Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 231 AM EDT Sat Apr 04 2020 Valid Apr 04/0000 UTC thru Apr 07/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Elongated frontal zone in Great Lakes today and through Northeast/Mid-Atlantic Sunday/early Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 07z update: No significant changes were noted to break from initial preference/thinking...so a general model blend remains preference. Very narrow, elongated frontal zone currently across WI to W AR will continue to press eastward but with main shortwave already angled into Canada, low level flow and mass field responses become weak over the Mid-MS valley into the Ohio Valley. The upstream kicker shortwave in MT will swing through and stretch across much of Canada. So while there may be sizable mass differences across the border to the north, the timing of the front south of it across the Great Lakes into the Northeast/Mid-Atlantic is solid to support a general model blend. By late Sunday/Monday, the shortwave energy in north-central Canada will press across Interior New England/Rooftop of Maine with some small timing differences and surface low placement differences. The ECMWF seems most out of place and uncharacteristically fast, while the strength in flow is stronger with the 00z NAM/GFS (being a tad south with the shortwave within the overall suite). Thinking a compromise it best here, to support a general model blend at slightly above average confidence. ...Frontal zone across TX/Lower MS valley, subtropical jet/weak shortwaves lingering through Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM, weight toward GFS then ECMWF Confidence: Below average 07z update: The 00z UKMET trended a bit faster than the 00z GFS and with the CMC/ECMWF both trending a bit faster further east with the 850-7H frontal zone/QPF axis will support a Non-NAM blend hedging toward the GFS/ECMWF compromise in timing/placement. Still given the uncertainty remains with mesoscale convergence fields and moisture/instability axes alignment, it remains below average. ---Prior Discussion--- The tail end of the elongated/narrowing frontal zone across the Mississippi Valley currently resides in the Coastal Plains of TX. This front will remain in place and influenced by subtle shortwave features emerging from the subtropical jet stream and eastern Pacific moisture stream. The 00z NAM is now particular fast and supports pressing low level confluent flow eastward with less overall forcing to lead to increased organization. The 12z ECMWF/CMC both continue to suggest greater moisture streaming through the Mexican Plateau allowing for reduced EML/capping bleeding off the terrain (though cap is very strong over the Plateau itself). With greater backed low level flow off the Gulf, better low level ascent/convergence both are more aggressive keeping convection west, which may be a bit too aggressive in coverage/magnitude. The UKMET/GFS are a nice compromise...and while the mass fields are only slightly different, the affects on QPF remain sizable. As such, a 00z GFS and 12z UKMET solution is favored but at below average confidence, given importance on meso/miso scale features/interactions which are too hard to lock down in a more chaotic flow regime. ...West coast shortwave(s) expanding to deep closed low by Sun off OR/CA coast, slowly wobbles to Central CA coast by Tues... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-GFS blend Confidence: Slightly above average 07z update: The 00z GEFS mean trended toward the operational GFS, but is still more in line with the NAM and the non-NCEP solutions. The UKMET trended back toward the ECMWF/CMC enough to support a non-GFS blend overall at this point. ---Prior Discussion--- Lead shortwave is well defined in WV suite from GOES-W just off the OR/CA coast. This wave remains well defined and agreed upon through the guidance suite as it devolves/shears into the flow across OR/ID into W MT by Sunday. The next wave will continue to amplify and become a closed low by later today just west of Vancouver Island and will drop into a position just of the NW coast of California by Sunday morning, with fairly good agreement in placement/depth of the associated surface low. It is expected the internal dynamics within the closed low will allow for a tricordial wobble, slowing its southward parent motion toward the central CA coast by Monday, again still in solid model agreement; however, this will change as models start to show slightly different magnitudes of internal instability with respect to the binary interaction. To stabilize this, the ensemble suite/means should help to push toward higher probabilities of outcome. This helps to eliminate the UKMET which has shown continued westward bias in the suite over the last few days. This also helps to eliminate the GFS which continues to further trend being fast, losing some energy northward, then eventually the remaining energy accelerating south and east by the end of day 3, generally weaker. This is not supported in the GEFS suite and matches typical negative bias. The 00z NAM is a bit tighter with the overall internal structure but looks solid with the GEFS/ECENS means as well as the ECMWF/CMC. As such a 00z NAM and 12z ECMWF/CMC blend will be preferred at slightly above average confidence. ...Shortwave/northwest Pacific jet crossing Northern Rockies Mon/Tues with associated northern Plains surface low... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 07z update: The CMC and UKMET both trended slower and overall the evolution of this portion of the trof and Pacific trof have a solid agreement to have increased confidence overall. There remains some uncertainty to the strength/sharpness of the downstream ridging by Day 3, but is small enough to support a general model blend at slightly above average confidence. ---Prior Discussion--- As the closed low digs and the eastern portion of the Canadian gyre slides under the deep blocking ridge over the northern Hudson Bay, the western portion of the gyre slide south across the Canadian Rockies. Additionally, strong Pacific jet tightens the trof and surges it east-southeast into northern MT supporting lee cyclogenesis in the High Plains by Monday afternoon. This wave/surface cyclone will press into the Northern Plains by the end of the short-term forecast. The 00z GFS depicted a sizable slowing trend and now appears comparable to the 00z NAM and 12z ECMWF in timing, though the 00z NAM appears to have some typical late Day 3 over-amplification suggestions perhaps with greater southern stream influences (broad southwesterly flow ahead of deep closed low). The 12z CMC seems uncharacteristically fast, though also has a more amplified downstream ridge given even greater blocking further downstream in eastern Canada..so will shift away from its influence. The UKMET also shows greater negative tilting/swinging the base of the trof through faster, severing the connection to the southern stream/closed low earlier...allowing for a deeper surface wave and mass/wind field response. As such will favor the ECMWF within a 00z GFS/NAM and 12z ECMWF blend. Confidence is average in this blend. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina