Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Tue Jun 05 2018 Valid 12Z Fri Jun 08 2018 - 12Z Tue Jun 12 2018 ...Heat to continue for the Desert Southwest to the south-central U.S... ...Pattern overview... The primary fixture in the forecast will be a sprawling upper ridge spread across the Desert Southwest eastward to the mid-South. Rather impressive mid-level height anomalies on the order of 2 sigma above climatology will be fixed over this region on Day 3/Friday. Eventual northern erosion of this anticyclone is expected given robust troughing moving through the Pacific Northwest during the weekend. An embedded compact upper low or strong open wave will evolve out of this negative height anomaly with a position over the Upper Intermountain West on Sunday morning. This is where the mentioned upper ridge will partially erode although many solutions favor it rebuilding by early next week once the Pacific system lifts into central Canada. Across the East Coast, a broad weakness should linger over the Florida Peninsula through part of the weekend before gradually shearing out. Meanwhile, the mid-latitude flow significantly amplifies over the northeastern U.S. by Sunday onward which will act to solidify the eastern extent of a developing omega block regime. ...Model guidance/predictability evaluation/preferences... With the 12Z CMC being more aggressive with some of the "ridge rollers" traversing the north-central U.S., it displays a more suppressed solution. This has been an issue over the course of the previous couple of days. Otherwise, the GFS/ECMWF/UKMET model suites have been supportive of building heights over the weekend in advance of the amplified northwestern U.S. flow. As the upstream shortwaves are able to cross the Great Plains, there are a variety of solutions at hand with mesoscale differences quite abundant. the 12Z CMC stood out with a powerful upper low across western Kansas on 10/1200Z which it has since backed off on. Eventually the upper ridge should begin to expand its spatial coverage into early next week with some spread amongst the guidance. By Day 7/June 12, this positive height anomaly will likely situate from the Desert Southwest eastward into the Southern/Central Great Plains. There has become fairly widespread agreement in carrying a rather significant upper trough into the Pacific Northwest/northern California this weekend with the core of lower heights crossing western Montana by midday Sunday. This will draw a strengthening surface cyclone up along the Alberta/Saskatchewan border prompting a significant increase in warm advection across the Great Plains. Where models have struggled is with run-to-run continuity, particularly when considering the 00Z/18Z solutions with upper low positions either over southwestern Saskatchewan or extreme northern Alberta, respectively, on 12/0000Z. It is safe to say the 12Z GFS solution is equally problematic with height contours offering stark differences from the ensemble spaghetti plots. Over the eastern U.S., models offer a bit more certainty to the medium range pattern. Global guidance agree on maintaining a mid/upper-level weakness around Florida through Saturday before gradual erosion into a more randomly oriented array of vorticity centers. To the north, ensembles favor a sharpening in the upper trough as heights lower into New England on Sunday. While the 12Z CMC was within this grouping, its 00Z run shifted toward a grossly different solution which appears outlying. Additional shortwaves within the longwave trough will help encourage further amplification in the eastern U.S. flow, thus shifting the surface front down to the Carolinas by next Tuesday. With similar output from the 18Z GFS and 12Z ECMWF/UKMET through Day 5/Sunday, favored a three-way blend of such solutions with more weighting toward the 18Z GFS/12Z ECMWF. Thereafter, quickly shifted toward a greater influence of the ensemble means, 18Z GEFS/12Z ECMWF/NAEFS, into the picture given growing model uncertainties. While synoptic-scale differences do exist, there are plenty of mesoscale influences which will be impossible to resolve at this time-scale. ...Weather highlights and hazards... Given the extended influence of ridging from the Desert Southwestward eastward into the Southern Plains, expect above average temperatures to persist as readings of 105 to 110 degrees are likely over the California, Nevada, and Arizona deserts. However, the more impressive departures from climatology will focus across the Northern Rockies into the Northern/Central Plains with readings in the 10 to 15 degree range through Sunday. These numbers should come down a bit as a frontal zone sweeps through the region early next week. The cool spot in the nation will be along the West Coast, generally focused over the Pacific Northwest, as temperatures should be 5 to 10 degrees below climatology this weekend. This supports highs only in the upper 50s along western sections of Washington/Oregon. Along the East Coast, persistent troughing should allow numbers to stick a bit closer to average for early/mid-June. Multiple mesoscale complexes will likely evolve from "ridge rollers" tracking from the Northern Plains downstream into the Great Lakes/Ohio Valley region. Of course these will be difficult to predict given their mesoscale nature. A particular focus is likely along a meandering west-east boundary where better low-level convergence will overlap enhanced moisture transport. Wet conditions are expect this weekend across the eastern U.S. as stronger dynamics descend from eastern Canada. Farther south, conditions should remain intermittently wet across Florida, especially this weekend with the weakness lingering overhead. Out west, an anomalously strong upper trough will spread abundant rainfall to the Pacific Northwest, particularly across the Cascades. Such conditions should lift north-northeastward into western Canada as conditions dry out over the northwestern U.S. Rubin-Oster 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