Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1127 AM EST Mon Dec 23 2019 Valid Dec 23/1200 UTC thru Dec 27/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preference and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed low across Southeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Slightly above average Deep, symmetric closed low over the Northeast Gulf and associated surface low will continue to wobble with continued eastward progression through the short-term forecast period. The 12z NAM has trended back into the fold, with a faster eastward track, on par with the 12z GFS and 00z ECMWF/UKMET. The 00z CMC continues to slow eject eastward, and so a non-CMC blend is favored for the remainder of the short-range. Confidence is slightly above average for this system. ...Deep latitude trof crossing Southwest, weakening through the Plains before consolidating in Red River Valley of the North Thursday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 06z GEFS and 00z ECMWF/UKMET/CMC blend Confidence: Slightly below average Current GOES-WV suite depicts a compact shortwave/small closed low well southwest of CA with trof extending to another small compact shortwave over NE Oregon. This initial shortwave pairing, will give way to the broadening larger scale trof that expands across the Intermountain West to 135W by Wed, in doing so, the older base shortwave, elongates and weakens moving through the Southwest and eventually into the Central Plains by midday Wed. The northern stream compact low is retained and slowly wobbles into S Alberta. By this point, the guidance is starting to decouple in solutions. The 12z GFS, as an initial omega block, with tail of shortwave energy lingering through the southeast Canadian Prairies and that along with a stronger/slower closed low in Alberta, allows for the shortwave emerging from the Plains to develop much further north and east from prior guidance and bulk of GEFS members (ensemble suite as a whole), which may be a good replacement to the operational. The 12z NAM, while more sensible in the locations of the shortwave pieces, shows typical day 3 over-amplification with much colder air surging through MT/ND in advance of the system (aiding the intensification). As such will favor an ECMWF/UKMET/CMC and GEFS blend but at slightly below average confidence given all the uncertainty in timing, magnitude of each of these pieces coming together over the Red River Valley of the north Thursday. ...Western side of large scale trof in Pacific...Developing SW CA Closed Low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS/00z ECMWF blend Confidence: Average By late Tuesday, a large scale global trof will be forming from 135W into the Intermountain West. A narrow vorticity strip will shear out in favor of the compact shortwave tracking through the southern Gulf of AK by Wed. The 00z CMC is the only solution that retains the vorticity strip through the length of the period with negative moisture flux transport through S CA, Wed into early Thursday. The upstream shortwave will enter the western periphery of the larger trof, enhance with strong meridional 110+ kt 25H jet, and slide along the length of California. Much of the guidance is in strong agreement of the energy's existence and orientation but the timing is a clear uncertainty. The ensemble trend is toward a closed low evolution and overall slower. This favors the mass fields of the ECMWF/CMC and ECENS mean. Interestingly, the 12z GFS slowed by the end of the forecast period, but because it is initially fast (early toward developing the closed low, further north) than the ECMWF/ECENS. Still, this slowing aids support toward the ECMWF/CMC timing overall and away from the 12z NAM and 00z UKMET. Both which have a tendency to be faster with closed lows. So overall, a 12z GFS and 00z ECMWF/CMC blend is preferred though the CMC should be removed for QPF concerns as alluded to initially. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina