Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EDT Mon May 13 2019 Valid 12Z Thu May 16 2019 - 12Z Mon May 20 2019 Tonight's forecasts were derived from a blend of the latest 12z ECMWF/18z GEFS Ensemble Means with less weight on the operational ECMWF and 12z Canadian global days 3-7. The GFS has made significant run to run changes 12-18z with the Midwest cyclone and also the east pacific cyclone, a pattern which has continued for the 00z run. this makes it difficult to have confidence in any one run of the GFS. Models have typical timing/amplitude differences with northern stream energy crossing the central Appalachians/mid Atlantic/coastal waters Thu, with potential low pressure development off the coast Fri 17 May. There remains latitudinal spread as to where low pressure develops on the surface front, along with typical timing differences, as the ensemble means are a few hours slower with the frontal progression across Maine. A blend of the 18z GEFS/12z ECMWF Ensemble means with less weight on the 12z Canadian/ECMWF models was used until better agreement develops. Agreement amongst the models/ensembles supports an amplified upper trough reaching the west coast Thu 16 May, the southern stream portion of which then moves inland into the Great Basin Fri 17 May and then ejects on to the high Plains Sat 18 May. Models differ on the exact timing and phasing of the trough. Big differences exist on the intensity of the possible embedded closed low Sat 18 May, as the GFS/ECMWF have shown considerable run to run changes. As a a result, greater weighting in the forecasts were given to the ensemble means, including the 12z NAEFS Mean. Height falls crossing the Rockies on to the Plains by late next week should induce surface cyclogenesis across the northern Plains Fri night-Sat, drifting slowly east northeast towards the upper MS Valley Sun 19 May. Good clustering among the 12z ECMWF/18z GEFS Mean/12z ECMWF Ensemble Mean/12z Canadian global led to a consensus of these forecasts. The 00z GFS trended faster and moved the low well up into Canada by 12z Mon 20 May, making it appear to be a low probability solution that differs form the 12-18z runs, other models, and its ensemble mean. Another low is forecast to develop on the high Plains as the next wave within the upper trough crosses the Great Basin to the CO/NM Rockies Mon 20 May. ...Weather Highlights/Threats... A surface wave and cold front should produce showers over the northeast Thu before moving off the coast on Fri. Showers/storms also develop in the vicinity of a stationary front lingering over the southern Fl peninsula in the middle to latter portions of next week. The upper trough should bring increased clouds/precipitation to the Pacific Northwest and CA, continuing inland into the Great Basin and northern Rockies Thu 16 May, and then the Plains on Fri 17 May and Sat 18 May. Higher elevation snows develop as colder air aloft moves into the northwest US, starting in northern CA/western OR Thu and then spreading into the higher elevations of the Northern Rockies and northern Great Basin next weekend. With low pressure moving slowly east across the Plains next weekend, return moist southerly flow provides fuel to enhance shower/thunderstorm activity, with potential for locally heavy rain to develop over portions of central and eastern Texas. Temperature-wise, highs/lows of 10-15 F above normal are forecast over the northern to central Plains Thu, progressing into the mid MS Valley Fri, Ohio Valley Sat/Sun, and central Appalachians next Mon 20 May. With the approaching trough, CA/OR/WA will see a reversal to cooler than normal temperatures in the middle to latter portions of the week. Temperatures several degrees above normal develop over the southeast late in the week and continue through the weekend and next Mon 20 May. 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