Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu May 09 2019 Valid 12Z Sun May 12 2019 - 12Z Thu May 16 2019 ...Overview... Most guidance shows the large scale mean flow transitioning to a trough-ridge-trough pattern aloft by the mid-late part of next week. On the way to this configuration, a strong upper trough will cross the eastern half of the country Sun-Wed with fine-scale details likely playing a role in exact evolution of surface low pressure and coverage/intensity of precipitation. Upstream energy feeding into the mean trough should push a front into the northern Plains early in the week and farther southeast thereafter. Over the West the pattern will take on a Rex block appearance through the first part of next week as an upper low tracks across extreme southern areas while a ridge drifts over the north-central latitudes. In principle the upper troughing that amplifies toward the West Coast has fairly good confidence but there is fair degree of detail uncertainty. ...Guidance Evaluation/Predictability Assessment... Guidance is still in the process of trying to resolve specifics of the upper trough crossing the East Sun onward, along with the corresponding surface evolution. Consensus has the trough core--most likely a closed low--progressing from the Upper Midwest through the Great Lakes and Northeast. The 18Z GFS/FV3 GFS strayed south of consensus with the upper low, ultimately leading to an offshore surface low to the south of most other models/ensemble members by Tue. The new 00Z GFS made a partial northward adjustment while the 00Z UKMET has trended south, at least maintaining support for a compromise relative to the farther north 12Z ECMWF/ECMWF mean. Based on guidance through the 18Z cycle the preferred multi-model/ensemble blend (using the 12Z run for the GFS component) yielded a surface low track between the 18Z GEFS/12Z ECMWF means by Tue. Current clustering has the upstream Upper Midwest wave becoming fairly suppressed after early Sun--as previously advertised by some ECMWF/UKMET/CMC runs. The next frontal system dropping into the northern Plains by Mon and then continuing southeast has good agreement on existence but differences aloft have been resulting in meaningful timing spread and run-to-run variability. This favors a model/mean blend as the best deterministic starting point. The average of current guidance yields a slower trend compared to the cycle 12 hours ago. Note that the 00Z GFS has a low-confidence Northwest U.S. shortwave around Mon which disrupts downstream flow in later days, so consensus of other models/means would appear better. Multi-day trends for the upper low forecast to be just southwest of Arizona as of early Sun have generally been slower along with maintaining somewhat better definition as the feature reaches the Plains. This favors somewhat more account for operational model solutions relative to the means late in the period versus preference for other areas of the forecast domain. For the amplifying upper trough approaching the West Coast, model/ensemble behavior thus far favors solutions in the middle to slower half of the spread. In contrast to GEFS mean runs that have been comparable to the ECMWF mean aside from modest details, the operational GFS has tended to be faster bringing in height falls over at least a part of the West--northern areas in some runs, farther south in others. This tilts preference toward the CMC/ECMWF and GEFS/ECMWF means. Based on above considerations the forecast started with varying weight of the 12Z models and minority 18Z GEFS/12Z ECMWF mean input for about the first half of the period and then rapidly adjusted to 60-70 percent weight of the ensemble means while keeping some ideas of the ECMWF/CMC. ...Weather Highlights/Threats... Expect a broad shield of precipitation over the East as frontal waviness lifts through the region on Sun followed by possible consolidation of low pressure off the northeastern coast. Best potential for highest totals with this system should extend from the central Gulf Coast through the Mid-Atlantic but with lingering potential for significant amounts a bit northwest of this axis. Confidence in specifics remains lower than desired in light of persistent model spread. At least some of the precipitation could fall in the form of snow over highest elevations of New England as the core of the upper trough passes through. Behind this system a cold front dropping southeast from the northern Plains may spread rainfall of various intensity through the northern Plains and Midwest/Great Lakes/Ohio Valley. The upper low tracking from the western U.S.-Mexico border into the Plains will be responsible for locally enhanced areas of precipitation, including some convection, over southern parts of the West and then likely extending into parts of the southern Plains and vicinity. Confidence is still modest regarding specifics of southern Plains activity, which will be important to monitor given recent and upcoming short-range heavy rainfall over the region. The upper trough nearing the West Coast should bring increased clouds/precipitation to the region after Tue. However distribution and intensity of precip are uncertain as guidance has yet to agree on the finer details of the trough. Short-range warmth over the Northwest will continue into the first half of next week, though with a steady cooling trend over Washington/Oregon as upper troughing approaches. Expect highs of 10-20F above normal over the Interior Northwest and northern Rockies Sun-Tue with coverage of plus 10F anomalies likely decreasing significantly by Wed-Thu. Cool highs of 10-15F below normal over the Southwest into southern High Plains will moderate closer to normal as well. The system affecting the East during the first half of the period will keep highs 10-15F or so below normal for one or more days from parts of the Ohio Valley/eastern Great Lakes into the northern Mid-Atlantic/Northeast. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml