Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1225 AM EDT Wed Mar 21 2018 Valid Mar 21/0000 UTC thru Mar 24/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Amplifying synoptic scale trof in Eastern US with development of Nor'easter off Delmarva coast today that will reach Nova Scotia late Thursday with significant winter weather impacts along the Northeast corridor... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend leaning 00z GFS in weighting Confidence: Average The 00z NAM has trended a bit more amplified with the return wave emerging from the Ohio valley undercutting the main inner pivot portion of the vorticity center currently progressing into WV. This leads to an eventual slowing and secondary tricordial wobble southeast of Long Island before transferring dominance to the newer triple point low near Nova Scotia by 18z Thurs. While the evolution is similar to a slower 12z ECMWF, this run takes it even further from the stronger clustering, and given the latent heat/convective influence already manifesting along the Gulf Stream currently, the likelihood would to favor a slightly earlier transfer and northeasterly drift Wed into Thurs as suggested by the 00z GFS, 18z GEFS and to a lesser extent the 12z ECENS mean. By Thurs into Friday, the 12z ECMWF looks more in line with GFS as well as the 12z UKMET/CMC which appear serviceable. As such will favor a Non-NAM blend leaning toward the GFS solution which continues to anchor/represent the ensembles well as well as maintains continuity. Small scale influences growing upscale a bit and their importance on mesoscale banding effects and impact weather conditions still lead to enough uncertainty to have high confidence in this blend, but is becoming stronger with time to be considered average at this time. ...Weak broad remaining mid-level troughing across the central US/Canada border that will dig and amplify through the Great Lakes toward NY by Friday in the wake of exiting Nor'easter... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend leaning EC/CMC toward end of Day 3 Confidence: Slightly above average Goes-WV mosaic depicts a broad loosely defined (multiple internal shortwaves) troughing conditions over ND/MN/Manitoba at this time. As the main eastern US trof exits, this wave will amplify across the Great Lakes corralled by a strong vort max near James bay that was the tail of old global scale trof exiting the Northeast American continent. The strength and position of this strong vort max is very important to the evolution of this and the upstream wave playing a large role in the medium range (please refer to PMDEPD for additional information) along the Northeast and Mid-Atlantic. The 00z NAM and GFS are quick to break down this corralling wave shearing it northeast; which in turn strengthens northern stream digging and jet streak through SE Canada into New England making it about 30-40 DM deeper. As such the wave is slightly faster toward southeast transition into the northeast Atlantic, but more importantly, has reduced upstream shortwave ridging which negatively affects Day 4 evolution. As this point, the spread is mild enough to support a general model blend but will be hedging weighting toward the more amplified upstream ridging presented by the ECMWF/CMC in collaboration with WPC medium range forecast preference. Confidence is slightly above average, and would be higher if there was stronger agreement weakening of the strong vort in Quebec/"corralling wave". ...Deep trough/closed low offshore the Northwest by late Wed into Thurs with accompanying surface lows... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend to 23/12z 00z NAM and 12z ECMWF/UKMET blend afterward Confidence: Slightly above average Models have locked into a very common solution for large deep, slightly elongated north to south closed upper low across the eastern Gulf of AK. Eventually, as the lead shortwave round the base midday Thurs phasing with shearing sub-tropical stream shortwave energy into the Pacific northwest (more specific details/preferences about this in section below), the upper low becomes more symmetric continuing to slowly drop southeast. At the surface, guidance is also quite agreeable in the mass fields especially through 12z Friday, the UKMET and CMC centers are a bit more flung further from the center of rotation but this appears minor at this time. However, after 12z Friday, the instability of the circulation begins to manifest as it is fully mature, ready to devolve into its constituent internal smaller scale waves. The strongest wave will rotate around the NW and western side and start angling toward OR/N CA by the end of the forecast period. The 00z GFS is faster to wind up the system as it rotates through, but also is faster than the remaining guidance, perhaps a bit too much to keep in the blend. The 00z NAM leads the ECMWF/CMC and GEFS mean slightly but remains tightly clustered to the strong consensus. By this time the main outlier is the 12z CMC, which has broken from the tight agreement favoring weakening earlier and swinging the wave faster and shallower around the parent closed low's base toward WA/OR. Specifically with mass, will support a general model blend through 23/12z and shift to a 00z NAM/12z ECMWF/UKMET blend thereafter at slightly above average confidence. ...Combination of sheared southern stream energy and height falls from internal wave of closed low enters Pacific NW Thurs into Friday with associated surface wave that transfers to deepening surface cyclone across SW Canada/Northern High Plains Fri into Sat... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend through 23/00z 12z UKMET/CMC blend after Confidence: Slightly above average through 23/00z Average thereafter As mentioned in the parent closed low section above, a combination of strong height falls/shortwave rotating around its base in combination with moisture and shearing shortwave energy from the subtropical stream will phase as it begins to enter coastal OR with landfall of weakening surface wave along the frontal zone very late Wed into early Thursday. This wave translates through E WA/ID/S BC leading to lee surface cyclogenesis in Alberta late Friday. However, there is a bit of discrepancy in the angle and latitude with the models bring the strongest height falls. The 12z ECMWF is uncharacteristically a bit faster and injects a but further north supported by a bulk of its ensemble members. This is opposed to the GFS and GEFS members that are slower (though the 00z is a bit faster than the 18z) and further southwest along the terrain. The 00z NAM favors the ECMWF solution but is even a bit faster making it a clearer outlier at this time. The UKMET shares similarities with the ECMWF and hedges toward its solution but is the clear most central solution. The CMC, like the UKMET is very central but is even slower than the GFS, but shades to that side of the guidance. Ensemble suites/trends would favor the ECMWF side of evolution/timing but not as far NW with the mass field solutions. As a compromise will favor a 12z UKMET/CMC blend mainly after crossing the higher terrain (Fri 12z) confidence is higher earlier but average by Fri into Sat. ...Trailing upstream shortwave energy of the shearing central Pacific closed low, focusing atmospheric river through landfall in Srn CA Friday morning. Crossing the Four Corners into the Central plains with lee cyclone late Fri into Sat ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Average The 00z NAM was generally weaker, north and therefore faster with the trailing shortwave energy entering southern California. This trend continues through late Friday into Saturday, with the resultant surface cyclone well east of the otherwise moderately tight clustering in the central Plains. The 00z GFS also shifted a bit weaker initially resulting in a slightly faster, weaker solution in the central Plains, but still within the moderate clustering of the 18z GEFS. The 12z ECMWF/CMC are nearly identical in timing/strength and evolution/shape with the UKMET a shade faster. All in all, will support a Non-Nam blend favoring the continuity of the ECMWF/CMC/UKMET and lower the weighting of the GFS (but reserve right to shift toward GFS with upcoming update as guidance may be trending this direction). 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