Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1211 AM EDT Thu Apr 19 2018 Valid Apr 19/0000 UTC thru Apr 22/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...With Final Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Closed mid level low over southern Great Lakes tracking across the Ohio Valley and Northeast Thu and Thu night with surface low in central Ohio Valley, with secondary development off the New England coast Thu... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average GOES-16 WV loop shows center of upper low nearing the southern portion of Lake Michigan at this time with the northern stream shortwave amplifying and starting to enter northern L.P. of Michigan. Guidance appears to have a much better agreement on the timing/spacing as the two waves interact and meld into a broader trof across the northern Mid-Atlantic into New England Thursday. The surface low has nearly fully filled across the central Ohio River Valley/Appalachians currently before it starts the transfer to a developing coastal low in the next 6-12hrs. Modest deepening and track look fairly tight in the deterministic guidance as well as ensemble suite to suggest a general model blend will suffice as WPC preference. Confidence is also above average in this blended solution. ...Long wave trough crossing the California coast, which closes off over the Great Basin and Southwest Thu into Fri...shifting slowly east reaching Red River Valley Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend through 21/12z Non-ECMWF blend 21/12z to 22/12z Confidence: Average GOES-WV suite shows a large scale trof centered over SW Oregon with a tight smaller scale shortwave just southwest of San Francisco Bay likely to reach Central to S CA early this morning. The combination of features further sink southwest into the Lower Colorado River Valley by late Thurs/early Fri with fairly tight model agreement. The ECMWF/CMC and their ensemble suites lag the GFS/NAM and GEFS ever so slightly through midday Sat; where it starts to increase slightly. The downstream ridge/surface high are quite strong to support the slower solutions initially. However, as the closed low exits and induces deep convection along return Gulf moisture/instability into Sunday...should allow for increased forward propagation under influence of the latent heat release in the convective complex(es). A surface inverted trof will translate across TX into the Lower MS River Valley with the 00z GFS and NAM both closing a surface low in LA by 12z Sunday. Here the 00z ECMWF's inverted trof is much slower even within its ensemble suite. The 00z NAM also shifts the low further south and east likely in response to the convection but also in response to the stronger high to the northeast; this does not seem implausible though it also feeds into the known Day3 bias of being a bit too strong (which it is in comparison to the remaining guidance). The 00z GFS slowed a bit and looks similar to the 00z NAM though a bit weaker overall and closer in strength to the more middle ground CMC and UKMET. As such a general model blend blend weighted to the UKMET/CMC/ECMWF is preferred early (in the Southwest through 21/12z Sat) but shifts to a Non-ECMWF solution by 22/12z. Confidence is average through increasing given reduction in ensemble spread and deterministic guidance better representing the middle of said spread. ...Strong shortwave approaching the Pacific Northwest and British Columbia late Fri into early Sat before shifting north of the US border Sat into Sun... ...Associated surface cold front crossing the Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Slightly above average A potent shortwave trof quickly approaches the Pacific Northwest coast by Sat with an attendant cold front. Last 4 cycles of ensemble spaghetti analysis depicts the 12z UKMET a bit offset along with the 12z CMC (faster/slower) but the UKMET has significantly come back toward the growing consensus. The CMC was a bit slow and perhaps a bit south with the base of the trof, showing more severing with the top of the larger scale trof. As the wave swings a bit more negatively through the US/Canadian Rockies along the border...the CMC's base finally breaks from the slower northern portion of the trof leading to a dual center of the mid-level wave on Sunday. Both the differences do not seem to have a great affect on CONUS weather but Canadian interests may support something closer to the 00z GFS/NAM and 12z ECWMF; otherwise WPC will support a general model blend at slightly above average confidence. ...Weak subtle shortwave with slight amplification as it reaches Pacific Northwest Coast by early Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-UKMET blend Confidence: Average In the wake of the prior potent shortwave a fairly flat zonal pattern remains upstream...but with the downstream amplification as well as rapidly northeast moving sharp wave pumping downstream ridging around 130W on late Sat into early Sun. A weak shortwave amplifies at the Oregon coast. Is effects are fairly limited by the end of the Day 3 forecast period. The 12z UKMET is uncharacteristically slow; about 5 degrees too slow. The 00z GFS, typical of its bias it a bit fast and therefore most sharp by 12z Sunday but at least is timed well with the 00z NAM and 12z ECMWF/CMC as well as the ensemble suite means. While the GFS will be weighted slightly less to match more of the consensus...it will remain in WPC preference: Non-UKMET. Given the amplification/evolution appears more influenced external to this wave; confidence is only average and has the potential to vary as the upstream and downstream waves evolve. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml GALLINA