Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 300 AM EST Sun Nov 24 2019 Valid Nov 24/0000 UTC thru Nov 27/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Negative Tilted Shortwave Mid-Atlantic / Northeast and Associated Surface Low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Current WV imagery shows shortwave trough over the Ohio Valley with the latest surface analysis having the parent low over West Virginia. The earlier 12Z guidance and now the incoming 00Z guidance all is in good agreement through 36-48 hours with the evolution of this system. The parent low will be replaced with a secondary low off the Mid-Atlantic coast and as the shortwave deepens/tilts more, the low will take a north/northeast turn up the coast. The global deterministic guidance is in very good agreement with little spread. Given the higher than normal confidence and little spread seen in the models, a general model blend should suffice for this system. ...Shortwave racing through Canadian Rockies Sunday before supporting Northern Tier surface wave Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average The fast Pacific flow continues to press the parade of low amplitude shortwave/jet speed maxima through Sat into early Sunday. The last will help break the zonal flow pattern as it amplifies across the Southern Prairies of Canada into the Northern Tier by early Monday. This will support a strong Clipper system. Given the flat/zonal flow, there is expected small timing differences that lead to latitudinal differences as it amplifies. The GFS is a bit flatter and a tad faster compared to the rest of the guidance while the CMC is a touch slower/deeper. The ECMWF/UKMET lies in between and is a good proxy, and with a general model blend, a reasonable solution should be appropriate for slightly above average confidence. ...Developing broad global trof over Northwestern North America... ...Lead shortwave in Great Basin Monday, shifting to Central Plains by Tuesday... ...Secondary broad shortwave digging along West Coast Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM, non-CMC blend Confidence: Below Average 08Z update: Latest trends still show a slightly slowing pattern with the new 00Z guidance, such that the GFS and NAM are fast solutions, with perhaps the NAM being an outlier at this point from the west coast toward the central Plains and Upper Midwest by day 3. The 00Z CMC still is a slower solution, but is not out of the ensemble spread as much as before. Therefore the initial preference is amended to include non-NAM across the board. In the wake of the last zonal shortwave, the overall large scale pattern starts its phase of amplification centered around Alberta after midday Monday. A strong leading jet and associated shortwave rips through the Great Basin and by 12z Tuesday is sharpening over the CO Rockies, developing a strong Central Plains surface low. Upstream, another broad shortwave hugging the Panhandle of AK will drop south along the Coast, starting to buckle into a larger closed low feature just after the period. For the Central Plains, Upper Midwest, and Great Lakes cyclone, the model guidance has come into slightly better agreement in the overall, synoptic pattern and now has reasonable spread for Day 3. The exception is the 12Z CMC, as it is too deep/amplified/fast with the shortwave trough by 84 hours, and thus its surface low is further east. The rest of the guidance shows some typical biases but overall the spread is within the ensemble spread, so a non-CMC blend should be appropriate for the large scale mass fields. The stronger, more anomalous closed low that arrives on Day 3 along the West Coast has more significant model spread. The NAM is considerably stronger with the closed low and subsequently has a much deeper surface low off the northern CA coast by 84 hours. There is also some latitudinal differences (ECMWF south, GFS north) but overall the setup is largely the same and within the ensemble spread. The ECENS mean and ECMWF are tightly clustered and the 18Z GEFS mean and 00Z GFS show fairly good agreement. So all things considered, a non-CMC blend overall is preferred (also non-NAM west coast day 3) for this cycle. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor