Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 237 PM EDT Wed May 09 2018 Valid May 09/1200 UTC thru May 13/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... ===== 12Z Model Evaluation with Preferences and Confidence ===== ...Trough lifting through the Mid Atlantic today, and past the Northeast on Thursday with associated weak surface low off the East Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average 12z NAM and GFS continue to show similarity for this system with recent few cycles of guidance/ensemble...so will continue a general model blend is preferred with high confidence. ...Northern stream shortwaves consolidating over the Upper Midwest today, and pushing into the Northeast on Thursday... ...Surface low moving through the Great Lakes with cold front sweeping across the northern tier of states... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average Satellite and local radar mosaic depict two well defined MCV signatures over the Midwest/Great Lakes region. Upstream a larger scale shortwave was located over the Dakotas with a even high level scale amplified wave over north central Canada. Guidance has come into very close alignment with these features as the smaller (southern) wave consolidate through the Great Lakes and lift north ahead of the sharpening height-falls in Canada. The associated surface wave in the Midwest and upstream cold front over the Dakotas/Southern Canadian Prairies will combine crossing through the Great Lakes and sliding across Southeast Canada and the Northeast on Thursday into Friday. While there are clear small scale details yet to resolve, the pattern/shape/timing and depth all seem quite reasonable and clustered to have high confidence in a general model blend. ...Upper level trough near the Pacific Northwest followed by general evolution over the western U.S. to zonal flow... ...Followed by redevelopment of closed low into Great Basin by late Friday with daily emerging surface waves in the Plains Fri and Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM Confidence: Slightly above average Goes-W WV suite shows a binary interaction between similar scaled waves along 50N west of Vancouver island, with the older wave stretching westward while the Gulf of AK wave under-cuts. This will lead to short-term zonal flow across the West but also broad surface pressure falls across the terrain. As the Gulf of AK wave reconsolidates over the Northwest late Thurs into Fri, it will descend in latitude and eventually stall over the Great Basin Sat ejecting a few shortwaves into the Central Plains...with lee cyclogenesis response (please see more in section below). The overall ensemble suite shows increasing convergence in solutions with small/reducing spread. Large scale agreement is strong enough to start scrutinizing the internal wave dynamics a bit more with some confidence. The 12z NAM remains a progressive leader with a stronger/faster rotational response with the closed wave and height falls surging south a bit faster. On the other side of the spectrum, appears slow and not as far south, but this is likely due to less binary interaction initially over the Northwest allowing for a deeper/faster closed low, but shifted north without the "tug" of the other strong wave (which seems less realistic given current setup). The 00z ECMWF is a bit slower than the 12z GFS and UKMET internally, but show much tighter overall evolution. Overall, a general model blend would likely suffice, but for higher fidelity of the internal wave signal (and response) will favor a 12z GFS and 00z ECMWF/UKMET blend at slightly above average confidence. 19z update: The 12z CMC trended much closer in shape/timing with the internal wave to suggest its inclusion. The ECMWF/UKMET continued with good agreement to prior runs/initial thoughts to support a non-NAM blend at slightly above average confidence. ...Subtle shortwave(s) exiting the broad western trof into confluence over Great Lakes...surface reflection(s) along stationary from from Plains to Mid-Atlantic by late Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~ Preference: General model blend Confidence: Average As described in the section above, as the large scale closed low develops in the west and a very large global scale cyclone evolves across northeast Canada, subtle ridging exists between into generally confluent flow over the Great Lakes and Northeast. A few subtle wave emerge and in concert with daily tidal convective maxima, allow for low amplitude shortwave features to grow upscale and have significant impact on precipitation across the Central Plains to Mid-Atlantic (focused on the Midwest) with multiple bouts Friday and Saturday afternoon. More details with respect to QPF will be highlighted in the QPFPFD...but with respect to the mass fields, the guidance is not too far out of line. The initial wave/height falls seem to sharpen the W-E stationary front before Saturday's shortwave tracks a more resolved surface wave along it into the Mid-Atlantic on Sat evening. The GFS has shown a southward shift relative to the consistency of the ECMWF/UKMET with a bit of typical faster bias while NAM/UKMET split the EC/GFS timing difference yet do not seem to adversely affect the mass fields too much. The 00z CMC seems a bit north but also not too adverse to the mass. As such will favor a general model blend for the mass, but will defer to the QPFPFD for more important blend/model preference. Given the meso-scale influence and subtle nature of the wave confidence is only average in a general model blend. 19z update: Slight adjustments of the 12z Non-NCEP solutions show closer clustering to the 12z NAM, while the GFS appears a bit more amplified crossing the Midwest on Friday...but again, the larger scale signal/mass field differences do not appear to be the main driver of QPF/sensible wx differences. So will continue a general model blend at average confidence. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina