Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Sun Jun 24 2018 Valid 12Z Wed Jun 27 2018 - 12Z Sun Jul 01 2018 ...Overview and guidance/predictability assessment... Latest guidance continues to show the mean pattern transitioning toward an amplified western ridge/eastern trough configuration. A notable trend over the past day has been toward greater northward amplitude of the eastern ridge by next weekend, with the center of the embedded upper high possibly drifting at least as far north as 40N latitude. This evolution would direct highest temperature anomalies originating from the central Plains/mid-upper MS Valley into the Northeast. The primary uncertainty with the western trough aloft is for its depth and the details of individual shortwaves. The latter will affect the forecast for what may be a convectively active period across the Northern Tier states. For the western trough, GFS/GEFS runs have been a bit on the stronger and deeper side of the spectrum but worthy of partial inclusion in the forecast given the strength of the upper ridge building over the eastern Pacific. Meanwhile a greater proportion of ECMWF/CMC runs and their ensembles suggest a somewhat more shallow overall trough due to faster ejection of leading energy in response to upstream flow--which is also plausible within the context of a multi-day mean trough. However recent ECMWF runs have been inconsistent with the evolution of trough energy by next weekend. In new 00Z guidance thus far the GFS has trended toward a 12Z ECMWF type of ejection late in the period albeit with differing upstream details, while the CMC maintains a broader mean trough on most days. Potential merit for varying clusters and GFS/ECMWF run to run variability favor a compromise approach for a single deterministic forecast. Farther east the best-behaved part of the forecast is for the system expected to be over the Great Lakes as of day 3 Wed. Guidance over recent days has tended to show moderate spread with a consensus approach maintaining good continuity. In the wake of this feature, as previously noted the recent trend in most guidance is for eastern ridging to extend farther northward. By day 7 Sun the past couple ECMWF runs show the farthest north position for the upper high, reaching New England. Teleconnections relative to the upstream pattern offer potential for somewhat flatter flow than advertised by the strongest/northern solutions with the eastern ridge, leaving a model/mean blend as a reasonable starting point late in the period. Guidance is suggesting development of a weakness aloft over the Southeast/Florida with possible surface reflection in the form of a trough. Predictability for specifics appears to be below average given diffuse nature of the feature. A blend among the 18Z GFS and 12Z ECMWF/UKMET/CMC represented consensus well during the first half of the period while guidance spread/variability favored a trend toward a more even model/ensemble weight by the weekend. The 18Z GEFS/12Z ECMWF mean provided the ensemble input for days 6-7 Sat-Sun while operational ECMWF variability led to including a small component of its past two runs by that time frame. The blend held onto modest weight of 18Z GFS/12Z CMC runs as well. ...Weather threats/highlights... The system initially over the Great Lakes will likely support some areas of moderate to heavy rainfall early in the period, most likely from eastern Great Lakes/upper Ohio Valley into western-northern New England. As the upper pattern becomes more amplified and a mean frontal boundary/low pressure persist over the Plains and vicinity, there will be the potential for multiple convective episodes and accompanying heavy rainfall from over or just east of the northern Rockies into the upper MS Valley/upper Great Lakes. Specifics will depend on less predictable details of shortwaves ejecting from the western trough aloft. Currently the best signal for highest 5-day rainfall totals exists over the upper MS Valley/upper Great Lakes. With lighter amounts, the western upper trough will bring scattered rainfall to other areas within the northern half of the region. The Southeast/Florida will see areas of locally heavy convection during the period. Enhancement to rainfall may come from a weakening front early in the period and then the developing upper weakness/surface trough. An expanding area of hot/humid conditions will likely cover a large portion of the central-eastern U.S. by late this week and weekend. Currently expect the axis of highest min/max temperature anomalies of plus 10-20F to extend from the central Plains through the Great Lakes/Ohio Valley and into the Northeast. Any daily records appear to be more likely for warm lows than daytime highs. Increased low-level moisture will also lead to very high heat index values over many areas. On the other hand the upper trough amplifying into the West will bring a pronounced cooling trend. Locations in the northern Rockies/High Plains should see highs as much as 10-15F below normal by Fri-Sun with less extreme anomalies elsewhere. Rausch WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities are 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