Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Tue May 14 2019 Valid 12Z Fri May 17 2019 - 12Z Tue May 21 2019 ...Overview and Guidance evaluation/preferences... Mean troughing aloft will bring a wet period to a large portion of the western U.S. and Plains states during the weekend to early next week. Much of the western US to the northern high Plains will turn colder, with much of the lower MS Valley and TN Valley to the southeast turning warm to hot. Guidance is steadily trending toward resolving the leading strong trough progressing from the northern Rockies onto the Plains by day 3 Fri and progressing across the northern Plains thereafter. By day 4 Sat the operational GFS runs become faster than the other models in moving the closed low sfc/aloft out of the Plains across the upper MS Valley and upper Great Lakes. The 12z ECMWF/00z UKMET/12z Canadian suggest that southern trough energy could form another upper low that reaches the Plains late Sat-Sun, which leads to slightly slower forward motion, which is preferred. There is good agreement that upstream flow will bring another round of strong height falls to the western states this weekend, continuing inland early week across the Great Basin to the northern high Plains. The 12z ECMWF and 12-18z GFS indicate a deeper low in the eastern Pacific early next week, which has potential to reach CA Tue 21 May. Models/ensembles increasingly diverge for northeastern/mid Atlantic U.S. flow, highlighted by a deep New England/Mid-Atlantic trough aloft in the 00Z UKMET versus a strong ridge in recent GFS runs. The 12Z ECMWF/12z Canadian had lower heights than the GFS, which were given a little more weight. Teleconnections relative to the negative height anomaly center associated with the West Coast mean trough favor modest cyclonic flow aloft, thus the GFS was given less weight. More weighting was given to the 12z ECMWF mean/18z GEFS Mean to avoid committing to any particular model solution given continuing spread. ...Weather Highlights/Threats... Over the West enhanced precipitation is expected with the mid level closed low over the northern Rockies and High plains Fri-Sat. The next significant precip event is expected Sat-Sun extending from favored terrain in northern-central California into the northern Rockies and then the northern great Basin to the the high Plains Mon 20 May-Tue 21 May. This period is expected to be unusually wet for so late in the wet season from CA to the northern Rockies. The greatest potential for high elevation snow should be in the northern Sierra Nevada. Progression of the initial western storm will generate areas of heavy rain/thunderstorms over the central to southern Plains over the weekend, with the Storm Prediction Center also indicating severe weather potential for Fri-Sat. Widespread rain is expected over the northern Plains to upper MS Valley this weekend. Well below normal highs (clusters of minus 10-20F anomalies) over California/ Nevada/Utah on day 3 Fri will spread northeast over the weekend into the northern Plains and remain in place through early next week. Clouds and precip should keep morning lows mostly a few degrees below normal with only isolated locations ten or more degrees below normal this weekend into early next week. Well above normal highs will progress from the Plains Fri and gradually relocate into the MS Valley and Oh Valley over the weekend and the southern mid Atlantic into the southeast early next week, with temps commonly 10-15 degrees above normal in the core of the warmest air. From Sat-Mon temperatures over the mid Atlantic states are very uncertain given a wide range of possibilities for frontal position within a region of strong temperature contrast. Petersen 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