Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 226 PM EDT Thu Apr 23 2020 Valid Apr 23/1200 UTC thru Apr 27/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Evaluation with Model Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Ohio Valley system today through Northeast Fri into early Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 19z update: The ECMWF slowed a bit, while the CMC/UKMET trended a bit faster. There remains a modest spread in placement of the coastal surface low, but overall a general model blend will suffice at slightly above average confidence. ---Prior Discussion--- Mature closed low across Tri-Rivers of W Kentucky continues to press east, with active warm conveyor belt well displaced from the cold front across the Lower MS valley. Guidance is fairly solid initially, but faster solutions of the 12z GFS/NAM both suggest a stronger MCV growth out of the Southeast supporting a coastal low to take over the filling surface wave through the Ohio Valley by later Friday. While the GFS appears to become a tad to fast (out of ensemble/continuity tolerance by 36hrs), a blend with the slightly slower 00z ECMWF and NAM should offset the negative bias. The 00z CMC is significantly slower and lingers across the Southeast and western Atlantic a tad too much in this progressive pattern. The 00z UKMET is also slow, but that appears to be due to the fact there is no MCV reflection/upscale growth moving off the east coast into Friday; so it favors a stronger inner core to the filling closed low, which appears increasingly less likely given observational trends. Confidence is slightly above average in a 12z NAM/GFS and 00z ECMWF blend. Conglomeration of shortwaves toward Mid-MS/Ohio Valley Sat/Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z GFS/ECMWF blend (incl GEFS/ECENS by D3) Confidence: Slightly below average 19z update: The CMC trended a bit weaker across AR and shifted north, closer to the ECMWF/GFS/NAM solutions, but eventually slow a bit. Given the ECMWF did trend slower/south as well, it is plausible the CMC has some sort of handle on this evolution; however, will continue to suggest its exclusion and remain with a GFS/ECMWF blend at slightly below average confidence. ---Prior Discussion--- Broad large scale ridge over the Eastern Pacific with very strong global scale vortex over the Arctic leave a very diffuse northwest flow pattern across the West into the Plains over the next day or so. A jet max and associated speed vorticity center currently over WA/ID will drop southeast across the Central Plains and support a surface wave out of the lee of the central Rockies. With the exiting wave there is subtle ridging and with upstream upper level jet energy; the shortwave is able to amplify across E OK/AR having tapped some return moisture. The 00z CMC being much slower to exit with the prior wave, allows for greater ridging between the systems; allowing for the greatest deepening and well as southward track. Given the CMC is unfavored downstream; it is equally discouraged with this evolution. While the wave is deepening on Friday, small northern stream shortwave energy will carve out across the Midwest and help to draw the growing base of the trof northward. This binary interaction is always a lower predictability, as timing of the interaction is critical. Additionally, with the amplification, additional jet energy in the northwest flow will further help to slow/amplify the wave. The UKMET/CMC both suggest upstream development over the northern Plains by late Saturday, which will act more of a kicker to the system than helping to amplify it. The 12z NAM has this feature but also shows a much stronger compact development through the Mid-MS valley on Sat...perhaps with increased latent heat release from stronger convection...eventually having a stronger wave and an upstream kicker; the NAM starts to press eastward through the Ohio Valley and Mid-Atlantic by Sunday. The 00z ECMWF and 12z GFS eventually slow relative to their ensemble suite, so perhaps trending a bit more toward the means particularly by Sunday is prudent given the progressive pattern, the CONUS has been experiencing. Still a 12z GFS/00z ECMWF blend is preferred with some inclusion of the 06z GEFS/00z ECENS on day 3 to account for the moderate uncertainty. Given the spread, lower predictability pattern (due to broad stream/binary interactions) confidence is slightly below average in the blend. Shortwave entering PacNW late Sat, clipping through northern tier by Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Above average 19z update: The UKMET remains a bit north of the overall suite, but has trended toward the CMC/ECMWF (which keep with continuity) and the GFS. As such, a Non-NAM blend is preferred at above average confidence. ---Prior Discussion--- Active southern portion of the Pacific northerlies will press a subtle jet max and shortwave through Vancouver Island by midday Friday...this helps to press the larger scale Pacific ridge a bit eastward in advance of a stronger shortwave that approaches late Sat. This wave is fairly compact but also a tad south. The 12z NAM is most aggressive with this solution and supports a compact closed low off the northwest tip of Vancouver Island. This is supported by the GFS, followed by the CMC and lesser so the ECMWF solutions, while the UKMET is well northeast with the inner core of the upper low. Still the timing of the trof/frontal zone is fairly solid with similar flow regime/strength minus the NAM. This continues as the shortwave presses through the Canadian Rockies Sat into Sunday with strong surface lee-cyclone pressing east across north-central Alberta/Saskatchewan. This clips the northern row of counties with sensible weather by day 3, but with little fanfare, having wrung out the moisture across the coastal range and into the Rockies of ID/W MT. So for affects in the CONUS, would support a Non-UKMET blend, but would tamp down or remove the NAM for likely best possible blend. Overall, the spread is fairly small for day 2-3 and the affects are fairly small to have above average confidence in this blend, across the CONUS. Frontal zone of Gulf of AK closed low approaching PacNW late Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Slightly above average 19z update: The UKMET trended a bit better but remains a bit too fast given the otherwise tighter cluster. The ECMWF also trended a bit slower but still well within the ensemble suite and near the 12z CMC, GFS. As such will keep with non-UKMET blend at slightly above average confidence. ---Prior Discussion--- The next strong shortwave helps to anchor the larger scale trough into the Gulf of AK by the end of the forecast period. While there are some residual differences due to the prior shortwave exiting (ie the ECMWF/CMC a bit north; GFS/NAM a bit further south), there is actually solid model agreement for the timing/strength and position of closed low off of Haida Gwaii by 00z Monday. While the front is still well offshore, its affects start to encroach the Pacific NW. Being a tad north, the moisture transport (QPF) is delayed in the CMC/ECMWF relative to the faster/south GFS/NAM. Only the UKMET is well too fast, too strong (likely due to timing with the prior system, accelerating it eastward earlier), enough to suggest a Non-UKMET blend this far out in the forecast period at fairly high confidence (slightly above 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