Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 203 AM EST Thu Jan 02 2020 Valid Jan 02/0000 UTC thru Jan 05/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation with latest Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Upper low over Mexico shearing to the northeast... ...Frontal wave impacting the Gulf Coast/TN Valley... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show good agreement in lifting the southern stream closed low over Mexico off to the northeast over the next 24 hours with the system shearing out across the lower Rio Grande Valley as it become embedded in deep layer southwest flow ahead of an amplifying trough over the southern High Plains. The guidance though does support the idea of a weak area of low pressure/frontal wave lifting northeast out of the northwest Gulf of Mexico and across MS/AL before dampening out. This will be a player in generating heavy to excessive rainfall across portions of the Gulf Coast states/TN Valley. A general model blend will be preferred. ...Amplifying longwave trough across the central U.S. ...Surface low from the Lower MS Valley to the Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00Z NAM/12Z ECENS mean Confidence: Below average A split flow upper trough with several small scale shortwave impulses will eject out across the Plains going through the end of the week, with initially the dominant amplification occurring down over the southern High Plains. However, by late Friday, multiple pieces of energy in the northern stream will dig down across the Midwest and lead to a more dominant northern stream trough and closed low evolution which will likely pick up the southern stream energy. This will lead to surface low development over the lower MS Valley which will lift northeast toward the OH Valley by Saturday. This system will cross the northern Mid-Atlantic by Saturday evening with low pressure redeveloping south of Long Island by early Sunday. The 00Z GFS had trended closer to the 12Z ECMWF and GEFS/ECENS mean cluster regarding the low and and mid-level trough evolution. However, with the remaining 00Z cycle of guidance, the 00Z UKMET and especially the 00Z ECMWF/CMC solutions have all trended a bit more progressive and farther south. This is related to the remnant southern stream energy being a bit more progressive and not completely phasing with the stronger aforementioned northern stream closed low. The 00Z GEFS mean is now out of tolerance with the GFS toward the end of the period and suggests the GFS is tucking low pressure in too close to southern New England and then the Gulf of Maine. The 00Z CMCE though suggests that the CMC/ECMWF deterministic solutions are perhaps over adjusting too far south. As a means of compromising in between the camps, a preference toward a blend of the 00Z NAM/12Z ECENS mean will be recommended. ...Shortwave trough/low impacting the Pacific Northwest... ...Energy ejecting out across the Plains/Midwest by Sat/Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average Model agreement is actually rather good with the first shortwave and attendant surface low that aims for central British Columbia by late Thursday which will help drive a warm front up across western WA Thursday night. After this system, fast upstream Pacific flow will drive another shortwave and strong surface low eastward to near the northern end of Vancouver Island by Friday night. A cold front associated with this will then cross the Pacific Northwest by early Saturday. The 00Z cycle of guidance has come into better agreement with the timing details of the second system with only the 00Z NAM now perhaps appearing a tad too slow. This system will advance very quickly downstream across the Intermountain West and across the northern Plains and Midwest Saturday and Sunday. The 00Z NAM by the end of the period does appear a tad too slow as low pressure and a frontal system crosses southern Canada and adjacent areas of the Plains/Midwest. Given the latest trends and model clustering, a non-NAM blend will be preferred with this energy. ...Split flow trough near the Pacific Northwest by Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Average The models show a new split flow upper trough and attendant front approaching the Pacific Northwest by Sunday. The 00Z NAM/GFS solutions are a tad faster than the 00Z UKMET/ECMWF solutions, but there is a bit better clustering with both the northern and southern stream components. The 00Z CMC overall appears a bit too slow with the entire evolution of the trough axis, so a non-CMC blend 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