Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Tue May 21 2019 Valid 12Z Fri May 24 2019 - 12Z Tue May 28 2019 ...Record/Dangerous Heat for the Southeast this weekend into next week... ...Heavy rainfall possible for portions of the central Plains/Corn Belt... ...1600 UTC Update... The latest forecast is generally a refinement of continuity, with a lead system ejecting from the northern High Plains and a deep upper low dropping southward along the West Coast before ejecting inland to produce another significant Plains system early-mid week. An operational model consensus still looks good through day 5 Sun. Then models/ensembles diverge regarding how much flow cuts into the eastern Pacific ridge aloft--in turn affecting progression of the western U.S. trough/embedded upper low into the Plains. At the moment the GFS/GEFS runs are much more aggressive than most other guidance to bring height falls into the ridge and thus become faster to eject the western trough. Given the pattern amplitude, farther westward ensemble mean trend for the upper trough/low into day 5 Sun (compared to 1-2 days ago), and typical GFS/GEFS biases, preference leaned 70 percent toward the ECMWF/ECMWF mean/CMC scenario by days 6-7 Mon-Tue. It is possible for energy on the western side of the overall trough to help eject the feature more quickly than the slowest solutions so partial inclusion of the GFS/GEFS idea is warranted at least over the CONUS though maybe less so over the eastern Pacific. Fortunately into early day 7 Tue surface differences over the central U.S. are not quite as dramatic as the spread aloft. At that time the model/mean blend yielded a slightly slower trend for the central Plains surface low. One other trend of note is for greater amplitude of energy crossing the Northeast next Mon-Tue. This led to a southward adjustment for the associated surface front over the East. Rausch Previous Discussion issued at 0632 UTC... ...Overview... Highly amplified upper level pattern will be dominated by deep troughing in the west and strong ridging in the Southeast. This is forecast to largely hold in place as reinforcing height falls sink into California on Saturday as the lead upper low lifts through the High Plains. The Northeast/Mid-Atlantic will lie in the confluence between the northern and southern stream as a wavy boundary oscillates north/south. The pattern favors well-below average temperatures in the west with rounds of precipitation but hot/dry conditions in the Southeast with record temperatures near 100F. In between, broad southwesterly flow aloft and south to southeasterly flow at the surface will favor rounds of convection and locally heavy rainfall with multi-inch totals over the period especially over the central Plains/southwestern Corn Belt. ...Guidance Evaluation and Preferences... Latest guidance proved to be a good consensus/starting point for the forecast, despite their continued differences right from Fri/D3 with the system exiting through the High Plains. Opted to rely entirely on the deterministic models to start given a good clustering of solutions though the ECMWF ensembles were still notably quicker. Incorporated more ensemble guidance by next Mon/Tue as the track of the upper low in the west loses focus as well as the strength of the upper high in the Southeast (ECMWF-led guidance was generally stronger than the GFS-led cluster). 18Z GFS and 12Z ECMWF offered some good details with the western system next week but were both quicker than their ensemble means by next Tue/D7 over IA/MN vs Nebraska. Ensemble trend has been much slower so favored the means by the end of the period. ...Weather Highlights and Hazards... Summertime heat will become increasingly intense in the Southeast though at least humidity levels may be only moderate (dew points in the 60s F) which would allow overnight lows to fall to the upper 60s to low 70s but would support heat indices of 100-105F during the day. Temperatures will approach daily and quite possibly monthly records in the 90s to low 100s over portions of northern Florida and Alabama/Georgia/South Carolina. ECMWF-led guidance has been quite hot and several degrees above the record hottest May temperatures for many locations ever observed, but seemed too hot (105+) given the time of year and climatological history. Regardless, precautions will need to be taken as successive hot days repeat. The west, by contrast, will see well-below average temperatures (about 15-25F below typical values) with abundant cloud cover and modest rain with high elevation snow. Temperatures around the wavy boundary from the central Plains eastward will rise/fall as the front lifts northward or slips southward. This will also provide a focus for rainfall and some convection. Rounds of rainfall with locally heavy amounts may add up to several inches over the period, some of which may fall within a short amount of time over areas that have seen quite a bit of rainfall recently. Greatest flooding/flash flooding risk seems to be centered over eastern Kansas northeastward through Iowa, but with a notable north-south spread in the ensembles. Fracasso Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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