Prognostic Meteorological Discussion
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000 FXUS10 KWNH 181831 PMDHMD Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 131 PM EST Sun Feb 18 2018 Valid Feb 18/1200 UTC thru Feb 22/0000 UTC ...See NOUS42 KWNO (ADMNFD) for status of the upper air ingest... ...12Z Model Evaluation with Final Preferences/Confidence... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Amplified Western US Trough Gradually Building Into the Central US With Several Shortwaves Rounding the Trough... ...Active Front on Leading Edge of the Trough and Lee Surface Cyclone, With Additional Weak Cyclogenesis in the Great Lakes... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z ECMWF, CMC, UKMET Confidence: Average ---18Z UPDATE--- The 12Z runs of the ECMWF, CMC, and UKMET show fairly good continuity overall with their previous runs, and maintain the idea of a faster surface low in the Great Lakes, and a slightly faster southeast progression of the cold front across the central US. Given that this aligns with the preliminary reasoning, and has good ensemble support, the preference remains the same. ---PREV. DISCUSSION--- On the synoptic scale, models are generally in good agreement on the overall position and structure of the large trough that will be situated over the western half of the contiguous United States, particularly through Tuesday. Therefore, they are all generally showing the same basic scenario with slightly different placement of sensible weather impacts -- emanating from small differences in timing of shortwaves or mesoscale details. The 12Z NAM is very similar to the other available models in its strength of both the trough, and the large mid-upper level anticyclone positioned just off the Southeast Atlantic coast. It does show the trough shifted slightly out of phase with the other models, with the trough axis by 21/00Z about 250km to the west. This also leads to slightly higher heights across the central U.S. (where much of the precipitation impacts will be). This bias does appear to affect its progression of the axis of heaviest QPF, and thus the preference is to not incorporate the NAM for this system. Looking at the global models, one more notable difference is between the 12Z GFS and the 00Z runs of the other global models (CMC, UKMET, ECMWF). The GFS is slower in ejecting a shortwave and associated sheared lobe of vorticity to the northeast across the Plains on Tuesday, and this results in a surface low that is about 6 hours slower than the other models. This affects the eastward progression of the northern extent of the surface front, and thus may affect precipitation patterns and other sensible weather elements. Given a lack of substantial support among GEFS/ECMWF ensemble clusters for this slower scenario, the preference is to lean toward the CMC, UKMET, and ECMWF. ...Shortwave Dropping South Along West Coast of North America from Monday Night into Wednesday... ...Associated Weak Surface Low Just Offshore... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z ECMWF, CMC, UKMET Confidence: Average ---18Z UPDATE--- The 12Z UKMET did trend to a slightly more amplified trough as it passes near the OR/CA coastline, and a slightly deeper surface low. However, the ECMWF and CMC remained relatively consistent, and the GFS continues to be the most amplified scenario. Therefore, the preference will remain the same, despite the small changes on the UKMET. ---PREV. DISCUSSION--- A low-amplitude shortwave dropping south along the West Coast is forecast to amplify slightly as it passes by the coasts of Oregon and California. The 12Z GFS (and secondarily the 12Z NAM) is the most amplified with the trough, and thus produces the strongest surface low just offshore. This also tends to focus the precipitation more offshore relative to the other global models, which show some light precipitation extending inland. The preference is to lean toward the lower amplitude solutions which at the moment have better ensemble support. However, with a southward digging wave, a future trend toward the GFS solution could not be ruled out. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Lamers $$

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