Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Thu May 23 2019 Valid 12Z Sun May 26 2019 - 12Z Thu May 30 2019 ...Record/Dangerous Heat for the Southeast will continue into next week... ...Heavy rainfall possible for portions of the central Plains and mid-Mississippi Valley... ...Overview and Guidance Evaluation/Preferences... Upper pattern will only slowly trend toward a bit less amplification but otherwise maintain troughing in the west and ridging in the Southeast and an active northern stream through Canada. The models/ensembles have struggled to fully resolve the flow out of the Pacific/high latitudes this weekend which keeps shifting either (or both) the track or evolution of systems in the eastern half of the CONUS. Ensemble means have wavered on either slower/more coherent upper shortwaves or closed lows vs quicker/strung out impulses which may or may not loosely phase with northern stream flow over southern Canada. Best approach has been that of avoiding extremes (too quick/slow or weak/deep) given the lack of continuity over recent days. The 18Z GFS and 12Z ECMWF/Canadian offered better clustering near the ensemble means and formed much of the starting point. By next Wed/Thu only the ECMWF was even somewhat close to the favored pattern though the GFS/Canadian could not necessarily be discounted in totality. This resulted in a bit quicker progression of the system out of the west toward the Great Lakes Mon/Tue as well as with a northern clipper system Tue/Wed. One constant has been keeping some frontal boundary over the southern Plains through the period, reinforced by upstream height falls and kept from pushing east due to the strong upper high in the northeast Gulf. ...Weather Highlights and Hazards... The heat is on. Widespread temperatures in the upper 90s and a modest area of 100-103F will challenge monthly records in the Southeast (AL/GA/n FL/SC). Heat indices over 100F are likely for many areas under the upper high with no chance of rain due to the stable atmosphere. Variable heat may extend into the mid-Atlantic, perhaps with a push into the low/mid 90s up to DC next Wednesday. Still a large spread in ensemble forecasts (60s to 100F) but as the position of the front and intensity of lower level warmth is still uncertain. Farther north, Temperatures will trend toward average along the US/Canadian border where rain chances will be greater as well. Well-below average temperatures in the west will slowly ease by the end of the period (next Thu) but abundant cloud cover and modest rain (with some high elevation snow) will support some record low max temperatures. To the east, temperatures around the wavy boundary from the central Plains eastward will average closer to normal and 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 during the period seems to be centered from eastern Kansas/Oklahoma northeastward through Iowa on different days. 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