Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 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 that should migrate from near the Aleutians into the northern Pacific before possibly wobbling back to the north somewhat. A couple Pacific shortwaves ejecting from the mean upper low will erode the western ridge during the forecast period, but by next weekend most guidance shows a building eastern Pacific ridge reaching at least as far as the West Coast. Downstream from the mean ridge expect shortwave energy to amplify over central U.S. and then move east along the northern periphery of a strengthening subtropical ridge that will serve to raise heights across southeastern states through the end of the week. ...Guidance and uncertainty assessment... There is still significant model/ensemble spread over the eastern Pacific/western U.S. from late week into the weekend but trends over the past 12-24 hours have at least led to better guidance agreement in principle for the time being. This is a result of ECMWF/CMC runs toning down the depth and closed nature of upper energy dropping into the West from Fri onward, ensemble means trending somewhat deeper/sharper with the upper trough, and GFS/GEFS guidance becoming less aggressive in trying to push upstream Pacific flow toward the West Coast--thus allowing for higher East Pacific heights and more western troughing as have been favored by teleconnection relationships relative to the negative height anomaly center between Hudson Bay and southern Greenland. Among latest runs the 00z GFS represents a scenario more similar to older ECMWF/CMC runs that were deeper and more closed over the West (but similar to consensus in concept) while after Fri the 06z GFS appears to stray to the fast side with the trough and upstream Pacific flow. The 06z GEFS mean also shows a somewhat faster trend versus the 00z cycle, so the 00z cycle would be preferred for any GFS/GEFS input. Farther east, a consensus forecast maintains continuity well for the system tracking out of the Plains into the Great Lakes early in the period. Toward Fri-Sat a multi-solution blend helps to temper the 00z ECMWF that is on the stronger side (albeit within the ensemble envelope) with the overall upper troughing that crosses eastern North America. By the latter half of the period the greatest uncertainty will likely be the position of a fairly strong and wavy front extending between Plains low pressure and the East Coast. The stronger dynamics aloft in the 00z ECMWF lead to a farther south backdoor front along the coast while wave details farther westward will be sensitive to shortwave specifics (impulses possibly ejecting from the developing western trough as well as the southern fringe of northern stream flow) that have low predictability several days out in time. Prefer to maintain an intermediate scenario with below average confidence. The updated forecast maintained continuity in principle while refining some details per the majority guidance cluster. Primary emphasis days 3-5 Wed-Fri was on the 00z ECMWF/00z-06z GFS with lesser input from the 00z UKMET/CMC and 00z NAEFS/ECMWF means. Ensemble mean weight increased for days 6-7 Sat-Sun with minority components of the 00z ECMWF/GFS/CMC retained through the end of the period. ...Sensible weather/threats highlights... The system crossing the Midwest/Great Lakes/Ohio valley Wed-Thu will likely produce scattered to numerous showers and thunderstorms. Some locally heavy/strong activity is possible but the probability of widespread heavy rainfall appears fairly low. Showers and storms may reach the Northeast by Thu night. Arriving Pacific energy will support a period of precip over the Northwest and northern Rockies around midweek followed by areas of rain/thunderstorms progressing across the northern half of the Plains and beyond. There is still uncertainty in the evolving pattern but some improvement in confidence toward the threat of significant rainfall over and downstream from the north-central High Plains. There is increasing potential for slow moving low pressure over the High Plains with the wavy front extending east-northeast serving as a focus for rain/storms as flow to the south of the front transports Gulf of Mexico moisture northward. For the overall period the full array of model/ensemble guidance highlights an area from the north-central Plains into Great Lakes as having the greatest probability of receiving the highest rainfall. Meanwhile the Florida Peninsula should see a steady increase in rainfall, some potentially becoming heavy, as flow brings moisture in from the east/southeast. Over the West expect well above normal temperatures to become more suppressed relative to Wed as shortwave energy erodes the western ridge aloft. Best potential for plus 10-20F or slightly greater anomalies during Wed-Fri, and scattered daily records, will extend from the Great Basin into southern High Plains. Cooler air will spread from northern areas into the Interior West/Rockies while the building ridge nearing the West Coast will promote a significant warming trend over WA/OR/CA next weekend with some highs 10-20F above normal. Farther east expect cool air to push southward from the northern Plains with some highs at least 10-15F below normal from the northern Plains/upper MS Valley into northern-central High Plains around Fri-Sat. Warmth moving into the East by Thu will persist through the rest of the period to the south of a sharp front setting up over north-central latitudes, with readings 5-15F above normal. 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