Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1243 AM EDT Sun Apr 26 2020 Valid Apr 26/0000 UTC thru Apr 29/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation Including Initial Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Shortwave and developing low pressure moving from the Ohio Valley through the Northeast ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS consensus Confidence: Moderate-High A well developed surface low currently over the Ohio Valley will gradually weaken on Sunday while a new surface low evolves near the Delmarva coast. This Miller-Type B cyclogenesis will result in a nor'easter near the 40N/70W benchmark by Monday morning, and slowly track towards the northeast and south of Nova Scotia through Tuesday. The main difference noted with the models is a warmer thermal profile on the 00Z GFS compared to the other guidance owing to a northward displacement of the 850 mb low. Trough axis affecting the Northwest into the Upper Midwest/Great Lakes through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: High Shortwave and developing surface low currently over Alberta will cross Southern Canada with a cold front clipping the northern tier of the U.S. northern Plains through Sunday evening. There is now enough deterministic model agreement to merit the use of a general model blend with this front. Shortwave energy affecting the Pacific Northwest on Monday into the Plains by Tuesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General blend through 12Z Tuesday, then 12Z ECMWF/ECENS/CMC/00Z NAM blend Confidence: Moderate The first of two noteworthy storm systems to impact the Pacific Northwest is expected to arrive on Sunday night and into Monday morning with a cold front quickly moving inland, and the parent surface low lifting northeast towards the Gulf of Alaska. Some of the shortwave energy from this trough crosses the Canadian Rockies and spawns a new surface low across southern Alberta by Monday evening and this reaches the northern Plains by Tuesday afternoon. Secondary cyclogenesis evolves around the same time across the Midwest states and eventually becomes the next major storm system to affect the eastern U.S. The models are in good agreement mass field wise through about 12Z Tuesday, after which the 12Z UKMET is farther north and much stronger with the Midwest surface low during the day 3 period Tuesday night. The 00Z NAM is a little faster with the low and the 00Z GFS a bit faster with the trailing cold front across the Mid-South. There is relatively good agreement among the ECMWF/ECENS/CMC/NAM to support this blend as an initial starting point in the forecast process. Cold front reaching Pacific Northwest late Tuesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z ECMWF/ECENS/00Z GFS Confidence: Moderate The next major storm system to approach British Columbia from the northeast Pacific will likely be Tuesday afternoon after a brief break from the prior storm. The cold front should reach Washington and Oregon by Tuesday evening, but should weaken after that since it will be going into an upper level ridge axis over the Intermountain West. The NCEP guidance is a bit faster than the non-NCEP guidance, but there are less in the way of latitudinal differences. The UKMET has been consistently slower with this system. A blend of the ECMWF/EC mean/GFS should work as a baseline. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Hamrick