Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 AM EDT Fri Aug 31 2018 Valid 12Z Mon Sep 03 2018 - 12Z Fri Sep 07 2018 ...Pattern Overview and Guidance/Predictability Assessment... Latest guidance maintains reasonable agreement/continuity for the overall mean pattern but continues to suggest greater than average uncertainty for some details. Expect the center of a strengthening upper ridge over the eastern U.S. and western Atlantic as of early Mon to settle over or near the Mid-Atlantic region, with highest heights aloft likely around midweek followed by some gradual weakening. Flow around this ridge may bring one or more bundles of energy (and possibly a weak surface reflection) across Florida and the Gulf Coast region. At the same time an evolving mean trough should cover western Canada and the northeastern Pacific. The greatest forecast uncertainty involves the shortwave details within and downstream from the mean trough, keeping confidence fairly low for specific frontal details across much of the northern contiguous U.S.. Details within flow to the south of the eastern ridge aloft are also fairly uncertain. Consecutive runs of the GFS/ECMWF as well as individual 12Z/18Z ensemble members are quite varied regarding the amplitude/timing of one or more shortwaves ejecting from the western Canada/northeastern Pacific mean trough. Trends through the 18Z cycle generally showed a slower trend with the best defined shortwave forecast to eject from western North America after Mon, though the 12Z UKMET/CMC were somewhat faster than the majority cluster. Reinforcing the idea of low predictability for this aspect of the forecast, the 00Z GFS has reverted back to a faster scenario for the shortwave. Issues with this shortwave energy will result in variations for timing and southward extent of the associated front over the northern states. There is somewhat better clustering of guidance for a leading front that progresses from the upper Great Lakes into New England during the first half of the period and likely lifts back as a warm front mid-late week. Ensemble means have trended stronger with the Mid-Atlantic ridge aloft over the past day, especially in forecasts valid around days 5-6 Wed-Thu. This trend is a little less pronounced in the operational runs. To the south of the ridge, the 12Z ECMWF made a notable adjustment toward a weaker surface reflection of a wave expected to emerge from the Bahamas and possibly affect Florida and the Gulf Coast region. The manual forecast maintains yesterday's WPC-NHC coordinated depiction of this feature through day 6 Thu. The new day 7 forecast shows an evolution toward a trough along/northeast of the Texas coast to incorporate latest weakening trends for any lingering reflection of the Gulf Coast wave, while also including low level circulation potential farther southwest near the south-central Texas coast as hinted at by some 12Z/18Z models and means. This aspect of the forecast will be refined later today based on newest guidance. In light of the array of guidance from the 12Z/18Z cycles, the early part of the forecast primarily consisted of the 18Z GFS, 12Z FV3 (parallel) GFS, and 12Z ECMWF with modest inclusion of the 18Z GEFS/12Z ECMWF means. Detail divergence in the models led to increasing ensemble weight to 50 percent or higher mid-late period. By late in the week the 12Z FV3 GFS compared better to the means than the 18Z GFS (possibly too amplified with its eastern Canada trough/southward with the front to the south) and 12Z ECMWF (in which the eastern Pacific trough sagged a little farther southward than the majority of solutions). ...Weather Highlights/Hazards... The strong upper ridge over the East will lead to multiple days of humid and very warm to hot conditions over much of the eastern half of the country. Highest anomalies of 10-15F above normal are most likely from the Great Lakes/Ohio Valley into the Northeast, with morning lows tending to have slightly greater anomalies--and better chance of achieving daily record values--versus the daytime highs. A few record highs may still be possible though. Multiple areas will see at least some potential for heavy rainfall around the periphery of the upper ridge. Florida and the Gulf Coast region should see an episode of enhanced activity with westward-moving energy aloft/possible surface reflection emerging from the Bahamas. A return of moisture to the southern Rockies/High Plains should yield periods of diurnally favored convection over that region. Locations from the northern-central Plains into the upper Great Lakes may see multiple heavy convective events in the vicinity of a mean frontal boundary expected to meander over this area during the period. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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