Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1206 PM EDT Fri May 03 2019 Valid 12Z Mon May 06 2019 - 12Z Fri May 10 2019 ...Overview... Much of the southern and central U.S. will remain in a mostly unsettled and wet pattern as the flow aloft amplifies toward the latter half of next week allowing a longwave trough to become reinforced over the western U.S. An upper level low approaching the California will track through the southern Great Basin/Southwest through the extended period and will northeastward over and beyond the Rockies. This may lead to additional heavy rainfall over parts of the central U.S. that have already received significant accumulations recently, which will keep the risk for flash flooding elevated during this period. Flat ridging aloft should settle over the East Coast/western Atlantic by next Wednesday-Friday. ...Guidance Evaluation/Predictability Assessment... Model guidance have struggled a bit with a shortwave feature that will track south/southeast from the southern tier of Canada into the northern U.S. states. As noted from the previous forecast, the GFS/GEFS mean were 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. The rest of the deterministic and means depicted flatter and faster flow in varying ways, resulting in a more suppressed wavy front over the Northeast quadrant of the U.S. next Tuesday-Thursday. The latest runs show a slight improvement with the details of this feature, however there is uncertainty. 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. Clustering was better with the latest guidance and is suggesting that it will track southwest/south of the Great Basin; the ejecting shortwave will likely produce a rainfall-enhancing frontal wave tracking through the Southern High Plains and into the Mississippi Valley and beyond by midweek. The preferred blend for the beginning of the extended forecast consisted of the ECWMF and its ensemble mean, the GFS, UKMET and the Canadian model. Weighting of the operational models decreased with time as the weighting of the ECWMF ensemble mean and GEFS Means increased. ...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. Campbell/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