Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 227 AM EDT Tue Mar 20 2018 Valid Mar 20/0000 UTC thru Mar 23/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model Evaluation...Including Model Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Lower Ohio Valley strong shortwave and associated surface wave in the TN valley transferring to a coastal low near VA by late Tuesday... reinforced by rapidly amplifying upstream shortwave evolving into deep eastern US trof...producing a secondary sfc low in the Carolinas late Tues becoming a Nor'easter paralleling the coast... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Average 07z update: The 00z Non-NCEP guidance appears to be showing stronger consolidation on a solution fairly close to the 00z GFS and while there are subtle but important differences (particularly proximity to the coast for the surface wave), the tricordial wobble is much better agreed upon. Differences in timing start manifesting through Day 3, with the ECMWF lifting faster and shifting further west, the CMC fast and center, while the UKMET slides east. All in all, this convergence is providing better confidence (now to average) in utilizing a non-NAM blend. ----Prior discussion---- Goes-16 WV loop shows strong compact vorticity maxims across the lower Ohio valley at this time with an eastward trof extension associated with more positive tilted warm conveyor belt. An associated surface wave is tracking across the TN Valley but with time/due to tilted warm conveyor/height falls will form a weak coastal low off the mouth of the Chesapeake midday today. The compact mid-level center will weaken across the upper Ohio Valley and give way to upstream development. This upstream feature is also detected in Goes-16 WV as a low amplitude shortwave emerging from the southern Colorado Rockies with an associated strong jet streak. In combination with weak/broad cyclonic shortwave energy currently in the central Missouri River Basin will become much more important in the evolution of an amplifying larger scale deep eastern US trof by Wed/Thurs. With this amplification a secondary surface low will rapidly develop out of the Carolinas becoming a Nor'easter. While these features are becoming a bit better resolved, small variations lead to moderate to large sensitivity toward the larger scale trof evolution. Hence there remains some run to run inconsistency in the longer term (last 2 days or so), keeping confidence quite low in the precise details, locations of thermal, precip and other important sensible weather conditions; however recent trends over the last day or so are providing much higher confidence in the eventual deep Nor'easter along the east coast. The 00z NAM which has been most aggressive over the last 2 days or so, especially with the lead wave staying strong in the Ohio Valley and building the eastern trof has backed off suggesting a flatter devolution of the lead height falls and slightly stronger coastal reflection on late Tuesday bringing it back into the fold of the more consistent consensus at least for the lead wave/surface low. However, in shifting the wave east, the secondary low has greater spacing and increased shortwave ridging between the systems resulting in a southward shift of the inner core of the trof as well as the surface low. As the surface low rapidly deepens off the Delmarva, the NAM shifts west and leads to an earlier wobble as upstream energy carves out the upper level trof. This provides increased confidence in other guidance solutions (mainly the ECMWF) that have presented a surface wobble toward the coast. While still plausible (especially with the dry air in place over the northern mid-Atlantic/New England filtering south), this the run departure from prior NAM runs as well as the increasing clustering in the ensemble suite makes it hard to make such a shift at this time. While the ECMWF had been inconsistent over the last day or so...the last 2 runs have set a trend with most guidance trending toward it. However, the ECENS cluster is well west toward the coast during a tricordial wobble and quite a bit stronger through the depth of cyclone (up to 30 DM). To offset this potential western, potential over-development, recent 00z GFS run shifts closer but along with 12z/18z GEFS members are set a bit south and east of the ECENS members (particularly on Thur into Fri); although with similar timing/evolution are consistent enough to have some confidence in a blend of the two solutions. The 12z UKMET is a bit too strong with the lead energy offshore which also reduces best shape of the upper jet pattern and disrupts the best rapid cyclogenesis off the coast with the second low. The 12z CMC, while being most consistent with the longer term ensemble trend, seems to have missed the recent shift in the last day cycles that has been picked up by the remaining guidance. All considered will suggest a 00z GFS/12z ECMWF blend, likely stabilizing the blend with some very low percentage of the GEFS/ECENS