Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sat May 05 2018 Valid 12Z Tue May 08 2018 - 12Z Sat May 12 2018 ...Pattern overview... Downstream from a mean upper low whose emphasis likely transitions from the Bering Sea southeastward into the northern Pacific, expect mean ridging to persist over western Canada through the period. Initial ridging also in place over the western U.S. should be eroded as an eastern Pacific system progresses/opens up and trailing energy arrives. Farther east cyclonic flow will prevail over Canada through the period while mean troughing over the eastern U.S. should trend toward a more zonal flow toward next Fri-Sat. By that time the dominant feature will likely be a mean closed low between Hudson Bay and Greenland. ...Guidance and uncertainty assessment... The most contentious/uncertain aspects of the forecast during the period exist over the eastern Pacific and western U.S. Guidance is continuing to have some difficulty in resolving specifics of the eastern Pacific upper low that most guidance now shows opening up as it approaches the western North America coast by Wed, with some influence by shortwave energy immediately upstream. Compared to 24 hours ago the majority consensus has trended farther northwest with the track of the upper low/trough core, which happens to be closer to GFS/GEFS scenarios from a couple days ago when there was much wider spread. By early day 5 Thu there is also reasonable similarity among guidance for the shortwave immediately upstream. However after early Thu guidance rapidly diverges over the eastern Pacific/western U.S. domain. The 00z CMC and past couple ECMWF runs quickly sharpen/close off this trailing energy into a western U.S. trough and compact embedded low in contrast to GFS/GEFS solutions that in varying ways lead to lower heights over the eastern Pacific and higher heights in the GFS/GEFS. Flow evolution by the D+8 time frame leads to greatest emphasis on a strong core of negative height anomalies centered just west of the southern tip of Greenland, associated with the upper low expected between Hudson Bay and Greenland. Teleconnections relative to this negative anomaly center suggest moderate western U.S. troughing and near to slightly above average heights over the eastern Pacific. The 00z ECMWF/CMC means and to some degree 00z GEFS mean (thus 00z NAEFS as well) offer the best template for this teleconnection-favored pattern by days 6-7 Fri-Sat. Confidence is not sufficiently high to incorporate much of the deeper operational ECMWF/CMC runs at this time. For the most part discrepancies are less significant over the central/eastern states. An intermediate forecast still looks best for the upper trough initially nearing the East Coast, between the deeper/slower 00z ECMWF-CMC and weaker/faster GFS/UKMET. A positive trend is better agreement that any waves along the leading frontal boundary should remain offshore. Meanwhile through about Thu-Fri a blended approach offers good continuity and downplays low predictability detail differences for the eventual interaction of amplifying Canadian flow aloft/leading cold front and the system tracking eastward from the Plains. Toward the end of the period the aforementioned issues with upstream flow will begin to play a role in low pressure/frontal positions over the central-eastern U.S. Thus forecast confidence declines considerably after Thu-Fri. Based on the above considerations, forecast input consisting of 80 percent operational guidance for Tue-Thu quickly trended to 70 and 100 percent ensemble weight (00z NAEFS/ECMWF means) for Fri-Sat respectively. ...Sensible weather/threats highlights... Confidence is gradually increasing that the majority of rainfall associated with the wavy front off the East Coast should remain offshore. Some probability still exists that some moisture could extend back to parts of the North Carolina/Delmarva/southeastern New England coasts for a time. At least in principle the forecast is fairly consistent for the system tracking eastward from the Plains Tue onward to generate an area of rain/thunderstorms with some locally heavy/strong activity possible between the Plains and central Appalachians. The initial eastern Pacific system and trailing energy will bring a period of rainfall to the Pacific Northwest/northern Rockies mid-late week. As the energy aloft continues along, expect another episode of rainfall/possible convection over some locations from the northern and possibly central Plains eastward. Uncertainty over the western U.S. pattern by late week/weekend lowers confidence in the distribution and intensity of any western U.S. precip at that time, as well as details of where heaviest rainfall may occur farther eastward. Expect very warm temperatures from the Interior West into High Plains during the period, but with gradual erosion of the warmth across northern areas in particular as upper ridging weakens. Within a broad area of plus 10-20F (and perhaps locally higher) anomalies, there will be some opportunities for daily record highs/warm lows. Pattern uncertainty by the end of the period decreases confidence in the temperature forecast. Readings over the East will rise from within a few degrees on either side of normal into solidly above normal territory with the gradual advance of the Plains system/front. By the latter part of the period the northern tier states are likely to be on the cool side and the South warmer than climo, separated by what could be a fairly sharp front extending from the Plains to East Coast. Thus toward Fri-Sat there is fairly high ensemble spread for temperatures over the middle latitudes of the central/eastern U.S. Rausch Associated WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation forecasts, and winter weather outlook snow/sleet probabilities can be 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