Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Thu Jun 07 2018 Valid 12Z Sun Jun 10 2018 - 12Z Thu Jun 14 2018 ...Pattern overview... At the end of the upcoming weekend, a robust upper trough will be in the process of moving across the northwestern U.S. with 500-mb height anomalies in the 2 to 2.5 sigma range. This will ultimately form the western extent of a high-latitude omega block. Gradually this negative height anomaly should lift across the Alberta border where a significant upper low will likely drift eastward during the following days. Its progress could nearly come to a halt by the middle of next week as a possible rex block sets up with a closed high pinching off north of Hudson Bay. Elsewhere, the other side of the omega block signature will allow the persistence of upper troughing across far eastern Canada down into New England. Eventually the forward progression of the synoptic-scale flow will allow this mentioned upper trough to slowly exit into the western Atlantic by the end of the period, Day 6/7, June 13/14. Considering latitudes south of 40 degrees north, an upper ridge should situate over the Desert Southwest/Four Corners region early on. Gradual eastward expansion of this anticyclone is likely to occur during the days ahead as the prevailing westerlies remain along the international border with Canada. By Day 7/June 14, the eastern extent of this upper ridge may encompass areas of the Lower Mississippi Valley. ...Model guidance/predictability evaluation/preferences... Beginning with the northwestern U.S. trough, multi-day ensemble spaghetti plot comparisons depict an eastward trend, particularly during the last 24 hours of model runs. Overall, global models and their ensembles are fairly clustered with previously divergent operational runs no longer evident. Where the models continue to struggle is just beyond Day 5/June 12 as the upper low begins its slow eastward march across south-central Canada. It is at this point where the guidance exhibits poor run-to-run continuity as evidenced by the latest 00Z GFS suggesting a completely different pattern from its previous run. As a whole, the ensemble spaghetti plots would at least suggest the idea of a closed low undercutting the downstream ridge as it is forced north of Hudson Bay. This ultimately either allows for the persistence of the high-latitude omega block or even forms a rex block across the region. Suffice to say, an ensemble-based approach is necessary by the middle part of next week. Given the downstream impacts on the flow across eastern Canada, many of the mentioned challenges will also apply across eastern North America as well. While longwave troughing is primed to impact the region through mid-week, the details amongst the guidance are somewhat uncertain, particularly given the timing differences between ensemble members. Things are a bit more clear-cut across the more southern latitudes of the nation as a broad upper ridge returns to the Desert Southwest by as early as Monday. As the active mid-latitude flow becomes more restricted to the northern tier, expect further expansion of the anticyclone toward the Southern/Central Plains, particularly shown by the 12Z ECMWF. The 00Z/12Z CMC/UKMET also support modest height rises across this region while the previous few GFS runs have been more subdued. One further issue the GFS/GEFS runs have been plagued by is the renewed tropical activity across the eastern Gulf of Mexico by Day 7/June 14. The 00Z/12Z CMC show a tropical disturbance approximately a day later but this model can be a bit too pro-active in development. Based on the WPC-NHC medium range coordination call, these scenarios will remain downplayed. Regarding preferences, favored a general combination of the 18Z/12Z GFS, 12Z ECMWF/CMC through Tuesday with subtle additions of the 18Z GEFS/12Z ECMWF ensemble means to the picture. Beyond this period, significant uncertainties led to more rapid inclusion of the mentioned ensemble solutions before going 50/50 between the two means by the end of the period. Would say confidence is below average during the June 13-14 timeframe. ...Weather highlights and hazards... Rather cool temperatures can be expected across much of the western third of the country to conclude the weekend. In response to the deep upper trough, readings should be around 10 to 15 degrees below climatology across areas of the Pacific Northwest into the Central Great Basin and Upper Intermountain West. Such anomalies will reduce in magnitude and gradually advance into the Northern Rockies by Monday before returning to near average thereafter. While not as anomalous in nature, the other cool spot will be the East Coast as numbers stay very near climatology through Tuesday. Otherwise, expect the rest of the country to be on the warm side, generally in advance of the slow-moving central U.S. cold front as well as under the expanding influence of the upper ridge. Within the former regime, temperatures well into the low/mid 90s are likely across the Northern/Central Plains on Sunday as numbers move into the 15 to 20 degree above average range. The true hot spot in the country will be in its usual location, the Desert Southwest as highs range from 105 to 110 degree each day. Active convection should generally accompany the southward moving front across the lower mid-Atlantic/Carolinas back toward the Ohio Valley with the threat lingering into early next week across the southeastern U.S. Conditions should remain unsettled across much of the Gulf Coast up toward the mid-South and Carolinas with the synoptic boundary lingering before shearing out by late in the period. Looking toward the middle of the country, showers and thunderstorms will be likely along the slow eastward moving cold front. Models vary in forecast amounts but perhaps slightly heavier amounts may be possible over the Central Plains by mid-week as the southern extent of the boundary stalls. There will also be a lingering threat for precipitation across the Pacific Northwest/Upper Intermountain West early on before clearing out once heights begin to rise. More precipitation may be on its way by Wednesday if the 12Z ECMWF pans out. 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