Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Fri May 18 2018 Valid 12Z Mon May 21 2018 - 12Z Fri May 25 2018 ...Guidance and Predictability Assessment... During the Mon-Wed period there is reasonable agreement in the large scale evolution. Expect gradual weakening of western U.S. troughing, with the most prominent embedded upper low center likely to drift from California into the Great Basin. At the same time latest guidance indicates that another piece of energy may drop southward from the Pacific Northwest. A persistently strong western Canada ridge will promote cyclonic flow over eastern Canada, perhaps amplifying enough to interact with initial Plains/MS Valley shortwave energy. Farther southeast an upper ridge northeast/east of the Bahamas will steadily expand southward. Within this evolution the greatest overall uncertainty seems to be with the Plains/MS valley shortwave and associated surface low pressure. Through the 12z/18z cycles there was a fair degree of operational model spread for timing and the ensemble envelope was even broader--ECMWF leaning on the fast side and the GFS a bit on the slower side. If anything model spread is greater with 00z runs thus far as the GFS has become weak/slow and the UKMET/CMC are slower than prior runs--at the very least supporting slower timing than the 12z ECMWF. Beyond the timing differences is the question of northern stream interaction that will influence the strength and track of low pressure. Within the western trough aloft the 18z GFS started out slower than consensus with the CA upper low as of early Mon before gradually catching up. Confidence is low with trailing energy, with a consensus blend downplaying its specifics until better clustering/continuity emerge. Based on these considerations the Mon-Wed portion of the forecast included various components of the 18z-12z GFS and 12z ECMWF/UKMET/CMC to reflect the most common ideas of guidance or intermediate solution as appropriate. Guidance spread for eastern Pacific flow that develops by Wed ultimately leads to increasing uncertainty for larger scale aspects of the eastern Pacific/North America forecast for late next week. In varying ways recent GFS runs have tended to split Pacific trough energy such that the northern energy breaks down the western Canada ridge while leaving a lower latitude upper low back toward 135-150W longitude. On the other hand the 12z ECMWF and recent CMC runs (including the new 00z version) maintain the western Canada ridge while bringing the core of Pacific trough energy closer to the West Coast. Highlighting the delicate nature of the forecast evolution, the 12z UKMET through the end of its run was close to the ECMWF cluster but the new 00z UKMET shows a GFS-type solution. Confidence is not particularly high in such a deep upper low approaching the West Coast as depicted in the 12z ECMWF and recent CMC runs. However the GEFS/ECMWF ensemble means lean more toward the overall ECMWF/CMC scenario of maintaining the western Canada ridge while nudging the Pacific trough toward the West Coast--which is closer to the established long-term mean pattern. The occasional bias of the GFS to break down ridges too easily further recommends leaning away from the GFS scenario at this time. Farther east the primary forecast issues will be details of northern stream cyclonic flow aloft and effect on surface fronts/waves, and at lower latitudes guidance signals for a possible tropical feature to emerge from the northwestern Caribbean. Both have fairly low confidence. Regarding the latter there is a dramatic amount of spread regarding timing/track/strength. GFS runs/GEFS members have tended to be on the faster side of the full envelope with the the 00z GFS slowing down to near the 12z ECMWF as of early day 7 Fri. The 00z CMC has strayed faster though, and with a track farther west. It may take a while for guidance clustering to improve significantly for this feature. For the days 6-7 time frame the forecast transitioned to half weight of the 12z ECMWF mean with remaining input spread among the 18z GEFS mean and 12z ECMWF/CMC. The 18z GEFS mean compared favorably to the other solutions over the western states and eastern Pacific but was likely too quick to lower pressures over the eastern Gulf/FL Peninsula. ...Weather/Threats Highlights... The early-mid week upper trough with embedded low(s) over the West will support areas of precip that should be best organized along and north of the best-defined circulations aloft--with highest totals likely to extend from favored terrain in northern-central CA through the northern/north-central Rockies. Locations downstream will see periods of enhanced rainfall as energy ejects eastward. A leading system tracking out of the Plains/MS valley will likely generate episodes of convection over the eastern half of the country during the first half of the week. The front trailing from this system should extend back to low pressure that becomes established over the central High Plains. This front combined with energy ejecting out of the West may promote one or more convective events that should produce highest rainfall totals over/near the north-central Plains. Meanwhile confidence is quite low in specifics of any defined feature that may emerge from the northwestern Caribbean and affect the Florida Peninsula, but there is at least potential for some enhanced moisture to reach the area given the flow around ridging at the surface and aloft to the east. The greater proportion of temperature anomalies will be on the warm side of the spectrum. Highest anomalies should be for min temps over the East Mon-Wed and for max/min temps from the Pacific Northwest eastward along the Canadian border Tue onward. Most of Florida will tend to see modestly below normal highs most of next week while upper troughing will keep the Southwest/Great Basin on the cool side for daytime highs Mon-Wed. 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