Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sun May 05 2019 Valid 12Z Wed May 08 2019 - 12Z Sun May 12 2019 ...Busy springtime pattern for much of the Lower 48... ...Overview... Expect active weather for one or more days of the period from portions of the central/southern West through a significant part of the central/eastern states. A storm emerging over the Plains on Wed and tracking northeastward thereafter will produce a warm sector severe weather threat and one area of potentially heavy rainfall from the Midwest into the Northeast. The Storm Prediction Center has outlined an area over the southern Plains to Missouri/Arkansas on Wednesday and then from the ArkLaTex northeastward to the Midwest on Thursday for the potential for severe weather. Locations from the eastern half of Texas through the Lower Mississippi Valley and possibly into the Tennessee Valley will be of greater concern for a significant multi-day heavy rainfall event. The front trailing from Plains' low pressure will generate initial convection and then likely stall near and northeast from the western Gulf Coast Fri-Sun, interacting with impulses embedded in southwesterly flow aloft and a steady flow of Gulf moisture. At the same time upper troughing over the West will promote terrain-enhanced precipitation from parts of California into the Rockies. Marginally cold air at higher elevations supports the chance for snow Wed-Fri as the lead upper trough moves through. ...Guidance Evaluation/Predictability Assessment... From the eastern Pacific into western North America, guidance is still beginning to establish western U.S. troughing aloft by midweek in response to a strong eastern Pacific ridge. However later in the week most solutions are showing a trend toward the ridge extending farther into southwestern Canada and the Pacific Northwest. This leads to a farther southwest track of a potential embedded upper low (advertised by some earlier GFS runs), with the added complication that this energy could interact with a separate southern stream upper low that starts the period around 130-140W longitude. The best consensus/average among models and means thus far would place the center of the upper low near southern Arizona/northwestern Mexico as of early day 7 Sun--slower than the 00Z GFS but faster than the 00Z CMC. Meanwhile after early Fri models and ensemble members continue to become chaotic in depicting exactly how northern Pacific flow will erode the eastern Pacific ridge. Spread favors a blended approach with increasing ensemble mean emphasis at later times. A consensus approach has held up fairly well for the storm tracking from the Plains into Midwest Wed-Thu, but the latest 00Z/06Z GFS runs were displaced a bit south/east of the ECMWF/UKMET/Canadian consensus early Thu into Fri. Preferred this non-GFS blend given the better ensemble clustering to its north but the GFS runs do come back into agreement as the low moves into Canada. The forecast after early Thu becomes dependent on more variable central Canada into northern U.S. upper trough details where the GFS was quicker than the rest with lead height falls preceding the system, which may tarnish its forecast thereafter. Overall trend still points to a stronger/farther northward low pressure reaching into Canada while a triple point wave may break off just east of New England. Aforementioned issues with incoming Pacific flow after Fri along with model/ensemble spread and run-to-run variability do not lend much confidence to details of trailing flow within and around the central Canada/northern U.S. mean trough aloft. Best clustering would have a Canadian wave anchoring a front that drops into the northern U.S. during the latter half of the period, but there remains modest spread in track/intensity of this front. The first half of the forecast reflected the most common ideas (or average depending on the system) among the 00Z ECMWF/UKMET/Canadian. The rapidly increasing detail uncertainty over some areas led to increasing emphasis on the 06Z GEFS/00Z ECMWF means by next weekend along with the previous forecast. ...Weather Highlights/Threats... The storm tracking northeast from the central Plains Wed onward will produce various hazards. These include: 1) heavy rainfall potential over the Midwest into the Great Lakes with meaningful rain likely extending into parts of the Northeast 2) thunderstorms with an embedded severe threat from the southern Plains into Lower Mississippi Valley that extends into the Ohio Valley/Lower Great Lakes as the system moves northeastward. Consult Storm Prediction Center products for latest info on the severe weather forecast. 3) mountain snow over Colorado Wed/Thu with several inches likely at higher elevations above 7000-8000ft but even some snow to 6000ft possible. More localized pockets of snow in the northern fringe of the precip shield are possible with best probabilities (albeit low in absolute terms) over the western UP of Michigan and northeastern Wisconsin but also over western Nebraska early in the period. Areas from eastern Texas through the Lower Mississippi Valley and possibly farther northeast will see a continued threat of periods of heavy rain into the weekend as low level Gulf moisture feeds into the trailing front that likely stalls near and northeast from the western Gulf Coast and southwesterly mean flow aloft carries along convection-enhancing shortwaves. For the full five-day period model/ensemble guidance shows very high probabilities of 2" or greater accumulations over this area and models are suggesting some 5-12" totals. Over the western U.S. some details will likely keep changing as consensus flow aloft evolves, but there is a consistent theme of enhanced precipitation from California through the southern half of the Rockies in association with the mean trough aloft. Currently expect highest precipitation totals (rain and higher elevation snow) over the Colorado/New Mexico portions of the Rockies. Significant totals are also possible over Utah/northern Arizona, while increased potential for an upper low to trace a wider arc compared to previous days has raised amounts to some extent over favored terrain in central/southern California. Some of the Rockies snow could briefly extend into the High Plains early in the period (please consult the latest day 4-7 winter weather outlook probabilities). Chilly air behind the midweek Plains storm will bring a broad area of minus 10-25F anomalies for highs through much of the central U.S. The most extreme temperatures within that range should be over the High Plains. On the other hand southerly flow ahead of the storm will bring warmth into the East, with greatest anomalies for morning lows (plus 10-20F). The Pacific ridge aloft building into the Pacific Northwest will lead to multiple days of highs 10-20F above normal over that region--possibly approaching record values at some locations especially Friday from Olympia southward to Portland and Salem (mid to upper 80s). Fracasso/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