Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sun Jun 30 2019 Valid 12Z Wed Jul 03 2019 - 12Z Sun Jul 07 2019 ...Overview and Guidance/Predictability Assessment... Latest guidance continues to show a typical summertime pattern aloft over the lower 48 during the period. Progressive and low amplitude flow will prevail across the northern states, between southern tier ridging (with some day-to-day fluctuations in strength) and an upper low tracking across central-eastern Canada. A Rex block will persist over the North Pacific/Alaska. A couple convection-focusing fronts over the northern states during midweek should consolidate into one boundary by late in the week, and then push somewhat farther south by the weekend in response to the westerlies dropping gradually southward over the eastern half of the country. While there is above average confidence in the overall pattern evolution, some of the embedded features have much lower predictability/confidence. The primary forecast problems involve how energy will eject from the upper trough over the Pacific Northwest and details of flow over/west of western Canada in response to the strong upper ridge/high that builds over mainland Alaska. Both will determine northern tier shortwave specifics influencing convective systems and frontal waves. Operational models have still not decided on whether the initial Pacific Northwest trough will eject as one coherent entity or in multiple parts, with the scale of individual impulses so small as to yield minimal confidence in any specific solution. The guidance spread for energy over/near western Canada by Fri-Sun has not improved over the past day. 00-06Z GFS runs (especially the 06Z version) are farthest east while the ECMWF/CMC display some degree of consistency in closing off a low near the British Columbia coast. However the ECMWF mean has backed away from the operational run's closed low. Teleconnections relative to the Alaska ridge's positive height anomaly center are fairly sensitive to ridge location but at least suggest holding some upper troughing westward of the GFS runs. Thus the primary emphasis would be to phase out GFS input by late in the period. The latest forecast starts with a consensus model blend early and then evolves toward a mix of 00Z ECMWF/CMC and 00Z NAEFS/ECMWF means late. GEFS means (and more so the 06Z run) offer potential for a stronger shortwave to produce lower surface pressures over parts of the northern U.S. during the weekend (like the 00Z ECMWF) but this feature has such low confidence 6-7 days out in time that the forecast uses the NAEFS to provide a more coherent blend. ...Weather Highlights/Threats... A fairly broad area of the lower 48 from the Rockies eastward should see one or more days of showers and thunderstorms. Best potential for a heavy rainfall threat would be from the northern Plains into Midwest, near an initial wavy front and second approaching front. Some convection will likely extend farther east/southeast toward the East Coast with locally heavy totals as well. The fronts should consolidate into one by Fri and then push southward/southeastward with time, providing a larger-scale focus for organized convection into the weekend. Strong to severe thunderstorms will be possible over some areas but with low predictability for any specifics this far in advance. Expect high temperatures to be 5-10F above normal near the East Coast mid-late week with a gradual cooling trend by the weekend. A few locations over the Southeast could approach daily records for highs/warm lows around midweek. Heat index values over 100F will be possible across much of the Southeast and southern Mid-Atlantic. On the other hand clouds/rainfall should keep daytime highs 5-15F below average most days over the northern High Plains. Northwestern areas should see highs 5-15F above normal Fri-Sun after near to below normal readings on Wed due to the upper trough over the region. 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