Extended Forecast Discussion NWS Weather Prediction Center College Park MD 122 AM EDT Sun May 06 2018 Valid 12Z Wed May 09 2018 - 12Z Sun May 13 2018 ...Pattern overview... An upper ridge will initially be in place across western North America, downstream from a mean upper low migrating from near the Aleutians into the northern Pacific. A couple Pacific shortwaves ejecting from the mean upper low will attempt to erode the ridge through the forecast period, but by late in the period most guidance show the ridge once again exerting itself along the West Coast. Downstream of the broad area of anticyclonic upper flow, the tendency will exist for shortwaves to amplify across the central U.S. and then move east along the northern periphery of the subtropical ridge, which will also attempt to raise heights across southeastern states. ...Guidance and uncertainty assessment... The WPC forecast was initially based on a blend of majority deterministic guidance, including the 12Z ECMWF/UKMET along with the 12Z/18Z runs of the GFS. Most emphasis was placed on the ECMWF and GFS. The forecast was transitioned to a majority ensemble means (12Z ECENS/NAEFS) during days 6-7 to account for substantially decreased confidence during that time frame. Model solutions have come into somewhat better agreement with respect to the trough initially along the Eastern Seaboard on Wed, and the blended approach described represents the current consensus solution. The same holds true for additional shortwave energy crossing the Midwest/Great Lakes Wed-Thu. Timing differences shown by the guidance at this time last night have decreased, and the ECMWF/GFS solutions were well clustered within the consensus with respect to a surface low tracking across the Midwest/Great Lakes Wed night/Thu, and the cold front reaching the East Coast by Fri. The most uncertain aspects of the forecast involve energy originating in the northern Pacific which moves into the Pacific Northwest/western Canada and attempts to weaken the upper ridge. The first of these systems should reach the Northwest on Wed. There has been somewhat of a trend over the past day of weakening this shortwave/upper low more quickly (the GFS was the model to lead this trend), but a number of ensemble members as well as the 12Z UKMET continued to suggest keeping the system closed for longer, which results in a slower inland progression and somewhat less weakening of the ridge. The WPC forecast was adjusted in the direction these latest trends, and a somewhat quicker progression of the shortwave and associated surface front inland Wed-Thu. A second shortwave reaches the coast Wed night-Thu, and the evolution of this features as it moves inland is the area of greatest uncertainty this forecast period. The ECMWF continues to quickly amplify this second wave across the northern Great Basin/Intermountain Region Fri-Sat. The GFS has trended a bit toward the ECMWF, but remains less amplified. At the same time both solutions show the upper ridge becoming reestablished along/just off the West Coast, with perhaps a highly amplified flow pattern developing from the north central Pacific into the western half of North America. ECENS/NAEFS means represented a good compromise between the ECMWF/GFS solutions, and a blend weighted toward these two means was favored by late in the week. The 18Z GEFS was a significant outlier in terms of the ensemble means, quickly bringing the mean upper low east across the north Pacific, which displaces the re-amplifying ridge farther east (across the Rockies) relative to the ECENS/NAEFS which keep it more along the West Coast. Based on the broad agreement shown between the latter to means (and a large amount of spread among GEFS members), the 18Z GEFS solution was discarded. ...Sensible weather/threats highlights... The shortwave/cold front crossing the Midwest/Great Lakes/Ohio valley Wed-Thu is expected to produce scattered to numerous showers and thunderstorms. Widespread heavy rainfall is not expected, but locally heavy rain will be possible due to the convective nature of much of the precipitation. Showers and storms may reach the Northeast by Thu night. Meanwhile, the Northwest and northern Rockies will see scattered precipitation Wed-Thu as the aforementioned Pacific shortwaves move inland. As the second wave amplifies across the Rockies by late in the week, the impacts associated with the system could increase. While confidence is low in any specifics for reasons described above, a surface low pressure area forecast to develop along a stalled frontal boundary across the central plains Fri-Sat has the potential to produce widespread convection and heavy rainfall for portions of the central/northern High Plains and Midwest into the weekend. The orientation of the upper flow by that time would be supportive of efficient transport of Gulf of Mexico moisture northward to the front, and a number of deterministic solutions show the potential for heavy rainfall across differing areas. Temperatures will start off the period well above average across much of the interior western U.S. beneath the upper ridge. Highs temperature anomalies of +10 to +20 deg F are possible Wed and Thu, before moderating slightly (but remaining above average) Thu-Sat as the ridge flattens a bit. Much of the central/eastern U.S. will begin the forecast period near or above average on Wed. The northern tier, from the northern plains to the Northeast will see cooling temperatures Thu-Sat in the wake of a cold front, and highs of 5 to 10 deg F below average are possible. By late in the week, a gradually expanding subtropical ridge will support increasing temperatures across the Southeast. High temperatures anomalies Fri-Sun of +10 to +15 deg F are possible. Ryan 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