Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1216 PM EDT Sun Apr 26 2020 Valid Apr 26/1200 UTC thru Apr 30/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Developing Northeast Coastal Low today through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Slightly above average The Miller-B coastal cyclogenesis is already well on its way along the East Coast/Lower Chesapeake as the Ohio Valley surface wave weakens under increasingly sheared mid to upper level pattern (as evidenced by GOES-E WV suite). The GFS continues to bulge slightly warmer 850mb temps across the Catskills/Poconos and NJ in exiting making it less favorable within increasingly stronger SW-NE gradient in the remainder of the guidance suite. Otherwise, a more traditional alignment of guidance members will occur with the wave lifting north. The GFS is fastest and generally weaker than the slower but stronger UKMET, and even deeper 12z NAM. The 00z CMC/ECMWF lag a little slower; but allows for a more traditional blending scenario. The trailing southern stream shortwave energy will enhance south of the 40N70W benchmark late Monday into early Tuesday, under the influence of the upstream (see section below) 'kicker' wave. The GFS remains fast/likely too fast, but the UKMET/NAM depict a slower, more reasonable similarity. The 00z CMC/ECMWF are too slow, allowing for the surface occlusion to linger likely too long. This allows for the ECMWF to shift well north of the other consensus solutions. While neither camp looks ideal, a compromise of the NAM/UKMET and the ECMWF/CMC seems prudent after 28.12z (48hrs); as such a Non-GFS blend is supported at slightly above confidence. Trough axis crossing Canadian Prairies, clipping Upper Midwest/Great Lakes through Monday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average GOES-WV suite denotes maturing cyclone across central Alberta/Saskatchewan, this will drop a frontal zone that crosses the Northern Plains later today into Monday. While the bulk of the shortwave energy will lift north over the downstream blocking ridge in Ontario, the base of the shortwave will slide underneath across the Great Lakes eventually acting as a kicker to the larger coastal cyclone (see section above). Until the wave is under-cutting the ridge on Monday, there is solid model agreement. The GFS starts to accelerate east-southeast thereafter but that is driven more by the downstream faster exit. Additionally, the frontal zone will drape across the northern Plains and act as a seeding of the approaching shortwave (see section below) Monday. Here the guidance is tightly agreed upon to at least start the next system on similar footing. So a general model blend can be afforded at above average confidence Pacific Northwest shortwave on Monday Digging Across Northern Plains Tues and into a closed Low over Midwest by Wed...Accompanying Front Thru Southern Plains (Tues) and Mid-MS to TN Valleys (Wed) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General blend through 28.12Z 12z GFS and 00z UKMET/ECMWF blend thereafter Confidence: Average Strong Gulf of AK gyre, will eject the base of shortwave and strong anti-cyclonic upper level jet features across British Columbia the end of Monday into Tuesday. Preceding the main wave, the left exit of the jet progresses through the Northern Rockies. This feature links up with the frontal zone from the prior shortwave (see section above) and progresses through S MT into the Northern Plains at the same time. Solid moisture from the Pacific stream will remain and intersect with return moisture across the Plains. This will spark some convection and spur surface cyclogenesis. Guidance has been flip-flopping on which solution will be over-aggressive with the convection. Yesterday's runs were the ECMWF followed by the UKMET. The last few cycles has been the GFS. This is still the case with the 12z run, but has tempered a bit but still showing stronger moisture flux convergence on the elevated boundary versus more surface rooted cells, which tends to be a weakness of the GFS. As the main wave exits the terrain early Tuesday, it will dig and lead to a binary interaction with the initial shortwave...the stronger the initial wave the further north and broader the trof will be. While, moisture is there, it is meager and latent heat release to build this northward solution is looking less likely. The 12z GFS/NAM and 00z UKMET are stronger with this than the ECMWF so are a tad north initially. However, the 00z CMC is much slower overall and quickly moves out of preference by 29.00z. As the upstream wave digs by Wed, the strong 12z NAM, tightens even further showing some typical day 3 over-amplification bias...that slows its overall progression. Otherwise, the weaker/south ECMWF is not too far from the stronger/north UKMET/GFS and the GEFS/ECENS solutions to support a blend of the three by the end of day 3. There are numerous internal interactions and importance of convective upscale enhancement via latent heat release to have tremendous confidence in a lower than normal predictability forecast. However the agreement is solid to support average confidence at this point in a 12z GFS and 00z UKMET/ECMWF blend. Glancing effects(front) of lead shortwave late Tues/Wed in PacNW...Full bodied shortwave reaching PacNW by 00z Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 29.00z Non-NAM thereafter Confidence: Slightly above average through 29.12z Slightly below average thereafter The next Pacific shortwave has trended a bit north, slower and therefore a bit flatter. Given the older Gulf of AK binary gyre will be eroding late Monday int Tuesday, the upstream wave's arrival late Tuesday will reset the larger scale trough but shift it eastward. This will allow for solid southwesterly moisture transport mainly into southwest British Columbia but is expected to clip the Olympic Peninsula by late Tuesday/early Wed. Another trailing Pacific wave will quickly approach on Wed and with the lingering frontal zone directed toward the WA coast...stronger moisture flux will enter W WA by 00z on Thurs. There is solid agreement in timing/depth and position of the initial wave/frontal zone through Wed, with perhaps the 12z NAM a tad too strong (typical by D3). This likely leads the NAM to be a tad too fast/south with the secondary push. However, the 12z GFS and 00z UKMET are quick on the heels, with the ECMWF/CMC both lagging in typical formation. Being a bit faster, the UKMET/GFS are a bit deeper and more mature in the cyclogenesis process, also typical. The ensemble suite shows moderate to high spread across the timing of this wave, to not really help determine which side to go toward...as such will split in a non-NAM compromise. Confidence is slightly above average through the first wave 29.12z but becoming slightly below average given the spread for the last few time-steps of the short-term forecast. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina