Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1239 AM EDT Fri Apr 03 2020 Valid Apr 03/0000 UTC thru Apr 06/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Western Atlantic closed low and offshore surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Above average Large NW Atlantic closed low has one stronger deep stacked low with surface center east of Cape Cod currently, while an undercutting shortwave currently is exiting the Mid-Atlantic coast per GOES-WV suite. The binary interaction will continue to be dominated by the deeper cyclone that will retrograde toward the Cape by early this morning, before drifting southeast toward Bermuda by late Sat/Sunday. The spread here will be mainly offshore by the time it reaches Bermuda, with the 00z NAM and lesser so the 12z UKMET followed by the CMC favoring greater elongation in the binary interaction and therefore further southwest by the end of the forecast period. The 00z GFS, 12z ECMWF and the bulk of the ensemble suite are a tad stronger/stacked and cyclonically rotate eastward earlier (north of Bermuda). So will favor a general model blend through 24hrs but shifting more GFS/ECMWF heavy thereafter at slightly above average confidence. ...Northern Rockies/W Canada Gyre breaking down and sliding through Northern Plains Fri/Sat into Great Lakes/Northeast by Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend 00z NAM/12z CMC/ECMWF blend after 05.12z Confidence: Slightly above average becoming average Very anomalous large scale gyre set up over northern Alberta has an elongated extension/trof crossing out of the northern High Plains across across the Northern Plains today. A secondary internal shortwave will follow late Sat into Sun. The overall gyre will continue to breakdown and these shortwaves will shear under the strong closed anticyclone over Nunavut, with the secondary wave accelerating across the northern Great Lakes clipping Interior New England late Sunday into Monday. The UKMET becomes an significant outlier within the suite by Sat, with a weaker/faster leading wave sliding up into Boreal Canada, allowing for a stronger more amplified secondary wave. This is not likely. The remaining uncertainty in the guidance will be the strength of the developing closed low/Rex block across Nunavut/Ontario and the amplitude of the wave into the Northeast. Here the 00z GFS is faster toward the closed low over N Manitoba and more concentric having less influence on the undercutting wave...so that wave remains well south supporting a surface low near the Rooftop of Maine by early Monday. The 12z ECMWF is a bit weaker through the entire length of the wave, perhaps too much, while the 00z NAM and 12z CMC are central in the overall ensemble suite as a nice compromise while depicting sensible evolution. So will support a Non-UKMET blend with reducing influence of the GFS particularly after 05.12z. Confidence is solid though the pattern may be less predictable in vortex breakdown stages; so it is slightly above average becoming average by Monday. ...Southern Plains/Lower MS valley --- Subtropical stream... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General blend thru 05.00z 00z GFS and 12z UKMET/CMC blend thereafter Confidence: Slightly below average Significant moisture/instability return across the southern Plains may present the potential for heavy rainfall/flash flooding conditions through the weekend. This will be driven mainly by mesoscale forcing/weak boundaries for a difficult mass field preference. Still, a very active/strong subtropical jet will exist and slide through northern Old Mexico into Texas and the Lower MS valley through the forecast period. The cold front currently dropping south of the Red River will continue to progress toward the coast, but with significant uncertainty in the position of the boundary due to the unknown depth of the cold air (ie slope of the frontal zone across TX). This is critically important on Day 2 and even more so on Day 3. The main mass field "certainties" and influences are upstream in CA (see section below). The 00z NAM is quite strong and south with this feature and pumps the downstream ridge across this area; as it is unfavored across the West, the NAM will be increasingly less preferred by the end of Day 2 into Day 3 across the southern Plains. The other differences, noted was the slower but also more amplified subtropical wave in the 00z ECMWF; this wave helps to back low level flow through the Rio Grande Valley increasing moisture flux off the western Gulf, while the overall pattern should support better southerly flow (than southeasterly), the GFS while showing some typical day 3 speed(fast) bias, is paired with the UKMET and CMC as well as GEFS to have some confidence toward this evolution unfolding over the 00z ECMWF. While confidence is always poor with the position of subtropical shortwaves and subtropical jet orientation, a 00z GFS and 12z UKMET/CMC blend is preferred by day 3 at slightly below average confidence. ...Pair of shortwaves along West Coast becoming dominant closed low Sun/Mon... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend for initial shortwave 00z GFS/12z ECMWF blend overall Confidence: Slightly above average A compact closed low currently sit atop of the larger scale Pacific Ridging across the S Gulf of AK currently, this wave is expected to drop south on the backside of the deep Canadian/Northern Rockies gyre reaching the Northern CA coast Sat. Upstream jet energy will support a developing shortwave quick on its heels that will dig/amplify along the BC coast at the same time. Models are in solid agreement on the lead wave as it stalls under the upstream development and eventually swings eastward through the northern Great Basin/Snake River Plain by Sunday, rapidly weakening. Perhaps the 00z NAM is a bit stronger and faster (along with a weaker but fast GFS) but is small to have higher than normal confidence in a general model blend for this feature. However, as the wave weakens and slides east-northeast; it leaving the Pacific moisture stream directed across the Northern/Central CA coast ahead of the secondary wave. Here the 00z NAM is a clear outlier being a bit stronger and further west (having less of an inner core, less cyclonic shear, more speed shear). The 12z UKMET becomes a bit unruly as an upscale feedback rapidly tightens and deepens the core of the developing/expanding closed low off the central CA coast...a negative bias when it manifests with this model. The 00z GFS remains solid agreement with the 12z ECMWF/ECENS mean and 18z GEFS solutions...given this tightness of agreement/continuity will favor them heavily. The 12z CMC seems ok within this blend if a tad weaker/south, but would be viable if desired in a blend. Confidence is slightly above average in a GFS/ECMWF blend. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina