Extended Forecast Discussion NWS Weather Prediction Center College Park MD 946 PM EDT Mon May 28 2018 Valid 12Z Fri Jun 01 2018 - 12Z Tue Jun 05 2018 ...Record heat expected across the southern Plains & western Gulf Coast... Overview ~~~~~~~~ The expected trough-ridge-trough mean pattern compares favorably to that expected from teleconnections relative to the core of positive height anomalies which multi-day means show over the northern Pacific near 150-160W longitude. This provides some degree of confidence in the large scale forecast but spread/variability for individual features within the mean flow suggest a fair amount of uncertainty in the specifics. Meanwhile relatively strong above normal height anomalies persisting across the North Atlantic and Southeast Canada through the period will correspond to blocky flow with the pattern possibly taking on the shape of a Rex Block for a time across southeast Canada and the East. Guidance Evaluation/Preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The guidance shows better agreement than seen this time yesterday. The main issue tonight is how quick/strong a pair of shortwaves or closed low will be as they track across south-central Canada Sunday and the Northwest on Monday. In both cases, the 12z ECMWF was a strong/quick outlier. The system moving through south-central Canada is likely to be weak as it moves over the top of ridging in southeast Canada and in the Northwest it flies in the face of its own ensemble mean, so used it minimally beyond Sunday morning. A compromise of the 12z Canadian, 12z ECMWF, 18z GFS, and 12z UKMET proved useful for the pressures, 500 hPa heights, and winds for Friday into Sunday. Thereafter, use an increasing percentage of the 12z ECMWF/12z NAEFS ensemble means and rely somewhat less on the 12z ECMWF than the 12z Canadian and 18z GFS deterministicly. For the remainder of the weather grids, used a more even blend of the deterministic and ensemble mean guidance. An early draft of the days 4-7 QPF uses a four way blend of the 12z ECMWF, 12z UKMET, 18z GFS, and 00z National Blend of Models as a starting point. If the 00z GFS proves useful, it could be included into the mix. Weather Highlights and Threats ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ An upper low/trough will likely drop southeast into the Mid-Atlantic States and Carolinas, setting the stage for wet weather for the Mid-Atlantic and Southeast states. 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 northern and central Plains. This vorticity aloft may play a role in rainfall distribution/intensity over the East early next week. The dominant focus of the temperature forecast will be on a multi-day southern Plains heat wave whose influence may extend at times westward into the Rockies and eastward into the lower MS Valley and western Gulf coast. Hot afternoon readings in the 100-110F range should be most common in the southern High Plains/lower Rio Grande, with Saturday setting up to be the hottest day. It is likely that record values for max/warm min will be reached/exceeded on multiple days, with some of the record-breaking heat also extending for a time farther north/northeast. There should be some southward suppression of the heat in Texas with time from Sunday into Monday as a front settles southward, though it should rebound by Tuesday. On the other hand, mean troughing aloft will tend to keep highs near to below normal near the West Coast and by the weekend/early next week over the East. Roth/Rausch 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