Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Apr 25 2020 Valid Apr 25/0000 UTC thru Apr 28/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Final Evaluation with Model Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Shortwave and associated developing low pressure moving from the Midwest through the Northeast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: ECMWF/ECENS/UKMET/NAM through D2, ECMWF/ECENS D3. Confidence: Slightly below average ...09Z Update... The 00Z UKMET is still a bit weaker with the mid-level trough structure in the Northeast, but the QPF and thermal profiles are still aligned with current thinking. The 00Z EC is also well aligned with the prelim preferences and its previous model run. While the 00Z CMC looks within tolerance from a mass field perspective, the run to run consistency has been lacking and the thermals are too warm. So, the model preferences have not changed based on the latest guidance. ...Previous Discussion... A shortwave moving through the broad cyclonic flow across the east will dig across the TN Valley Saturday before interacting with several other impulses to amplify across the Mid-Atlantic/Northeast. The lead shortwave is likely to weaken as secondary energy digs into the mean trough and becomes dominant. This will cause the surface low across the OH Valley to transfer towards the Tidewater of VA with new cyclogenesis, which will then lift E/NE through Monday into the Gulf of Maine. Through 48 hours, the mass fields, with the exception of the CMC, are in generally good agreement. However, in the 48-72 hour time frame, the NAM becomes much deeper with the closed low and degree of negative tilt from Ontario into Maine. The GFS/ECMWF and its ensembles are in generally good agreement in the upper trough Monday/Monday night as it closes off east of Massachusetts, but timing and intensity differences of the coupled jet structure lead to significant placement discrepancy with the surface low. The GFS is a bit slower and west of the ECMWF, which is slower than the CMC. The GFS is also warmer with its thermal structure, which seems unlikely in the presence of cold Canadian high pressure to the north. The ECMWF being in the middle, along with its good ensemble support, lends credence to this solution, and a heavy weight of ECMWF/ECENS is preferred in the blend. The UKMET and NAM are also usable, especially through 60 hours, but should have less weight on D3. Trough axis affecting the Northwest into the Upper Midwest/Great Lakes through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Average ...09Z Update... The 00Z EC/UK have trended toward model consensus. However the 00Z CMC trended a bit faster, but is not outside the model spread window. Thus a general model blend should still suffice. ...Previous Discussion... A long wave trough axis anchored along the Gulf of Alaska will help to pivot mid-level energy east into the west coast of Canada. This fairly weak trough axis will traverse the Canadian Rockies through Sunday, moving into Manitoba and Ontario by Monday. The impact of this trough axis will be minimal across the northern Rockies and Plains until it interacts with a bit more moisture, eventually becoming negativity tilted (strong forcing for ascent) across the Upper Midwest and Great Lakes region. The models are in fairly decent agreement, though there are some timing and amplitude differences. An example being the 00Z NAM where the mid-level energy shifts east toward the trough moving off the East Coast which shifts a bit farther east into the OH Valley. These variabilities are still within the model guidance clustering with only modest impacts to QPF/thermal differences overall. Shortwave energy affecting the Pacific Northwest on Monday into the Plains by Tuesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z GFS/12Z GEFS/12Z EC/ECENS Confidence: Average ...09Z Update... 00Z EC is very similar to its previous run and falls in line with the aforementioned model preference of choice. The 00Z UKMET is much more in line with the consensus and could be incorporated within the blend. ...Previous Discussion... A second Pacific shortwave will translate along the base of the consolidating closed low over the Gulf of AK. This wave is drawn more northward with only the base rapidly swinging more negative through Vancouver Island late Sunday into early Monday. The trailing cold front and moisture plume press across the Coastal Range and into the Northern U.S. Rockies. The timing and amplitude represented by the models do vary with the GFS a bit too fast and the UKMET entirely too slow based on the spaghetti plots. The 00Z NAM also remains a fairly dry outlier at this point as well. As the system continues to track east, the mass field difference remain consistent and do not diverge greatly. In addition, the 12Z UKMET was too slow and the CMC too inconsistent with its amplitude. Based on QPF alone, there are some wild differences that are not as tied to the typical mass field structure but rather the impulses that ride within the mid-levels. Regardless, based on the mass fields alone, felt better consensus among the 00Z GFS, 12Z EC and their respective 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 Pagano