Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sat Jun 29 2019 Valid 12Z Tue Jul 02 2019 - 12Z Sat Jul 06 2019 ...Overview and Guidance/Predictability Assessment... Downstream from a strong Rex block covering Alaska and the central/east-central Pacific, the pattern over the lower 48 will be close to the summertime average with progressive flow across the northern states and a southern ridge aloft that will become more expansive with the departure of an initial central-southern Plains weakness after midweek. Eastward progress of a central Canada upper low will be fairly slow. The pattern will favor a couple wavy fronts meandering over parts of the northern half of the country and a focus for convective systems from just east of the northern half of the Plains to central Appalachians and possibly East Coast. One of the primary forecast challenges will continue to be timing the ejection of the upper trough/closed low that starts the period near the Pacific Northwest coast on Tue. This will depend in part on exact evolution of energy over/offshore British Columbia in response to the strong ridge/upper high building into mainland Alaska. Confidence will likely be low for a while since evolution of energy to the east/south of a closed upper high tends to be one of the lowest predictability situations for extended-range synoptic scale forecasts. A compromise for Pacific Northwest trough ejection looks reasonable at this time, between the faster 00Z ECMWF and slow/deep 00Z CMC. As for the elongating BC/northeastern Pacific energy, teleconnections relative to the Alaska ridge's positive height anomalies do offer support for an embedded upper low that could settle near Haida Gwaii per the 00Z ECMWF/ECMWF mean and 06Z GFS--but there's high sensitivity in specifics depending on exact location of the positive height anomaly center. The main guidance discrepancy elsewhere involves the 00Z GFS becoming a slow extreme with the Canadian upper low during the latter half of the period. The updated forecast maintains the approach of starting with an operational model consensus with eventual incorporation of the 06Z GEFS/00Z ECMWF means by mid-late period (reaching 60 percent total weight by days 6-7 Fri-Sat, while keeping some 06Z GFS/00Z ECMWF input) given typically increasing detail uncertainty. The one notable adjustment in the majority cluster is a southward trend for the front reaching the Great Lakes toward the end of the period. ...Weather Highlights/Threats... One wavy surface front extending across the northern U.S. Tue-Thu and a trailing front dropping into the northern tier from Canada will provide a multi-day focus for showers/thunderstorms across a broad area from just east of the northern Rockies through the Great Lakes/Ohio Valley into the central Appalachians, with some activity possibly reaching the East Coast at times. Some storms may be strong to severe but with low confidence in determining any favored areas this far in advance. Some rainfall will likely be heavy and best potential for highest five-day rainfall totals should be over the northern Plains/Upper Midwest. Locations along the southern Rockies/High Plains may see some showers/storms at times, parts of the southern Plains/western Gulf Coast may see some rainfall with the upper weakness over the southern Plains as of Tue, and scattered showers/storms will be possible over the Southeast/Florida. High temperatures of 5-10F or so above normal over the Southeast could get close to daily record values at a few locations on one or more days. Humidity should be sufficient to produce fairly high heat index values, with some max heat index values possibly reaching at least 105F. Potentially hazardous heat/humidity could extend as far north as the central Mid-Atlantic. Meanwhile the northern High Plains should see below normal highs most days, perhaps by 10F or so at a few locations. The front(s) to the south should push farther south late week into the weekend, extending the cool highs toward the central High Plains by Fri-Sat. Expect the Pacific Northwest and northern California to see a fairly rapid change from below normal highs to plus 5-12F anomalies with the eventual departure of the initial upper trough. 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