Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 246 PM EDT Thu Apr 02 2020 Valid Apr 02/1200 UTC thru Apr 06/0000 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... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Above average 19Z update: The 12Z ECMWF/UKMET/CMC offer very similar solutions to the earlier GFS/NAM with the evolution of the closed upper level low and associated surface low through 48 hours. No change in the model preference is needed for the final preference. ---previous discussion--- Several lobes of vorticity will pivot around a large closed upper level low currently centered near 40N/62W, with one particular piece of energy seen in recent water vapor over Quebec/Ontario. In the highly amplified and blocky pattern, the upper level low will be slow to move east over the next 24-36 hours across the Western Atlantic Ocean. The surface low will retrograde and drift southwest, making its closet approach to Cape Cod by 12Z Friday. Model differences this cycle are fairly minimal and the most recent GFS/NAM offer very similar solutions over the next 36 hours. Between 00Z and 12Z Saturday, the upper level low begins to exit the area. At the surface, the 00Z UKMET is the furthest west while the 00Z CMC is on the eastern end of the model spread, but overall there is minimal differences. Given the increased consensus and model tightening, a general model blend should suffice for the model mass fields through the next 48 hours. ...Upper Midwest/Great Plains/Mississippi Valley... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Average 19Z update: No significant changes with the rest of the guidance now available. The 12Z UKMET remains an outlier with its evolution of the mid/upper level energy over the northern Rockies day 2/3, as it is more progressive with the initial shortwave and then results in upstream ridging while the other models are much slower. Will still lean on a non-UKMET blend at this time. ---previous discussion--- Large scale troughing over Alberta will push a series of shortwaves across the northern Rockies into the Upper Midwest and Great Lakes regions over the period. With a parent surface low developing well south in the central Plains, a secondary/weaker low is expected to develop over the Upper Midwest Friday into Saturday. Aloft, the shortwave is well agreed upon by the latest model guidance. The GFS and NAM to some degree are a bit faster than the rest of the guidance, but its differences are fairly minor. The notable difference however is in the UKMET, which is not nearly as amplified with the trough axis Friday/Friday night. QPF differences are most apparent with the NAM, which is further west with its heaviest QPF axis for the 6-hour periods late tonight into Friday morning. But overall with the mass fields, the UKMET remains the biggest outlier and will go with a non-UKMET blend for this area. ...Northern Rockies... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 19Z update: The differences between the GFS and the rest of the 12Z guidance now appears minimal enough to change the preference to a general model blend. ---previous discussion--- Large closed low situated over north-central Alberta will have a couple of shortwaves rounding the base through the northern Rockies over the next couple of days, one Friday and another Saturday morning. Models are in fairly good agreement with the passage of these features with the one notable exception being the 12Z GFS which shows a faster progression than the rest of the models. The surface low that moves through the area is fairly well agreed upon as well spatially and temporally. QPF amounts during this time frame are fairly light and similar between the GFS/ECMWF/UKMET/CMC but given the timing issue with the GFS, will continue with a non-GFS blend. ...West Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z ECMWF and 12Z GFS Confidence: Average 19Z update: Better agreement with the rest of the 12Z guidance now available. The UKMET and CMC came in with a flatter and slightly more progressive initial shortwave, which is trending toward the earlier ECMWF, ECENS, and GEFS means. Now, the spread between the faster GFS/NAM and the rest of the guidance is less, and more of the deterministic models can be incorporated. For the secondary shortwave/impulse that closes off late in the weekend, the guidance also trended together and the overall spread seen in the deterministic models toward 84 hours is much less. A blend toward the GFS/ECMWF would be a consensus approach and is preferred now. The forecast confidence will be pumped up to average now. ---previous discussion--- A pair of shortwave troughs will drop southeastward from the eastern Pacific toward the California coast over the next few days. The first impulse arrives on Saturday and there are some sizable model differences noted. The 12Z GFS and NAM show a faster and flatter progression as the energy moves onshore. Meanwhile, the UKMET/CMC offer a more amplified solution which is slower. The 00Z ECMWF lies in between, more pronounced than the GFS/NAM camp but slower timing more similar to the UKMET/CMC side. The deterministic ECMWF actually mirrors fairly well the 00Z ECENS and 06Z GEFS means. So given the timing and spatial spread, the initial preference for this feature is favored toward the ECENS and GEFS means along with the deterministic ECMWF. The second impulse that rotates through is larger and closes off the California coast by late in the weekend and early next week. Model differences are notable, with a sizable latitudinal spread in the low center ranging from the further north UKMET to the NAM which is on the southern end of the model spread. Here again, the ECENS and GEFS are fairly agreeable through 84 hours and show similarities to the deterministic ECMWF. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor