Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 149 PM EST Sun Dec 29 2019 Valid Dec 29/1200 UTC thru Jan 02/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with preferences/confidence intervals ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Dominant Central Plains through Great Lakes and Northeast closed low/surface cyclone(s) including coastal Low Tues/Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend (less weight to GFS) Confidence: Slightly above average 19z update: The ECMWF/UKMET/CMC trended toward the GFS, but the GFS still stands out with warmer thermal profiles to have some question. However, the mass fields are now strong enough to support a general model blend. However, please refer to WPC winter weather discussion for more thermal profile preferences/discussion WPC QPFHSD. ---Prior Discussion--- Mature/vertically stacked low over IA/MN will give way to the shortwave starting to take shape/amplify across NE TX at this time. This wave takes over the larger closed low as it develops the new triple point low through the MS Valley, Central Ohio Valley into the Great Lakes late tomorrow. The 12z GFS continues to be slightly off-balance favoring a northern/eastern track relative to the overall guidance, additionally the thermal profiles show continued warmer solutions particularly int the transition between the next shortwave lifts up and starts to develop a coastal surface wave on late Monday into Tuesday, which affects precipitation type across PA into the Northeast Mon into Tuesday. The mass fields are not too bad even as the former inner core rotates through supporting the Coastal Low into the next permutation of the closed low center hub in Maine/Southeast Canada by midday Wed. Here, the GFS is very agreeable in a tightly packed solution at the lowest layers. So generally, mass fields could be generally blended together, but given the thermals a still more important a non-GFS blend is preferred through the forecast period at slightly above average confidence. ...Pacific shortwave diving along the West Coast Sun-Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average 19z update: No significant change with the evolution along the CONUS west coast, so keeping with the general model blend through 31.12z. ---Prior Discussion--- GOES-W WV denotes compact closed low core off the NW tip of Vancouver Island with an elongating warming associated with the stretching shortwave aloft, a few degrees west of the US Coast. This area continues to elongate/stretch and the base is expected to reorganize to the new closed low off the CA coast by later today/early tomorrow with an associated surface low. Guidance continues to be very strong with the evolution and timing along the CONUS area to support a general model blend. However, by 01.00z there is some model differences that will play a role in longer term forecast differences as it rotates into the Sea of Cortez/Northern Mexico (so please refer to WPC PMDEPD for more details). ...Atmospheric River sinking into Pac NW by Late Tues... ...Amplifying into broad trof across Northern Rockies/northern Plains by late Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Slightly below average 19z update: Like the 12z GFS, the 12z ECMWF trended a bit stronger initially with better surface reflection further north toward Vancouver Island, though as it translates across the Canadian Rockies it remains slightly south and faster than the NAM and GFS, respectfully. The 12z UKMET, which still remains very strong with the Pacific surface wave, at least trended southward and into the High Plains...showing the difficulty in locking down timing. So a non-CMC blend is preferred but at slightly to below average confidence. ---Prior Discussion--- In the wake of the closed low near the coast currently, very vigorous Pacific flow, noted upstream in the GOES-West WV suite, becomes very zonal into the Pacific NW but is very strong with internal smaller scale waves rippling fast through it. Combine this with deep anomalous moisture tapped from the warm central Pacific aids a strong Atmospheric River that slowly drops south into the CONUS by early Tuesday. As such, predictability is quite difficult especially with timing. Yet, the larger evolving pattern as the main wave/jet energy enters the Pacific Northwest and bleeds over into the Northern Rockies is taking shape fairly agreeably as a broad but modestly deep longer wavelength into the Northern Plains by 00z Thursday. Here the ensemble suite shows the broad pattern but given the spread, effectively loses the signal and washes it out. Still, deterministically, the 12z GFS and 00z ECMWF are very central to the ensemble suite providing confidence but also show a more realistic depth of the wave and surface low track to suggest a blend of the two. The 00z UKMET/CMC are both much to strong with strong surface reflections in this stream that transfers into S Canada well broader and well north of current preference/thinking. The 12z NAM is a bit slower and stronger therefore encapsulate some of uncertainty in the ensemble suite and will be included in the blend to help account for some of that "positive" uncertainty, even if not ideally desired...perhaps showing some typical day 3 more amplified typical negative bias. Overall confidence is slightly below average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina