Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 204 PM EST Fri Mar 09 2018 Valid Mar 09/1200 UTC thru Mar 13/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation with final preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Surface low near wrapping back west across Nova Scotia tonight... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Trends in the ensemble data have been to bring the surface low more to the west on Saturday over New Brunswick, closer to eastern Maine. The latest 12Z model suite shows good enough agreement with this system regarding strength and location to support a general model blend, however, the 12Z CMC did edge a a bit farther to the east and currently lies on the eastern end of the ensemble scatter low plots. ...Shortwave(s) dropping southward through the interior Northwest U.S., with the southern portion reaching the Plains Saturday... ...Surface low developing over the southern Plains late Saturday and moving toward the Southeast on Sunday... ...Coastal low development/strengthening on Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 06Z/09 GFS, 00Z/09 CMC blend Confidence: Below average A complex interaction of a few vorticity centers leads to a complicated forecast with higher than average uncertainty. Minor timing differences with vorticity centers located off of the Oregon coast and over British Columbia early in the forecast period grown downscale and lead to large differences downstream at the surface. Regarding a surface low forecast to form over the southern Plains and move into Tennessee/Mississippi/Alabama Sunday morning, ensemble scatter low plots have trended south 4, 3, and 2 cycles back, with the latest cycle coming back north. By 12Z/11, the center point of the latest ensemble scatter low plots puts a low over the northern Mississippi/Alabama border, which says the 12Z NAM/ECMWF/UKMET are a little too far south. Beyond 00Z/12, ensemble low spread increases dramatically and most deterministic run to run consistency has been poor except for the past 4 runs of the CMC, ending with the latest (12Z/09). The GFS had been rather consistent, but changed its 12Z run to the far edge of the ensemble guidance at 500 mb and the surface. The 12Z UKMET/ECMWF trended north and toward the idea of a deepening surface low tracking up the east coast on Monday, but the 12Z UKMET is quite strong and west, due in part to unfavorable evolution of a vortex dropping south through Ontario. Believe the 12Z ECMWF still has room to adjust northward and/or eastward based on the latest ensemble distribution through Monday. The preference is to be near the better ensemble clustering, considering trends, and near the better consistency of the CMC model along with the GFS, through the 06Z/09 cycle, with the 12Z GFS considered less likely to verify. The below average confidence preference is for a 06Z GFS/00Z CMC blend. The 12Z CMC is thought to have brought the mid-level shortwave too far south across the Southeast Monday morning, and is considered slightly less favorable compared to the 00Z/09 CMC. Unfortunately, none of the 12Z guidance looks close enough to WPC thinking to use in a blend, and prefer to stay closer to the previous preference for the final recommendation. ...Low amplitude shortwave approaching California from the late Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/12Z ECMWF blend Confidence: Average The stronger ECMWF has trended weaker while the weaker GFS has trended stronger with a low amplitude shortwave moving across California into the interior West through Sunday morning. Significant spread remains in the amplitude and timing of this wave as seen in the latest ensemble spaghetti plots. Currently, a blend of the GEFS and ECMWF means is the preference which appears to be where the model trends are converging. At the surface, a surface low is forecast to reach the west coast of California Saturday night/early Sunday morning. Only the CMC from 12Z holds onto a stronger low and doesn't appear to be supported in strength by the ensemble means. A 12Z GFS/12Z ECMWF blend appears to best fit for this system, with the 12Z ECMWF trending a little faster with the leading edge of the upper trough. ...Large closed low off of the West Coast Monday night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS/00Z ECMWF/12Z UKMET blend Confidence: Average While individual vorticity timing within a large 500 mb is uncertain at the day 3 time range, the large scale flow shows good agreement for a large closed low off of the West Coast by Monday night. At the surface, a low will strengthen and move north on Monday with an occluded/cold front extending to the south. While large scale flow is similar with strength and timing, ensemble scatter low plots show large spread. However, the 12Z NAM does not fit well with the low plot clustering and a blend of the 12Z GFS/00Z ECMWF and 12Z UKMET seems to better fit the ensemble low clustering at this time, with the 12Z ECMWF 500 mb height pattern less representative of the ensemble means by 00Z/13 compared to the 00Z ECMWF. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto