Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 316 PM EDT Wed Apr 24 2019 Valid Apr 24/1200 UTC thru Apr 28/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation...with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Southern Stream Weakening Shortwave Phasing with Amplifying Northern Stream Across Eastern US by Fri... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend weighted to ECMWF/CMC Confidence: Slightly Below Average Update: Overall the 12z Non-NCEP guidance trended favorably toward a consensus between the 12z NAM/GFS, with the ECMWF trending most favorably. The ECMWF broke from the continuity to trend faster lifting the wave out of the Southern Plains and looks like a slower 12z GFS. The 12z CMC shifted to match the NAM quite well while the UKMET is a good split here. While the overall evolution toward a concentric closed low (ECMWF/CMC/NAM) or a more elongated negative tilt trof (GFS/UKMET) by the end of Day 3, the effects in the lower level mass fields and QPF seem to be resolving better. Still, while there are converging solutions, there remains significant potential for solutions to split again given this timing/phasing differences. As such, a general model blend (weighted to the ECMWF/CMC) is preferred but at slightly below average confidence. ---Prior Discussion--- Models are struggling to come to a consensus even in the short term mainly driven by above average chaotic convective environment across Northern Mexico and Texas this morning with the southern stream system. Given the down stream phasing across the Ohio/Tennessee Valley by Fri is highly contingent on timing and upscale enhancement magnitude differences, there is very high uncertainty across the Lower MS Valley into the Coastal Carolinas by late Friday. The 12z GFS favorably shifted from the 06z run, by being even faster lifting north and phasing with the northern stream leading to a less sharp/further south amplification. This does slow/deepen the northern stream cyclone relative to the ECMWF but given where it was relative, this is more useful, yet still on the lower end of weighting. As alluded to, the ECMWF also has some timing issues but on the opposite side of solutions, continuing to be well south and slow with the surface wave in the lower MS valley, falling in line with its bias. The 12z NAM is a nice middle ground solution to both the GFS/ECMWF especially in the initial phasing process on Fri. By the end of the forecast period, the negative bias of the NAM appears to manifest with an over amplification of the base of the trof through the Northeast, as such will be reducing weighting of the NAM toward Sat relative to the ECMWF/ECENS mean. Reasoning for this higher weighting is driven mainly by the run to run consistency seen with the ECMWF/ECENS mean over the last 3-5 ensemble cycles (though falling into typical slow bias). The 00z CMC continues to be very weak in the southern stream and as such shifts the trof east a bit faster than preference and is least favorable in the operational suite given the magnitude of expected upscale enhancement in the southern stream. The UKMET, while generally in line with the northern stream is very strong but very fast in the southern stream. As such will favor a 00z ECMWF/ECENS mean blend with the 12z NAM;weighting the NAM equal through Fri but reducing after 60hrs on Sat; and the 12z GFS but at reduced weighting relative to the other guidance throughout the forecast period. ...Pair of Northeast Pacific Shortwaves Crossing the Southern Canadian Rockies and Entering Northern Plains 00z Sat and High Plains of MT by 00z Sunday, Respectively... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z NAM/GFS/ECMWF blend Confidence: Slightly above average Update: The ECMWF, with a faster downstream solution, also trended a bit faster with the lead shortwave into the Midwest Sat. While the CMC/UKMET both trended north and stronger, they are clearly south of the tighter clustering of the NAM/GFS and ECMWF. Upstream, similarly the NAM/GFS and ECMWF remain in solid agreement; and while the CMC did start to shear more, it also was weaker with the Arctic vortex allowing cyclogenesis in the lee of Central Alberta Rockies leading to a northward shift to the otherwise solid agreement. As such there is no change in preference of a NAM/GFS/ECMWF blend for both systems (but the UKMET could be included in the second wave). Confidence remains slightly above average ---Prior Discussion--- The downstream evolution of the phasing trof will play a great role on the timing/amplification and track of the lead shortwave that consolidates over the Northern US Rockies by late Friday and emerging into the Northern Plains by early Sat and Midwest by 00z Sun. The faster solutions downstream across the east, flatten the flow and accelerate the system further south with the 00z CMC and UKMET. The UKMET having a stronger bias is also noted, which leads to it being a tad slower than the CMC but still south of the other guidance/ensemble suite making both unfavorable in the blend. The 12z GFS showed an increase in speed and therefore reduced deepening of the wave through the Midwest relative to the 00z/06z and other remaining guidance, due to less downstream ridging, this also reduces the cold air wrapping around the western side of the cyclone reducing snow rates/efficiency. The ensembles would favor the slower more amplified solutions of the 00z ECMWF and 12z NAM. The NAM may have some typical cold bias toward the end of Day 3, but still is quite useful in the blend. Further, upstream the closed low over the Northeast Gulf of AK will quickly fall behind the aforementioned system, as it is elongated/sheared below amplifying Arctic/Polar Vortex. Here the CMC is very slow and most reticent to shear. The UKMET is well timed but a bit stronger, it can be blended in with this system but given the initial system... a 12z NAM/GFS and 00z ECMWF blend is preferred overall at Slightly above average confidence. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Gallina