Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 128 PM EDT Fri Apr 17 2020 Valid Apr 17/1200 UTC thru Apr 21/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z NAM/GFS Evaluation with Preliminary Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Short wave energy tracking from the Rockies to Southern New England... ...Surface low pressure crossing the Ohio Valley into the northern Mid Atlantic... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 00Z ECMWF, 00Z UKMET blend Confidence: Slightly above average The models are in reasonable agreement with this system but minor differences with the 12Z GFS having a stronger push of low level warm air to the north into the Northeast early Friday night. Beyond that point, the 00Z CMC becomes a bit slower and south of the model consensus. A 12Z NAM, 00Z ECMWF, 00Z UKMET blend appears to be the most reasonable. ...Short wave energy in the southern stream... ...Surface low tracking from the Southern Plains to the Mid Atlantic coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z UKMET/ECMWF blend Confidence: Slightly below average An initial shortwave is expected to track east from southern CA early on and weaken across the Four Corners region through Saturday evening. A second shortwave will follow in the wake of the first and track from the Southwest into the Southern Plains early Sunday. Eventually as the shortwave approaches the East Coast, surface low development is expected with deepening offshore into Sunday night. The models begin to diverge on timing with this feature, mainly noted with the 850-700 mb layer during the day on Saturday. The 12Z GFS ends up more progressive compared to the remaining consensus but the 12Z NAM is a bit fast as well. The 00Z ECMWF/UKMET appear a better fit to the ensemble and remaining deterministic consensus. However, as the surface low intensifies over the western Atlantic, The track of the ensemble means is better suited to the 00Z UKMET with the 00Z ECMWF track farther west and the 12Z GFS more offshore given its more progressive lower-level wave. The 12Z NAM may have some utility along the East Coast as well, but time periods leading up to Sunday night are less desirable. ...Short wave energy in the northern stream from Manitoba into the Upper Great Lakes... ...Attendant cold front crossing the Northern Plains into the Upper Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/UKMET blend Confidence: average Both the 12z NAM/GFS are displaced in timing from the middle ground with the 12Z NAM a bit slower and 12Z GFS a bit faster as a cold front sweeps across the Great Lakes region. The 00Z ECMWF/UKMET appear to be a better compromise and are in agreement with the preference for the southern stream system discussed in the above section. Short wave over the Pacific Northwest Sat into Sun ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z NAM, 12Z GFS, 00Z ECMWF blend Confidence: Slightly below average Large differences develop with an approaching shortwave into the West Coast on Monday with model agreement poor. The ensemble means, however, show a similar depiction and so deterministic guidance matching the means will be favored. At this time, a blend of the 12Z NAM, 12Z GFS and 00Z ECMWF are considered closest to the ensemble means. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Otto