Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Fri Jun 22 2018 Valid 12Z Mon Jun 25 2018 - 12Z Fri Jun 29 2018 ...Overview and guidance/predictability assessment... The majority of model and ensemble guidance agrees fairly well with the large scale evolution during next week. Expect upper ridging initially over lower latitudes of the contiguous U.S. to expand its influence over the eastern half of the country with time, first with the departure of a Northeast/Mid Atlantic trough and then behind an upper low/trough tracking northeastward from the High Plains. A vigorous system entering western Canada will continue eastward while a ridge building over the eastern Pacific by the latter half of the week will support an upper trough that develops along or more likely a bit inland from the West Coast. For the days 3-5 Mon-Wed period an operational model blend (18Z GFS/12Z ECMWF and somewhat lesser weight of the 12Z UKMET/12Z CMC) either represented consensus or provided a good intermediate solution in the case of more pronounced differences. The latter situation applied to the departing East Coast trough. The 12Z ECMWF and many of its ensemble members showed a faster departure than other guidance while the GFS had adjusted to the slower side of the spread. Both models have displayed some run to run variability so a compromise appeared best. New 00Z runs thus far seem to support that idea with the GFS trending faster than its 12Z/18Z runs but the GFS/UKMET/CMC remaining slower than the 12Z ECMWF to varying degrees. Guidance behavior and spread holds confidence down to moderate levels for this feature. Differences for the system tracking from the Plains into the Great Lakes and upstream southern Canada low pressure/front trailing into the northern Plains-western U.S. were generally within typical guidance error for days 3-5 forecasts. By days 6-7 Thu-Fri the forecast incorporated 30-60 percent ensemble guidance by way of the 18Z GEFS/12Z ECMWF mean as detail uncertainty increased, though the operational runs were close enough to the means to maintain some weight through the end of the forecast. The 12Z CMC was phased out after Thu due to developing a stronger than consensus eastern Canada/Great Lakes upper trough--a trait that has carried into its new 00Z run. Regarding the developing western trough, recent ECMWF runs have tended to show the axis near the West Coast but teleconnections relative to the upstream ridge's positive height anomaly center suggest the trough axis could end up a little farther eastward. There are still shortwave detail questions that lower confidence in specifics within the more agreeable mean trough. ...Weather threats/highlights... An area extending from the northern-central Plains into the western Great Lakes will be one focus for heavy rainfall during the period. The majority of convection will be supported by the system tracking from the Plains into Great Lakes Mon-Wed. Some moisture will eventually extend into the East but rain should trend lighter as upper dynamics weaken/lift farther north. The front moving into the West early in the week may bring light/scattered precipitation to extreme northern parts of the West and then some light-moderate rainfall over the northern Plains. As this front stalls, it may become a more prominent focus for convection over the northern tier states toward the end of the week. This latter potential episode will have to be monitored for any overlap with locations receiving significant rainfall earlier in the week. Meanwhile some areas over the Southeast/Florida may see one or more days of locally heavy rainfall in diurnally favored convection. A cold front dropping into the Southeast and eventually weakening will likely enhance showers/thunderstorms at times. After eastern U.S. surface high pressure departs into the Atlantic after Tue, the expected pattern evolution at the surface and aloft should promote an expanding area of heat/humidity over the central/eastern states. Greatest anomalies for min/max temperatures (plus 10-15F) by Thu-Fri should extend from parts of the Central Plains through the MS Valley into portions of the Great Lakes/OH Valley. Dew points reaching well into the 70's F over some areas may contribute to high heat index values. Another area of persistent warmth albeit with lower humidity should be over the Great Basin/Four Corners region, with min temps tending to be somewhat more above normal than maxes. The initial High Plains system will lead to cool daytime highs on Mon. The Northwest will see near to below normal highs behind the early week front and then with upper troughing moving in late in the week. 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