Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 AM EDT Sat May 18 2019 Valid 12Z Tue May 21 2019 - 12Z Sat May 25 2019 ...Overall Weather Pattern... The upper level pattern across the continental U.S. will continue to be rather amplified and slow to change next week. The upper level ridge over the southeast U.S. becomes anchored over the region and gradually becomes stronger for the second half of the upcoming work week. Across the western U.S., a large scale trough will be dominant as multiple shortwave perturbations keep conditions cool and unsettled. This will also be slow to move given the expected development of an omega block pattern over the eastern Pacific. The main weather feature of interest will be a strong low pressure system across the Plains and Upper Midwest for the Tuesday and Wednesday time period, and then followed by a weaker low and frontal boundary for the end of the week. ...Guidance/Predictability Assessment... The models and ensemble means indicate the greatest agreement with respect to the building upper level ridge over the southeastern U.S., with the ECMWF and the EC mean supporting the strongest solutions. The greatest differences involve the evolution and track of individual closed lows within the western U.S. trough. These become evident at the beginning of the forecast period on Tuesday, with the 00Z UKMET notably more amplified with the upper low over New Mexico, and its surface low tracks well to the west of the model consensus. The CMC becomes more progressive with the next closed low over the Desert Southwest and this becomes more apparent by Thursday. Even greater differences exist with the surface low that eventually reaches the Upper Midwest and southern Canada by Thursday morning, with the ECMWF and EC mean over Ontario and the GFS much more delayed in bringing the low out of the Upper Midwest. The track of the ECMWF best fits continuity and recent storm tracks. Towards the end of the forecast period, there is an increasing model signal for a third trough axis to close off near the California coast by Friday and Saturday, with support from the GFS, FV3, and ECMWF. Taking these factors into account, the WPC forecast was based on a blend of the ECMWF/GFS/FV3/ensemble means, along with some of the previous WPC forecast for continuity. ...Weather Highlights and Hazards... Numerous showers and thunderstorms continue to be in the forecast through the middle of next week from the central plains to the Upper Midwest. Multiple rounds of MCS activity are likely with both of the low pressure systems that are progged to track across the Plains. This will have the potential to produce very heavy rainfall capable of producing flash flooding, and severe weather is also likely across much of the Midwest states based on recent SPC forecasts. Additional heavy rain is also possible from the next storm system tracking eastward from the Intermountain West later in the week, although details on timing and placement continue to remain unclear at this time. There will also be a dichotomy of temperature anomalies given the amplified flow pattern in place. Temperatures are expected to be well above normal across much of the southeastern U.S. owing to the strength of the upper ridge, and this will also serve to keep rainfall chances minimal. Afternoon high temperatures may approach 100 degrees for parts of Georgia and South Carolina by later in the week. The opposite will be the case across much of the western U.S. and also the northern Plains with readings on the order of 10 to 20 degrees below climatology, which will be under the influence of the upper level trough and to the north of the low tracks. Heavier late season snows are also likely for the Sierra Nevada to the northern Rockies for the middle of the week. D. Hamrick 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