Prognostic Meteorological Discussion
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000 FXUS10 KWNH 200636 PMDHMD Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 236 AM EDT Fri Apr 20 2018 Valid Apr 20/0000 UTC thru Apr 23/1200 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 00Z Model evaluation including preference and forecast confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Exiting Northeast Closed Low with lingering troughiness through Sun... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 00z GFS/ECMWF/CMC blend Confidence: Slightly above average Goes-16 WV loops depict strong vort center over the mouth of the Hudson River starting to swing north-north east... while the surface reflection is now well out of US waters...southeast of Nova Scotia. Strong model agreement remains with this vorticity center and the lingering trof that remains across New England through the weekend. As the trof exits, a shortwave out of the northern stream drops into New England and reinforces the broad larger scale troughing into Monday. The 00z NAM however, appears to show typical day 3 over amplification producing a very deep trof that drags south and southwest to elongate the closed low over the Southeast. This is not likely given the overall agreement in the remaining guidance and ensemble suite. A such a general model blend is good through Sunday but a Non-NAM blend should be employed into Monday at above average confidence. 07z update: While the CMC/UKMET and ECMWF all remain agreeable with the exiting trof, the UKMET has broken from the tighter faster solutions and initial preference with respect to the shortwave crossing New England late Sunday into Monday, in the same vain as the 00z NAM just not as deep. Still believe this is an aberration than a trend, though confidence is slightly reduced in the day 3 period in a 00z GFS/ECMWF/CMC blend to slightly above average. ...Strong closed low aloft in Colorado River Valley pushing to the Four Corners by Friday, and into the South-Central US this weekend and East South Central US Mon ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly above average Goes-WV mosaic depicts a strong and fairly concentric closed low centered over the UT/NV/AZ tri-state junction with a fairly broad warm conveyor shield dominating the US Rockies and high plains with broad diffluence across NM shifting to TX. The strong surface low over the Great Basin will fill across the central Rockies and shift toward new lee cyclogenesis across the Permian Basin. Models remain in good agreement mass-wise through this transition. By late Sat into early Sun, the low as opened to an inverted sfc trof across eastern TX. It is at this point, the upper low is starting to break down into its constituent internal parts/shortwaves. This leads to larger model variation with the CMC showing some weakness...the 00z NAM showing some stronger/tighter vorticity centers, as the closed low migrates eastward. Convective feed back leads to differences in timing/intensity and depth of the internal waves; yet larger scale mass fields remain remarkably stable which manifests with good placement/strength and timing of the surface wave/frontal zone across the East South-Central US states by Monday. The 00z NAM and 12z UKMET are a bit broader E to W while the CMC remains generally shallower with the 00z GFS and 12z ECMWF the closest guidance members tight to the ensemble suite. All considered, a general model blend is supported weighted to the 00z GFS/12z ECMWF but given internal variation, convective/upscale influences...confidence remains slightly above average as smaller scale run to run variation is expected. 07z update: The 00z ECMWF shifted to even tighter agreement with the 00z GFS...further supporting a higher weighting to these common solutions. However, the UKMET and CMC both, like the NAM, are well within the tightening ensemble suite to keep at lower weighting to account for this run to run, smaller scale inter-variability seen in the deterministic runs. Generally, the CMC is further south through the Lower MS into the Central Gulf states...the UKMET is a shade faster and the NAM is north and a bit deeper. Given the tighter agreement of the UKMET and CMC to the GFS/ECMWF...a Non-NAM blend is now preferred though still weighted toward the GFS/ECMWF at slightly above average confidence. ...Strong shortwave approaching the Pacific Northwest and British Columbia late Friday before shifting north of the US border this weekend...Surface cold front crossing the Pacific Northwest... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average Models continue to show strong agreement with this system especially with the timing/strength of the front crossing the Pacific Northwest. There is some model discrepancy much further north, as the 00z NAM is stronger further north (how the top of the trof crosses the Northern Canadian Rockies) that lead to differences in mass/surface low placement across boreal regions of the Central Providences, but does not affect the CONUS timing/placement/strength of the mass fields. As such a general model blend is preferred. 07z update: No significant change here, especially in the CONUS, with the 00z ECMWF/GEFS/UKMET or CMC. As such a general model blend remains preferred at slightly above average confidence. ...Lower amplitude shortwave reaching the Pacific Northwest early Sunday...amplifies and pushes into the Northern US Rockies Monday... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: 1/3 00z GFS & 2/3 12z ECMWF blend Confidence: Average In the wake of the strong shortwave lifting into South-central Canada...a low amplitude wave emerges from the Pacific and with upstream ridge strengthening, is also able to amplify as it enters the Pacific Northwest (SW OR/N CA specifically) late Sunday into Monday. Guidance is a bit uncharacteristically ordered with respect to timing... as the 00z GFS continues to be the slowest piece of guidance but also is a bit more amplified and narrower than the slightly faster 12z ECMWF/CMC. The 12z UKMET is fastest and is a bit broader over through the wavelength, perhaps influenced by the prior wave...as it is further north comparatively within the suite. The 12z NAM is slower than the ECMWF like the GFS, but is also much narrower and injected a bit south of the main clustering. This leads to significant deviation with respect to QPF (please refer to QPFPFD for additional details) though lesser in the mass field. Still the NAM should not be considered given these factors. The 12z CMC is a bit broader and faster than the ECMWF and is connected to the northern stream wave a bit more. All considered, will favor a 1/3 GFS to 2/3 ECMWF blend. Confidence remains average given large variation in the spread and influence of downstream and upstream stronger features. 07z update: The 00z ECMWF showed very little change from the initial preference, though the UKMET/CMC both trended slightly narrower, slower and a bit south...there remains enough departure from the initial preference to keep them excluded from the blend at this time. However, their changes provide some additional confidence in the initial preference and higher weighting toward the ECMWF. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Gallina $$

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