Sunday, January 11, 2015

Model Discussion - January 10, 2015

ESRL Ensemble Guidance Analysis
Prepared January 10 2015

Short-Medium Range (00hr to 144hr)
Guidance retracts strong upper level low from South Canada into Greenland, a textbook signal of the North Atlantic Oscillation (NAO) active in its positive phase. This is seen well in sustained ridging across much of the country, primarily the northern half, in most of the period. Positively tilted shortwave drops into the Southwest and slowly exits into the Southern Plains under the ridge, permitting for a southern track. This may allow for precipitation development in the mid-section of the country, though exact placement and precipitation type is in question. Have a good feeling wintry precipitation will not be favored due to generally seasonal temperatures in the aforementioned North US, where ridging will be present. Regardless of storm evolution, overall warm temperature pattern is favored.

Long Range (144hr to 360hr)

Strong ridge cell then evolves in the West US and shifts east with the departure of the aforementioned trough along the East Coast. Ridge continues pushing east and amplifies in the Central US with the assistance of a longwave trough pattern moving into the Pacific Northwest, allowing for above-normal warmth in much of the land east of the Rockies. As this ridge slowly departs into the Canadian Maritimes, troughing pattern in the Gulf of Alaska is decimated and replaced by a ridge, forming a connection to the other ridge in the Maritimes. Intensification of the ridge along the West Coast of North America permits the evolution of the active Pacific North American (PNA) index in its positive phase, likely reflected later in the period as a negative East Pacific Oscillation (EPO) signal. Aforementioned ridge that moves east of the Canadian Maritimes has some ensemble support to form an east-based negative North Atlantic Oscillation (NAO) signal, agitating a much more wintry pattern to end January compared to that in the short term. Some indication of subtropical jet stream amplification is evident, though uncertainty is too high to evaluate such a potential at this time.