Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1209 PM EST Thu Nov 29 2018 Valid Nov 29/1200 UTC thru Dec 03/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preliminary Preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Central U.S. Storm and Downstream Pattern into the East... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z NAM/GFS and 00Z ECMWF A compact shortwave trough is expected to briefly take on a negative tilt as it ejects from the Rockies. It will then go through a fairly quick life cycle as a mature low - driving winter weather impacts in the northern High Plains and central Plains to Upper Midwest. The NAM/GFS/ECMWF are in seemingly good agreement, especially through most of the impact period, Days 1 and 2. The ECMWF tucks the surface reflection up under the low a little farther west and a little stronger than some guidance. The NAM thermal fields are slightly colder than other guidance, with cooling taking place at a more rapid rate, such as over western Nebraska. On the one hand these are consistent with biases of these models, but on the other hand the synoptic pattern supports such trends as they are depicted in the NAM and ECMWF, so their solutions do look very valid. ...Western U.S... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 00Z ECMWF / Canadian Overall when considering the full CONUS and the eastern Pacific, the forecast carries quite a bit of uncertainty with respect to shortwave details. Consistency run to run has not been great. NCEP guidance wants to flatten the flow on Days 2 and 3, with broad cyclonic curvature over the west, and generally warmer heights than the non-NCEP guidance. Looking at trends in the ensemble spaghetti plots has not proven too helpful in this case, as the ECMWF and Canadian have become more emphatic in their lower heights in the Southwest on Day 3, whereas the GEFS has become more emphatic on warmer heights and absence of a marked vorticity center there at that time. This is all related to a somewhat jumbled mess of shortwave interactions in multiple streams over the Pacific. Based on synoptic thinking, we would argue for continuity of the shortwave digging into the Southwest at 02/00Z so that a distinct low center / ECMWF-Canadian solution / is preferred. This also sticks with the previous WPC preference from overnight. Would not be shocked, however, if the full 12Z suite reveals some other preference, so please check back for the final PMDHMD around 19Z. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Burke