Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1234 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: Blend of the 00Z GFS and 12Z ECMWF Confidence: 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, the 12Z UKMET and 12Z CMC align farthest to the east, with the 00Z GFS and 12Z ECMWF farther west, but east of the NAM. 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 GFS and ECMWF which have excellent support from the latest ensemble means and also are very close to the model consensus. So, a blend of the GFS and ECMWF will be preferred for the model mass fields. ...Reinforcing shortwave digging over the Great Lakes Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00Z GFS, 12Z ECMWF and 12Z CMC Confidence: Slightly 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 12Z UKMET appears to be just a little on the weaker side. The 00Z GFS, 12Z ECMWF and 12Z CMC are 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 Blend of the 00Z GFS/12Z ECMWF...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 12Z 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, 12Z CMC and 12Z 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. The CMC is also little south of the model consensus but is a bit weaker. 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 GFS/ECMWF 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 GFS, 12Z UKMET and 12Z ECMWF Confidence: Slightly above 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 12Z CMC for its part is a little slow with the ejecting shortwave trough but also appears to be a tad too weak. The 00Z GFS trended slower from its previous run and is close to the reasonably well clustered 12Z UKMET and 12Z ECMWF solutions. A blend of the GFS, UKMET and ECMWF will be preferred at this time with this system. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison