Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Sat May 19 2018 Valid 12Z Tue May 22 2018 - 12Z Sat May 26 2018 ...Guidance and Predictability Assessment... Latest model/ensemble solutions show reasonable similarity for the overall mean flow but still present some meaningful uncertainties for specifics over some areas. One of the major question marks has been how eastern Pacific flow splits by the middle of next week and what the downstream impact on flow details will be. At the moment latest runs signal a compromise of sorts relative to the wide spread from 24 hours ago. This compromise idea involves a closed low evolving over the Pacific farther offshore than in some ECMWF runs before the 12z cycle but not as far west as some earlier GFS runs, while northern energy briefly dents the western Canada mean ridge to a greater extent than in the prior ECMWF/ensemble mean cluster but without the more significant breakdown that had been advertised in some earlier GFS runs. Among GFS runs through 18z, the 18z version compared much more closely to the majority cluster than the 12z version. New 00z runs maintain decent continuity in principle but still display significant spread in how quickly height falls approach the West Coast, with recent trends toward a compromise (and ensemble means showing a partial trend toward operational runs) seemingly recommending an intermediate solution. Forecast confidence has improved from yesterday but there is still enough sensitivity in the evolution to suggest significant changes in guidance are still possible. Ahead of this evolution there is decent continuity/consensus with the initial western U.S. upper trough and embedded upper low weakening while it ejects northward and then eastward, likely to head into the northern Plains by late in the week. In the wake of this feature heights aloft will rise over the West with a strong ridge also building over Mexico. With understandable detail differences there is a decent signal in the guidance that some combination of the late week northern Plains shortwave and energy progressing through western Canada will promote a general area of low pressure over southern Canada and the vicinity of the upper MS Valley by early day 7 Sat. Over the eastern states, guidance is still having difficulty latching onto specifics of weak waviness over the OH Valley/lower Great Lakes/Northeast Tue-Wed. Recent trends have been toward a weaker reflection with a general model blend of 12z/18z guidance offering the best starting point given its fairly small scale/lower predictability. An upper trough passing through eastern Canada and brushing the Northeast will push a cold front into parts of the East immediately behind the initial wave. Southward extent of this front will depend on timing/amplitude of the upper trough, with models/ensembles not yet decided on specifics and no coherent multi-run trends seen thus far. Latest guidance still offers very diverse possibilities for low pressure that may track out of the northwestern Caribbean. GEFS members have tended to be on the fast/eastern side of the spread while CMC runs have also been fairly progressive but on the western side of the envelope. Combined uncertainties including degree/depth of development determining what level of steering flow will have greatest influence at a particular time, along with evolution/position of forecast upper troughing over the southern U.S./Gulf of Mexico, lead to continued very low confidence for details of this feature. Based on the composite of forecast considerations through the 18z cycle of data, the early part of the forecast incorporated mostly operational model input with greater weight on the 18z GFS/12z ECMWF. Increasing uncertainty with time favored increased ensemble mean weight (slightly more than half of the overall blend) by mid-late period but with more ECMWF mean versus GEFS due to the latter not being favored over Florida and vicinity. ...Weather/Threats Highlights... The upper trough/low ejecting out of the West and into the northern Plains will likely promote areas of precip through the period, from portions of the northern half to two thirds of the West and into the central U.S. This primary feature along with leading impulses will likely interact with a surface front anchored by central High Plains low pressure to produce multiple convective episodes. Best signal for highest rainfall totals during the period extend from the northern/north-central Rockies toward the upper half of the MS Valley. Near the West Coast late in the period, the upper trough/low offshore may get close enough to generate mostly light/scattered precip but confidence is low. Meanwhile the Tue-Wed system over the East will likely be accompanied by areas of rainfall of varying intensity. South of the front that likely stalls over the mid latitudes of the lower 48, there will be periods of diurnally favored showers/storms. Forecast specifics for Florida and vicinity remain very uncertain given low predictability of the feature that may emerge from the northwestern Caribbean. There is still somewhat better confidence that the overall pattern should promote an increase of moisture and potential for some areas of locally moderate to heavy rainfall. Much of the country will see above normal temperatures during the period. Greatest anomalies should be for min/max readings from the Pacific Northwest eastward along the Canadian border and for min temps over parts of the East during the first half of the period. Another band of relatively high anomalies may set up over the central Plains/MS Valley. Upper troughing will promote below normal highs over the Great Basin/Southwest early in the period while clouds/rainfall will tend to keep Florida highs slightly below normal. 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