Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Sun May 10 2020 Valid 12Z Wed May 13 2020 - 12Z Sun May 17 2020 ...Record low temperatures possible across the East through Wednesday morning followed by a gradual warming trend... ...Heavy rain possible over portions of the Plains and east-central U.S. as well as South Florida... ...Overview and Weather/Hazard Highlights... Temperatures 10-15F below normal over some areas from the Northeast into the Northern Plains on Wed, including some possible record lows in the East to start the day, will be the last vestige of the current March-like regime affecting areas to the east of the Rockies. Then the CONUS upper level and temperature pattern will be more typical of the latter half of spring with lower amplitude mean flow aloft downstream from a Rex block pattern that prevails over the North Pacific through northwestern North America and into the Arctic. An upper trough (likely incorporating at least some energy from the upper low off the Pacific Northwest as of Wed) should migrate across the West and into the Plains during the latter half of the week and weekend while a flat mean ridge/modestly above normal heights aloft become established over the East. This trough and leading shortwave energy flowing along the U.S.-Canadian border should support at least a couple waves along a front that becomes established between the northeastern U.S. and central/southern Plains, and for a time extends into the West. The interaction of these features and a persistent low-level flow of moisture from the Gulf of Mexico will increase the potential for multiple episodes of moderate to heavy rain/thunderstorms between the central-southern Plains and locations to the northeast. Currently the most coherent signal for highest five-day rainfall totals extends across the Midwest and Lower Great Lakes/northern Ohio Valley region but there are meaningful probabilities over the Southern Plains as well. The Storm Prediction Center is monitoring the severe weather potential. Check for upcoming outlooks as important details become better resolved. From late week into the weekend the wavy front will separate near to below normal temperatures to the north/west from above normal readings to the south, in most cases within 10F on either side of climatology. The trailing end of a stationary front near South Florida will eventually dissipate. Moisture and instability focused along this front will promote scattered to numerous showers and thunderstorms through late week with at least locally heavy rain possible over the area through Fri-Sat. The West will see two primary precipitation events during the period. The first should be during Wed-Fri with rain and some high elevation snow extending from the northern half of the West Coast into the Rockies as the the upper low initially off the Northwest coast and trough energy to the south gradually move into/through the West. Depending on the evolution of this system, there could be locally enhanced precip extending into parts of the northern-central High Plains. After a brief dry period behind this precip area, expect a stronger system with a robust front to approach the West Coast next weekend and bring locally enhanced activity to northern California and the Pacific Northwest. There is some uncertainty over the eastern/southern extent of moisture especially by next Sun. Anticipate the highest five-day totals along the most favored terrain of extreme northwest California and the Pacific Northwest. ...Guidance/Predictability Assessment... Latest models and means agreed well for the overall pattern evolution to leave mostly embedded details up for some debate. An operational model blend of latest 06Z/00Z runs into day 5 Fri followed by a trend to a model/ensemble mean/continuity mix represented consensus well for significant features while downplaying low-confidence specifics for smaller scale aspects exhibiting low predictability. One particular guidance issue of note is that by day 7 Sun the 00Z ECMWF became fairly extreme versus the full ensemble spread for the southeastward amplitude of the upper trough reaching the West Coast--ultimately leading to an embedded upper low path well south of most other solutions just after the medium range period. The leading western trough has been fairly diffuse in most guidance thus far but the new 12Z ECMWF deepens it to a greater extent than the majority cluster and by next weekend becomes considerably slower. Meanwhile guidance has been providing some various alternatives along/offshore the southeastern coast during the latter half of the period. The 00Z CMC/CMC ensembles were generally on their own with weak waviness and associated moisture lifting from Florida into the Mid-Atlantic. Farther offshore the GFS/GEFS mean and now the 12Z UKMET show development of better defined low pressure while latest ECMWF runs have been slower and weaker with such development. Rausch 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