Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Mon May 28 2018 Valid 12Z Thu May 31 2018 - 12Z Mon Jun 04 2018 ...Record heat expected across the southern Plains & western Gulf Coast... ...Post-Tropical Cyclone Alberto tracks from the Great Lakes into Canada... ...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... Already in the first half of the period there are some continuity adjustments/spread with the combination of ejecting West Coast trough energy and incoming flow that amplifies toward the West Coast. There is a strong signal from the 00Z/06Z models/means that the leading feature will track inland faster than previously forecast. A compromise among the latest available operational runs (06Z GFS and 00Z ECMWF/UKMET/CMC) reflects this scenario without getting too far on the faster side of the envelope that is represented by the 00Z GFS/ECMWF. Given the lack of consistency in the guidance over the past couple days, a similar approach provides the best risk-managed forecast for the incoming eastern Pacific trough--between the deeper 00Z GFS/CMC and flatter 06Z GFS/00Z ECMWF. During the latter half of the forecast expect upper troughing to deepen somewhat as it slowly settles over or just inland from the West Coast, with a model/ensemble mean blend representing consensus well. By day 7 Mon the 06Z GFS becomes the fast extreme with its embedded upper low. Farther east the main question marks will be to what extent and where eastern U.S. trough energy forms a closed low and then how the ejecting western shortwave may interact with the initial energy over the East. Thus far the GFS/CMC are more aggressive with a closed/westward feature by the weekend and also allow the upstream energy to hold the overall feature fairly far westward into early next week. Teleconnection-favored flow suggests the GFS cluster could be a little too far west with the overall mean trough but the blocky pattern could lead to a slower/less open evolution than advertised by the ECMWF/ECMWF mean. Thus an even blend among the two clusters would appear best at this time. ...Weather Highlights and Threats Some combination of Post-Tropical Cyclone Alberto's energy aloft and an approaching shortwave will likely drop southeast into the Mid-Atlantic States, setting the stage for potential heavy rainfall over parts of the Mid-Atlantic region and vicinity. Uncertainty over whether and where upper flow may close off a low tempers confidence in location/extent of any heavy rain. Abundant moisture along/ahead of a frontal boundary will also provide a focus for some episodes of locally heavy rainfall over central/southern latitudes of the South and East. A shortwave ejecting from the West Coast should generate some rainfall over northern parts of the West late this week and locally heavy convection over the Plains/MS Valley Fri into the weekend. This energy aloft may play a role in rainfall distribution/intensity over the East toward Sun-Mon. The upstream trough arriving at the West Coast should bring some areas of precip to the Northwest. The dominant focus of the temperature forecast will be on a multi-day southern Plains heat event whose influence may extend at times westward into the Rockies and eastward into the lower MS Valley. 100-110F readings should be most common in the southern High Plains/lower Rio Grande. 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. 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. Rausch/Roth 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