Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 143 PM EST Wed Feb 14 2018 Valid Feb 14/1200 UTC thru Feb 18/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... ...12Z Model evaluation with latest model preferences/confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low initially offshore Southern California... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models take the upper low offshore southern California gradually off to the east and inland across the Desert Southwest over the next 12 to 24 hours. As the energy moves inland, it will be deamplifying, and it is expected to shear out across the Four Corners region on Thursday. The latest guidance is in good agreement on the details of this, and so a general model blend will be preferred at this time. ...Shortwave digging down across the Pacific Northwest today... ...Energy digging into the Southwest by Thursday... ...Ejecting rapidly east across the Southern U.S Friday.... ...Southern stream shortwave over Tennessee Valley by Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS/12Z UKMET and 12Z ECMWF Confidence: Slightly above average The models allow shortwave energy currently digging south down across the Pacific Northwest to drop down into the Southwest U.S. by Thursday which will effectively help kick the current energy over the Southwest downstream in a sheared fashion. The new shortwave focusing over the Southwest will then tend to capture some of the shortwave/vort energy in the subtropical stream to the west of the Baja Peninsula and then begin rapidly eject downstream across the Southern U.S. by Friday. A southern stream shortwave remnant will then quickly arrive across the Lower Mississippi Valley and Tennessee Valley region by Saturday while attempting to also develop a weak surface low. The 12Z NAM is more sheared and a bit weaker with the southern stream energy and tends to place more emphasis on northern stream energy instead across the Midwest. The 12Z GFS, 12Z ECMWF and 12Z UKMET are better defined and support a progressive system, but the UKMET may be a tad too strong with its surface low development at the end of the period over the Tennessee Valley. The UKMET also is showing a bit more northern/southern stream interaction by Saturday upstream over the the Midwest which the ECMWF is showing as well, but with the UKMET a bit slower. Based on the latest ensemble means from the GEFS and ECENS, a compromise of the GFS, ECMWF and UKMET should still suffice, and this consensus is generally supported by the latest CMC as well. ...Shortwaves crossing the Great Basin, Midwest and Great Lakes... ...Stronger energy over the Pacific Northwest by Saturday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS/12Z UKMET and 12Z ECMWF Confidence: Slightly above average Multiple northern stream shortwaves are expected to traverse the Northern U.S. over the next few days. The models show good agreement with the first shortwave that crosses a portion of the Northwest today and then quickly ejects east across the Plains and Midwest through Thursday. This energy then crosses through the Northeast by Friday. Then a second shortwave traverses the Northwest on Friday and ejects out across the Midwest on Saturday. The models are in good agreement with this second shortwave minus the 12Z NAM which is a somewhat slower and more amplified outlier by Saturday over the Midwest. A third and stronger shortwave impulse then drops south down coastal areas of British Columbia on Friday and into the Pacific Northwest on Saturday. A well-defined low center will accompany this next surge of energy. The 12Z NAM is the slowest with this system, with the 12Z UKMET and 12Z ECMWF the fastest. The 12Z GFS and 12Z CMC split the difference with timing between camps. Will lean overall toward the more progressive consensus with the idea of more vigorously digging energy arriving toward the Pacific Northwest given the strength of the mean layer ridge nosing up across the Gulf of Alaska by Saturday. Thus, collectively a blend of the GFS, UKMET and ECMWF will be preferred with the northern stream shortwaves. Previous model trends at www.wpc.ncep.noaa.gov/html/model2.shtml Previous 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Orrison