Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Apr 01 2020 Valid Apr 01/1200 UTC thru Apr 05/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Western Atlantic closed low and offshore surface low through Friday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Any of the operational models Confidence: Above average A maturing large scale closed trough, with intense surface low pressure dipping down to near or below 980 mb, will affect the east coast and western Atlantic with strong winds. The general circulation pattern now features this and another developing high amplitude, closed trough over the Intermountain West / northern Plains. So the pattern is slowing down, and the intense cyclone off the coast will be slow to exit. The global models are in strong agreement in their handling of this system. If any difference could be noted, the GFS shows its typical bias by Day 3 with the trough swinging farther forward and out to sea. ...Intermountain West / Great Plains... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS/ECMWF/UKMET/Canadian... less weight on GFS after 04/00Z Confidence: Average As a large scale closed trough develops over the mountains and parts of the Plains, elongated low pressure will slip out through the Plains and Upper Midwest along a cold front, with wintry weather impacts felt across the northern Rockies to the Upper Midwest. In the 12Z cycle the global models trended toward better agreement as to most of the details here. The ECMWF/UKMET/Canadian are best synced in terms of the timing and location of individual shortwaves, and blending them will help form a consensus around the amplitude differences. The NAM is the first to drop out of the larger scale consensus. Its forecast is largely in step with the other models through 03/12Z, so only 48 hours into the forecast. But even then the solution is slightly fast and depicts a weaker pressure gradient along the front. It then becomes more unrealistic as it significantly erodes the southern flank of the ridge in Canada, whereas typically a low would become trapped beneath the ridge as depicted in the other models. THE GFS is not far off from model consensus for the northern U.S. weather system throughout its life cycle. The only reason we suggest reducing weight given to the GFS over time is that in broader terms the GFS upper air pattern turns a little out of phase given some upstream problems over the eastern Pacific. Another aspect of this broad scale trough is the increasing likelihood of Gulf return moisture interacting with a front and seasonably strong westerly flow to yield thunderstorms in the southern Plains. We note that the NAM QPF intensity drops off with southward extent. The other models do display some spatial spread, but all are more indicative of robust precipitation, so the NAM appears to be a dry outlier, especially across south Texas. ...West Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z ECMWF/UKMET/Canadian Confidence: Slightly Below Average Challenging pattern on the back side of the developing larger scale trough over the Intermountain West. Two noteworthy shortwave troughs come into play along the west coast. The first into California shows the telltale signs of a remarkably fast GFS solution that stands out as an outlier. On the other hand, the ECMWF suddenly went flat with this wave, dampening it on Day 3... but it still does not catch up to the very fast GFS. The second, larger wave shows a tendency to form a closed circulation and expand its influence as it approaches Washington/Oregon on Day 3. The NCEP guidance was first to become more bullish in strengthening this feature and driving it farther south and west offshore. This is especially true of the NAM. Spaghetti plot show that this farther south and west position has lost favor in the ensembles over time, but not entirely. Consensus definitely points toward the 12Z UKMET/Canadian solutions (and 00Z ECMWF/UKMET), but perhaps with some legitimate trending toward a deeper solution farther south and west...per the 12Z ECMWF and GFS. Overall, the GFS problems with the lead wave into California make it less desirable along the west coast, and the NAM is so much deeper with the upstream wave off WA/OR. So our preference remains with the EC/UK, now joined by the 12Z Canadian. But if one wanted to include the NAM for the California wave and GFS for the WA/OR wave, they may have some utility. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Burke