Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1222 PM EDT Sun Mar 24 2019 Valid Mar 24/1200 UTC thru Mar 28/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation With Preferences/Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Plains closed Low shearing into Ohio Valley early Mon across Southeast Tues ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Above average The 12z NAM remains slowest to break down the inner core of the closed low that progresses through the Ohio Valley into the N Carolinas relative to the other guidance. The UKMET is generally weaker, but overall the mass fields have a good look in timing/evolution to support a general model blend at above average confidence. As the wave exits, upstream shortwave (see below) will interact with the trailing frontal zone. Shortwave moving through the Great Basin/C Plains to Southeast Sun-late Tues ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-ECMWF blend Confidence: Average Models are starting to come into better agreement as the stretched trof from NW to SE, shifts through the mean ridge into NW flow. The ECMWF continues to inject the wave a bit more concentric and much further south relative to the other deterministic guidance; as such, the wave translates faster and reaches the lingering base of the downstream positive tilt trof. This allows the wave to amplify and wind up a bit greater than other guidance about 6-12hrs earlier too. The 12z NAM is closest to this amplification especially at 72hrs but this would fall in line with a typical over-amplification negative bias at the end of the forecast period. The 12z GFS and 00z UKMET/CMC are a bit slower initially eventually catching the slowing ECMWF with the UKMET passing it (closer to the NAM). While the ECMWF is not well out of phase, will allow the NAM to represent it (slightly weaker and slower) in the blend. As such a non-ECMWF blend is preferred at average confidence. Pacific Gyre's First Wave Entering Northwest Monday Sliding across Southern Canada Enhanced by Digging Northern Stream Trof Wednesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM through 26/12z 12z GFS/06z GEFS afterwards Confidence: Slightly above average through 26/12z Below average thereafter GOES-W WV suite depicts the first phases of the developing broad Pacific gyre that reaches concentric maturity around 06z Monday (though still off balance to the west). By late Monday into early Tuesday, the gyre elongates W-E with lead height falls grazing the OR/CA coast and driving dual stream of mid-latitude and subtropical moisture streams into NW CA. After this time period, there is a fairly sizable timing differences though the evolution of shape, depth of the cyclone is fairly agreeable. The one exception is the 12z NAM which, unlike the other deterministic guidance has two smaller scale waves about 6-9hrs separated that are out of phase with the one more amplified wave solution of the GFS/ECMWF/UKMET/CMC. This cascades into larger model differences thereafter breaking away from ensemble support. While the remaining guidance still shows timing differences, the 12z GFS slowed a bit relative to the 00/06z runs matching fairly well with the CMC, while the ECMWF continues to lag. By 00z Tues, guidance is fairly solid in splitting the wave's energy, westward back into the gyre with the trailing/southeastern portion of the wave sliding through the col into SW Canada. The UKMET, GFS and CMC being faster generally, favor shedding more energy west, with the UKMET significantly amplifying with a strong surface reflection; while the ECMWF favors energy into Canada. Being slower and a tad stronger the ECMWF and a bulk of ECENS members, phase with northern stream energy and accelerate just north of the US/Canada border. Additionally, the northern stream digging trof fed by a small but very cold pocket of arctic air, allows for uncharacteristically fast timing of the shortwave as well as the digging trof. This leads the ECMWF to stand out relative to other guidance. The 12z GFS and 00z UKMET show greater/faster northern stream development than the 00z CMC. The UKMET/CMC both favor very strong surface cyclogenesis east of the Rockies. A compromise in timing near the 12z GFS seems best but the wave may be a bit too weak (typical of the GFS recently). Would include some of the UKMET, given better timing/track of the surface low, but the strength of the surface wave, both here and west of WA give pause to is inclusion in the blend. As such will support the 12z GFS as best solution to represent current thinking mainly in timing, yet without a better representative, confidence is below average after 26/12z. Main Core of Pacific Gyre Nearing Northern CA by Late Tuesday/Wednesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Average As noted above, the 12z NAM is already off balance initially (please refer to first paragraph is section above for more details) and remains so with the main core of the upper low as it approaches the NW CA/SW OR coasts Wed; the NAM is too fast and well east of the other guidance. The 12z GFS has trended a bit slower initially, which lead to a closer balance in the binary interaction with the weaker lead wave and the main core Tuesday; however, by Wed, it is a bit fast, typical of bias but not tremendously so. The GFS is partnered well with the UKMET (which does not seem to show some of the over-amplification issues seen with the lead wave). The CMC and ECMWF show good stability though perhaps a bit slower of the entire ensemble suite mean...so will favor a Non-NAM blend for the main core of the gyre and surface wave. Again, confidence in the evolution of this portion of the gyre is fairly high, yet moderate spread in timing and smaller scale interactions (esp. with terrain/QPF) yields only average confidence overall in a non-NAM blend. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Gallina