Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Thu May 13 2021 Valid 12Z Sun May 16 2021 - 12Z Thu May 20 2021 ...Heavy rainfall possible over parts of the Southern Plains into the Mississippi Valley from early through at least middle of next week... ...Overview... The majority of guidance continues to show the establishment of a western trough/eastern ridge upper pattern and persistent flow of Gulf moisture around East Coast surface high pressure, leading to a multi-day threat of significant rainfall over portions of the Southern Plains into Mississippi Valley. Locations and magnitude of heaviest rainfall may vary on a day-to-day basis and some specifics may take until well into the short range to be resolved, but there is a consistent signal for a wet pattern over the central U.S. Some variation of this pattern could extend beyond the medium range period based on the latest 8-14 Day forecast from the Climate Prediction Center. In addition to this area of focus, locations from the Central Plains/Midwest east-southeastward into the Mid-Atlantic may see one or more convective systems aided by a wavy front and shortwave energy aloft, and improving consensus for the upper low tracking through the Southwest toward the Plains is suggesting greater potential that the central Rockies/High Plains may see a period of enhanced precipitation. An upstream trough reaching the Northwest will likely bring unsettled weather to that region but with lighter totals. ...Guidance Evaluation and Preferences... The preferred consensus forecast maintains good continuity with only minor detail adjustments, though individual solutions continue to show some divergence and run-to-run changes. The northeastern Pacific upper trough reaching the Northwest had been trending toward better clustering after a period of significant spread, but the 12Z ECMWF suddenly adjusted slower to resemble some GFS runs from a couple days ago. After day 4 Monday a composite of latest GEFS/ECMWF means and GFS runs, and old 00Z/12 ECMWF represented the established consensus. Among the new 00Z runs, the UKMET becomes somewhat strong/fast but the GFS/CMC and GEFS mean continue to provide an intermediate cluster. Meanwhile the models agree fairly well for the upper low tracking through the Southwest into the southern Rockies early-mid period. As this feature likely opens up as it reaches the Plains, some spread develops due in part to differences with the upstream trough. Farther east there are still notable differences for what becomes of shortwave energy that progresses from the Northern Plains/southern Canada. Among the possibilities include the Plains energy lingering for a time over the Midwest as an upper ridge passes by to the north, while the new 00Z UKMET uses Canadian energy to form a Great Lakes upper low (another aspect seen in some GFS runs from a couple days ago and then abandoned). Ensemble means and remaining models wait until this energy reaches New England and the Canadian Maritimes to amplify. The 12Z ECMWF is underdone with New England troughing relative to the means. If/where an upper low forms is still up for debate, with CMC runs dropping a low just off the East Coast while other solutions have been well eastward or northward. Guidance comparisons led to following an operational model blend early, replacing the 12Z ECMWF with the prior 00Z run by day 5 Tuesday, and then transitioning to a model/mean mix for balancing the consensus pattern with some operational detail where possible. In most respects the new 00Z ECMWF has come in with a better comparison to consensus/preference. ...Weather Highlights/Hazards... The overall forecast is consistent in highlighting the potential for a multi-day wet pattern over the central U.S., most likely over parts of the Southern Plains into the Mississippi Valley, as persistent low level flow from the Gulf transports moisture into the region. Areas of heavy rainfall may be somewhat more organized/focused around Tuesday-Wednesday as the upper system crossing the Southwest approaches and reaches the Plains. Some pockets of meaningful rainfall should continue into Thursday but perhaps with less focus depending on how the energy from the Southwest system weakens/progresses. Parts of this area have seen much above normal precipitation over the past several weeks, which may increase flooding concerns. Meanwhile the guidance signal appears to be strengthening for a period of enhanced precipitation over the central Rockies/High Plains during the first part of the week, from the combination of low level upslope flow and favorable location relative to the Southwest upper low's track. The trailing upper trough reaching the Northwest should bring mostly light to moderate precipitation to the region from about Monday night onward. Highest elevations could see some snow. A wavy front and initial Northern Plains shortwave energy may promote one or more convective systems that could produce locally heavy rainfall from the Central Plains/Midwest east-southeastward. However there is still considerable uncertainty over the magnitude and location of rainfall within this axis. On Sunday parts of the Northeast may see one more episode of mostly light mid-late day rainfall under a departing upper trough. Ahead of the Pacific front that moves into the West next week, an area of highs 10-20F above normal will progress from the Northwest U.S. into the northern High Plains and eastward near the Canadian border. The cooling trend behind the front should bring modestly below normal highs. Meanwhile moderately below normal highs will accompany the system initially tracking through the southwestern states and then weakening as it ejects into the Plains. Most of the East will see slightly below normal readings on Sunday followed by a gradual warming trend to near or slightly above normal temperatures. 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