Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1245 PM EDT Sat Mar 21 2020 Valid Mar 21/1200 UTC thru Mar 25/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Initial Model Evaluation with Latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Phasing streams over the Mississippi Valley Mon, supporting East Coast/Northeast storm Tues ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z ECMWF/UKMET blend with 12z GFS only thru 23.12z. Confidence: Slightly below average Active southern stream is ejecting a shortwave across the Four Corners region into the Plains today and this wave will continue to draw deep moisture both off the Gulf, but the subtropical Pacific to support an inverted surface trof/wave through the Mississippi Valley Monday. The northern stream will also be digging across central Canada into the Northern High Plains to help broaden a deeper latitude/phased trof across the Great Lakes by late Monday. Timing and interaction between streams with the inclusion of latent heat release in certain portions of the wave will continue to be lead to large uncertainty to precise impactful sensible weather. This includes the development of the coastal surface cyclone, both in location and depth by Tuesday. The 12z GFS continues to be slightly faster in the northern stream and so does not dig as sharply, yet also shows a bias noted this winter of being a bit too warm with the warm conveyor belt/isentropic ascent channel which has the potential to lead to greater uncertainty with precipitation type across the Northeast. The 12z NAM continues to show a bias of being a bit too strong through the depth, likely due to increased latent heat release in the mid-level column through the Midwest/Ohio Valley, which supports a stronger cyclone as well as stronger trailing anti-cyclone. This is a clear outlier in mass fields; yet has seen the ECMWF/UKMET trending towards it in the development of the surface cyclone. The 00z CMC is slow, even more so than normal in the southern stream. So with reduced northern stream phasing; the northern stream exits faster allowing for favorable upper level divergence to deepen the cyclone over southern New England on Tuesday. The UKMET/ECWMF and ECENS have seen the trend northward and stronger with the cyclone as a whole which continues to be most central in the evolution. Given the uncertainty inherent in phasing streams; will go with the more central evolution of the UKMET/ECWMF as preference; though inclusion of the GFS is likely supportive until around 54hrs (23.12z) when the northern stream/southern stream are most decoupled and the coastal cyclone develops well south of the cluster (though supported by the 06z GEFS). Confidence is slightly below average given the lower predictability of the combining streams. Closed low off SW CA today, tracks across Southwest into Mid-MS Valley, Mon and Tues. respectfully ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS and 00Z UKMET/12Z ECMWF blend Confidence: Slightly above average The large closed low currently near 35N135W will start to advance eastward in the next this evening reaching Central/Southern CA by early Monday. However, even by this point this is a solid difference in the mass fields, with the 00z CMC already significantly slower, which continues through the rest of the short-term forecast period. The 12z NAM is also a bit stronger but also faster. The concern though with the NAM is the poor downstream setup, as the over-strong precursory ridge leads to stronger return flow, further north and while the surface low level mass fields jive well, the effective boundaries/QPF are not, and so like the CMC is not supported. The 12z GFS saw a sizable slowing especially as the wave enters the Plains late Monday into Tuesday, which makes it a tad slower than the remaining guidance but is still not as slow/deep as the GEFS mean/bulk of solutions which also lag the UKMET/ECMWF. Still this fairly small for day 3...and a 12z GFS and 00z ECMWF/UKMET is preferred at this time at slightly above average confidence. Pacific NW closed low, Mon/Tues ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM through 24.00z 12z GFS and 00z UKMET/ECMWF blend thereafter Confidence: Slightly above to above average The northern stream strong shortwave bowling through Bristol Bay/AK peninsula will continue to hug the AK panhandle and BC coast closing off into a small but compact closed low reaching the Pacific NW by Monday. Only the 12z NAM continues to show a broader inner core/ring of energy, which eventually slides eastward relative to the other guidance as the wave elongates from the upstream jet pulling the larger trof pattern southwest into the Pacific by 00z Tuesday; while the inner core stretches east of the Coast Range in BC and accelerating faster/stronger into the southern Canadian Prairies just north of the US border by the 00z Wed. The CMC is once again also slow, possible due to the downstream blocking/slower exiting wave. So a Non-NAM blend preference through 24/00z becomes a 12z GFS and 00z UKMET/ECMWF blend thereafter at slightly above to above average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina