Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EDT Thu May 24 2018 Valid 12Z Sun May 27 2018 - 12Z Thu May 31 2018 ...Potentially significant rainfall events possible over the lower Mississippi Valley/Southeast and northern Rockies/Plains... ...Overview... The fairly blocky pattern expected through the first part of the period shows signs of change at least over western areas as Pacific trough energy heads toward the West Coast, serving to dislodge the slow moving upper low expected to be over the Great Basin to start the period. Teleconnections relative to positive height anomaly centers over the Pacific (south of Alaska along 45-50N latitude) and south of James Bay in recent D+8 multi-day means offer support for a mean trough settling along or just inland from the West Coast. Away from northern latitudes the slow evolution of flow will persist somewhat longer over the East, as surrounding ridges aloft will likely allow for only a gradual northward drift of a Gulf of Mexico/southern U.S. upper low. ...Guidance and Predictability Assessment... In principle there is decent agreement and continuity (at least within the past day) for evolution from the western Pacific into central U.S. There are still some significant differences in the specifics. Among recent solutions the 12z ECMWF becomes the lowest confidence scenario from about Mon onward as it is quite progressive with Pacific height falls moving into the western North America and thus may be somewhat quick to eject energy from the Great Basin upper low into the northern Plains. Thus for the purpose of an overall blend the old 00z/23 ECMWF run had to be given greater weight for the operational ECMWF component. Other models/means compared better to each other. The 18z GEFS mean was a tad slower with the incoming trough versus most of the remaining guidance leading to greater preference for the 12z version. Aforementioned teleconnection support adds confidence in the idea of a mean trough becoming established near the West Coast but ensemble spread is high enough to suggest a fair degree of uncertainty in the details. Significant track spread continues for the system expected to emerge from the northwestern Caribbean and track into the Gulf of Mexico during the short range time frame. Latest solutions generally continue to occupy their historical sides of the east-west envelope thus far, GFS/GEFS/UKMET on the east (though GEFS west of the GFS) and ECMWF/ECMWF ensembles/CMC ensembles on the west with the operational CMC a bit eastward of the latter cluster. A westward adjustment in the new 00z GFS versus the 18z/12z runs has narrowed the spread though. Multi-day trends toward slower northward progression persist. Trends that have seen a trimming of the eastern side of the full guidance envelope over recent days along with the strengthening ridge aloft to the east of the system seem to continue support for a track in the middle to western portion of the envelope. By day 7 Thu it remains to be seen how much of a surface reflection remains, while the new 00z GFS may become a little quick to eject the upper feature in response to flow upstream. After some recent continuity changes, the forecast for southern Canada/northeastern U.S. flow aloft and associated surface evolution over the Great Lakes/Northeast/Mid Atlantic have held fairly steady over the past day. Based on above preferences or representation of consensus, the updated forecast incorporated aspects of the past two ECMWF runs (more 00z/23 than 12z/23), 12z CMC, 12z-18z GFS, and 12z GEFS/ECMWF means. Operational model weight decreased from 80 percent early to about half by the end of the forecast. ...Weather/Threats Highlights... The possible system tracking toward the Gulf Coast along with broad moist flow to its east will bring the potential for a multi-day heavy rainfall event to parts of the Gulf Coast and Southeast--most likely beginning this weekend and persisting into the middle of next week. Preferred solutions would bring some of the heavy rainfall as far west as the lower Mississippi Valley. On the other hand the lower probability scenario of a farther east track could direct more moisture into the Mid Atlantic where a frontal boundary could provide an added focus for significant rainfall. Farther northwest, expect the upper low tracking out of the Great Basin to generate areas of locally heavy rainfall from the northern Great Basin across the northern Rockies and northern to possibly central Plains. In both of these areas of focus some locations have already experienced significant rainfall recently and will be sensitive to additional rain. Portions of the Great Lakes/Northeast may see some showers/thunderstorms and locally heavy rain with an upper shortwave and associated surface wave(s)/frontal system. The Plains will likely see the highest temperature anomalies during the period with many areas experiencing readings 10-20F or so above normal for multiple days. Some daily max/warm min records are possible. Cool highs over the Great Basin will trend closer to normal as the initial upper low lifts to the northeast. Over the Southeast persistent cloudiness/areas of rainfall should result in below average highs while early in the period onshore flow from the Atlantic will keep New England on the cool side. The pattern will be favorable for warm lows over much of the East though. Rausch 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