Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 157 PM EST Mon Feb 19 2018 Valid Feb 19/1200 UTC thru Feb 23/0000 UTC ...See NOUS42 KWNO (ADMNFD) for status of the upper air ingest... ...12Z Model Evaluation with Final Preferences/Confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Amplified Western US Trough with Primary Shortwave Lifting Northeast from the Four Corners Today to Ontario by Tuesday Night... ...Associated Weak Surface Low Tracking into the Great Lakes... ...Trailing, Slow-moving Front Stretching from the Great Lakes to the Southern Plains through Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM, Non-CMC Blend Confidence: Slightly Above Average ---18Z UPDATE--- The 12Z CMC has been removed from the model preference. Its heights aloft are much lower than other models over the South-Central US, while still having one of the strongest upper level ridges on the east coast. The remaining global models' 12Z runs were relatively consistent with their previous runs. ---PREV. DISCUSSION--- From a synoptic scale, the models are in very good agreement on the big picture, which is to be expected with a relatively stable 1-wave pattern over the CONUS -- an amplified trough in the West and an amplified ridge in the East. There are still some small detail differences on the mesoscale, or with timing of individual shortwaves embedded in the broader flow pattern, that could make an impact on the forecast. The 12Z NAM kicks the primary shortwave out of the base of the trough a little earlier than the other models, and thus is about six hours faster with the progression of the wave and the associated surface low. With a lack of support for such a scenario from other models or ensemble members, the preference is to exclude the NAM at this time. The 12Z GFS, meanwhile, continues to be on the slower end of the model spread, which was also the case yesterday. However, the difference between the GFS and the other global models has narrowed sufficiently, that it is a reasonable possibility. The 06Z GEFS was slightly faster with the forward progression of the surface low, indicating that the GFS may converge further to the other models over the next couple cycles. Otherwise, the preference is for a broad-based blend to account for mesoscale uncertainties. Along the southern extent of the front in particular, convection may be the primary driver of certain things, such as how quickly the front advances to the southeast. A broader blend would account for the possibility of small mass field variations having an effect on the sensible weather impacts. ...Secondary Shortwave Dropping South Along West Coast of North America through Wednesday, Reinforcing the Larger Trough over the Western CONUS... ...Shortwave Kicking into the Four Corners on Thursday with Front Possibly Returning North Across the South-Central US... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Average ---18Z UPDATE--- No change to the preliminary preference. 12Z models remained relatively consistent with their previous model runs. ---PREV. DISCUSSION--- Models have come into better agreement with the shortwave digging along the West Coast on Tuesday and Wednesday. The GFS had previously been much more amplified with the shortwave trough aloft, and had a much stronger surface low. While the other models have trended slightly more amplified than they were a couple model cycles ago, the GFS has trended substantially less amplified with the trough aloft. The resulting model mass fields are relatively similar. The primary difference appears to be timing, with the 12Z GFS and NAM slightly slower than the 00Z ECMWF, CMC and UKMET. The GFS and ECMWF both have support from their respective ensemble means. The differences through the short term period would be relatively subtle, but a faster solution (closer to the ECMWF) would produce height falls over the Southern Plains sooner, with a corresponding earlier low-level mass field response. Thus, the return flow and surface front may begin to push north from the northwestern Gulf a little sooner. That being said, there is not a clear preference between the small timing difference, so a general model blend is preferred with this system. ...Third Shortwave Also Digging South Along West Coast Wednesday Night and Thursday and Further Reinforcing the Trough... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z GEFS Mean, ECMWF Confidence: Slightly Below Average ---18Z UPDATE--- Substantial differences remain on the 12Z model cycle for this system, although the GFS does still seem to be the only model that digs the shortwave so far south. The 12Z CMC also maintained a more concentrated vorticity maximum relative to the other models, despite not closing off a low any more. The preference will remain with the GEFS Mean and ECMWF as they remained relatively consistent. ---PREV. DISCUSSION--- More significant differences emerge with the mid-week shortwave digging into the West Coast. The 12Z GFS digs the primary shortwave into southern California and has the primary mid-level vorticity maximum in that area by 23/00Z. The other models have it further north, and this is supported by the vast majority of the GEFS/ECMWF Ensemble clusters. The 06Z GEFS mean has lower heights near WA/OR, with higher heights in the Desert Southwest, so the GEFS mean seems like a reasonable alternative to the GFS. The 00Z CMC closes off an upper level low very quickly and has a much more concentrated vorticity maximum relative to the other models (which have an elongated/sheared appearance along the base of the trough). These differences in the GFS and CMC seem substantial enough to not be included in the preference at this time. Therefore, will lean more in the direction of a blend of the ECMWF, NAM and GEFS Mean. ...Large Mid-Upper Level Anticyclone just off the Southeast Coast with East Coast Ridge Axis... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 00Z ECMWF, ECMWF ENS Mean Confidence: Average ---18Z UPDATE--- No change to the preliminary preference. 12Z models remained relatively consistent with their previous model runs. ---PREV. DISCUSSION--- The 00Z CMC and UKMET have one of the stronger depictions of the upper level ridge in the first 24 hours of the forecast, and this eventually grows over time by mid-week. The 00Z UKMET eventually has higher heights aloft from the Tennessee Valley into the Upper Midwest, and has the biggest deviation from the model average. Meanwhile, the 12Z GFS is much flatter with its depiction of the Ridge by Wednesday and Thursday, and has lower heights over much of the Northeast. The preference is for a depiction in the middle of these extremes, and closest to a blend of the 00Z ECMWF and 00Z ECMWF Ensemble Mean. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers