Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Mon May 10 2021 Valid 12Z Thu May 13 2021 - 12Z Mon May 17 2021 ...Overview... Latest guidance continues to show moderate progression to flow aloft across the lower 48 and vicinity. Aside from fairly minor details, there is reasonable consensus for a broad mean trough to cross the eastern U.S. and emerge into the Atlantic while a trailing ridge over the West as of Thursday should reach into the Mississippi Valley by next Monday. There has been more spread and variability for details of upstream Pacific energy that should amplify into an upper trough that reaches/crosses the West during the weekend, as well as for details of flow behind this feature. Aside from a front dropping through the Gulf and Florida Peninsula late this week, waves/fronts will primarily affect the western/central U.S. as high pressure prevails over the East. Thus the best focus for precipitation during the period will likely extend from the northern Rockies through a majority of the Plains and into the Middle/Upper Mississippi Valley. Contrasting temperatures on Thursday, with parts of the West quite warm and the South/East much cooler than normal, should steadily gravitate toward climatology with time. ...Guidance Evaluation and Preferences... With current model runs still showing minimal surface response to the upper trough crossing the East aside from waviness well offshore, the dominant forecast difficulties involve the shortwave energy expected to move into the West during the weekend and upstream flow that will affect the pattern behind this energy. After a couple days of increased spread for where/when features could try to separate, 12Z/18Z model/ensemble runs (and followed up in the new 00Z runs) have returned to some variation of 2-3 days ago when there was better clustering toward the idea that any separation that occurs would wait until energy reaches near the West Coast. There are still significant differences for what individual pieces of energy will do though, and predictability is low due to their small scale. A blended approach (more models than means into day 5 Saturday followed by a more even mix by day 7 Monday) continues to provide the best way to navigate the spread and variability that the models/ensembles have exhibited so far. Farther west, recent GFS runs trend out of phase with North Pacific flow from the weekend onward--a result of western Pacific flow separation which other guidance does not depict. In contrast to the flat flow to ridging that the GFS shows near the West Coast by next Monday, ECMWF/CMC runs and their ensemble means have been fairly consistent so far in bringing another trough toward the West--albeit with detail and amplitude differences. Latest GEFS means fall between these two extremes, with a weaker and delayed trough approaching the West. Current preference is to tilt the forecast somewhat more in the direction of the ECMWF cluster relative to the GFS/GEFS. ...Weather Highlights/Hazards... Expect a gradual increase in coverage and amounts for rain and thunderstorms over the central U.S. and northern Rockies from late this week into early next week. Initial activity will be focused mostly by weak northern stream shortwave impulses and there should be a relative lack of deep moisture. Then Gulf moisture should begin to contribute more and interact with one or more waves/fronts as eastern U.S. high pressure nears the East Coast. With uncertainty in the specifics, shortwave energy emerging from the West late in the period may provide another ingredient in the rainfall recipe. Currently expect the highest five-day totals over portions of the Central/Southern Plains and Middle/Upper Mississippi Valley. Some precipitation could be in the form of snow over highest elevations of the northern Rockies. Any precipitation farther west should be fairly light and scattered. The upper trough crossing the East late this week may produce some areas of mostly light rainfall over central/northern latitudes, with greater coverage during the latter half of each day. Locations near the Southeast coast and over the Florida Peninsula could see somewhat more organized rainfall for a time before a drier trend, as a wavy front initially over the Gulf and Florida Peninsula pushes southward. On Thursday the southern tier will see well below normal highs with minus 10-20F anomalies over parts of the Southeast and minus 10-15F anomalies over parts of Texas. Some locations in the Southeast could challenge daily records for cool highs. Less extreme below normal readings should prevail late this week over the remaining majority of the central/eastern U.S. Meanwhile temperatures are likely to be up to 10-20F above normal over areas from Oregon/California into the Great Basin late this week with more moderate anomalies stretching into the central Rockies/High Plains late this week into the weekend. During the weekend upper troughing that moves into the West should bring temperatures back down toward normal across the region from west to east. The moderating trends for both cool and warm areas from late this week may yield highs within a few degrees on either side of normal over much of the lower 48 by next Monday. 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