Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri May 03 2019 Valid 12Z Mon May 06 2019 - 12Z Fri May 10 2019 ...Overview... The pattern evolution expected for next week may lead to additional heavy rainfall over parts of the central U.S. that have already received significant accumulations recently. Unsettled to active weather will also be possible across other portions of the Lower 48 for at least part of the period. Guidance suggests flow aloft will amplify toward the latter half of next week, with a building eastern Pacific ridge helping to develop a western U.S. trough. Flat ridging aloft should settle over the East Coast/western Atlantic by next Wed-Fri. The other major feature of note will be an upper low that should reach California at the start of the week and then open up/eject northeastward over and beyond the Rockies. ...Guidance Evaluation/Predictability Assessment... One of the less confident aspects of the forecast involves shortwave energy progressing across southern Canada and the northern U.S. from day 3 Mon onward. Among 12-18Z guidance, the GFS/GEFS mean were on their own in being slower and stronger with the upper dynamics--leading to surface low pressure tracking through the Upper Midwest/Upper Great Lakes and wrapping up over Canada. Remaining solutions were flatter and faster in varying ways, resulting in a more suppressed wavy front over the Northeast quadrant of the U.S. next Tue-Thu. Continuity/clustering have not been very good over the past couple days, plus either scenario is plausible from the multi-day mean point of view. Preferred to blend toward the majority scenario, which also had support from the 18Z FV3 GFS. Most of the new 00Z solutions still favor the non-GFS idea with the GEFS mean showing a partial trend in that direction as well. Within the amplifying western U.S. trough there have been signals for an embedded closed low by the latter half of the week but with a fair amount of spread for location. Based on the trough axis in the ensemble means and average of the past couple ECMWF runs (the most compatible guidance), it appears the most likely path of an upper low would be over or just southwest of the Great Basin. The upper low moving through the Southwest will open up and eject northeastward as the western trough begins to amplify. The ejecting shortwave will likely produce a rainfall-enhancing frontal wave tracking through and beyond the Plains mid-late week. Clustering has been fairly good though the new 00Z GFS becomes slower than most other solutions after early Tue. Based on above considerations the first half of the period started with an operational model blend (including two ECMWF runs given the lack of confidence with northern stream details and associated surface forecast). Then the forecast rapidly adjusted to a blend of the 18Z GEFS/12Z ECMWF means with minority input from the two ECMWF runs through day 7 Fri and the 18Z GFS through day 6 Thu (after which time the GFS ejected part of the western trough faster than desired). ...Weather Highlights/Threats... Portions of the Plains and Mississippi Valley should see the highest potential for heaviest total rainfall during the 5-day period, especially across southern areas. A wave embedded within a north-central U.S. front should promote one area of enhanced rain/thunderstorm activity over the Plains/Midwest during the first half of the week with lesser amounts of moisture extending farther east. Meanwhile moist flow from the Gulf will intersect a front initially draped near Gulf Coast, and then the ejecting Southwest U.S. feature will bring a surface wave and trailing front into the southern Plains. The threat for heavy rainfall may persist over/near the southern Plains through Fri as the southern Plains front likely decelerates/stalls as it becomes parallel with flow aloft while low level moisture continues to feed in from the Gulf. The combination of fronts/waves may bring some significant rainfall amounts to the Northeast/Mid-Atlantic but likely with lower totals than expected over the central U.S. The western states will see a broad area of precipitation (from California through the Great Basin/northern Arizona and into most of the Rockies) in association with the upper low tracking through the Southwest early in the week and the upper trough/possible embedded upper low developing over the West later in the week. Some of the precipitation may be in the form of snow over high elevations of the northern-central Rockies. Expect highest precip totals to be over favored terrain of the Great Basin and central Rockies. Snow potential could extend into the extreme northern Plains early in the week. The greatest temperature anomalies during the period should be with cool air pushing south from the northern High Plains, leading to decent coverage of highs 10-20F below normal by Wed-Thu. Highest warm anomalies, of similar magnitude, should be for morning lows in the warm sector from the eastern/southeastern Plains to Mid-Atlantic Tue onward and for highs over the Pacific Northwest. 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