Extended Forecast Discussion NWS Weather Prediction Center College Park MD 359 PM EDT Thu Apr 18 2019 Valid 12Z Sun Apr 21 2019 - 12Z Thu Apr 25 2019 ...Overview and Guidance Preferences... Mean flow aloft across the Lower 48 will become somewhat less energetic with higher heights after the departure of the deep but gradually weakening system over the East Sun into Mon. Teleconnections relative to a persistent upper low in northern Canada favor relatively flat flow--or at least decent progression of any amplified features--but over the past day guidance has gravitated toward GFS/GEFS/UKMET solutions that have been fairly slow and amplified with the initial western U.S. upper trough drifting into the central states. In general a model/mean blend reflects latest consensus well and mitigates detail uncertainties within the trough aloft and corresponding wavy surface front. Farther upstream there is reasonable agreement on strong flow coming into southwestern Canada from early in the week onward, with the southern periphery of this flow gradually sagging southward into the extreme northwestern U.S. 00Z CMC/CMC ensembles are more aggressive to bring height falls into western North America versus other guidance that agrees among each other rather well. CMC issues may adversely impact its forecast downstream as well. Over the East, recent trends have been slower for the upper low departing Sun-Mon--favoring progression closer but maybe not completely to the 06Z FV3 GFS/00Z ECMWF or a compromise with the 00Z CMC. The 00Z GFS becomes a fast extreme very early in the period while the 06Z run has adjusted to a compromise. Dependence on smaller scale details will limit confidence in details of a wavy front dropping into the region during the latter half of the period. Latest majority solution has backed away somewhat on the front's southward extent. Based on the current array of guidance the updated forecast emphasized the 06Z GFS/FV3 GFS and 00Z ECMWF/UKMET during the first half of the forecast with minor edits for the departing eastern system. Then the blend transitioned to an ensemble mean/model composite of the 06Z GEFS/00Z ECMWF mean and aforementioned GFS/ECMWF runs to mitigate increasing detail uncertainties within an overall agreeable pattern evolution by days 6-7 Wed-Thu. ...Weather Highlights/Threats... The system initially crossing the West will bring rain and some high elevation snow to parts of the Great Basin and north-central into central Rockies. A period of low level upslope flow may enhance precipitation over some northward and eastward facing slopes of the Rockies during the first half of the period. From about Mon night onward the most prominent threat will be for heavy rainfall over portions of the southern half of the Plains as the upper trough approaches/reaches the area while persistent low level Gulf inflow interacts with a slow moving and wavy front. Improved model agreement for the large scale evolution compared to yesterday is helping to increase confidence in the overall event but important smaller scale details will likely take into the short range time frame to be resolved better. Currently expect highest rainfall totals through the end of the period to be over Texas. Rain of varying intensity should extend northeastward along a front that reaches the eastern states. Ahead of this moisture, the departing eastern system may produce a day or so of showery weather over the Mid-Atlantic/Northeast Sun-Mon. A series of fronts will bring moisture to the Northwest but with mostly light to moderate amounts. The best defined front should push into the region Wed-Thu with a corresponding band of moisture drifting east/southeast with time. Clouds and precipitation on the cool side of the central U.S. front will tend to focus lowest anomalies for high temperatures over the High Plains. Expect warmest anomalies over the West Tue-Thu as upper ridging builds over the region, with fairly broad coverage of morning lows and daytime highs 10-20F above normal. At least a few daily records may be possible. Meanwhile another area of warmth will progress from the central U.S. into the East with some plus 10F or greater anomalies possible. 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 Central/Southern Plains, and the Middle/Lower Mississippi Valley, Tue-Thu, Apr 23-Apr 25. - Flooding possible across portions of the Mid-Atlantic, the Mississippi Valley, the Central/Southern Appalachians, the Northern Plains, the Tennessee Valley, the Great Lakes, the Northeast, the Southeast, the Southern Plains, and the Ohio Valley. - Flooding occurring or imminent across portions of the Mississippi Valley, the Northern Plains, the Tennessee Valley, the Northeast, 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