Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 242 PM EDT Sat Apr 18 2020 Valid Apr 18/1200 UTC thru Apr 22/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12z Model Evaluation with Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 19Z Update: The non-NCEP guidance has trended a little stronger with the shortwave crossing the Deep South and this continues when the system exits the East Coast, with the 12z NAM remaining on the more progressive side of the 12z model suite. There was also a slightly stronger trend with the Great Lakes trough by Day 3, with the 12z UKMET indicating the most amplified scenario, so the CMC replaces the UKMET as part of the model preference. Out west, the 12z CMC, and to a lesser extent the 12z ECMWF, trended faster with the upper level low over the Desert Southwest by the end of the forecast period. However, the CMC still trails the majority of the guidance with this feature. Storm intensifying off the Northeast Coast this weekend ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: High A potent shortwave tracking across the Mid-Atlantic region today will support an intensifying surface low east of New England and south of Nova Scotia through Sunday. There is now enough model agreement to support a multi-model deterministic blend. Southern stream shortwave with surface low tracking across Gulf Coast states ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z ECMWF/12z UKMET/12z GFS Confidence: Average The southern stream shortwave exiting the southern Rockies will give way to lee cyclogenesis across the western High Plains this evening, and a well developed surface low is progged to track eastward across the southern Plains and Deep South through Monday morning, and then exiting the southeast coast. The 12z NAM is a bit faster and a little farther north compared to the model consensus by Sunday night, and the 00z CMC is a little slower when the system exits the East Coast. Other than some small mesoscale differences among the UKMET, ECMWF, and 12z GFS, an initial blend of these models would suffice as a good starting point in the forecast process. Amplifying vortex over southern Canada with cold front across Great Lakes region ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z ECMWF/12z CMC/12z GFS Confidence: Slightly above average The large synoptic scale trough over central/eastern Canada and the northern tier of the U.S. gets reinforced with a strong shortwave tracking southward across Manitoba, and then evolving into a cold upper low over southern Ontario and Quebec. This will sustain a cold front crossing the Great Lakes and Ohio Valley, and then reaching the East Coast by Tuesday evening. While there is generally good model agreement in the evolution of this deep upper level trough and resulting surface low, the 12z NAM becomes more progressive with the cold front across the Ohio Valley region and quicker with the arrival of height falls across southern Canada. Although the UKMET is a little stronger with the surface low and front, it should still work as a part of the model blend. Upper low and trough reaching the Desert Southwest by Tuesday morning ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM/12z ECMWF/12z UKMET blend Confidence: Slightly below average The next short wave in the flow tracks toward the central California coast after Monday morning. There is still some model spread concerning the timing of this short wave, as it undercuts mid level ridging across the Pacific Northwest. Though there is a loose consensus taking the mid level system over Nevada and Arizona by the end of the forecast period, each solution takes a slightly different route getting there. In terms of the models, the 12z GFS appears more progressive with the mid level system, and the 00z CMC is much slower. With the UKMET, ECMWF, and NAM being relatively well clustered through day 3, a blend of those models should work well. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick