Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sun May 20 2018 Valid 12Z Wed May 23 2018 - 12Z Sun May 27 2018 ...Guidance and Predictability Assessment... In general, most models are in line with the overall synoptic pattern with the typical differences in detail. On Day 3/4 (Wednesday/Thursday), shortwave energy will begin to eject into the northern Plains while the upper ridge weakens and shifts east toward the Great Lakes. An upper trough in eastern Canada will dip into the Northeast states by Thursday, Meanwhile, the next upper trough will also begin to approach the West Coast. By late week into next weekend, the upper trough in the Pacific will begin approaching the West Coast. To the east, an upper trough will slide across the Great Lakes once again while an upper trough moves northward into the central Gulf of Mexico. The GFS/GEFS, for the most part, continues to be the slower solution in regards to the upper trough that will move toward the West Coast late next week. The ECMWF and its mean is staying the course on being more progressive with this system with the CMC also showing this progressive solution. However, the 00Z GFS/GEFS aligned more with the ECMWF/CMC with being faster--with the 06Z GFS/GEFS flipping back to being slower. By Day 7 (Sunday), the 00Z GFS/GEFS was much farther north with the trough moving into the Pacific Northwest while the 06Z GFS/GEFS was farther south like the ECMWF has consistently been. Both the 00Z GFS/ECMWF and their means also agreed with the evolution of the shortwave energy moving into the northern Plains midweek and eventually toward the Great Lakes by the weekend. Farther East, there are some differences in timing with regards to the upper trough in eastern Canada dropping south over the Northeast region. However, the 00Z GEFS/ECENS in general showed the upper trough over New England on Thursday. As the shortwave energy moving from the northern Plains enters the Great Lakes/Ohio Valley, models are consistent showing an upper trough over the Southeast/central Gulf Coast. There continues to be a signal that a weak surface low will evolve from the upper trough by next weekend. In addition, the GFS has consistently shown the low from the northwest Caribbean taking an eastward path with the ECMWF continually showing a westward trend. The CMC also favors the westward path but continues to be quite aggressive with this surface low. In general, stayed with continuity by favoring the eastward (GFS/GEFS) solution. The WPC blend for the first part of the medium range forecast was a blend of the 00Z ECMWF/GFS/CMC. By Day 5 and 6, weighted more of the blend with 00Z ECENS/ECMWF and some 00Z GEFS/GFS. By Day 7, because 06Z GEFS/GFS was trending toward the ECMWF and its mean, used the 00Z ECENS/ECMWF and the 06Z GEFS/GFS. ...Weather/Threats Highlights... The shortwave energy moving over the northern Plains and into the Great Lakes will bring precipitation across the Intermountain West and into the Plains by midweek and will increase over the Mississippi Valley by Thursday/Friday. By the weekend, rainfall will spread throughout the Great Lakes and the Ohio Valley/Northeast. With both the surface low from the upper trough in the Gulf along with the surface from the northwest Caribbean, the central and eastern Gulf Coast region, Southeast and Florida can expect showers and thunderstorms throughout the weekend. Rainfall will also increase through northern/central California and the Pacific Northwest as the upper trough approaches. Most of the country can expect above normal temperatures during the medium range period. The biggest exception will be Florida as cloudy conditions/rainfall will keep temperatures below normal. On Thursday, due to the eastern Canada upper trough, temperatures will dip below normal but will recover back to near normal by Friday and last through the weekend. 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