Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Tue Aug 14 2018 Valid 12Z Fri Aug 17 2018 - 12Z Tue Aug 21 2018 ...Pattern Overview and Preferences... Guidance continues to agree on a steady amplification of the large scale pattern toward a central/east-central North America trough. This evolution will be in response to a deep upper trough/low that develops to the south of the Alaska Peninsula and the downstream ridge which then builds over western Canada and vicinity. Within this agreeable scenario a fair amount of spread arises among individual model runs and ensemble members for specifics by the latter half of the period. During the first half of the forecast (Fri into early Sun) a general blend of latest operational models provided a good starting point to represent consensus ideas-- eastward progression of an initial Great Lakes to Plains shortwave pushing along a wavy front, a West Coast shortwave aloft emerging into the High Plains by Sun and beginning to support a defined surface wave, and Canadian energy starting to push a frontal boundary into the northern Plains. After early Sun the preferred guidance blend rapidly trended toward the 06Z GEFS/00Z ECMWF means with a pronounced increase in guidance spread and run to run variability for details within the amplifying North American trough as well as across portions of the West. Over the central/eastern states the big question will be how northern stream flow pushing south from Canada interacts with shortwave energy/surface low pressure expected to be over the Plains around Sun-Mon. For the time being the ensemble means offer good agreement/continuity with low pressure reaching the Great Lakes by early day 7 Tue. However the full spread of ensemble members/model runs is sufficiently great to keep confidence in specifics in the middle to lower portion of the spectrum. Over the West Coast and farther inland, the ensemble means agree fairly well with teleconnection-favored flow relative to upstream features. In particular they show the split flow configuration suggested by negative height anomalies that become established near the central Aleutians in the D+8 multi-day means. After passage of the weekend shortwave there is not a lot of support from the favored pattern for additional shortwave energy over the Interior West such as seen in the 00Z parallel (FV3) GFS or a minority of ensemble members. ...Weather Highlights and Hazards... Great Lakes/New England low pressure during the first half of the period and trailing frontal boundary that eventually stalls over the Plains then returns northeast as a warm front will provide a multi-day focus for showers and thunderstorms. West Coast shortwave energy that likely reaches the Plains by Sun should provide additional encouragement for organized convection. Over the five-day period there will be a fairly broad area with some degree of heavy rainfall potential from the High Plains into parts of the East. Less available moisture should equate to lighter rainfall along one or more fronts dropping into the northern states from Canada. Elsewhere, the aforementioned West Coast shortwave may produce some rainfall over parts of the Northwest/northern Rockies early in the period. Terrain/diurnal heating will promote showers/storms over parts of the Southwest with extreme southern areas likely to see the highest rainfall totals with greater available moisture. Florida and the Gulf Coast region should see diurnal activity as well. The large scale pattern evolution will bring an expanding area of below normal temperatures to the central U.S. by Sun-Tue. Currently expect the core of coolest high temperatures (10-15F below normal) to be centered over the north-central High Plains Sun-Mon. Before this trend expect warm readings (up to 10-15F above normal) across extreme northern areas late this week. Daytime highs over the Interior West will trend gradually warmer with time as ridging aloft rebuilds while min temps should be above normal through the period. The East will likely be within a few degrees of normal for highs on most days but with periods of clouds/rainfall helping to keep min temps warmer relative to normal. 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