Extended Forecast Discussion NWS Weather Prediction Center College Park MD 953 AM EDT Tue May 15 2018 Valid 12Z Fri May 18 2018 - 12Z Tue May 22 2018 ...Cluster of showers/storms near fronts in the central Plains/ mid MS Valley and northern mid Atlantic to southern New England... ...Guidance, Predictability Assessment, and Preferences... The models show a deep layer over the Great Basin ejecting to the north across the Pacific northwest through Sat and becoming ill-defined on Sunday. Heights increase in response to the next upper trough moving onshore from the Eastern Pacific into CA. The models have trended a little slower and deeper with the upper trough, with the 06z GFS showing the strongest upper level low. Teleconnections showing a modest upper trough downstream from the closed low south of AK. With better clustering of the 00z GFS with the respective ensemble means of the 00z GEFS and 00z ECMWF Ensemble Means, the 06z GFS was not used in favor of the 00z run. The 00z ECMWF was on the southern edge of the cluster of solutions and thus the mean was given more weighting Mon and Tue 21-22 May, especially considering better than average agreement among the means. The 06z run of the GFS also is more amplified with the Great Lakes to northeast trough next Mon-Tue 22 May. Given higher heights in the southeast and off the coast with the persistent anticyclone in place to abut trough development, the 06z GFS looks to be unlikely. The days 3-7 height/sea level pressure forecasts were derived from a blend of the 00z GFS/ECMWF/00z GEFS Mean/00z ECMWF Ensemble Mean days 3 through 5 with more weighting on the 00z GEFS/ECMWF ensemble means days 6-7. ...Weather/Threats Highlights... A few waves of lower heights/pressures emerge from the Rockies across the high Plains of southeast WY and northeast CO across the central Plains, with several models showing locally heavy rain in the central high Plains across the rest of the central Plains Fri into Sat 19 May. Anomalously deep moisture from the Gulf of Mexico and western Atlantic will likely spread showers and storms from the Southeast to up the Appalachians and Mid-Atlantic/Northeast later week into the weekend. The models forecast widespread showers when the moisture intercepts the front across Long Island and adjacent southern New England/nearby islands Fri into early this weekend. In this pattern, late in the week record high temps are expected in parts of Texas to the mid-lower MS valley before the airmass modifies and cools early next week. Record warm mins are possible for the Mid-Atlantic in the warm, humid airmass. An amplified mid-upper level trough and frontal system slowly work north over the Great Basin Fri and Rockies through the weekend, with showers developing in areas of combined terrain lift and deformation. The next upper trough coming onshore early next week produces showers in CA/NV/OR/ID. A persistent and wavy frontal zone extends east from the Plains to the MS Valley, Oh Valley, and Northeast. Hot and moist conditions persist to the south of the front. Instability and moisture pooling along the front will support showers and storms with potential for locally heavy downpours. Other showers/storms develop in the TX/OK panhandles near the dryline and return low level southerly flow advects moisture into the region. Also, FL has diurnal showers/storms developing with the GEFS Mean showing 2 inches of precipitable water, supporting locally heavy downpours where sea/lake breeze convergence occurs. 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 Petersen