Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Sun May 20 2018 Valid 12Z Wed May 23 2018 - 12Z Sun May 27 2018 ...Guidance and Predictability Assessment... Most models and ensemble means are gradually trending closer together for the upper low expected to close off over the eastern Pacific around midweek. However there are still meaningful timing differences as this feature drifts toward the West Coast and possibly inland. Consistent with prior runs, ECMWF/CMC-based solutions show greater eastward progression than the GFS/GEFS. UKMET runs have been holding the system back but generally faster trends over the past day or so in most other guidance (including 18z/00z GFS runs being faster than the 12z version) seem to favor leaning away from the slow side of the spread. While these trends recommend leaning toward the ECMWF/CMC side of the envelope, the fairly strong ridge that builds downstream over the West provides lingering concern that the fast scenario could be just a little too fast, or a little more deflection could occur such as suggested by the 00z GFS. Based on data through 18z the preference was to lean 2/3 toward the ECMWF-ECMWF mean scenario relative to the 18z GFS/GEFS mean, with more ensemble mean emphasis later in the period. Downstream there is still better than average agreement regarding the ejecting western U.S. energy that will likely lead to a shortwave trough crossing the northern Plains/southern Canada and heading into the upper MS Valley/Great Lakes late week through the weekend. The greater uncertainty that comes into the picture late in the period is with separate shortwave energy reaching western Canada by Thu. Models/ensembles still differ on timing and amplitude of this energy as it progresses into/over the western Canada mean ridge, and then on how much the ridge rebounds after shortwave passage. Thus by next weekend confidence is low in how much this energy may amplify and interact with the shortwave crossing the northern states. An intermediate solution should provide the best starting point as North Pacific troughing should promote a fairly rapid rebound of the western Canada ridge and thus allow for some downstream troughing--but perhaps not quite to the depth of recent GFS runs. The same approach applies for depicting the overall area of surface low pressure associated with the upper dynamics. Over the East, during the first half of the period there are still differences in shortwave details for cyclonic flow across the Northeast along with corresponding spread for the southward extent of the cold front that pushes southward and then stalls. The shortwave details of concern have fairly low predictability so prefer an average of guidance at this time. Farther south there is still a lot of spread regarding a potential feature that could track out of the northwestern Caribbean, including influence from an increasingly well defined southern U.S./Gulf of Mexico upper trough whose center may settle near the central Gulf Coast region. Aside from the past couple ECMWF runs adjusting westward, the forecast is generally in a holding pattern as GFS/GEFS runs remain on the east side of the spread and CMC runs show their characteristically aggressive development. Confidence in any particular solution for such a feature is minimal but there is a slightly more coherent signal that a weak surface low could evolve under the upper trough/low by next weekend--albeit with different ways of getting there. A forecast blend tilted 60-70 percent toward the 12z ECMWF/ECMWF mean relative to the 18z GFS/GEFS mean through the period (more operational model input early, ensemble means late) provided a starting point that generated only a modest nudge to continuity. ...Weather/Threats Highlights... Shortwave energy ejecting out of the West and continuing eastward across the northern Plains toward the Great Lakes will spread rainfall from the Great Basin/northern Rockies through the Plains and into the East. Showers/storms may be locally heavy. Trailing Pacific upper low approaching and possibly reaching the western states may generate another episode of unsettled weather toward late week/weekend. Coverage/intensity of activity will depend on exact path and timing of the upper low. Meanwhile expect diurnally favored convection over the Southeast during the period. Evolution of any defined Caribbean/Gulf surface system that could promote significant rainfall enhancement remains very uncertain, but the overall pattern with a deepening upper trough centered near 90W longitude will likely promote a heavier trend to rainfall with time over the Southeast and vicinity. Toward the end of the period and beyond the interaction of this moisture with a front approaching from the central U.S. will have to be monitored. Expect above normal temperatures to prevail over a majority of the lower 48 during the period. Anomalies exceeding plus 10F for min and/or max readings are most likely from the Northwest into the northern-central Plains and OH/TN Valleys. Such anomalies may extend into parts of the southern Rockies/Plains by mid-late period as well. Scattered daily records for max/warm low may be possible. Exceptions to this warmth will be Florida with below normal highs due to clouds/rainfall and California and vicinity with the upper low/trough approaching from the eastern Pacific. Cyclonic flow aloft may keep New England near to slightly below normal for a time as well. WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook 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 Rausch