Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 310 AM EST Sun Dec 29 2019 Valid Dec 29/0000 UTC thru Jan 01/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation with preferences/confidence intervals ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Central Plains to Great Lakes closed low/surface cyclone(s)... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Slightly above average Large troughing over the central Rockies and a surface low deepening in the central Plains will track toward the Upper MS River Valley through Sunday. In the near term, there is excellent/above average forecast confidence in the large scale synoptic setup however there remains some differences in the timing/strength of individual shortwaves embedded in the closed low. The GFS continues to show a weaker inner core dissolving faster than the rest of the models. This overall puts the low pressure further north/east by early next week, making it less favorable. Otherwise, the rest of the guidance is sufficient enough to include so will continue to prefer a non-GFS blend. ...Remains of closed low/development of coastal low into Northeast Mon-Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average 08Z update: The remaining 00Z guidance showed a trend toward the 00Z GFS, with a faster surface low progressing off the Northeast and New England coasts in the Mon/Tues time frame. As a result, will make a change to the preference and go with general model blend through the period. Upstream deep closed low will continue to break down across the Great Lakes by late Mon into Tuesday and the former pivot/inner core center accelerates eastward under the newer mature center. Through about 31.06Z, the latest model guidance shows high enough agreement to go with a general model blend. Beyond that time frame, the GFS accelerates the mid/upper level low (and surface low) much faster than the rest of the guidance and becomes unusable beyond 01.00Z across the Northeast US. Forecast confidence does drop off beyond that time frame as well, but given the relatively good agreement with the rest of the models, will side with a non-GFS blend beyond 31.06Z. ...Pacific shortwave diving along the West Coast Sun-Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average 08Z update: No big changes seen with the rest of the 00Z guidance now available. With the above average agreement and good run to run consistency, the general model blend is still reasonable. The Pacific continues to be very active but progressive and with an upstream sharp ridge that blocks the flow and rapidly stretches it southward along the entire West Coast of the US. There is growing consensus in the timing and shape/evolution of the wave including the timing of the wave as it closes aloft and spins up a surface low near San Francisco Bay late Sunday into Monday. Overall, the guidance is very strong that a general model blend is supported at above average confidence. ...Atmospheric River sinking into Pac NW by Late Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00Z ECMWF/CMC Confidence: Slightly below average 08Z update: The 00Z ECMWF/CMC show a slowing trend with the arrival of the shortwave and atmospheric river, which puts the GFS even more of a faster outlier (along with the UKMET). Given its typical fast bias, will continue to side toward the slower ECMWF/CMC solutions seen in the 00Z runs. A northern Pacific stream / atmospheric river will push into the Pacific Northwest late in the forecast period (Tues/Wed) on the southern periphery of a strong Gulf of Alaska closed low. In the big picture sense the latest deterministic models show favorable similarities. Once the system moves onshore, more notable differences develop. The ECMWF dives the shortwave energy furthest south as a longwave trough is carved out over the Rockies. This is supported by the ECENS mean as well as the 12Z CMC. The GFS shows this setup as well but is shallower with the depth of the trough that develops. The UKMET is considerably stronger with the entire system as it moves onshore. Overall, leaning on the previous preference, will continue to suggest a ECMWF/CMC blend at this point. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor