Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Mon May 21 2018 Valid 12Z Thu May 24 2018 - 12Z Mon May 28 2018 ...Guidance and Predictability Assessment... Over the eastern Pacific and western U.S., recent ECMWF/ECMWF mean runs along with the 18z GFS/GEFS mean provide a decent cluster for the handling of the eastern Pacific upper low that will likely track inland by next weekend. Over recent days typically slower GFS runs have been steadily catching up to the ECMWF scenario which has tended to include the CMC as well. Once again today the 18z GFS has advanced relative to the 12z version to yield the improved consensus. There is still a significant question of exact track once the feature moves inland with recent CMC runs tending to take a farther southeast track than the ECMWF. UKMET runs and the new 00z GFS are also farther southeast. For now prefer to lean somewhat more in the ECMWF direction as the fairly strong downstream ridge may lower potential for an upper low track on the southeast side of the envelope. By days 6-7 Sun-Mon upstream Pacific flow may complicate the forecast as well. Models and individual ensemble members are quite diverse for specifics with the most common signal being some degree of cyclonic flow centered farther offshore than depicted in the 12z GFS by the end of next Mon. Confidence is not especially great in 18z/00z GFS runs either, with a rather sharp/amplified trough versus broader cyclonic flow in most other models/means. From the general consensus perspective the forecast is fairly stable for the shortwave forecast to track eastward from the northern High Plains and corresponding low pressure that may reach the Great Lakes/Northeast by the latter half of the period. Primary detail uncertainties by later in the period involve the extent to which the northern tier energy could try to feed into the upper trough to the south by later in the period (6-hourly GFS runs steadily backing off) and interaction of potentially amplifying southern Canada flow (trends toward more in the ECMWF and less in the GFS). Ongoing adjustments in the guidance favor an intermediate solution that yields fairly good overall continuity. The forecast over lower latitudes continues to provide significant uncertainty, with surface forecasts over the Gulf of Mexico ranging widely for strength and track of any potential low pressure. CMC runs have been characteristically well-developed while UKMET runs thus far have showed no low pressure at all. For track recent ECMWF runs have been on the western side of the spread and GFS/GEFS on the east. 18z and new 00z GEFS runs have adjusted noticeably westward with their overall area of low pressure versus earlier runs, which provides more support for the idea of a central/west-central Gulf track. By day 7 Mon there is actually decent agreement from the means toward low pressure gravitating to the central Gulf Coast region under the core of the upper trough that deepens over 90W longitude. Consensus also sides with the ECMWF scenario of the upper trough being somewhat sharper versus recent GFS/GEFS runs that have showed more broadness on the eastern side. Forecast considerations continued to favor a blend that tilted 60-70 percent toward ECMWF/12z ECMWF mean guidance relative to the 18z GFS/GEFS mean. By mid-late period the forecast incorporated both the 12z and old 00z runs for the operational ECMWF component of the blend to downplay lower confidence detail differences. ...Weather/Threats Highlights... Areas from the northern Plains eastward will see rainfall/convection in association with the shortwave energy and surface low pressure/frontal system initially emerging over the northern Plains on Thu. Some activity may be locally moderate to heavy. Areas from the lower MS Valley through the Southeast including Florida will likely see a fairly wet period with the upper trough deepening along 90W longitude promoting a northward flow of moisture aloft and surface low pressure possibly tracking toward/settling over or near the central Gulf Coast. Later in the period there will be a question of how much of the moisture over the Southeast spreads northward ahead of the front approaching the eastern states. Heavy rainfall potential will require close monitoring given the significant totals already observed over some locations in the past week. Areas from the central/northern West Coast into the Rockies will likely see another episode of rainfall of varying intensity with the upper low forecast to come inland from the eastern Pacific. Much of the lower 48 will see above normal temperatures during the period. Exceptions will be over the Southeast/Florida where highs will be lower due to clouds/rainfall and California into the Great Basin under the eastern Pacific upper low that tracks inland. The most prominent warm anomalies should exist ahead of the system emerging from the northern Plains, and then from the Northwest through most of the Rockies and eventually into the Plains in association with upper ridging. Within both of these areas scattered locations may see daily record values for highs/warm lows. 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