Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 253 PM EST Sat Dec 14 2019 Valid Dec 14/1200 UTC thru Dec 18/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation with Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Deep surface low over the Northeast today... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Above average There remains very good agreement and consistency with the deepening surface low over the Northeast U.S. today and as the low tracks north/northeast, the most recent guidance shows little variability with the depth/track/position through 48 hours. A general model blend is preferred. ...Mid-level trough amplifying into the western U.S. this weekend and reaching the central Plains Monday... ...Surface low organizing east of the Mississippi River Monday into Tuesday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-NAM blend Confidence: Slightly below average 20Z update: No real significant changes noted with the rest of the 12Z guidance. The 12Z NAM still remains the outlier solution and is not included in the overall blend preference. There are some timing differences with the low lifting through the OH Valley by Monday evening but a blend of the other deterministic guidance suffices for now. With the next shortwave trough digging through the southwest U.S. and then reaching the central Plains early next week, the deterministic guidance is beginning to show less spread this cycle. There is some latitudinal differences in the low track across the Mid-South as well as some timing differences (a run to run analysis shows a slowing trend) but overall there is good agreement. The exception is the 12Z NAM, which is considerably stronger with the vorticity maxima swinging through the Southwest U.S. on Day 1/2. This leads to a much more amplified wave across the central Plains and contributes to higher QPF. With little support from the rest of the guidance and its solution outside of the ensemble spread, it was discarded for this cycle. Otherwise, the rest of the deterministic provide a good blend for the large scale mass fields. ...Mid-level shortwave nearing the Upper Mississippi Valley Tuesday morning... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend Confidence: Slightly above average In the wake of the southern stream shortwave that tracks across the central Plains, a secondary shortwave trough will drop through the northern tier and Great Lakes region Tuesday. Model differences are minimal, but the one notable difference is the 00Z UKMET which is a bit more amplified/deep compared to the rest of the guidance. The 00Z CMC is also stronger compared to the rest of the guidance. These differences do not appear to make much difference in the QPF, so overall a general model blend is sufficient for this system. ...Trough Approaching Western U.S. Tuesday Evening... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Non-CMC blend Confidence: Average Toward the end of the forecast period (Tuesday evening), a trough will approach the western U.S. coast. Overall setup is well agreed upon for the Day 3 forecast with the only notable difference is the 12Z CMC which is faster than the rest of the guidance. Looking at the ensembles, there is relatively similar clustering of low positions at 18.00Z, so a non-CMC blend is preferred. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor