Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Fri May 04 2018 Valid 12Z Mon May 07 2018 - 12Z Fri May 11 2018 ...Pattern overview... Latest guidance continues to show a less amplified mid-late spring variation of the western ridge/eastern trough mean pattern that has tended to be present across North America in recent months. Within this pattern expect Pacific systems to weaken as upper support heads into the mean ridge aloft, while downstream low pressure will likely track from the northern-central Plains into Great Lakes/Northeast. Shortwave energy within the eastern mean trough may encourage wave development along a front lingering off the East Coast. ...Guidance and uncertainty assessment... Confidence for details of the upper trough initially over the East is still fairly low. Among current guidance, energy within stronger northern stream flow crossing New England is slower/sharper in the 00z/06z GFS. The 06z GEFS mean hints at the slower timing but is less amplified as of early day 3 Mon and then compares well to the non-GFS consensus for the overall pattern by early Tue. Meanwhile the diffuse nature of the portion of the trough over central/southern portions of the East lowers predictability for important details. The guidance average has trended a bit deeper with the core of the feature aloft, resulting in a somewhat better defined wave along the leading surface front. From midweek onward guidance diverges significantly as 00z ECMWF/CMC runs offer fairly rapid northeastward progression while 00z/06z GFS runs are quite slow while wrapping the surface wave back to the East Coast. Overall prefer an intermediate solution that is closer to the ensemble means given the lack of confidence in details aloft and potential for the general evolution of flow aloft to support somewhat faster progression than seen in the GFS. There is generally better agreement/stability in principle for the shortwave energy initially crossing the West and heading into the eastern U.S. mean trough. Aside from a slightly faster trend over the past couple days there has been reasonable model/ensemble clustering for associated low pressure to track from the northern/central Plains into or near the Great Lakes from Mon into early Thu. After that time solutions show more divergence with increasing influence from Canadian/northern U.S. troughing aloft and leading cold front. Over the past day guidance has greatly improved its clustering for the path of the upper low initially over the eastern Pacific, with the guidance majority now showing a track across Washington state in contrast to some ECMWF runs that were farther southeast and GFS runs that were farther northwest. Timing still becomes a problem by the latter half of the period as the 00z/06z GFS stray faster than other operational models and to some degree the GEFS mean as well. This is at least partially due to faster arrival and/or greater southeastward amplitude of upstream energy in the GFS. Toward the end of the period the 00z GFS becomes particularly pronounced with height falls reaching just off the West Coast. A number of 00z GEFS members lean in that direction but the 06z GFS/GEFS have backed off some. Remaining guidance keeps higher heights/possible shortwave ridge over the extreme eastern Pacific. By late in the period the combined teleconnections relative to negative height anomalies south of the Alaska Peninsula and near southern Greenland in multi-day D+8 mean charts offer better support for some variation of the ECMWF/CMC and their means which bring modest troughing into the West--though perhaps with a weaker anchoring upper low than depicted in the operational runs. Based on guidance comparison/considerations, an operational model blend represented preferences well while downplaying any less confident details of an individual run. Preferences over both the West and East after Wed led to trending the days 6-7 forecast toward more 00z ECMWF mean relative to the 06z GEFS mean along with a lingering minority 00z ECMWF/CMC component. Any adjustments from continuity were generally in the realm of typical run-run variability. ...Sensible weather/threats highlights... Detail uncertainty within the upper trough initially over the East keeps confidence low regarding coverage/intensity of any potential rainfall along the East Coast. The highest probability scenario is for most significant activity to remain just offshore, but there is still minority potential for moisture to wrap back as far as the coast or slightly inland. This evolution will require monitoring over the coming days as specifics gradually become more clear. Farther west expect moisture initially over the northern Rockies and vicinity to progress along with surface low pressure/frontal system into and through the northern/central Plains and eventually reach the East. Some Plains into Great Lakes/Ohio Valley convection could be locally heavy and/or strong. Meanwhile the system arriving from the eastern Pacific will bring a period of enhanced rainfall to the Pacific Northwest/northern Rockies around midweek. The Interior West and portions of the High Plains should see the warmest temperature anomalies during the period--plus 10-20F and locally higher. There may be some daily records for highs/warm lows. The Northwest will see a cooling trend Wed-Fri with the arrival/passage of the system tracking out of the East Pacific. Eastern states will see near to slightly below readings Mon-Wed followed by a modest rebound to warmer conditions. 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