Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1050 AM EDT Fri Jun 29 2018 Valid 12Z Mon Jul 02 2018 - 12Z Fri Jul 06 2018 ...Extended period of above average temperatures expected for much of the central and eastern U.S... ...Heavy rain threat for the northern Gulf Coast... ...Overview... Models and ensembles remain in good agreement next week with the upper pattern that will feature an expanding upper high that will likely stretch from southern California northeastward to the Appalachians. This will support well above average temperatures over the eastern Great Lakes/Northeast/Mid-Atlantic with daily record highs likely to be broken. One upper low will linger over the Pacific Northwest and another off the Southeast coast with yet another to the southwest of the upper high over the Gulf Coast. A surface frontal boundary will lie astride the north/northwest side of the expanding 594-597dm (maybe 600dm?) upper high -- roughly from the Pacific/Interior West through the northern Plains into the Great Lakes. ...Guidance evaluation/preferences... Similar to the overnight issuance, the latest deterministic models formed a good starting point for the forecast through a majority of the period. The forecast for days 3-6 (Mon-Thu) was based on a blend of the 00Z ECMWF/UKMET and 06Z GFS. Ensemble mean (00Z ECENS/NAEFS) weighting was gradually increased from days 5-7 to account for some increasing spread among the guidance. Although, spread with respect to large scale features was relatively low for that time period. A few points of contention existed by later in the forecast period. The first was the track/timing/strength differences with upper low off the Southeast coast, and a compromise solution was favored at this time. Another was the amplifying upper low off the Pacific Northwest by Wed-Thu as energy slides south on the eastern side of the North Pacific ridge. Models have shown a trend toward more amplification of this feature, with the 06Z GFS leading the way. Given the amplified ridging across the North Pacific and the central/eastern U.S., this trend makes sense conceptually, and was included in the latest forecast update. Finally, the timing/amplitude of shortwave energy crossing central Canada next Thu-Fri has implications for frontal timing/position across the CONUS northern tier. A trend among some of the guidance toward somewhat greater amplification here resulted in a faster frontal timing across the northern plains/Midwest for days 6-7 relative to the overnight issuance. ...Weather threats/highlights... Ascendant upper high will force precipitation around its circulation, firstly in the Northwest in conjunction with southwest flow ahead of the upper low atop the surface boundary. This will extend eastward across Montana into the Plains and Great Lakes as the surface low wanders through southern Canada. A lead front into the Northeast next Mon/Tue will do little but knock temperatures a few degrees and perhaps touch off a shower/storm as it dissipates by Wednesday. The Southeast into Florida may see an increase in precipitation should the Atlantic upper low wobble westward toward the coast later in the week. Finally, the most significant precipitation may lie along the Gulf Coast from LA/MS westward to the upper Texas coast as a weakness drifts westward. ECMWF/UKMET seemed a bit aggressive/wet but with plenty of moisture to tap (PW anomalies about +2 to +3 sigma) heavy rainfall is certainly a possibility. Temperatures will be near/above records (15 to 20 degrees above average) in the Northeast early next week with a slow downward trend (but still above average) by late in the week as the core of the upper high moves southwestward. High temperatures are expected to be at least 5 degrees above average for most of the contiguous U.S. through much of next week, with widespread 90s for many areas. Meanwhile, cooler than average temperatures are expected for the Northwest and also along the Gulf Coast (especially max temps) given extensive cloud cover and rain. Fracasso/Ryan 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