Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 130 AM EST Wed Mar 06 2019 Valid Mar 06/0000 UTC thru Mar 09/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation Including Preference and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep trough over the Great Lakes/Northeast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average WV loop shows broad trof crossing the Great Lakes currently that will continue the progress through New England with little fanfare. Likewise, an amplifying kicker shortwave currently forming in N Manitoba will be the last vestige of the large scale/global trof over the Northeast American continent to affect the CONUS late Wed into the far NW Great Lakes. Like the wave before, the guidance is very agreeable and sensible weather is fairly quiescent with exception downstream of open lakes (i.e. Lake Effect snows). A general model blend is supported at above average confidence. ...Deep closed low center near 32N 143W... ...Arriving across CA by Wed... ...Shearing downstream across the central Plains on Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend (weight to ECMWF/CMC) Confidence: Slightly above average 07z update: No significant change in the 00z non-NCEP suite to suggest changes to the initial thinking/preference. ---Prior Discussion--- Currently a solid shortwave trough is affecting the West Coast with a strong warm conveyor directed at southern CA. Guidance is in fairly solid agreement with maintaining the strength of the warm conveyor though the Great Basin into the Central Rockies eventually leading to an axis of warm advection snows across the Central Plains by Thursday, as the shortwave shears into increasingly confluent/zonal flow across the Ohio Valley by late Thurs/Fri. The 00z NAM continues to be most amplified and is resistant to shearing the inner core of the wave, making it less desirable in the blend. By Friday evening into Saturday, the GFS and UKMET both show reduced phasing (spaced further away) with a strong polar stream wave clipping the Great Lakes compared to the CMC/ECMWF and bulk of ensemble guidance (and longer term trend). As such would favor higher weighting toward the ECMWF/CMC in a non-NAM blend for this system at slightly above average confidence. ...Closed low in Pacific Northwest ...Progressive Trof reaching West Coast late Thurs/early Fri... ...Phasing through Rockies with deep Plains surface low Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average 07z update: The UKMET trended weaker with the western portion of the closed low, allowing the southern stream to trend faster and match the better timing of the wave ejecting into the Plains. While the 00z CMC/ECMWF were slower, with increased spacing in the trof in the southern stream; leading to a broader wave ejecting out of the Southern Rockies relative to the faster GFS and UKMET, there is a growing consensus to provide increased confidence even though the waves are still upstream and smaller scale internal iterations continue to internally vary. As such a Non-NAM blend is supported at slightly above average confidence. ---Prior Discussion--- Concentric closed low over the Northwest will track eastward while an under-cutting shortwave/jet carves out a trof off the west coast by late Thurs into Friday. Models have come into fairly solid agreement with the timing/orientation of the wave, though both the 00z NAM and the 12z UKMET are a bit sharper and deeper with this trof. As the wave progresses ashore, the closed low also starts stretching with a deepening of the eastern side as it crosses the Southern Canadian Rockies. The UKMET and NAM, remain very strong with the upstream (western) portion of the closed low and start some binary interaction/stretching of the northern portion of the southern trough, delaying its crossing of the Central and Southern Rockies Sat. The orientation of the western portion of the closed low with the southern stream support solid model consensus of the wave ejecting with negative tilt and highly favorable upper level pattern for rapid surface cyclogenesis by 12z Sat in KS. While the ECMWF shows some slower northern stream wave increasing the stretching of the southern stream trof, weakening the SE portion of the wave, relative to the GFS/CMC... it is fairly minor given all the internal mesoscale features, jet structure. So while there is fairly strong GFS/CMC/ECMWF agreement, including a tightening ensemble (GEFS/CMCE/ECENS) suite, there is only average confidence in the blend. This is due to much of the energy features remain upstream of the best data network in the Pacific at this time, as well as upscale synoptic mass fields are contingent on the smaller scale internal interactions that are quite uncertain and vary with each model run. ...Next Pacific trough and surface low nearing Central West Coast Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 07z update: The trended faster, though like the ECMWF/UKMET eject the shortwave as two separate pulses into the easterly flow, reducing the compact nature manifest in the slightly faster GFS/NAM. This significantly reduces the overall spread and shows some increased overall agreement suggesting a general model blend would suffice at this time into the future, though there are some concerns with some traditional negative biases in the 00z GFS, 00z UKMET both strong and fast and ECMWF/CMC ejecting slower. All in all, a general model blend can be supported at slightly above average confidence. ---Prior Discussion--- On the heels of the last system, another shortwave is shed eastward out of the strong/northward lifting flow across the Alaskan Peninsula and Northwest Gulf of AK. Ensemble trends have been favoring eventual slowing, supporting a broadening more concentric trough that sets up off the Central West Coast by the end of the forecast period, supporting a fairly strong surface reflection as well. The 12z CMC is the clear outlier, supporting a very strong/compact closed low on the NW periphery of the guidance suite. While the ECMWF/UKMET both are slightly north (with the UKMET faster) of the GFS and NAM. The overall cycle to cycle trend favors the GFS/NAM slightly. As a slight hedge, will support a 00z GFS/NAM weighted, non-CMC blend at average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina