Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1202 PM EDT Mon May 07 2018 Valid 12Z Thu May 10 2018 - 12Z Mon May 14 2018 ...Pattern overview... Wavy, low amplitude flow will initially set up over the U.S. with a pair of shortwaves situated over the Great Lakes and Pacific Northwest, respectively. While the former should carry a cold front off the Eastern Seaboard by 11/1200Z, the latter height falls will likely continue digging into the southwestern states into Saturday. The mid/upper-level flow over the eastern Pacific and western U.S. is forecast to become quite amplified from Day 5/May 12 onward. Each such 500-mb anomaly center will likely depart around 1.5 to 2 standard deviations from climatology. This strong upper low positioned across the vicinity of Utah on Saturday morning should gradually trek eastward in time possibly reaching the Mississippi River valley by next Monday/May 14. In its wake, some remnant ridging is expected to slide over the Pacific Northwest with weak troughing potentially sitting back over the Four Corners region. ...Guidance/uncertainty assessment/preferences... While the available guidance exhibits decent agreement with the initial shortwave/attendant frontal zone crossing the East Coast, more variability exists with amplifying flow over the southwestern U.S. Multi-day operational/ensemble trends have shifted toward a stronger and more closed off solution. As a result, there has also been a push toward a more pronounced upper ridge across the eastern Pacific. Through Day 5/May 12, with the exception of the 00Z CMC, the models are in reasonably good agreement. Uncertainty begin to build thereafter as a wavy frontal zone and attendant dry line are depicted differently depending on the model. There is a notable difference in continuity between the 06Z/00Z GFS runs with the latter being quicker to eject an embedded shortwave into the Upper Midwest by Sunday morning. From this point onward, the forecast becomes more chaotic per ensemble spaghetti plot analysis. While multi-cycle trends of such plots have shown a decrease in spread, uncertainty still plagues the Day 6/7, May 13/14 period. The 00Z/12Z ECMWF runs have favored a more progressive scenario while generally maintaining the western ridge into Day 7/May 14. Meanwhile, the 06Z/00Z GFS solutions favor a bit more shearing of this trough with residual troughing possibly lingering back toward the Desert Southwest. These general scenarios are within the realm of possibilities but do reside on the more extreme side of the ensemble plots. This would support a push toward ensemble mean usage late in the period. Through early Saturday, was able to generally use a multi-operational model blend led by the 06Z GFS and 00Z ECMWF/UKMET solutions. With spread growing markedly thereafter, quickly pushed toward an ensemble mean based approach with more emphasis on the 06Z GEFS mean relative to the 00Z ECMWF ensemble mean given the preference for modest ridging over the western states by Day 7/May 14. ...Sensible weather/threats highlights... Throughout much of the forecast, below average temperatures should situate over the Upper Intermountain West eastward into the Upper Great Lakes region. During the first few days of the period, multiple chances for overrunning precipitation will likely keep numbers on the cooler side. This would generally favor readings in the 50s as departures from climatology sit around the 5 to 10 degree mark. Even some departures approaching the 10 to 15 degree range are possible on Saturday across the central High Plains as the upper low nears the region. Except for cool weather likely across interior New England, much of the rest of the country should anticipate above average readings into early next week. Numbers approaching the century mark are possible over the southern High Plains through Saturday, generally juxtaposed with the meandering dry line. Such temperatures would suggest 20 degree departures from average. The initial threat for moderate to locally heavy precipitation will reside across the north-central U.S. while eventually shifting focus toward Wyoming as the upper low approaches. Cold temperatures aloft would support some snowfall accumulations across the higher elevations of the northern/central Rockies. As this system advances toward the Great Plains, expect another swath of convection to break out over the middle of the country this weekend. This threat would gradually migrate eastward depending on the timing of the frontal zone. Elsewhere, a sufficient surge of high precipitable water air will increase the heavy rainfall threat over Florida. The 06Z GFS showed such values reaching the 2 inch mark, possibly higher. Rubin-Oster 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