Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1242 PM EDT Sun Mar 25 2018 Valid Mar 25/1200 UTC thru Mar 29/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Southern stream portion of positively tilted amplified trough over the West, slowly moving east to the Plains by Wednesday... ...Surface low over the Southern Plains through Monday, getting overtaken by a cold front on Monday Night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET Blend Confidence: Slightly Above Average Models have come into much better agreement over the past couple model cycles, and now offer very similar forecasts of the various mass fields as this trough pushes slowly to the east. Previously the GFS and NAM advanced the trough faster to the east, which led to substantial differences in height fields aloft over the Southern Plains in the Monday-Tuesday time frame. They have now converged to the consensus of the other global models, which leads to increasing confidence and a preference for a general model blend overall. The NAM and GFS do try to advance the trough a bit faster toward the end of the diagnostic period (28/12Z to 29/00Z), but not enough that they are excluded from the model preference for this system. The one exception is the 00Z UKMET which has more substantial differences with its height fields aloft by Wednesday. It shows the trough becoming negatively tilted and beginning to eject to the northeast with a more substantial wave digging into the Intermountain West (discussed below). These lead to lower heights across much of the West and higher heights across much of the East, but such a scenario is not supported by any of the other operational models. ...Southern stream portion of the trough in the West, kicking east into the Great Lakes Tuesday and Wednesday... ...Faster eastward motion of northern portion of surface cold front with possible weak surface low in the Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 00Z ECMWF Confidence: Slightly Below Average Through Monday Night, the models look relatively similar with how they handle this shortwave ejecting off to the northeast from the Intermountain West. The differences begin to emerge after that and are primarily related to initial amplification of the shortwave, and interaction with vorticity lobes extending north off the closed low over the Atlantic (discussed below). The 12Z NAM and 12Z UKMET show the wave amplifying as it begins to reach the Great Lakes and Ontario on Tuesday. From a conceptual standpoint, this seems less likely than the flatter wave offered by the other global models, given that it will be heading into a region of confluent flow aloft which tends to not favor amplification. The 00Z CMC offers differences from the ECMWF and GFS particularly with how it has a secondary shortwave arriving in the Great Lakes on Wednesday that is stronger than on any of the other global models. This leads to a surface low in the vicinity of Lake Erie (where other models only have weaker hints of a surface trough), and greater downstream amplification of the ridge axis over the Northeast. As a result, the preference is to lean toward a blend of the GFS and ECMWF, which are in good agreement overall and are reasonably close to ensemble means. ...Closed low developing off the Northeast today and Monday... ...Accelerated development of a weak Carolinas coastal low well offshore on Monday and Tuesday... ...Associated strong surface high settling into New England... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 00Z ECMWF Confidence: Average The model performance with this system may be affected somewhat by the forecast for the shortwave ejecting through the Great Lakes (discussed above), and as a result the preference for this system is partially tied to that. The 00Z UKMET, for example, offers a forecast reasonably close to other global models into Tuesday morning, but then begins to diverge due to effects from the approaching shortwave to the north and northwest. The 00Z CMC continues to show a slight high height bias just offshore, which leads to a slightly weaker low overall and increasing differences with time. The 12Z NAM has a closed low position forecast that is south of the other operational models and ensemble means, and so at this time it seems like a less likely solution. The net result is that the preference is the same as with the shortwave pushing into the Great Lakes -- a blend of the GFS and ECWMF. ...Shortwave rounding a Pacific ridge through the Gulf of Alaska on Monday and digging into the Northern Rockies Tuesday... ...Surface low developing over Alberta and Saskatchewan Tuesday with trailing cold front reaching the north-central U.S. Tuesday Night... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12Z GFS, 12Z NAM, 00Z ECMWF Confidence: Average Differences with this system will be more subtle given the low-amplitude nature of the wave. The surface low forecasts are relatively similar by Tuesday Night, but there are small differences related to how the models handle the associated shortwave. The 00Z UKMET seems to be strongest with the shortwave, and thus deepens the surface low a little faster with an eventual position to the northwest of the other operational models. Meanwhile, the 00Z CMC seems to be weaker with the initial shortwave, but stronger with a secondary shortwave, and that leads to a height pattern aloft that almost seems out of phase with the other models over the North Pacific on Monday. Eventually the weaker shortwave leads to a weaker surface low over southern Canada. The preference is to lean toward a consensus of the remaining models (GFS, NAM and ECMWF) which provide a surface low position fairly close to the 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