Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sat May 19 2018 Valid 12Z Tue May 22 2018 - 12Z Sat May 26 2018 ...Guidance and Predictability Assessment... Overall, there is not large differences in the overall synoptic pattern with models. On Day 3/4 (Tuesday/Wednesday), a large closed 500 mb low will move over the Great Basin/Desert Southwest and weaken while a shortwave skirts across the Great Lakes. The upper ridge still stays in tact across the central U.S. with very strong ridging in western Canada. By Day 5 (Thursday), mid-level energy will move from the Great Basin to the northern Plains as the upper ridge breaks down and the upper trough over eastern Canada will also dip into the Northeast. By late week and into the weekend (Friday/Saturday), another trough will meander toward the West Coast as a low from the Caribbean moves into the Gulf of Mexico. There are not significant differences in the 06Z GFS/00Z ECMWF/00Z UKMET/00Z CMC on Days 3 and 4 at the 500 mb level. All agree on the strength and timing of the closed low over the Great Basin evolving into an open trough by Wednesday. Meanwhile, a shortwave will quickly slide across the Great Lakes. WPC blended all of these with a little bit of the 06Z GEFS/00Z ECENS. By Thursday, models begin to differ on the timing of the upper trough approaching the West Coast. The 00Z ECMWF has consistently been the faster solution and the 00Z CMC along with the means also back up this faster solution. The GFS has been much slower with the GEFS leaning toward the faster solution of the ECMWF and its mean. the 06Z GFS/00Z ECWMF and their means are in relative agreement with the shortwave energy over the northern Plains. By Friday and into Saturday, the GFS/GEFS has consistently been on the eastern side of the spread with respect to the tropical low in the Caribbean moving northward into the Gulf of Mexico. The CMC runs also continue to be very aggressive with this low and being on the western side of the spread. The ECMWF had been in agreement with the GFS/GEFS but the 00Z run is faster and on the western side of the spread. For now, confidence remains low with the details of this system but trended toward slightly west of the GFS/GEFS solution. WPC took a blend of the 00Z ECMWF/ECENS and 06Z GFS/GEFS from Days 5-7 but put much more weight on the ECMWF and its mean due to its treatment of the evolution for the Pacific trough late in the week. ...Weather/Threats Highlights... With the upper trough over the western U.S. and the shortwave ejecting toward the northern Plains, expect an increase in precipitation from the Great Basin to the northern Plains through Thursday. By Friday and into Saturday, precipitation will move eastward into the MS and TN Valley/Great Lakes as the shortwave continues moving eastward. As the second Pacific trough moves toward the West Coast, California and perhaps parts of the Pacific Northwest can expect rainfall through the end of the week. Rainfall will likely increase by the weekend as the trough approaches closer to the coast. Though confidence of the timing and position of the low in the Gulf of Mexico, the Gulf Coast region can expect more coverage in convection as there will be an abundant amount of moisture in place. Most of the country will experience above average temperatures throughout the medium range period. The biggest exception is Florida which will have continued precipitation throughout the week; thus, maximum temperatures will be nearly 5 degrees below normal. The California coast will also be below average for much of the medium range period. As the upper trough dips into the Northeast from eastern Canada, cooler conditions will follow which will be felt on Thursday. Temperatures are expected to recover back to near normal by Friday. 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