Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Fri Aug 17 2018 Valid 12Z Mon Aug 20 2018 - 12Z Fri Aug 24 2018 ...Pattern Overview and Preferences... An amplified pattern over the northeastern Pacific and northwestern North America from the short range period into early next week will encourage deepening of a trough over the eastern part of the continent with greatest depth likely to be achieved around midweek. An upper trough with embedded low will linger over the northern-central West Coast during the first part of next week, with energy forecast to eject eastward mid-late week as upstream troughing arrives into the Northwest U.S. and western Canada. Teleconnections relative to the strengthening east-central Pacific ridge during the latter half of the period provide support for the consensus trough position by next Fri as well as the eventual filling of the eastern trough. The mean ridge over the Rockies, anchored by a high over or near New Mexico for one or more days, should become a little weaker/more suppressed with time. During the first half of the period guidance continues to signal a Midwest through Great Lakes into eastern Canada system that may be quite vigorous compared to typical August surface lows. However solutions that had been on the strong side of the envelope (ECMWF/UKMET and at times the CMC) have either backed off a bit or delayed development by a few hours. This reduces the percentage of guidance showing the potential for surface low pressure to be deeper than current records for the month over the central Great Lakes around Tue. Even the ECMWF mean has raised its central pressure by a few mb versus its prior run as of 12Z Tue. On the other hand the 06Z GFS and parallel FV3 00Z GFS are closer to consensus than the weak/suppressed 00Z operational GFS. This has been the case over at least the past 2-3 days. Ongoing uncertainty over exactly how Canadian flow will interact with the central U.S. shortwave recommends maintaining an intermediate solution--which for this run is slightly weaker than in the previous cycle. Continuity looks good for the trailing cold front that should reach the East Coast/Gulf Coast by Wed-Thu. The front will likely stall by Fri as eastern U.S. heights aloft rise, but with low confidence for exact position and details of any embedded waves. Regarding the trough/upper low initially over the West Coast, scatter among solutions appears typical for the time frame involved. Mean ridging over the Interior West/Rockies may argue against the most amplified side of the envelope (00Z UKMET and some 00Z ECMWF ensembles) as the shortwave ejects eastward but there is an overall signal that the feature will maintain better definition than in previous cycles as it continues eastward across the Northern Tier through Thu-Fri. This trend leads to a more defined surface reflection across the northern Plains/upper MS Valley late in the week. Upstream, as mentioned earlier the incoming Pacific trough settles into a day 7 Fri position (western Canada into Northwest U.S.) that compares well to teleconnections relative to the upstream ridge. An average among model/ensemble mean solutions should provide a good starting point for the forecast here. The 06Z GFS and 00Z ECMWF/ECMWF mean/CMC provided the input for today's mass field forecast during the first half of the period, yielding the desired consensus or intermediate solution as appropriate. This blend leaned toward a more even blend of model/ensemble mean solutions later in the period as both clusters of guidance seemed to provide reasonable ideas but with operational model detail specifics becoming less uncertain. ...Weather Highlights and Hazards... The developing system tracking from the Plains through Great Lakes into eastern Canada as well as the trailing cold front will likely produce a broad area of showers/thunderstorms--some with heavy rainfall--from the eastern Plains/MS Valley into the eastern U.S. during the first half of the week. Rainfall/convection may also focus at times along a leading front that initially extends from the western Atlantic back to the surface low. Passage of the cold front will bring a drying trend to many areas in the eastern half of the country but areas from the Mid-Atlantic to the extreme Southeast and along the Gulf Coast may see rainfall persist through late week as the front stalls. Meanwhile the trough/upper low initially near the West Coast may produce some scattered rainfall over northern parts of the West during the first half of the week. Then some showers/storms may emerge over the northern Plains/MS Valley later in the week as the shortwave energy ejects eastward. Most precipitation with the upper trough arriving from the Pacific late in the week should remain north of the Canadian border. Meanwhile the Four Corners states should continue to see periods of diurnally enhanced convection. The most extreme temperature anomalies during the period will likely be of the negative variety, with cool temperatures over the northern-central Plains during the first half of the week. Some locations may see one or two days with highs around 10-15F below normal. This cool air will modify as it progresses eastward. The West will tend to see above normal min temperatures most of next week but with above normal highs tending to be early in the week. The overall evolution toward less amplified flow aloft over much of the lower 48 by late in the week should lead to many areas seeing temperatures within a few degrees of normal by next Fri. 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