Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Wed May 23 2018 Valid 12Z Sat May 26 2018 - 12Z Wed May 30 2018 ...Potential multi-day heavy rainfall event possible for the lower Mississippi valley and Southeast... ...Overview... The flow pattern across North America will become a bit more blocky during the medium range, with persistent and relatively slow-propagating height anomalies. Negative height anomalies are expected to persist across northeast Canada and Greenland, with strong positive height anomalies across northwestern Europe. Teleconnections associated with these features are a bit contradictory, however the stronger anomaly over Europe supports the idea of persistent subtropical ridging along the U.S East Coast, and across western U.S./Canada, with potential for Pacific shortwave energy to undercut the ridge - which fits the dominant pattern through the medium range. An area of low pressure expected to be in place across the Gulf of Mexico on day 3 (Sat) will struggle to find a quick path northward given the ridging north/east of the system, keeping the westerlies confined relatively far to the north. A cutoff upper low crossing the Great Basin on Sat is also expected to slowly drift east-northeast across the Rockies through early next week before likely becoming absorbed in the westerlies and moving east across the northern plains. Finally, additional shortwave energy is likely to approach the West Coast by next Tue-Wed. ...Guidance and Predictability Assessment... With respect to the area of low pressure crossing the Gulf of Mexico during the period, models/ensembles show two distinct camps. The 00Z ECMWF/CMC along with the ECENS/CMCE means take the system on a northwesterly track, nearing the mouth of the Mississippi River by Sat night/Sun. The GFS/GEFS on the other hand take the system more northward or even slightly east of north, crossing the Florida Peninsula and the southeastern states. At this time, given the upper ridging prevalent north and east of the system, WPC preferred the more northwesterly ECMWF/CMC solutions. Additionally, ensemble guidance has shown a broad trend over the past couple cycles toward a slower northward progression of the system by days 6-7 (next Tue-Wed). As a result this forecast update reflected a slight trend in that direction, keeping the system a bit farther south by that time, and showing a slow westward propagation along the Gulf Coast. Given the divergent model ensemble camps, forecast confidence at this time is moderate at best. Farther north, model spread is lower, and forecast confidence is a bit higher. The GFS/ECMWF/CMC all handled the evolution of the western U.S. upper low during days 3-5 similarly. Some differences emerge by days 6-7, with the GFS/ECMWF bringing the system under the influence of the westerlies and moving it east into the northern plains, while the CMC continued to hang much of the energy back across interior West (while also bringing part of the energy east into the northern plains). Additional shortwave energy skirting the northern Great Lakes Sat-Sun is also handled reasonably well by the GFS/ECMWF/CMC. Finally, additional shortwave energy looks to approach the West Coast by the middle of next week. Models show general agreement on the existence of this feature, but run-to-run variations exist with respect to the exact character of the feature (e.g. a full-latitude upper trough/upper low or a more progressive open wave). Given these considerations the WPC forecast was initially based on a multi-model deterministic blend including the 00Z ECMWF/CMC/GFS (with a solution more toward the ECMWF/CMC for the Gulf of Mexico system) during days 3-5. During days 6-7, weighing of ensemble means (00Z ECENS/NAEFS) was boosted to account for increased uncertainty. ...Weather/Threats Highlights... Regardless of the final track that low pressure in the Gulf takes, portions of the Gulf Coast and Southeast will see the potential for a multi-day heavy rainfall event beginning this weekend and persisting into the middle of next week. Given the preferred ECMWF/CMC solutions, much of the Gulf Coast, lower Mississippi valley, and Southeast would see the potential for heavy rains spread across several days, with the heaviest rains potentially falling near the mouth of the Mississippi River. If the GFS solution were to verify, heavy rains would be much farther east, with a significantly reduced heavy rain threat for much of the Gulf Coast. A farther east scenario would also potentially direct the transport of higher moisture content air into the Mid-Atlantic, where interaction with a weak surface front would potentially result in heavy rainfall (as shown by the GEFS mean). Elsewhere, the upper low crossing the West could produce locally heavy rains across the Great Basin or portions of the northern Rockies, and the shortwave skirting the Great Lakes/Northeast Sat-Mon could produce scattered showers and thunderstorms with locally heavy rain possible. Temperatures are expected to be well above average across much of the central U.S. during the medium range. High temperature anomalies of +10 to +20 deg F are forecast Sat-Mon across much of the Plains, with the greatest anomalies from the northern plains to the Upper Midwest. A number of record high temperatures could be in jeopardy. Ryan 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