Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 AM EDT Mon May 07 2018 Valid 12Z Thu May 10 2018 - 12Z Mon May 14 2018 ...Pattern overview... A relatively amplified flow will be present across the northern hemisphere during the medium range. Negative height anomaly centers will persist across the north central Pacific and northeast of Hudson Bay. Meanwhile, ridging will attempt to expand northward along the U.S. and Canadian west coast. Meanwhile, positive height anomalies will also be prevalent across the southeastern/Mid-Atlantic states as the influence of the Atlantic subtropical ridge grows. A series of shortwaves ejecting from the broad north Pacific upper low will erode and/or undercut the western ridge. These waves will have a tendency to amplify as they cross the Rockies and enter the central U.S., in a broad region of cyclonic upper flow in between the ridging along the two coasts. Teleconnections based on a few of the stronger and persistent height anomalies across the northern hemisphere support the described North American pattern, and thus would expect this pattern to remain relatively stable for some period of time. ...Guidance and uncertainty assessment... The WPC medium range forecast was based heavily on a blend of deterministic guidance, including the 12Z ECMWF/CMC and 18Z GFS, during days 3-4 (Thu-Fri). Guidance consensus during this time period was relatively good, with substantial improvement noted among ensemble members over the past couple cycles, and a shift among the GEFS/CMCE members toward the ECENS mean. This was particularly true with respect to the rapid amplification of shortwave energy across the Great Basin/Rockies Thu night-Sat. The trend has been towards more amplification, as was shown by the ECMWF and its ensemble last night. Spread associated with the western system still increases by day 5 (Sat), with a significant degree of spread between the 12Z UKMET/CMC which continued to dig the trough/upper low across the Great Basin, while the ECMWF/GFS begin to move the system eastward. Something a bit closer to the GFS/ECMWF solutions here was preferred, although ensemble means (especially the GEFS) did allow for a solution somewhere in between the two deterministic camps. As the shortwave/upper low ejects east into the central U.S. by Sun-Mon, the GFS/ECMWF solutions remained relatively close to one another in terms of timing, but some significant spread emerges farther north as a shortwave of Arctic origin (moving along the western side of the Hudson Bay upper low) begins to interact with the Pacific energy across the central plains/Midwest. Differences in the phasing of these systems have major implications for if and how quickly amplification would occur and heights would fall across the Great Lakes. The past couple runs of the ECMWF have generally been more aggressive with amplifying a long wave trough axis from Hudson Bay south to the Great Lakes by day 7 (Mon), while the GFS keeps the systems more separated, and the CMC still hangs the upper low back across the Great Basin by that time, obviously precluding any northern stream interaction. Farther west, guidance are in general agreement that another potential shortwave/cold front will approach the Northwest by early next week. While most solutions agree on the existence of this feature, deterministic models and ensemble member spread is quite large with respect to the amplitude, which appears largely dependent on how much energy remains consolidate in the Gulf of Alaska versus a more open wave. Given the quickly increasing uncertainty from day 5 onward, the ensemble means (12Z ECENS/18Z GEFS) were given increasing/majority weight during that time period, with 70 percent means by day 7. ...Sensible weather/threats highlights... A frontal boundary expected to stall through much of the period from the central plains to the Great Lakes/Northeast will produce scattered to numerous showers and thunderstorms, with areas of potentially heavy rainfall possible through much of the forecast period. The greatest potential for heavy rains will focus across portions of the central/northern plains and Midwest from Thu onward. As moisture surges farther northeastward, heavy rains will also be possible from portions of the lower Great Lakes to the Northeast Sat-Sun. The trailing end of this frontal boundary will also produce locally heavy areas of rainfall across portions of the central Rockies, with the heaviest amounts in favored upslope areas between the surface front and high pressure to the north. Farther south, additional showers and thunderstorms will be possible Sat-Sun across the southern plains ahead of the surface front and dry line. A lingering washed out frontal boundary will also bring increasing coverage and intensity of showers and storms to the Florida Peninsula by Fri-Sun. Temperatures are expected to be above average through much of the period from the Great Basin/Southeast eastward across the central/southern plains to the Eastern Seaboard. The greatest temperature anomalies Thu-Fri are forecast from the Southeast to the southern High Plains, where high temperature anomalies of +10 to +20 deg F are expected. By Sat-Sun, as the subtropical ridge builds, the greatest high temp anomalies will shift a bit eastward, from the southern plains to the Southeast/Mid-Atlantic, where +10 to +15 deg F anomalies are possible. Finally, north of the surface front, areas from the northern Rockies to the Great Lakes will see below average temperatures through much of the period, with highs 10 to 20 deg F below average. Ryan Associated WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation forecasts, and winter weather outlook snow/sleet 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