Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Fri May 18 2018 Valid 12Z Mon May 21 2018 - 12Z Fri May 25 2018 ...Guidance and Predictability Assessment... Overall, especially in the first half of the medium range period, there continues to be fair agreement in models on the overall pattern. In the beginning of the week, a robust upper trough will move over the West Coast on Monday and become a closed off low as it meanders toward the Desert Southwest by Tuesday. The 00Z ECMWF/CMC/06Z GFS are in agreement with the evolution of this trough; however, the GFS both the 06Z and 00Z runs have showed the primary closed low breaking off along with another piece of energy sliding southward from the trough. With the 00Z ECMWF/CMC in agreement and the GFS and UKMET showing these two closed lows, decided to blend all four solutions with slightly more weight given to the ECMWF and CMC solutions. By Day 5/Wednesday, models are still in agreement with the overall pattern with an open wave trough over the western U.S. and the upper ridge beginning to weaken across western Canada and the central U.S. Much like what was occurring on day 4/Tuesday, there are difference in the details especially between the 00Z ECMWF/CMC and 06Z/00Z GFS. Both the 00Z GEFS and 00Z ECENS are trending toward the 00Z ECMWF/CMC in terms of the orientation of the trough. Consequently, by day 5, dropped the operational GFS and began to increase the usage of the 00Z GEFS/ECENS while keeping the 00Z ECMWF/CMC. By Day 6 and 7/Thursday and Friday, both the 00Z ECENS/GEFS and 00Z ECMWF/CMC show another trough approaching the West Coast. The 06Z GFS and 00Z GFS have both been inconsistent with itself and other models with handling this trough. The 00Z ECMWF is much faster with with the trough--whereas the 12Z ECMWF matches fairly well with the 00Z CMC. Farther east, the ECENS/GEFS were comparable in handling the northern stream cyclonic flow and the effects on surface fronts/waves. For the tropical wave, the 00Z CMC was faster and more aggressive with it--as was the 00Z GFS. The 06Z GFS has trended slower and having a more broad wave as the ECMWF has consistently suggested. Thus the blend was based on 00Z (12Z on day 7)ECMWF/00Z CMC along with the 00Z ECENS/GEFS. ...Weather/Threats Highlights... With the first upper trough moving across the West Coast, scattered precipitation will occur across the Great Basin and Intermountain West. An increase in precipitation can be expected especially in the higher elevations of north and central California and through the northern and central Rockies. In addition, convection could occur along the southern Plains on Monday and track eastward along the warm front lifting into the central Plains/MS Valley. As the next trough approaches the Pacific Northwest, an increase in precipitation can be expected especially in eastern OR/WA. Confidence still remains low with the tropical feature in the Caribbean and how it will impact Florida--but an increase in moisture from this feature could occur and thus would increase precipitation coverage. In terms of temperatures, most of the country will slightly warmer than normal. There is a potential to break high minimum temperature records across the Tennessee Valley and into the Mid-Atlantic in the beginning of the week. Florida and the immediate coastline along California will remain below average in terms of high temperatures. WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 Reinhart/Rausch