Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Apr 04 2019 Valid 12Z Sun Apr 07 2019 - 12Z Thu Apr 11 2019 ...Overview... Ongoing progression of Pacific features into and across the Lower 48 will bring active weather to a decent percentage of the country for one or more days of the forecast period. The southwestern U.S./southern Rockies appear to be the one area that should have fairly dry weather. A leading upper trough over the central U.S. on Sun and its associated surface system will spread a broad area of precipitation across the eastern half of the country Sun-Tue with some areas of heavy rainfall possible. During that same time frame expect a strong jet aimed at the northwestern states to bring areas of enhanced precipitation to the northern half of the West Coast into the northern Rockies. An upper trough forecast to reach the West Coast around early Tue and continue inland should then push the best focus for precipitation farther east into the northern-central Rockies and then portions of the Plains/Mississippi Valley and eventually Ohio Valley and/or Great Lakes. This trough may support a fairly strong central U.S. storm system by Wed-Thu. Trailing trough energy reaching the northeastern Pacific late in the period may approach the West Coast by next Thu. ...Guidance Assessment/Preferences... Not a lot has been resolved yet for the overall upper trough progressing eastward from the Plains. There is still good agreement on its existence and general timing (especially in the means) but individual model runs and ensemble members still vary widely with respect to how energy is distributed within the trough. Over the past day or so there is been a moderate signal toward at least a period of separation between flow crossing the southeast quadrant of the country and that crossing the northeast quadrant. The southern part of the trough would support a frontal wave that could provide localized focus for heavy rainfall. Different variations of stream interaction persist as well. Continue to prefer a model/mean blend to reflect the most agreeable aspects of the forecast, in particular low pressure crossing the Great Lakes/Northeast, while offering a more conservative approach where greater spread exists. Fast Pacific flow crossing the Northwest Sun-Mon will likely contain multiple embedded impulses whose small scale and fast motion will make them difficult to resolve until the short range time frame. For the upper trough reaching the West Coast around early Tue, GFS/GEFS runs through the 18Z cycle were still a bit slower than than the ECMWF/ECMWF mean. For the 12-18Z cycles the full array of guidance was not quite as sharply clustered as in the past couple days when there was a more pronounced majority in favor of faster ECMWF timing. This led to a somewhat more even blend for some aspects of evolution, but ultimately the ejecting trough and corresponding surface low end up closer to the ECMWF/ECMWF mean late in the period. The 18Z GFS was similar as of early Thu. New 00Z guidance thus far, including the GFS, is providing a boost in confidence toward the progressive scenario. At the same time the 00Z GFS/CMC have trended much stronger with Plains/Mississippi Valley low pressure Wed-Thu. Farther upstream models/spaghetti plots still depict a lot of spread for northern into northeastern Pacific details into Wed. In spite of this spread through Wed there is actually some trending toward better large scale agreement by Thu with a trough evolving along or offshore the northern half of the West Coast. Combining forecast considerations across the Lower 48, the updated forecast started with a blend primarily emphasizing operational guidance early but 50 percent means (12Z ECMWF mean and lesser 18Z GEFS/12Z NAEFS weight) already by day 5 Tue. The rest of the forecast tilted a little more toward the means but kept some attributes of the 18Z GFS/12Z ECMWF to enhance ensemble mean detail. ...Weather Highlights/Hazards... Strong flow aloft across the Northwest will promote areas of terrain-enhanced rain and higher elevation snow from northern California and the Pacific Northwest into northern Rockies from Sun into early next week. Current guidance suggests best potential for heaviest activity will be centered along the West Coast near the California/Oregon border and the Oregon Cascades. By Tue the upper troughing moving into the West will begin to shift the best moisture focus eastward into the Great Basin and northern-central Rockies. Continued progression of this system should spread precipitation across the north-central Plains, Mid-Upper Mississippi Valley, and Ohio Valley and/or Great Lakes by Wed-Thu. Some activity may be heavy and areas with ongoing flooding concerns will need to monitor this system. A period of low level upslope flow may add to totals over the eastward/northward facing slopes of the Rockies. In addition stronger trends for Wed-Thu low pressure in latest guidance raise the potential for strong winds to be an issue over some areas. Finally, if the system is strong enough there may be some snow in the northern part of the precip shield to the east of the Rockies. The eastern half of the country will see a broad area of precipitation with the system progressing eastward from the Plains Sun onward. Ample flow of moisture from the Gulf will promote areas of heavy rainfall. Best potential for highest totals will be over/near the Lower Mississippi Valley and central Gulf Coast region and southern Appalachians. Convection may be strong to severe over southern areas. Check SPC outlooks as specifics become better resolved over the coming days. Significant rainfall is also possible from the Ohio Valley into northern Mid-Atlantic/Northeast. There are still significant detail uncertainties with this system so confidence in location/magnitude of heaviest rainfall remains somewhat lower than desired. Snow should be confined mostly to New England. The warm sector of the central-eastern U.S. system during the first half of the period will contain above to much above normal temperatures. Morning lows will be particularly anomalous with many locations 15-25F above normal Sun into Tue and some daily record warm low values may be challenged. The upstream system will bring a brief period of warmth across the Southwest into the southern Plans/Southeast over the course of the period, and at the same time spread below normal readings from the Northwest into the Plains during Tue-Thu. 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 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