Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1139 AM EST Sun Nov 24 2019 Valid Nov 24/1200 UTC thru Nov 28/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Mid-Atlantic/Northeast shortwave(s) and Associated Surface Low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The last vestiges of the nor'easter across New England occur through the rest of Sunday. The CMC is a bit weaker initially, but overall its slower development reaches the level of the other guidance within 3 to 6 hours by early Monday, that is not a major difference. So a general model blend can be supported at above average confidence for its remainder in the CONUS. ...Shortwave racing through Canadian Rockies Sunday before supporting Northern Tier surface wave Monday and shearing out along the Great Lakes/SW Quebec by Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average GOES-W WV staring to show the amplification of the northern stream shortwave over BC at this time in preparation for crossing the Canadian Rockies. The guidance suite has tightened over the last 24hrs though there remains a small latitude difference in the QPF swath with the NCEP guidance a bit north and the UKMET/CMC/ECMWF suite a tad south (along/just south of the border Mon into early Tues). This is fairly minor and in line with typical biases...to support an equal blend of the guidance to split the difference, especially as the guidance remains agreeable through the shearing phase through the Great Lakes into Quebec. Confidence in a general model blend is slightly above average. ...Lead shortwave in developing global-scale trof in Four Corners by 00z Tuesday... ...Development of strong/broad surface low in Plains Tuesday and Great Lakes by Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS/00z ECMWF blend Confidence: Average Pattern changing shortwave/jet speed max over-topping the Central Pacific Ridge is expected to dive southeast into the Great Basin by 25.18z. In the weak, broad cyclonic flow, this wave starts to buckle through the Central Rockies and start to spin up a broad surface cyclone across the Central High Plains Monday into Tuesday. The ECMWF is a bit slower with this and shows a bit greater ridge amplification across the Rockies increasing upslope moisture flow. While the 12z GFS is in a typically slightly faster transition matching very closely the centroid of the ECENS and GEFS means. While the 12Z NAM is in fairly good agreement, there is greater depth and more concentric appearance to the developing closed low by Wed and is about 40-60dm deeper than the ECMWF/GFS. While on the opposite side, the 00z CMC is very weak and the surface wave is a bit south initially. The 00z UKMET while also fairly agreeable to the ECMWF, is even slower with the wave moving out of the Rockies (atypical) but then rapidly accelerates to match the GFS by the end of the forecast period. Overall, there is growing confidence with the evolution of the system, but with importance of timing and angle of amplification to the orientation of the terrain in the Rockies, will utilize the ensemble centroid and suggest a 12z GFS/00z ECMWF blend as a best representation, though confidence is average in this blend. ...Strong secondary shortwave supporting with very deep surface low near N CA Tues, expanding across northern CA/N Great Basin by 00z Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend (reduce/remove NAM after 27.00z) Confidence: Average The next shortwave quick on the heels of the prior, over-tops the Central Pacific Ridge by 00z Tues. However, being a bit stronger, it tamps down the ridge, and as it slides southeast along the eastern portion of the ridge, a small fraction of northern stream energy aids in a broader wave development eventually toward a closed low. The CMC is the only guidance, that suggests greater influence from the northern stream which leads to it being further east and weaker overall by the end of the forecast period, and is not preferred in the preference. The remaining guidance, all now support a very strong/compact leading edge to the shortwave reaching the N CA coast 00z Wed (late Tuesday), with a vigorously deepening surface cyclone. After landfall, the upper low, broadens and slides south. The greatest agreement exists with the 12z GFS/00z ECMWF but the 12z NAM and 00z UKMET are solid enough in the ensemble suite to have good agreement to incorporate into a preferred blend. The NAM is a bit slow on Wed, that would reduce/remove the weighting with time on Day 3. While there is good alignment of the wave development to provide confidence, its current location/source and influences of the Ridge and northern stream still provide some reduced absolute certainty to this blend. So, while the confidence has improved substantially since yesterday, it is still 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