Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1244 AM EDT Mon Mar 19 2018 Valid Mar 19/0000 UTC thru Mar 22/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Potent shortwave entering Southern Plains crossing to the Mid-Atlantic Coast by Tuesday... ...Surface low developing in the Southern Plains, moving into Tennessee Valley Mon night, transferring to Coastal Low Tuesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Average The 00z NAM once again shifts toward a slower initial compact shortwave moving out of the Plains in the Ohio valley possibly under some increased downstream ridging but more importantly a much faster secondary shortwave (see section below) that leads to the initial wave stalling over the central Ohio valley, allowing for ideal warm sector upscale growth of the coastal wave as it lifts northeast. For these reasons, the NAM is not preferred at this time The 00z GFS continues it trend northwest crossing the Ohio Valley through the Mid-Atlantic, and as such the surface wave likewise shifts and is on the northwest side of the modest clustering. The 12z UKMET/CMC are a bit slower than the GFS/GEFS members and while paced well with the ECMWF are both a bit further south, with the ECENS mean appearing to a be the most middle ground. At this point, the differences are small enough for this initial wave to be within a preferred blend, a non-NAM blend. However, as the secondary wave interacts very late Tuesday into Wed...preferences will change (please see section below). Confidence is average in this initial wave's blend. ...Low amplitude shortwave entering California now then amplifying over southern states Monday-Tuesday... ...Secondary Surface Cyclogenesis Along Coastal Carolinas by Tuesday Night and Possibly up the East Coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS with 18z GEFS/12z ECENS mean Confidence: Slightly below average Initial setup across the eastern third of the US plays a critical role in handling this wave and its secondary coastal cyclogenesis late Tuesday into Wed. Given the large departure, both in stronger depth and further west for the anchoring pivot of the deepening eastern trof over the Ohio Valley, the 00z NAM is not preferred here as well. The remaining guidance shows fairly solid agreement in timing/orientation of the wave as it cross the MS Valley and orients more positively crossing the Gulf states late Tuesday into Wed. After this point the depth/snap acceleration of the base of the trof across S GA/N FL and feedback from Gulf Stream energy start to split the guidance. An additional player in the evolution is the strength and placement of the tail end of northeastern trof/shortwave energy that sets up in SW Quebec and potentially acts as the fulcrum to accurate the wave northeast while amplifying/consolidating the inner core. This is most evident in the 12z ECMWF operational run...which shows a very strong/compact solution that is further south than much of the ensemble suite, leading to a rapid northeast acceleration and surface low in the Gulf of ME by Thurs. This is a bit too aggressive at this time. The 12z CMC and UKMET were both set up by the initial wave a bit further south and east than the GFS/ECMWF and as the wave swings around the base and opens a modestly diffluent pattern aloft allowing for a strong deepening off the Mid-Atlantic coast and slower northeast shift, than the ensemble suite. The CMC may also be a bit too close to the coast initial given the slower mid to upper level trof over the Ohio valley with the initial wave; however it is still bit more favorable in mass than the UKMET which hangs back and is slow to lift out but may be still slow to the overall trend set in the ensemble suite. The 00z GFS having shifted a bit NW with the initial wave starts closer to the coast but given the height falls from this wave, lift quickly and match quite well to the 12z ECENS mean. Additionally, there is a bit more agreement toward a stronger pivot/fulcrum wave in Quebec like the ECMWF but not as drastic/influential, leading to a more off-shore track off New England Thurs. All considered the 00z GFS best represents the favorable 18z GEFS and 12z ECENS means. Confidence is slightly below average in this blend given fairly high spread and sensitivity to small run to run fluctuations. ...Large closed low over the Northwest rapidly breaking down today and tonight into small distinct waves with limited motion of the waves into collapsing shear Axis Mon-Tue with remnant vort maxes feeding into East Coast trough Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Average The semblance of the old upper low has faded into its constituent shortwave elements with the strongest being located along the Alberta/Montana boarder at this time and a few weak swirls associated with the old northwest remnants of the sheared closed low earlier this weekend, that linger from the Black Hills to eastern Sand Hills of NEB on the upwind side of the sharp ridge from ND to the Ohio Valley. These features along with an amplifying shortwave over N Saskatchewan form into a weak loosely organized shear axis from S Canada stretching southeast into the western side of the building eastern US trof by late Tues/Wed. As expected with such weak features, there is modest variation in locations but the overall influence on sensible weather and flow regime appear to come to a general consensus and therefore a general model blend is supported. Lower scale influence may play a highly sensitive role if ideal situated to grow upscale, and while unlikely this uncertainty makes confidence in a general model blend only average overall. ...Closed Pacific low near 35N/145W drifting east and eventually opening into a wave near 135W by Wednesday morning with increasing deep SW flow and atmospheric river into Southern CA... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z NAM and 12z ECMWF/CMC blend Confidence: Slightly above average Goes-W WV depicts a consolidated closed low near 35N145W with a warm conveyor belt east of 140W pointing toward triple point further north. A compact shortwave is elongating eastward along 45N with a strong vort center on the western periphery helping to influence a slight northeastward imbalance to the closed low. The 00z GFS and 12z UKMET are most aggressive in accentuating this northeastward shift but 00z analysis would suggest slight displacements of strongest internal shortwave features aligning better with the ECMWF/CMC and 00z NAM. The latter all of which do eventually show northeastward shearing but more influenced by the strengthening/digging shortwave and eventual deep layered cyclone off BC extending south to 40-45N by late Wed (see section below). At this point continuity favors the ECMWF/CMC and NAM...though the NAM having a deeper/faster digging northern system ejects the surface wave northeast a bit faster than the ECMWF/CMC. This is minor in the blend that a ECMWF/CMC and 00z NAM blend is preferred at slightly above average. Of note: SW flow/AR event is favored in all deterministic guidance to have fairly high confidence in a very dangerous heavy rainfall/snowfall event in S California/Sierras. Please refer to WPC QPFPFD and QPFHSD for additional details for QPF/Snowfall amounts and confidence. ...Shortwave Rounding Pacific Ridge over near Alaska by Tuesday digging sharply south and developing a closed low over the Gulf of Alaska by Wednesday with possible surface low development near Vancouver Island... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General Model Blend Confidence: Slightly above average Deterministic and ensemble suite show very strong agreement in the development of a strong/latitudinal deep closed low over the eastern Gulf of Alaska that will extend into the northeastern Pacific by late Wed into Thurs. Of course there are differences but most of it is related to interaction/timing difference with other features particularly the closed low near 35N/145W and how/when it is absorbed into this deepening trof. The 00z GFS continues to be fast lifting the southern wave and is absorbed about the time the jet reaches the base of the trof slinging the surface low up coastal WA toward Vancouver Island by Thurs. likewise the UKMET is faster too and is a bit narrower with the overall trof shape in the Eastern Gulf of AK. The 00z NAM digs a bit deeper to eventually draw the surface low north, and is also generally narrower with the overall trof shape. The 12z ECMWF/CMC both are a bit southeast initially with the developing closed low centroid and by Thursday are a bit broader. Still, the differences seem very small given the otherwise good timing/strength. As such a general model blend is preferred but given the preference toward the NAM/ECMWF and CMC for the southern wave...likewise weighting will favor these in the blend for this system as well. Confidence is slightly above average. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina