Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1241 PM EDT Thu May 03 2018 Valid 12Z Sun May 06 2018 - 12Z Thu May 10 2018 ...Pattern overview... The overall pattern during the period will consist of a mean ridge covering a majority of the western U.S./Canada with varying degrees of mean troughing downstream over the eastern half of North America. Within this pattern, a shortwave near the West Coast will pass through the mean ridge and most likely reach the Plains/MS Valley by next Wed-Thu. A stronger eastern Pacific system will approach the West Coast by Tue-Wed while upstream flow may could begin to have some influence by next Thu as well. Most of the eastern U.S. mean trough will be fairly diffuse on average, more characteristic of the warm season, with the strongest jet energy tending to be confined mostly to Canada. ...Guidance and uncertainty assessment... Among latest/recent guidance the most significant differences with respect to sensible weather effects exist over the west as solutions diverge for the Pacific system after early Tue. Over the past couple cycles ECMWF runs have adjusted southward with the upper low, bringing the feature into OR/ID by next Thu. The 00z ECMWF mean keeps the core of the upper trough/implied upper low center farther northwest, while most other guidance takes a significantly faster/northern track with remaining trough energy being less amplified as it heads into the western states. Complicating the forecast is the variety of potential solutions for upstream flow as well. The 00z GFS/CMC are particularly fast relative to most other guidance for this upstream energy so preference would be to lean away from those solutions by the latter half of the period. As for resolving differences among the remaining guidance, offsetting considerations seem to favor an intermediate/blend scenario at this time. The long-term mean ridge over the West should tend to weaken any upper trough heading into the western states, while teleconnections relative to a core of negative height anomalies expected to be to the south/southwest of Mainland Alaska toward the end of the period have tended to favor at least some trough energy reaching the West. These teleconnections favor this to occur as part of a split-flow regime. For the leading shortwave initially near the West Coast, remaining 00z guidance and the 06z GFS/GEFS suggest lower probability for the slower and/or more amplified 00z GFS/multiple 00z GEFS members as the feature continues into/through the West. Otherwise for both this feature and the leading shortwave energy affecting the East, small scale details lead to low predictability and thus a blended/ensemble mean approach. This applies to the associated surface reflections as well, given low confidence for exact wave details along leading surface fronts. There is reasonable agreement on Canadian/northern tier U.S. flow supporting a series of fronts reaching the northern states. The one notable trend in the latest consensus is an earlier arrival of the front currently forecast to push south from Canada Wed-Thu. Based on the above considerations the updated forecast incorporated an operational guidance blend (06z GFS/00z ECMWF and lesser weight of the CMC/UKMET) for about the first half of the period. Then the forecast fairly quickly adjusted toward more 06z GEFS/00z ECMWF mean weight versus their operational counterparts. ...Sensible weather/threats highlights... Portions of the East Coast may see some showers/storms on Sun with a wavy front progressing off the coast. Expect the heaviest rainfall to stay offshore but it may be a close call for the North Carolina coast. There may be some rainfall along a front pushing southeast from the Great Lakes/Plains Sun onward as well. Meanwhile shortwave energy initially coming into the West will likely generate areas of rainfall over the Northwest early in the period, followed by areas of rain/convection spreading across the northern-central Plains into the Midwest. Some of this activity over the central U.S. from Mon onward may be locally heavy and/or strong. Given the small scale of important details it may take some time for specifics to come into clearer focus. The next Pacific system should bring another episode of precip to the Northwest within the Tue-Thu time frame but with a fair degree of uncertainty over timing and southward extent. Continue to expect the warmest anomalies for temps to be over the interior West and along the eastern slopes of the northern Rockies into northern Plains, with multiple days 10-20F above normal. Readings may approach/reach daily record values at some locations, somewhat more likely for warm lows versus daytime highs. The East will start off warm early Sun but near to slightly below normal air will filter southeastward from the Great Lakes during Sun-Tue. Eastern states should rebound to moderately above normal temps by Wed-Thu. Southern Texas may see near to slightly below normal temps during most of the period. 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