Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Sun Aug 02 2020 Valid Aug 02/1200 UTC thru Aug 06/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Final Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Mid-level shortwave lifting from the Lower Great Lakes into southeastern Canada through tonight with accompanying surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show similarly with this system. No significant changes were noted in the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...Tropical Storm Isaias... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 03Z NHC advisory package Best model proxy: through 12Z/04: non 12Z NAM blend 00Z/05 - 12Z/05: 12Z ECMWF/UKMET/CMC blend 00Z/06: near the 12Z ECMWF ...19Z update... Only minor timing changes were noted with the speed/track of Tropical Storm Isaias in the latest 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. The 12Z ECMWF/UKMET remain slower than the 15Z NHC advisory, while the 12Z CMC is generally faster and a bit weaker than the deterministic consensus. A blended approach of non-NCEP guidance is reasonably close to the 15Z NHC advisory after Monday night, with the 12Z ECMWF closest at the end of the short range period...or 00Z/06 (Wednesday evening). ...previous discussion follows... The 15Z NHC forecast track of Tropical Storm Isaias is overall closest now to a multi-model blend consisting of the 12Z GFS, 00Z ECMWF, 00Z UKMET and 00Z CMC. The model's proximity to NHC varies over the course of the next 3 days, but the 12Z NAM stands out the greatest with a timing that is 6 to 12 hours slower than the NHC track through Wednesday morning. Individual model timing varies considerably compared to the NHC track, where some models that are initially slower than NHC (00Z UKMET), speed up their timing to be faster than NHC by Wednesday afternoon. However, all of the models support the system rapidly lifting through New England by the end of the period. Please consult the latest NHC forecast advisory package for more details. ...Northern stream trough setting up over the Great Lakes on Monday/Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average ...19Z update... Adjustments in the latest 12Z non-NCEP suite of guidance make for two camps of solutions regarding the eventual closed low that sets up near the Great Lakes region for Tuesday. The 12Z NAM/GFS/CMC represent a more consolidated and symmetric closed low at 500 mb, while the 12Z ECMWF/UKMET show an elongated mid-level low with two main vorticity maxima (one north and one south). However, the recent trend by the 12Z ECMWF/UKMET is getting the models close enough that the impacts due to these differences are mainly into Canada. Even then, the differences are shrinking so that a general model blend can now be used here. ...previous discussion follows... Northern stream energy will be digging down across the Great Lakes through Sunday with likely development of an elongated closed low a bit farther north over Ontario by Monday and Tuesday. The 00Z ECMWF/UKMET and 00Z CMC differ with the handling of two main vorticity maxima within the larger scale trough. The 00Z ECMWF stands out the greatest from the deterministic/ensemble guidance but there is growing support for a solution edging in the direction of the 00Z ECMWF noted by its mean and the 00Z UKMET/CMC. However, a blended approach near the 12Z NAM, 12Z GFS and 00Z UKMET is nearest to the preference of the ensemble means at this point, with the ensemble means considered best. ...Upper low/trough crossing the Pacific Northwest tonight... ...Energy dropping down over the Northern Plains/Midwest for Tuesday/Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 12Z GFS/ECMWF Confidence: Slightly below average ...19Z update... Trends in the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles showed either slowing or strengthening with the mid-level vorticity maximum tracking near the U.S./Canadian border. However, the UKMET remains a bit weaker with its 12Z cycle while the 12Z CMC continues to appear as a near outlier with its strength on Wednesday. The 12Z ECMWF trended a little slower and south with the track of the vort max, putting it close enough to a favored middle ground...when combined with the 12Z GFS...to allow including as part of the preference. ...previous discussion follows... An upper trough will cross in through British Columbia and the Pacific Northwest tonight and then track well inland through Monday and Tuesday with the main focus of the height falls remaining across western and central Canada. However, the guidance supports some energy in the base of the trough separating out across the far northern Rockies and then advancing southeast by Wednesday across the northern Plains and Midwest. The 12Z NAM/GFS remain on the stronger side of the guidance with this energy over the northern U.S. Monday night through Tuesday...as is the 00Z CMC. This is reflected in the latest ensemble spaghetti plots which indicate the NAM/GFS/CMC on the edges of the latest member distribution. However, the 00Z ECMWF/UKMET are weaker and faster with this system tracking near the U.S./Canadian border. The 00Z UKMET and 00Z CMC are on the edges of the latest deterministic guidance, and so the preference is to equally weight the 12Z NAM, 12Z GFS and 00Z ECMWF. The thinking here is that a blended approach of these models will be closest to verification, lying in the middle of the model spread and near the middle ground considering trends. ...Southern stream trough moving into CA by Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average The models show similary with this system. No significant changes were noted in the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. ...Mid-upper level trough nearing the Pacific Northwest late Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average Only minor timing differences were noted, but agreement is fairly good for a day 3 forecast. No significant changes were noted in the 12Z ECMWF/UKMET/CMC compared to their previous 00Z cycles. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml This product will terminate August 15, 2020: www.weather.gov/media/notification/pdf2/scn20-58pmdhmd_termination. pdf Otto