Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 240 PM EDT Sun Sep 30 2018 Valid Sep 30/1200 UTC thru Oct 04/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Midwest to Northeast; Late Mon to Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-ECMWF blend Confidence: Average 19z update: The less favorable slowness noted with the 00z ECMWF seems to have resolved a bit better, and while slower than the remaining guidance...the 12z ECMWF falls into a more traditional day 3 difference in timing with the NAM/GFS and UKMET to favor a General model blend. Modest timing spread especially on day 3 remains and so confidence remains average in the general model blend. ---Prior Discussion--- Shearing shortwave over OR into tightly packed confluent flow over the Northern Plains, supports a fast moving surface wave through the Great Lakes into early Tues. The 00z ECMWF amplifies and slows a bit too much compared to the ensemble suite including a bulk of the ECENS solutions. While it does catch up to the remaining guidance toward Wed as the surface wave amplifies near Gulf of Maine. While the latitude/track of the wave is agreed upon, the modest timing spread leads to some reduced confidence, to about average in a non-ECMWF blend. ...Northwest Pacific Sun/Mon and Closed Low approaching Southern CA Wed... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS/CMC and 00z ECENS mean blend Confidence: Slightly below average 19z update: Very little change noted between the 00z and 12z runs of the UKMET/ECMWF or CMC or GEFS mean for that matter. As such will keep with initial preference of 12z GFS/CMC and 00z ECENS mean blend. ---Prior Discussion--- Compact open shortwave near N Vancouver Island and associated surface low will start to elongate shear and connect to the approaching compact closed low in the Central North Pacific late Monday into early Tuesday. There still remains some minor timing differences which compound to larger binary iteration differences as the two features meld into a larger closed low by Wed. The 12z GFS is a bit faster as it typical of bias and the UKMET/NAM both are a bit weaker and sheared out in comparison. As such the NAM/UKMET remain weak and therefore drift eastward a bit too fast compared to overall ensemble suite/trend. The 00z ECMWF, GFS and CMC all generally agree until late Wed, when it slows a bit too much even compared to the ECENS mean. The 00z CMC is a bit weak overall compared to the GFS/ECMWF but all in all a blend of the 12z GFS/00z CMC and 00z ECENS mean is preferred. The uncertainty and small timing differences compounding to moderate sized spread keeps confidence slightly below average for this wave. ...Northern stream from Pacific NW Tues into Northern Plains/NW Great Lakes by Thurs... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS and 00z ECMWF/CMC blend Confidence: Slightly below average 19z update: While all guidance has trended towards a sharper shortwave along the US/Canadian boarder...the ECMWF/CMC slowed a bit toward the 12z GFS; and while the UKMET also slowed, it remains stronger further north and faster like the less favorable 12z NAM. So will stay with the 12z ECMWF/CMC/GFS preference and slightly below average in confidence given spread in timing, and model run to run variability. ---Prior Discussion--- Solid cross polar cold air advection and associated jet streak helps to sharpen a strong shortwave over western NW Territories into BC by early Tuesday. This wave replaces the one currently in place and tightens the surface pressure gradient. This wave then swings through the base of the larger scale trof on early Wed, there is large spread in the timing/acceleration of the wave through the Northern Rockies and Northern Plains Wed, mainly due to the timing/amplification of the core of cold air/center of the global polar vortex sharpening upstream. The 12z NAM and 00z UKMET quickly become very amplified supporting a strong surface wave in MT to S Manitoba, due to the orientation of the 5H waves, while possible, this seems a bit too aggressive without extreme cold and further south placement of the lingering frontal zone... especially given the placement/emergence of the low/mid-level energy of Rosa. The ECMWF is a bit uncharacteristically fast compared to its ECENS mean but still is in good agreement with the 12z GFS, 06z GEFS and 00z CMC... enough to support these models in the preferred blend at this time. Strong flow and influence of multiple streams/shortwave and longwave features leads to modest to high uncertainty and likely run to run variation. As such confidence remains slightly below average in the 12z GFS and 00z ECMWF/CMC blend. ...Tropical Cyclone Rosa (Southwest US)... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ See NHC forecast Best Proxy: General model blend (12z NAM closest) 19z update: The 12z ECMWF and lesser so 12z CMC both trended a bit faster with the center of Rosa...so a General model blend is a good proxy for the 15z NHC official forecast with the 12z NAM closest though not perfect/ideal throughout the duration of the forecast period. ---Prior Discussion--- Overall the global dynamic guidance has been coming into better agreement, though the GFS/NAM/UKMET are all generally faster with bringing the cyclone into Baja than the 00z CMC/ECMWF. The official NHC forecast is more in line with the faster clustering and so a best proxy can be a blend between the 00z UKMET and the 12z NAM/GFS. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina