacific and Bering Sea contribute to differences that arise downstream late in the period. These issues keep forecast confidence below average--especially by the Fri-Sun. Especially with the arrival of the 06Z GFS and agreement from latest GEFS means, there is better clustering for the trough/possible embedded upper low tracking eastward across the southern tier states. The old 12Z ECMWF had jumped somewhat toward more phasing than prior runs but the new 00Z run has reversed back to more separation and now the 00Z CMC is the only model to show faster progression. Ensemble means offer reasonable support for the 06Z GFS/00Z ECMWF in principle for most of the period. Note that while the probability for maintaining separation per latest majority cluster is greater, it is a close enough call whether northern stream flow affects the southern feature that significant changes in future guidance are still possible. Meanwhile guidance for the midweek system crossing southeastern Canada and New England has gravitated toward a concentrated upper low and fairly vigorous surface system. There are two major question marks in the forecast over the eastern Pacific into western North America. First is the degree of separation that develops well offshore the West Coast during the latter half of the week. ECMWF/ECMWF ensemble runs still show considerable separation leading to a southern stream closed low but are a bit faster than 24 hours ago. The 00Z UKMET/CMC provide added support for the ECMWF idea. The 06Z GFS adjusted to a solution about halfway between the ECMWF cluster and faster 00Z GFS. These considerations provide greater support for tilting the forecast more toward the ECMWF cluster. The second issue is rapidly increasing spread that occurs over the Bering Sea (ranging between deep GFS troughing and strong ECMWF ridge) and farther south over the Pacific (differences in closed low position). Respective ensemble systems have tended to reflect hints of their operational counterparts but actually agree remarkably well on broad cyclonic flow over the northwestern states--in contrast to the deep trough/upper low which 00-06Z GFS runs show over the West Coast by next Sun. Thus prefer to emphasize the ensemble mean/ECMWF ideas for this part of the forecast. Based on above considerations the updated forecast started with a blend of operational guidance for days 3-5 Wed-Fri and then transitioned toward primary weight on the 00Z ECMWF/ECMWF mean and 06Z GEFS mean. ...Weather Highlights/Threats... Widespread showers and thunderstorms are likely with the system crossing the Southern Plains and Lower Mississippi Valley mid-late week and models/ensembles continue to show a strong signal for heavy rainfall from portions of central Texas east toward the Mississippi River. Severe thunderstorms are also possible. Please see convective outlooks issued by the Storm Prediction Center for further details on the severe weather threat. Expect scattered to numerous showers and thunderstorms to progress eastward into the Southeast as well as the Tennessee/Ohio Valleys (where Gulf moisture may interact with a front) by late Thu into Fri. The heavy rainfall signal is less clear over these areas--as well as the Southeast Coast/Florida into the weekend--but would expect continued potential for at least locally heavy rainfall given the recent gravitation of guidance toward the slower half of the spectrum. Farther west, the energetic upper-level trough entering the northwestern U.S. late in the week should produce rain and mountain snow from the Northwest to the northern Rockies. Rain and possibly even snow (though mostly along/north of the Canadian border) may spread into areas from the Northern Plains to the Upper Midwest/Great Lakes as energy aloft and associated low pressure move into the central U.S. by next weekend. Multiple uncertainties over details of flow aloft by next weekend lower confidence in coverage of scattered precipitation over parts of the West next weekend. Above average temperatures are likely through much of the period across central and southern portions of the western U.S. Expect broad coverage of high temperatures 10 to 20 deg F above average from California to the Great Basin Wed through Fri or Sat. The most likely daily records will be for warm lows Wed-Fri but a few record highs are possible as well. The warmth should spread into the Rockies and portions of the High Plains by Fri-Sat while cooler temperatures overspread the Northwest/northern Rockies and eventually northern Plains in association with the arriving upper trough/cold front. Rausch/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