Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1214 PM EST Mon Mar 02 2020 Valid Mar 02/1200 UTC thru Mar 06/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z evaluation including model preferences and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Amplifying shortwave over the northern Plains/Upper Midwest by Tuesday with surface low drawing weak surface wave crossing the OH Valley and deepening over northern New England by Wednesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend hedging toward UKMET Confidence: Slightly above average GOES-E WV depicts leading shortwave energy currently over SE Saskatchewan moving into Manitoba, increasing in sharpness at the left exit of strong but fairly zonal (from NNW to SSE) upper jet, but also at the left entrance to strong confluent 150kt+ over the Ohio Valley. The northern stream will support a strong surface wave to run along 50N through morning Wed. Concurrently, the broad upglide from the subtropical eastern Pacific stream provides ascent in the warm conveyor provides moisture and instability to enhance a weaker surface wave out of OK/MO today through the Ohio Valley tonight. Eventually, the combination of streams supports the Ohio Valley wave to deepen across the Lower Great Lakes and become the dominant low over the Canadian Maritimes by later Wed, as the upper level shortwave sharpens negative tilt. There has been significant model alignment over the last few cycles here, to have increased confidence in a generally lower predictable regime (combination of streams). The 12z GFS has tempered but remains the sharpest, fastest with the negative tilting as so appears to shift the wave a tad faster, but not significantly so. Influence of the next shortwave/jet streak exit is the greatest factor toward model spread after Wed, this suggests a general model blend will suffice for this system until the next wave's influence takes grip, but this is also generally into Canada and only affects ME/Gulf of Maine with more uncertainty. So overall, the UKMET is favored a bit heavier in a general model blend, as it is more central/reasonable within the overall suite with the approach of this next wave. Confidence is increased to slightly above average due to the overall mass field alignment through Wed, though given sizable but still not stable model trends over the last few days cycles in a lower than normal predictable pattern. Shortwave approaching British Columbia on Tuesday, significantly amplifying across the Northern Plains/Upper Midwest by early Thursday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend; reduce weight of UKMET at 84hrs. Confidence: Average Progressive northern stream flow out of the Gulf of AK continues through late Tuesday into Wed, the timing, strength of the subtle upstream wave crossing the Midwest/Great Lakes on Wed, leads to some uncertainty in increased ridging across the Northern Plains. Here the CMC is clearly south/deeper more amplified leading to stalled flow and a further southward digging of this trof in the Northern Plains/Midwest; while even the slightly slower ECMWF, deepens/amplifies a bit further north and east, even further north than the slightly faster GFS/NAM. The UKMET is a bit stronger but also faster, showing some typical late Day 3 negative bias, but could be kept in the preference through the short-term (84hrs) but should start to be weighted a bit lower. All considered at non-CMC blend is preferred but will weight a toward the GFS/NAM/ECMWF after 72 into 84hrs. With the good mass agreement, but harder to predict flow regime (and eventual binary interaction with the southern stream wave), confidence is considered average in this blend. Southern stream closed low system moving from northern Mexico through the Southern Plains into South Wed into Thurs ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly below average Sizable model difference over the last week continue with this closed low and has gone from nearly equally phased with the northern stream to barely connected with this current suite. The 12z GFS continues to suggest further slowing and less influence of the northern stream and is in an usual place within the overall suite: Slowest and therefore strongest coming through the Tennessee Valley late Thursday. This is opposed by the ECWMF, which had shown the first inclinations to a slower trend but has stopped the trend and is now vacillating run to run, with larger jumps over the last half-week, though seems to have stabilized around a common solution over the last few cycles including a fairly tight ensemble suite. Currently the closed low has settled in the California Bight with some upstream subtropical kicking influences, suggesting slightly faster progression may start sooner than later, tipping toward the ECMWF solution. The 12z NAM is slow/strong and north overall in the suite, and given this is in line with its typical negative bias, will discard its solution at this time. The ECMWF/CMC also being further south, shifting toward the instability provides some additional confidence. but influences out of the northern stream (particularly on Day 3 in the Southeast) provide very low confidence overall. So will favor the ECMWF/CMC a bit more in a non-NAM blend preference but cannot fully discount the GFS/UKMET given the influences in the northern stream that seem to favor them over the ECMWF/CMC at this time. Closed low in Northern Gulf of AK with deep latitude trof along the Pacific NW by Thursday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z ECMWF/CMC blend Confidence: Slightly above average Toward the end of the forecast period, finally a larger scale, closed low develops across the northern Gulf of AK late Wed. A strong upstream shortwave will swing around its base leading to further southward building and broadening of the trof over the entire Gulf of AK by Thurs. The 12z GFS is faster to do so in line with typical negative bias, but the eventual location/center of the closed low solidifies NW of Haida Gwaii by 06.00z and is supported by the other guidance members. This will direct strong southwesterly, moist flow regime into BC and the Pacific NW by the end of the forecast period. However, the stronger/compact but slower solutions of the NAM/UKMET favor a deeper surface low further north, delaying the cold front. The ensembles support the ECMWF/CMC timing placement of the mass fields and given the aforementioned issues in the GFS/NAM/UKMET, will support an ECMWF/CMC blend for mass fields at slightly above 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