Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 147 PM EST Sun Mar 04 2018 Valid Mar 04/1200 UTC thru Mar 08/0000 UTC =========================================== Final 12z Model Evaluation and Preferences =========================================== ...Mean Trough Exiting the Mid-Atlantic / Northeast States through Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~ Preference: Blend of the 12z operational models, excluding the UKMET Confidence: Average In the wake of the deeply occluded storm off the east coast, a small scale shortwave trough is forecast to dive across the Northeast and Mid-Atlantic today, reinvigorating the base of the larger scale trough offshore before the next upstream system starts to take center stage. Generally speaking, there are some reasons to favor the NCEP camp of models regarding many of the shortwave details within the blocky northern latitudes and semi-progressive more southerly latitudes (see Central U.S. section below). When the 12z NAM trended slower and deeper with the afore-mentioned shortwave diving across the Northeast, and the GFS then fully supported this trend, there appeared to be plenty of reasons to favor a NAM/GFS blend for this system. The 12z ECMWF and Canadian then trended in line with the NCEP guidance. Only the UKMET stands out as being too fast with this back-side shortwave. ...Central U.S. Storm Passing Off to Developing Nor'easter by Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS...supported by 00z/06z GFS Continuity, 12z GEFS Mean, and to lesser extent the NAM Confidence: Average A medium wavelength system ejecting at a negative tilt into the Plains will drive a winter storm and, in some cases, blizzard conditions, across that region through Monday night / Tuesday. The storm then goes through a fairly interesting evolution which the models agree should lead to a hand-off to coastal low pressure development and a potential Nor'easter mid week. The mature cyclone over the Plains/Midwest by Tuesday will sit beneath a blocky pattern in place across the full breadth of Canada, but within what could loosely be termed a progressive flow pattern at the latitudes of the United States, describing the flow from roughly British Columbia to the central U.S. to the western Atlantic. The mature cyclone is forecast to be urged along by energy dropping down the back side of the upper trough, pushing height falls to the south and east, while a sort of unconventional kicker system opens up the trough from the north side / Ontario and Great lakes / allowing it once again to pivot around to a negative tilt as it reaches the east coast on Wednesday. Models agree to these concepts, but when considering the details, WPC preferred the more pronounced cyclonic flow over the central High Plains states on the back side of the cyclone, as well as sharper troughing into the height weakness over the offshore waters of British Columbia. This tended to remove the 00z ECMWF from consideration, and WPC thinking was more in line with the NCEP guidance... GFS/NAM camp. Fortunately, the GFS showed good continuity between 00z, 06z, and 12z runs, as well as good support from the 06z GEFS mean and now 12z GEFS mean. Furthermore, the 12z global model consensus was toward the WPC preferred pattern over the High Plains and off of British Columbia, including the 12z ECMWF. This helps boost our confidence in recommending the GFS as a majority basis for the short term forecast. The 12z NAM, while usually not the best model for the details beyond Day 1, is reasonably close to the GFS for many aspects of the flow, and could be used, although we would more strongly recommend the GFS. The 12z run of the ECMWF looks like a much more viable solution than the 00z run had been, but is still displaying a systematic tendency to be quick in dropping the kicker system southwestward through Ontario, and a tendency to indicate a very rapid progression of the deepening east coast cyclone on Day 3. These tendencies still have a degree of support from the UKMET, but the overall trend has been toward a more GFS-like evolution. While there are no true outliers and a consensus approach could be used, we think there are some reasons to prefer the GFS. ...West Coast / Northwest / Northern Rockies... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12z operational models Confidence: Average Synoptic reasoning applied to the flow regime from the eastern Pacific to the Atlantic led WPC to favor the GFS as a basis for the short range forecast (see previous sections). In the West, the 12z GFS may be over-amplifying a shortwave within the eastern portion of the positively tilted trough by the end of Day 3, as it approaches the area off northern CA and southwest Oregon. Blending with the other 12z operational guidance, all of which was "in the ballpark" at the surface and aloft, is recommended to derive a more confident consensus solution. Previous model trends at www.wpc.ncep.noaa.gov/html/model2.shtml Previous 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Burke