Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 232 PM EDT Thu May 02 2019 Valid May 02/1200 UTC thru May 06/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Southern stream trough/wave(s) crossing the South lifting northeast across the OH/TN Valleys... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Average 19z update: The leading wave shows much better overall agreement in timing and orientation crossing into the Upper Ohio, Mid-Atlantic and New England regions (w/exception of the 12z NAM). The 12z ECMWF and CMC both trended a bit stronger and therefore a bit slower with the shortwave coming out of TX tonight and through the TN Valley into Mid-Atlantic by the end of the weekend. This leads to a separation between the faster UKMET/GFS which drop the front south through the Mid-Atlantic faster than the ECMWF/CMC. Obviously, the resultant upscale enhancement from convection tonight over TX is mildly uncertain, but each guidance member falls into typical timing differences to suggest that the GFS/UKMET are too fast and the ECMWF/CMC are too slow (deep) and a blend in timing and depth would seem logical supported by continuity and ensemble suite and trends. Still confidence is average in a Non-NAM blend. ---Prior Discussion--- A series of southern stream shortwave impulses will continue to lift northeast from the southern Plains up across the OH/TN Valleys and into the Mid-Atlantic region going into the weekend. This will allow multiple waves of low pressure to lift up along a frontal zone draped across the region. While the initial wave (currently in AR) weakens and shears through New England, the 12z NAM continues to be over amplified, slowing a bit. The 00z UKMET also appears to be a bit more amplified and delays the frontal boundary through the Mid-Atlantic and New England by early Sat. The second wave, that is currently tracking out of Northern Old Mexico finds a favorable environment to enhance slightly across TX into the Mississippi River Valley by Sat, but the 12z NAM once again is a clear outlier supporting a very strong upscale response that it maintains through the Appalachians with associated deep surface inflection. The 12z GFS is a shade fast, typical of day 3 bias relative to the CMC/ECMWF, but in this setup, it is typical the EC/CMC may be a bit slow too. While the UKMET is more inline (middle-ground) with this wave in timing relative to other guidance making it more preferable, but the location (south) of the front due to a stronger but slower wave across the Mid-Atlantic makes it a bit more uncertain for a high inclusion. All in all, a 12z GFS and 00z ECMWF/CMC blend is preferred with potentially some UKMET if the boundary/QPF axis is shifted a bit further north through day 3. Given weak waves and the importance/placement of convection and its affects on the boundaries/waves confidence is only average in this blend. ...Low to mid-level trough impacting the FL Peninsula... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average 19z update: the 12z ECMWF/CMC trending slower with the wave coming out of the TN valley, pulls this wave a bit north and more in line with the NAM. Still the UKMET/GFS are still very close in proximity and given the spread remains fairly small will incorporate a general model blend at slightly above average confidence to keep with continuity but start to shift slowly to this small faster trend. ---Prior Discussion--- Tropical wave rounding the western side of Subtropical High continues fairly good continuity and overall guidance agreement. However, as the wave lifts into the northern stream, the over-amplified NAM deepens it and draws it further north. Thinking this may be a bit too much and would support a non-NAM blend at slightly above average confidence. ...Southern stream closed low approaching CA... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average 19z update: The 12z UKMET/CMC/ECMWF and GEFS mean showed little variation from the prior run or initial preference. As such will keep general model blend preference at above average confidence. ---Prior Discussion--- Closed low off the southwest US coast continues to advance under strong model agreement. There is a bit of model difference into late Sunday with the magnitude of stretching toward the deepening trof in the northern stream...affecting some locations across the Sierra Nevada. However, given the agreement it is difficult to discern a particular positive/negative trend that a general model blend will be a solid choice. Given the overall agreement, confidence remains above average ...Height falls traversing the northern Plains/Midwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average 19z update: The 12z UKMET trended back to a slightly slower (but on the faster side of guidance), more concentric solution. This leaves the 12z NAM, the clear outlier given the ECMWF/CMC and GEFS mean tightened the overall solution spread with their 12z runs. Confidence remains slightly above average ---Prior Discussion--- Broad vortex over NW Canada will shed some energy southwest into south central Canada by late Fri, under influence of strong anticyclonic jet streak out of S AK. The cyclone rapidly deepens and is fairly well agreed upon through Sat, but by Sunday, the 12z NAM has begun to open the circulation indicating greater elongation/stretching, opposed by the GFS/ECMWF and slightly slower CMC. The UKMET is a bit faster and while more compact than the NAM, it draws upstream energy to lead to larger spread in the lower levels. As such will favor a 12z GFS and 00z CMC/ECMWF blend. Confidence is slightly above average ...Digging trough across Northwest/Northern Rockies Sunday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 12z ECMWF/CMC/UKMET blend Confidence: Average to slightly below average 19z Update: the trends suggested by the 12z GFS/NAM toward a faster, more amplified solution in the Northern Rockies by the end of the forecast period (84hr) were not supported by the later non-NCEP solutions. The UKMET is faster than the CMC/ECMWF, to be expected but unlike the GFS, the flow remains generally flatter and not amplifying by 84hrs. The 12z ECMWF/CMC also remained fairly stable to their 00z runs as well. All this suggests the GFS is too fast as well, and will remove it from the preference of ECMWF/CMC/UKMET. Confidence remains average to slightly below average. ---Prior Discussion--- Another AK jet streak and associated shortwave from the Pacific stream will undercut the larger scale weakening vortex over SW Canada by the end of the weekend. The 12z NAM was faster and very amplified in a fairly sizable shift in timing across the Southern Rockies by the end of the forecast period. The 12z GFS also showed some increased amplification, and given the orientation of the remaining polar vortex retrograding into the N Yukon, this does make some meteorological sense. Will incorporate some the 12z GFS to hint at this timing with the slower 00z ECMWF/CMC. All in all there is average confidence to slightly below average confidence in a 12z GFS and 00z ECMWF/CMC blend for this region on day 3. Model trends at www.wpc.noaa.gov/html/model2.html 500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml Gallina