Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1151 PM EDT Tue May 29 2018 Valid 12Z Sat Jun 02 2018 - 12Z Wed Jun 06 2018 ...Record heat expected across the southern Plains... Guidance and uncertainty assessment ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Model and ensemble guidance offer a reasonably well clustered range of solutions in the day 3-4 (Saturday to Sunday) time frame. Guidance spread increases thereafter, with the 18z GFS becoming a notable outlier, so it could not be used beyond Sunday. The ECMWF idea of a stronger system moving through southern Canada seen on previous days is one most of the guidance now agrees with, so shifted our solution with that one system and its associated cold front quicker/stronger when compared to continuity. Elsewhere, continuity was able to be better maintained. Despite the 12z Canadian model being agreeable with the means in the mid-levels, its surface pattern was a bit less desirable, so it could not be used from Monday onward either. For Saturday and Sunday, the 12z Canadian, 12z UKMET, 12z ECMWF, and 18z GFS made for a good starting point. From Monday onward, the forecast was primarily derived from the 12z ECMWF and 12z NAEFS mean. The 12z ECMWF ensemble mean could be used as a reasonable alternative, but it was only partially available for use internally. For QPF, the plan is to use a four way blend of the 18z GFS, 12z ECMWF, 12z Canadian, and 00z National Blend of Models through the period with the GFS being used to account for some uncertainty in the forecast. Should the 00z GFS shift to a more favorable solution, it could be included into the mix. Weather Highlights and Threats ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Shortwaves reinforcing an upper trough in the East should set the stage for wet weather for the Mid-Atlantic and Southeast states from Saturday into next Wednesday. A lingering lead/wavy surface front over the from the southern plains through the southeastern U.S. into the western Atlantic will also provide a focus for deeper moisture pooling and some heavier local convective downpours. Upstream, a general trough over the Northwest composed of a few active shortwaves should generate some late season rainfall over northern parts of the West late this week and locally heavy convection over the Plains/Midwest into mid next week. Mean troughing aloft will tend to keep highs near to below normal near the West Coast and over the East. The dominant focus of the temperature forecast will be on a multi-day Texas heat wave expected to persist into Sunday whose influence may extend at times northward into the central Plains and eastward into the western Gulf coast. Hot afternoon readings in the 100-110F range should be most common in the lower Rio Grande, with Saturday setting up to be the hottest day. Record warm values for high and low temperatures will be reached/exceeded during this time frame. The southward suppression of the heat in Texas with time from Sunday onward occurs as a front settles southward and mid-level ridging over the area slowly weakens. Roth/Schichtel WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4