Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 239 AM EDT Tue Mar 13 2018 Valid Mar 13/0000 UTC thru Mar 16/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z model evaluation...including model preferences and confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Nor'easter impacting New England... ...Deep closed low evolution over the Northeast through Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average The models gradually phase the strong northern trough digging down over the Great Lakes/northern Mid-Atlantic region with the negatively tilted shortwave trough lifting up off the East Coast through the late Tuesday and early Wednesday time frame. The result will be strengthening nor-easter that lifts up off southern New England by midday Tuesday and then crosses over Nova Scotia by early Wednesday. The low will reach peak intensity east New England Tuesday afternoon and then gradually weaken thereafter as the deeply occluded cyclone slowly begins to fill. Model mass field spread begins by 36 hours as the 00Z NAM begins to take its surface low west of all of the global models. Of the global model guidance, there is a little more tightening of the spread with the 00Z cycle as the GFS, UKMET, CMC and ECMWF approximate the model consensus and as a cluster is very well supported by the 00Z GEFS mean. As a deep closed low evolves over the Northeast, the initial low with the nor'easter will eventually weaken and dissipate by midday Thursday, however, new cyclogenesis farther offshore will lead to a new and rather strong surface low that lifts up across the Canadian maritimes Thursday into early Friday. The better model clustering at this point favors the non-NAM suite of guidance, and thus a non-NAM blend will be preferred by the model mass fields. ...Reinforcing shortwave digging over the Great Lakes Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00Z GFS, 00Z ECMWF and 00Z CMC Confidence: Above average The models all take another northern stream shortwave and dig it southeastward across the Great Lakes region by Thursday which will reinforce the larger scale trough and closed low over the Northeast through Friday. The 00Z NAM is perhaps just a tad stronger than the global models with this feature, and with respect to the global models, the 00Z UKMET has become a bit of a progressive outlier. The 00Z GFS, 00Z ECMWF and 00Z CMC are very well clustered in between these solutions and a blend of these models will be suggested as a compromise for the time being. ...Large closed low/trough moving into the West... ...Surface cyclones offshore the West Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend...through 36 hours Non-NAM blend...after 36 hours Confidence: Average A large deep layer closed low offshore the West Coast will be moving east over the next few days and gradually into the Western U.S. as multiple shortwaves round the base of the trough. One shortwave will focus a surface low pressure up off the Pacific Northwest for Monday, and then with a new area of low pressure likely consolidating by Wednesday offshore as additional energy digs into the base of the trough and arrives across the West Coast. This second low will drop southeast and approach coastal areas of northern CA or southwest OR by early Friday. The 00Z UKMET is a deep outlier with the initial surface low lifting up off the Pacific Northwest through Tuesday, with the remaining guidance including the 00Z NAM, 00Z GFS, 00Z CMC and 00Z ECMWF all better clustered with timing and depth. Regarding the second surface low evolution, there are some modest differences noted. Most notably the NAM with its low placement south of the model consensus by early Friday, and also its depiction of a sharper shortwave trough arriving across coastal areas of central/northern CA. Given the details of the first low evolution and then the second one, the model preference will be a non-UKMET blend through 36 hours, and then a non-NAM blend after 36 hours. ...Shortwave ejecting out across the central Plains by Friday... ...Surface low evolution off to the east across Kansas... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00Z UKMET and 00Z ECMWF Confidence: Average A lead shortwave progressing downstream from the large upper trough/closed low moving into the West by Thursday will eject out across the central Rockies and over the central Plains though early Friday. This will foster cyclogenesis over the High Plains which will be advancing east across western KS by the end of the period. The 00Z NAM is a notably faster and weaker outlier solution with this system and will be discounted. The 00Z CMC for its part is weaker with the shortwave and also farther north which fosters a surface wave a little north of the model consensus. The 00Z GFS trended slower from its previous run and is at least close to the well clustered 00Z UKMET and 00Z ECMWF solutions. However, there are now some concerns that the GFS could be a tad too strong by the end of the period and perhaps a little too far north with the surface low. The 00Z GEFS mean is more supportive of the UKMET and ECMWF, and thus that mode cluster will be preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison