Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 148 PM EST Thu Jan 30 2020 Valid Jan 30/1200 UTC thru Feb 03/0000 UTC ...See NOUS42 KWNO (ADMNFD) for the status of the upper air ingest... 12Z Model Evaluation ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...Evolution of Trough over Eastern Half of the U.S. through Sunday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~ Preference: Blend of 12Z ECMWF/UKMET...supported by 12Z GEFS Mean and 00Z ECMWF Ensemble Mean Confidence: Average Downwind of a sharp western U.S. ridge, northwesterly flow with southern and northern stream components will absorb a low shearing out of Mexico, forming a full-latitude trough over the eastern U.S. through Saturday. The trough does not consolidate and swing into a neutral or negative tilt, with associated cyclogenesis, until moving offshore over the weekend. At the surface, low development occurs Friday night along the North Carolina coast in the left exit of a strong southwesterly jet stream. Embedded upper-level impulses in the trough vary among the guidance, but there is decent agreement on position and movement of the large scale features. The tilt of the southern portion is more positive than in previous runs, and the upper-level energy skirts across the Southeast states a tad more quickly in the GFS/ECMWF compared to prior runs. The UKMET hangs back a bit more, looking more like continuity and the older ensemble means, but still clustered fairly well with the ECMWF, GFS. The now quicker and more open southern stream wave slows the cold frontal progression over the FL peninsula Friday night and results in a west to east swath of rainfall over the FL peninsula on Day 2. The 12Z CMC is an outlier in producing such staunch ridging in the immediate wake of the system as it gets past the lower Mississippi Valley. The 12Z GFS may be used, but with a little bit of caution as it looks overly developed with the very short wavelength system in the northern stream over the Great Lakes / Northeast Saturday. The NAM has somewhat opposite the issue seen in the CMC, maintaining colder heights over the southern Plains / Lower Mississippi Valley, with little ensemble support. ...Pacific Northwest Atmospheric River Leading a Longwave Trough Pushing into the West on Sunday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of 12Z ECMWF / UKMET...supported by 12Z GEFS Mean and 00Z ECMWF Ensemble Mean Confidence: Average An atmospheric river reaches the Washington coast later today and waivers north/south through Friday night before shifting down the Oregon coast Saturday. This occurs in advance of a longwave trough that reaches to central California as it pushes inland Sunday. The dilemma comes with the exact axis of heavy precipitation on Day 2 and the timing of the trough that moves ashore on Day 3. The ECMWF remains a steady 6 to 12 hours behind the rest of the guidance with the trough axis, so it has QPF inland on Day 3, especially over the northern Rockies. Looking at how ensemble spaghetti plots have evolved over multiple model cycles, the model camps have been steady in their predictions, suggesting this is just a case of normal biases. The ECMWF is probably a bit too slow, but the GFS errs on the fast side. In particular, it looks like the GFS is too quick in phasing cutoff energy over the Southwest U.S. back into the zonal flow. The UKMET offers a nice middle ground, so we recommend an ECMWF/UKMET blend, although the GFS is really only minimally faster. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Jackson/Burke