Model Diagnostic Discussion NWS Weather Prediction Center College Park MD 1243 PM EDT Tue Jun 30 2020 Valid Jun 30/1200 UTC thru Jul 04/0000 UTC ...See NOUS42 KWNO (ADMSDM) for the status of the upper air ingest... 12Z Model Evaluation Including Preferences and Forecast Confidence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Closed low anchored over New England...transitioning off Mid-Atlantic by Friday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: general model blend day 1/2; non-UKMET blend day 3 Confidence: Above average Model guidance is in excellent agreement through day 2 with the evolution of the closed low that has been anchored over New England. By 48-60 hours, troughing from the north dips down which weakens the closed low and gradually pushes it eastward off the coast. A portion of that drops southeastward with a broad but weak trough by day 3. Here, the UKMET is slower and displaced westward of the model consensus. Therefore, a general model blend is preferred for days 1/2 and a non-UKMET blend for day 3. Deep upper trough/closed low across the Intermountain West ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: non-CMC blend Confidence: Average Upper level low over the Intermountain West is expected to lift northeast/north toward the northern Rockies while another shortwave trough drops southward over the Pacific NW. The phasing of this energy will keep the close low in place through much of the period with some tendency to lift further north into the Canadian Rockies late in the day 2 period and into day 3. Another compact shortwave trough then swings onshore the Pacific NW coast to finish the forecast period. In the large scale sense, there is relatively good model agreement but with individual lobes of vorticity rotating through the flow, some timing and strength differences creep up. The CMC remains a bit of an outlier, especially by day 3, as it's too fast and too deep with the shortwave moving onshore Washington State. Otherwise, guidance is in good shape to go with a blend of the other deterministic guidance. MCV crossing the middle and upper MS Valley Tuesday/Wednesday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: Blend of the 00Z GFS/CMC/UKMET Confidence: Below average A convectively enhanced MCV ongoing across the mid-MS river valley, stuck between the dominant closed low flows on either side, will gradually drift eastward/southeastward over the next couple of days. By day 2, with building heights through the central US, the bulk of this energy is pushed eastward and gradually drops toward the Gulf States region (see section below). Model guidance has improved such that a general model blend can be used for this area now. Mid-level closed low over the Gulf Coast by Friday ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Preference: General model blend- Confidence: Above average Mid-level troughing and a developing closed low over the Gulf Coast states is expected by the end of the week, with weak shortwave energy dropping from the Mid-South. There is decent agreement with the available deterministic guidance, the only exception being the 12Z NAM is spatially displaced further north from the other guidance, but it isn't much. Sensible weather differences are likely to be driven from more mesoscale processes, so a general model blend is preferred from a mass field perspective. Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml 500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml Taylor