mean and 00z NAM to account for the uncertainty. Confidence is high for a strong Nor'easter and eastern US trof to develop but slightly below to below average as the precise details and the model selection to accomplish these details remains elusive. ...Deep trough/closed low offshore the Northwest by Wednesday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Large closed upper level ridge over the western and central Gulf of AK will retrograde Tuesday and two shortwave features: one lingering along the SE AK coast and an upstream wave over western AK begin a binary interaction early Wed in the eastern Gulf of AK beginning to carve out a deep layered closed upper low (with the prior wave becoming the core, while the other digs southward. As the digging wave rounds the base of the other, a weak surface wave drops south and begins to merge with the frontal zone lifting north from a closed central Pacific system (see section below). Overall, the guidance has been consistent and agreeable with respect to the evolution of the pair with exception of the 12z UKMET which is much stronger (typical of bias) and more elongated N-S, i.e. narrower W-E. The other departure is more related to the interface/interaction with the southern stream wave/frontal zone which is better described below. As such a Non-UKMET blend is supported at Slightly above average confidence. 07z update: The 00z UKMET trended much broader with the width of the developing low, mainly now that it is a bit slower bottoming out the base of the trof on Thurs, generally similar to the ECMWF/UKMET. As such a general model blend can now be supported, though confidence remains slightly above average given the remaining small (but important) scale differences. ...Closed Pacific low near 34N/140W drifting east breaking down under influence of digging/northern stream closed low providing deep layer southwest flow/atmospheric river into Southern CA with associated surface low and frontal zone. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/ECMWF blend Confidence: Slightly above average A broad closed low around 33N142W is consolidating with strong inner core and secondary/triple point/interface of warm and cool conveyor belts near 37N138W...starting to elongate a bit under some upstream ridging dropping south. This will flatten the overall flow but also strengthen it providing an excellent corridor of moisture flux from the subtropical/tropical regions of the central Pacific supporting a very strong atmospheric river event directed at south-central to Southern California from early Wed to Fri. As mentioned in section above, strong deep closed low will dig south out of the Gulf of AK influencing the shortwave as well, potentially shearing it into the northern stream toward late Wed into Thurs. However, there remains mild disagreement in the timing, magnitude of the shearing of the low toward the northeast (with associated surface wave along sharpening frontal zone). The 12z UKMET digs a bit further south with northern stream system leading to increased shearing early, but given this narrow solution is not favored for the northern system, its effects on this system appear equally dubious. The 00z NAM also showing increased influence of the northern stream but is also uncharacteristically quick to weaken/shear this system, allowing it to migrate toward the West Coast a bit too quick too. The GFS and continued with the 00z run, has been trending slower to interact with the northern stream, nearing in the ECMWF/CMC timing, but this is mainly due to the aforementioned triple point shortwave rotating much further west, then south to be affected by the northern stream eventually rotating back as a weaker wave (than the ECMWF) and therefore faster than the ECMWF/CMC toward the West Coast by late Thursday. The 12z operational ECMWF is very strong/consolidated and very slow to break down and advance eastward with time, even more so than a bulk of its ensemble counterparts. Additionally, the ECMWF resolves a very strong/deep surface low. The 12z CMC seems a more reasonable solution compared to the ECENS members and the mean as a whole. Still, a blend of the 00z GFS along with the 12z ECMWF/CMC should represent a good blend of the GEFS/ECENS means which are preferred. Given spread of timing, confidence is average in the mass fields though quite high for the resulting atmospheric river, placement and strength. 07z update: The faster northeast shearing toward the West Coast has been limited as the UKMET dramatically slowed. In fact the UKMET is now slowest (north and west) with the tail end surface cyclone and frontal zone pressing ashore. Likewise the ECMWF/CMC both trended faster, with the CMC now outpacing the GFS. The shift of the ECMWF leads to remarkable mass agreement in shape/timing with the 00z GFS and ensemble means. As such will support a 00z GFS/ECMWF blend to increase tightness of the blend and bump up overall confidence to 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