Prognostic Meteorological Discussion
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000 FXUS02 KWBC 181411 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1010 AM EDT Tue Sep 18 2018 Valid 12Z Fri Sep 21 2018 - 12Z Tue Sep 25 2018 ***Pattern Overview and Model Guidance*** A 500 mb trough is forecast Friday just off the Pacific Northwest coast. As the system moves onshore and inland, timing differences become greater, with a large spread among the 06z GFS/00z ECMWF/00z Canadian models (ECMWF fastest, Canadian slowest), with a wide spread in the ensemble members. The differences stem from the handling of a trough over the Bering Sea Sat, with the 00z ECMWF deamplifying it and moving it faster downstream, in tandem with the ejection of the Pacific northwest trough. As the trough moves east, the ECMWF also becomes the most amplified with the southern Canada trough, as the 50 ensemble members have lower amplitude than the operational run Tue 25 Sept. With large run to run changes and little support from the ECMWF ensembles, little weighting was given to the 00z operational ECMWF. Manual progs blended the 00z ECMWF ensemble mean with the 06z GFS, with less weighting to the 06z GEFS Mean through the forecast. Confidence is a little below normal given the large spread of solutions early to middle portions of next week. On Fri 21 Sept, the models track a 500 mb trough that moves out of the upper MS Valley and Great Lakes, deamplifying as it moves quickly across the northeast Sat. Surface high pressure drives the cold front south through the Lakes and into the Mid-Atlantic region and Ohio Valley, where it stalls. The models show a 700 mb trough developing in CO/NM as a southern extension of the northern stream trough and move this trough into the southern Plains, with a surface wave resulting. ***Sensible Weather and Hazards*** The next heavy rainfall event during the medium range period is expected across the south-central Plains for the upcoming weekend as tropical moisture surges northward from Mexico and the western Gulf, with lift from an approaching 700 mb trough. The potential exists for several inches of rain across portions of eastern Oklahoma into adjacent portions of southeast KS, southwest MO, and north Texas. Lesser amounts are forecast eastward across the Midwest and the Ohio Valley region in the vicinity of a frontal boundary. Another area of moderate to locally heavy rainfall will be possible across the Upper Midwest/Upper MS Valley towards the middle of next week as a surface low develops over the northern Plains. Likewise, as the western trough progresses across the northern Rockies and adjacent high Plains, showers are expected in advance of the upper trough. Expect temperatures to be well above average on Friday across much of the eastern U.S. ahead of the cold front, with areas of 10 to 15 degrees above normal forecast. Temperatures moderate to near normal after the cold front passes. Above normal temperatures are forecast across the southwest Tue 18 Sept. and Wed Sept 19, and also over the southeast US underneath the upper high. Once the upper trough passes, cooler than normal temperatures develop next week from the northern Rockies to the northern Plains. Petersen WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.