Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Mon Jun 29 2020 Valid 12Z Thu Jul 02 2020 - 12Z Mon Jul 06 2020 ...Overview and Guidance/Predictability Assessment... Latest guidance continues to show a late-week evolution to a fairly typical summer pattern that should last into next week. A Plains/southern Rockies/northern Mexico upper ridge and separate ridging over the Northeast Pacific/Alaska will position a mean trough over the eastern Pacific/West Coast. A departing short-range New England upper low/trough will be replaced by a somewhat broader eastern Canada/Northeast trough. Ridging initially over central/east-central Canada should gradually weaken and allow for increasing progression across southern Canada and the northern tier U.S. later in the period. This evolution should lead to flatter Northeast flow aloft by early next week. Farther south the guidance still suggests the development and persistence of a diffuse weakness/trough between the lower half of the Mississippi Valley and Southeast--a feature having low predictability for specifics due to its scale but still important due to its potential to support some areas of significant rainfall. The mean trough aloft near the West Coast is agreeable in principle but significant detail differences persist, due in part to short-range differences in how flow around an eastern Siberia upper low ejects across Alaska and beyond followed by how much ridging extends into Alaska. Guidance is mixed over whether an upper low may settle near Vancouver Island, though the 12Z CMC mean offered support for recent GFS/GEFS mean runs while multiple ECMWF runs also had such a low before the 12Z run changed to a having more of a weakness aloft over Alaska versus a ridge. The new 00Z ECMWF returned to showing more Alaska ridging. For the eastern Canada/Northeast trough, the 18Z GFS made a favorable adjustment from the 12Z run that had been on the amplified side of guidance. The new 00Z run has a reinforcing push of energy within the trough during the weekend but support from other guidance for its depth is tepid at the moment. The guidance average has adjusted a bit east/southeast for the weakness that settles over/near the Southeast, with individual models exhibiting typical spread and variability for such a diffuse feature. It will likely take a while for specifics to achieve a clearer focus. The updated forecast started with an operational model consensus during the first half of the period and then transitioned to an even model/ensemble mean weight by day 7 Mon. The ECMWF component was split between the 00Z/28 and 12Z runs as the former was closer to the majority over the Northeast Pacific and vicinity while the latter was reasonable over other areas. The end result was fairly good continuity for the large scale forecast. ...Weather/Hazard Highlights... The expected weakness aloft over or near the Southeast will support potential for areas of enhanced rainfall. However guidance is still in the process of trying to resolve this diffuse feature so confidence remains low for locating the most favored areas for highest totals between the Lower Mississippi Valley/central Gulf Coast region and Southeast/southern Mid-Atlantic coast. Trends over the past day have been a bit east/southeast for the upper feature and associated rainfall but further adjustments one way or the other appear likely. Some showers/thunderstorms are possible with the front reaching New England late this week as well as with one or more fronts over the northern tier, with mixed signals regarding intensity of rainfall. From late week onward expect scattered diurnally favored convection over the central/southern Rockies. The forecast pattern will promote a persistent area of warm temperatures up to 10-15F above normal centered over the Upper Mississippi Valley and Great Lakes. At times some of this warmth will extend back into the Northern Plains, while eastward extent into New England should be limited to early and late in the period--ahead of the front dropping south from Canada and then after its eventual return as a warm front. The rest of the Plains will start the period moderately above normal but then trend closer to normal. At the same time most of the West will trend toward near/slightly above normal readings after starting with single-digit negative anomalies for highs on Thu. The Pacific Northwest coast may remain slightly below normal due to proximity of the upper trough. Portions of the Southeast and vicinity may also be a little below normal due to periods of clouds/rainfall but the Florida Peninsula could see one or more days of slightly above normal readings. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml