Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sat Aug 18 2018 Valid 12Z Tue Aug 21 2018 - 12Z Sat Aug 25 2018 ...Pattern Overview and Guidance Evaluation/Preferences... Ridging and associated positive height anomalies aloft building over the Pacific, aligned near 150W longitude, will become an increasingly prominent feature over the course of the period. As supported by teleconnections, model/ensemble mean consensus shows the establishment of a mean trough from western Canada to near the northern California coast by late week. Meanwhile fairly amplified eastern U.S. troughing Tue-Thu should fill rapidly thereafter due to the combination of upstream evolution and an Atlantic ridge whose western periphery builds closer to the East Coast. Embedded within this overall flow evolution which has fairly good agreement/consistency in the guidance, there are multiple uncertainties for specifics of the forecast. The upper ridge covering southern portions of the West will persist though the period albeit with a slightly weaker trend after midweek. Even as of the start of the period guidance shows some spread and continuity adjustments for the details of the elongated trough/upper low over the Northwest as of early Tue. As a whole, latest runs appear a little more diffuse with the feature than in forecasts from 24-36 hours ago. Latest GFS runs lean on the diffuse side of the guidance spread, ultimately ejecting energy in at least a couple separate pieces while the 00Z CMC is the most concentrated and reflective of yesterday's consensus. Scale of the energy and some degree of ridging expected to the north early in the period lower predictability so confidence in specifics is not very high. On the positive side there is at least continuity in the idea that ejecting energy will support modest surface waviness across the extreme Northern Tier mid-late period. Behind this ejecting shortwave there is reasonable agreement/consistency with the arrival of a northeastern Pacific trough around day 5 Thu. After this time however model/ensemble spread steadily increases due to uncertainty of specifics in upstream shortwave energy and how it will affect the day 5 Thu trough. Guidance behavior thus far lends itself to low confidence but 06Z GFS/00Z ECMWF solutions that bring another shortwave into the mean trough position by day 7 Sat are certainly plausible and merit inclusion with the GEFS/ECMWF means through the end of the forecast. Farther east, modest guidance adjustments are ongoing for the early period surface low tracking northeast from the central Great Lakes, with an evolution more commonly seen in the cool season. The past couple days of runs have been trending gradually slower while the guidance spread for depth--at least while over the central Great Lakes--has been narrowing in on the middle or slightly weaker portion of the spread from recent days. The most noticeable trend is in the ECMWF whose operational runs have trended about 10mb weaker over the past 36 hours to yield a solution much closer to latest GFS runs. The 00Z UKMET/CMC still show a surface low that could be strong enough to challenge August low pressure records though. An intermediate depth still appears to be the best option for this system. After system passage, solutions increasingly vary with shortwave details over the eastern U.S. as well as over Canada (affecting any fronts that could reach New England) especially after day 4 Wed. As eastern U.S. heights rise expect the cold front to stall near the southeastern and Gulf coasts with uncertain specifics aloft determining the details for embedded waves. Based on the latest array of guidance the forecast started with the 06Z GFS/00Z ECMWF and lesser input from the 00Z CMC and 06Z GEFS/00Z ECMWF means early in the period, followed by phasing out of the CMC and increasing total ensemble mean weight to 60 percent by days 6-7 Fri-Sat. ...Weather Highlights and Hazards... Low pressure tracking northeast from the Great Lakes and its trailing cold front will bring showers and thunderstorms into the eastern states early in the period, with some pockets of strong and/or heavy convection possible. Deceleration of the front toward the southern Mid-Atlantic/southeastern coasts and Gulf Coast will likely allow for persistence of rainfall over those areas. Current guidance suggests best potential for highest 5-day rainfall totals along the NC/SC coast. South of this front there will be periods of convection over the peninsula of Florida. Trough/upper low energy initially over the Northwest may generate some scattered rainfall over northern areas of the West early in the period, followed by potentially heavier late week activity from the northern Plains into upper Great Lakes. There is also the possibility that some of the Northwest energy could round the lower latitude western ridge and generate rainfall over parts of the southern half of the Plains. Most rainfall associated with the upper trough nearing the Northwest by Thu should remain north of the Canadian border. Depending on how this feature progresses with time, it may contribute to Northern Tier convection toward Fri-Sat. Expect continued episodes of diurnally favored convection over the Four Corners states. Pattern evolution will tend to have a moderating effect on temperatures toward late week and early weekend with many locations seeing min/max readings within a few degrees F of normal by Fri-Sat. Parts of the northern-central Plains may see rather cool temperatures Tue-Wed with some localized anomalies in the minus 10-15F range. Some of this cool air will extent eastward but with less extreme anomalies. 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