Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1139 AM EST Thu Feb 08 2018 Valid Feb 08/1200 UTC thru Feb 12/0000 UTC ...See NOUS42 KWNO (ADMNFD) for status of the upper air ingest... ...12Z Model Evaluation with Preliminary Preferences/Confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Low-Amplitude Shortwave Rotating Around Large Upper Level Hudson Bay Low, Through the Great Lakes and Northeast... ...Associated Elongated Front From Northern Rockies Into the Great Lakes Along Northern Stream Jet Core... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC Blend Confidence: Above Average A low-amplitude shortwave currently digging into the Canadian Prairies will continue to rotate around a large upper level low near Hudson Bay. For Friday and Saturday, that will mean the shortwave pushing into the Great Lakes on Friday and getting gradually sheared out, quickly passing the Northeast on Saturday. An associated front will be draped on the south and southeast periphery of an Arctic high from the Northern Rockies into the Great Lakes. Model differences in this region are relatively minor in the mass fields. The one exception would be the 00Z CMC which appears to have a high height bias through much of the CONUS domain relative to the other models. Therefore, a general model blend which excludes the CMC is the preference at this time to account for small uncertainties related to the minor amount of remaining model spread. ...Developing Front From the Arklatex Region Northeastward to the Northeast on Saturday and Sunday... ...Eventual Cyclogenesis Along the Front in the Eastern US... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GFS, 06Z GEFS, 00Z EC ENS Confidence: Average Several other shortwaves will also rotate into the CONUS in the broad cyclonic flow around the Hudson Bay low. One will dig from the Pacific Northwest into the Central Plains by Sunday, and another will dig from the Canadian Arctic into the western Great Lakes in the same time frame. Both waves should have a relatively low amplitude and mainly contribute to a gradually developing broad trough over the central US. A low-level front will continue to take shape along the leading edge of the trough as an already-existing baroclinic zone slowly re-orients to a more SW-NE direction. Models do eventually show weak surface cyclogenesis this weekend into portions of the Northeast along this front, in the right rear quadrant of an upper level jet streak. Initial model differences are rather small through the entire region, with the exception of the 00Z CMC. As mentioned in the previous section, it appears to have a high height bias through much of the CONUS domain relative to other models. Later in the period, greater differences emerge as the surface low begins to develop in the Northeast. The 00Z ECMWF and UKMET have stronger surface lows that track further northwest and inland over portions of the Northeast, while the 12Z GFS has a weaker surface low that tracks further east. Given the broad nature of the trough and lack of any focused region of very strong PVA and height falls aloft, the preference is to lean in the direction of a weaker surface low. The ECMWF Ensemble mean and GEFS mean are relatively close, and both lie to the east of the ECMWF/UKMET as well (but not as far east as the operational GFS). To account for a few of the more westerly solutions, the preference is a blend of the GFS with the ensemble means. ...Shortwave Undercutting Northward Extension of Ridge in the Gulf of Alaska and Pushing Into Western NOAM by Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF Confidence: Slightly Below Average Models show the most variability with this system, as there are notable differences with how they handle the shortwave as it undercuts the northward ridge extension in the Gulf of Alaska. This is primarily related to what latitude that occurs, and how much digging occurs prior to the arrival of the wave on the west coast of North America. The 12Z GFS and 12Z NAM show a much stronger wave digging into Washington and Oregon, while the 00Z ECMWF, UKMET and CMC are nearly 10 degrees latitude further north. The preference is to lean toward the models that have the wave further north. As the shortwave will be progressing through a ridge axis, it seems less likely that a wave would strengthen or dig as that process is occurring, and as the GFS/NAM show. Therefore, a flatter wave that ends up further north seems more likely. The 00Z CMC does have some high height bias relative to the other models through much of the domain, so it is not included in the preference. The 00Z UKMET is relatively close to the 00Z ECMWF, but is on the northern end of the spread. Therefore, the preference is to stick closest to the 00Z ECMWF. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers