Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Tue May 11 2021 Valid 12Z Fri May 14 2021 - 12Z Tue May 18 2021 ...Overview... The best clustering of guidance shows an evolution toward some degree of mean troughing over the West while mean ridging progresses across the central U.S. into the East behind a broad upper trough that departs from the East Coast this weekend. There is a small minority that trends out of phase with this idea by late in the period though. As the most likely pattern develops, expect a gradual increase in coverage and intensity of rainfall over the central U.S. with time. Some rain/thunderstorms could eventually extend into parts of the East while parts of the Rockies may see some precipitation as well. Contrasting temperature anomalies late this week with warmth over the West and cooler than normal readings to the east of the Rockies should generally moderate with time, leaving many areas fairly close to normal by early next week. ...Guidance Evaluation and Preferences... Aside from the more open UKMET, 12Z/18Z guidance was showing decent agreement with forming an upper low over/near California from the shortwave energy nearing the West Coast as of early Friday. The upper low would then eject into the southern Rockies or High Plains by early next Tuesday. However there has been a fairly wide spread for what becomes of separate northern stream shortwave energy. New 00Z runs decrease confidence in specifics with the UKMET holding onto an open trough and the new ECMWF changing to a more open shortwave as well. As was seen 24 hours ago, significant divergences develop for the large scale pattern by late in the period. Although the GFS appears to have fixed some of its prior issues with its western/northern Pacific forecast, by the latter half of the period it still becomes slower and more amplified with flow from the eastern Pacific into North America. This leads the GFS to show a ridge over the West by early next week while developing a fairly deep closed low over or near the Great Lakes. In contrast most remaining guidance shows some degree of cyclonic flow arriving into the Northwest U.S. while above normal heights/ridging prevail over the East. Latest GEFS means reflect a compromise, being a little slower than the ECMWF/CMC and their means with troughing that heads toward the Northwest while depicting only a shortwave reaching the Great Lakes and vicinity. Most D+8 multi-day means have recently been signaling a trough-ridge pattern, so preference continues to trend the initial operational model blend more toward the ECMWF/CMC and the ECMWF mean/GEFS mean by the latter half of the period. ...Weather Highlights/Hazards... Expect central U.S. rainfall to focus initially over parts of the Central Plains and Midwest ahead of a wavy warm front. With time, rain and thunderstorms should expand southward and become heavier as return flow behind eastern U.S. high pressure brings in more Gulf moisture and western U.S. shortwave energy/possible upper low approaches the Plains. Some of the activity over the Midwest could eventually reach east-southeast into the eastern states as the focusing warm front extends farther eastward. There may be some enhancement of precipitation near the central Rockies/High Plains late in the period depending on the evolution of the feature emerging from the West. In most cases any precipitation to the west of the Rockies should be fairly light and scattered. Meanwhile the upper trough moving through the East late this week into the weekend should promote areas of mostly light rainfall from the Appalachians into the Northeast. Well above normal temperatures centered over the Great Basin and vicinity will extend into Friday with plus 10-20F anomalies for highs. Then an upper trough/potential closed low should promote a cooling trend over the southern half or so of the West, with parts of the far southwestern U.S. possibly seeing a day or two with slightly below normal highs. A trailing upper ridge may promote a brief rebound over the Interior Northwest on Sunday followed by cooling trend and southward suppression of remaining above normal readings as cyclonic flow aloft approaches/arrives. Note that a minority scenario could have a stronger ridge persisting over the West with warmer temperatures. Over the eastern half of the country expect below normal temperatures to prevail over many areas late this week. Locations near the southeastern coast could see highs up to 10F or so below normal on Friday. Then a moderating trend will likely yield near normal highs and slightly above normal lows over the eastern half of the lower 48 by early next week. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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