Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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... An upper-level trough forecast to exit the southern Rockies will likely open the door for a persistent stream of moisture from the Gulf of Mexico northward into much of the southern and central Plains, leading to a threat of significant rainfall for these areas through the medium-range period. Locations and magnitude of heaviest rainfall will likely vary on a day-to-day basis and some specifics may take until well into the short range to be resolved, but the model signal for a wet pattern over the central U.S. has been quite consistent. 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 Midwest eastward into the Mid-Atlantic may see one or more convective systems aided by a wavy front and shortwave energy aloft. An upstream trough reaching the Northwest will likely bring unsettled weather to that region but with lighter totals. ...Guidance Evaluation and Preferences... Latest model and ensemble solutions generally show good agreement on the above-mentioned synoptic settings, and have slowed down the east/northeastward progression of the upper trough across the central U.S. midweek next week. The GFS continues to depict a broader trough configuration in contrast with the more compact configuration depicted by the ECMWF and CMC. A slight preference was given to the ECMWF/EC mean solutions. A consensus of these models yielded a stationary boundary extending across the mid-section of the country toward the Mid-Atlantic region. The boundary appears to weaken with time by midweek as it begins to lift northward as a warm front. Nevertheless, convection appears to remain most active south of this boundary over the southern Plains into the latter part of next week. Over the Pacific Northwest, an upper trough from the Pacific is forecast to approach the region by next Wednesday to Thursday, thereby introducing higher degrees of uncertainty although the associated precipitation amounts appear to be modest. The latest (12Z) ECMWF now depicts a much amplified trough digging into the West Coast during this period. Another feature of note is the potential for trough fracture or cut-off low formation over New England midweek next week. The GFS is more progressive as opposed to the deeper solution of ECMWF and the most aggressive CMC solution. The WPC medium-range forecast package was based on the consensus of the 00Z ECMWF/00Z EC mean, the 06Z GFS/GEFS mean, as well as some contribution from the 00Z CMC and CMC mean. This yielded a solution close to WPC continuity. ...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. Kong/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