Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 138 PM EST Sat Jan 27 2018 Valid Jan 27/1200 UTC thru Jan 31/0000 UTC ..See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z model evaluation along with latest model preferences/confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Weakening shortwave crossing the western Gulf Coast today... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models handle the system similarly today through tonight as it moves east and dampens out with time. ...Shortwave trough/cold front over the Great Lakes/Midwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show a northern stream shortwave trough crossing portions of the northern Plains and the upper Midwest through tonight and then crossing the Great Lakes region through Sunday. Model spread is rather minimal with this system, and so a general model blend will be preferred. ...Evolving longwave trough over the Eastern U.S. by Tuesday... ...Surface low evolution offshore the Mid-Atlantic/New England... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GEFS mean and 00Z ECENS mean Confidence: Slightly below average The models take a relatively weak southern stream trough out of the southern Plains and allow it to cross the Gulf Coast states through Sunday which will help to drive an area of low pressure along a cold front advancing into the Eastern U.S. Meanwhile there will be an intrusion of northern stream shortwave energy crossing the Midwest that will be digging southeast through the Ohio and Tennessee Valleys through Monday. The two streams will attempt to phase along the East Coast by later Monday and will likely lead to the development of a longwave trough over the Eastern U.S. that will take on a neutral to negative tilt. The aforementioned low pressure wave which will be over the Gulf Coast states should lift northeast and redevelopment over the Mid-Atlantic coastal plain Sunday night and then move offshore the Delmarva before then intensifying east of New England Monday night. The 12Z UKMET and 12Z CMC have tended to trend somewhat toward the 00Z/12Z ECMWF solutions which depict a bit more of a negatively tilted trough along the East Coast. This added energy in the base of the trough also allows for the deepening low pressure off the East Coast to be a bit slower to lift away to the northeast. The 00Z ECENS mean supports this camp. Meanwhile, the 12Z NAM and 12Z GFS along with the 12Z GEFS mean support the idea of low pressure lifting a bit more progressively away from the Mid-Atlantic and New England. Given the two model camps and relatively increased model spread, an attempt to compromise will be suggested by blending the GEFS mean and ECENS mean. ...Large upper trough/closed low impacting the Northwest... ...Surface low near the U.S./Canadian border moving east... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS and 12Z ECMWF Confidence: Above average The models bring a robust upper trough into the Pacific Northwest by later Monday and Tuesday as a strong closed low wraps up over the Gulf of Alaska. The upper trough and associated cold front will move inland and cross the northern Rockies before then ejecting east across the northern Plains. A strong area of low pressure with this system will traverse southern Canada just to the north of the U.S./Canadian border. The 12Z UKMET appears to be just a tad too progressive, with the 12Z NAM and 12Z CMC a bit too slow. The 12Z GFS and 12Z ECMWF best approximate the model consensus and are well clustered, so will prefer a blend of these two solutions. ...Shortwave approaching the Pacific Northwest late Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM consensus Confidence: Above average The models allow another shortwave to approach the Pacific Northwest and adjacent areas of British Columbia by late Tuesday. The 12Z NAM is somewhat of a stronger outlier as the energy approaches British Columbia, with the remaining models pretty well clustered with timing and depth. Thus a non-NAM consensus will be preferred at this time. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 MB forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison