Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun May 05 2019 Valid 12Z Wed May 08 2019 - 12Z Sun May 12 2019 ...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. 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. ...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 as of early day 7 Sun--slower than the 18Z/new 00Z GFS runs but faster than the new 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. At the very least the 12Z ECMWF mean has made a favorable adjustment toward slower weakening of the ridge versus its run from 24 hours ago. The wide spread favors a blended approach with increasing ensemble mean emphasis. A consensus approach has held up fairly well for the storm tracking from the Plains into Midwest Wed-Thu. Faster solutions from 1-2 days ago (CMC/UKMET and to some degree ECMWF) have trended slower while the GFS has trended a bit faster from its prior slow timing. The forecast after early Thu becomes dependent on more variable central Canada into northern U.S. upper trough details. Trends have been faster within this northern stream--leading to a more phased evolution aloft and stronger/farther northward low pressure reaching into Canada while a triple point wave may break off east of New England. This evolution was becoming evident in yesterday's 00Z guidance and subsequent runs have been increasing the confidence in such a scenario. 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. The first half of the forecast reflected the most common ideas (or average depending on the system) among the 12-18Z GFS and 12Z ECMWF/CMC. The rapidly increasing detail uncertainty over some areas led to increasing emphasis on the 18Z GEFS/12Z ECMWF means by next weekend. The CMC and 18Z GFS contributions were phased out before day 7 Sun. ...Weather Highlights/Threats... The storm tracking northeast from the central Plains Wed onward will produce various hazards. These include heavy rainfall potential over the Midwest into the Great Lakes with meaningful rain likely extending into parts of the Northeast, locally heavy convection from the southern Plains into Lower Mississippi Valley, and a threat of severe weather that may extend from the southern half of the Plains into the Ohio Valley/Lower Great Lakes. Consult Storm Prediction Center products for latest info on the severe weather forecast. Localized pockets of snow are possible in the northern fringe of the precip shield with best probabilities (albeit low in absolute terms) over the western UP of Michigan and northeastern Wisconsin. 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. 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. 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