We also have every reason to believe that the generally favorable pattern will continue thanks to a weakened jet in the Pacific and a large ridge over western Canada. The first will be the shifting of that deep trough along the west coast of Canada south into California, and then moving north and east into Montana, as this GFS Ensemble forecast shows on the morning of this coming Sunday. Consequentially, snowstorms pose threats to the Northeast and East Coast, in addition to the cold weather. When you bring the Typhoon Rule into play after accounting for such storminess and cold in Japan, the risk of a cold and snowy winter suddenly becomes much higher. We’re kicking off this winter with a pattern change update for North America. I took a look at the NWS GFS model’s ensemble forecasts from 00 UTC last evening to get a feel for when the NH pattern at 500 mb might change.

It is expected that a pattern change will impact the United States in the next week or two, leading to substantial air mass shifts across North America. This volatile pattern has wedged the United States into a rather stagnant pattern, with warm weather over-spreading the Central United States, making pushes north and east as time has gone on. The closing part of the month is likely to feature a return to some cooler conditions, primarily in the eastern third of the country as a new MJO wave forms in phases favorable for a chill in the East. A weak front is now approaching the coast, and the thermal trough, which has been sitting over or just to the east of the western lowlands is rapidly moving eastward. Plus lots of low clouds along the coast, with a sliver entering the Strait of Juan de Fuca. You will be flabbergasted at how lots of people drive from an area where there is no snow and then panic when they’re needing to drive in it with bumper to bumper cars all along the interstate.

For the record I will say 1-3 degrees below normal on temperatures and 260-280 inches of snow (which is just to the above side of normal). AMO tends to enhance the likelihood of a negative North Atlantic Oscillation, which then helps cold and snow enter the Northeast and overall Eastern US. Blustery and mild conditions will then prevail during the day Tuesday followed by showers and perhaps even a rumble of thunder Tuesday afternoon as a cold front attacks the region. If the weather conditions are questionable before you leave to the terminal you will need to call first. We are now resuming winter weather posting, which means we’re back in business for the next several months! In January 2009, a city like London usually enjoys milder winter chills compared to the rest of the country but due to its urban heat trend, was struck with heavy snowfall! The NWS map of current watches and warnings (above) continues to be a colorful display this morning (Saturday, January 4th). Another storm is developing over the Plains, as a new push of arctic air heads south and eastward. If you live with a person who works night shifts and you do morning ones, this combination will be unhealthy for both his/her or your sleep.

Indeed, the early WRF-NAM forecast from Atmo this morning indicates PW increasing to around an inch by 11 pm Thursday night the 21st (below). A look at the GFS ensembles from last evening shows that 11 of 12 members forecast precipitation for southern Arizona during the Friday to Saturday time frame. The situation will likely be complicated by a strong cold front and high pressure that is forecast to plunge south over the Plains and Front Range. Naturally, some breeds and breed mixes are built for the cold but even these dogs should be monitored for icy paws and frozen ear tips. Above graphic shows the 500 mb height averages and departures from normal valid at 48-hours – dominant features are the strong, west coast ridge and the cold lows over the Great Lakes and north Atlantic. The forecasts above and below are the surface and thickness patterns from the ECMWF and GFS valid at 5 pm MST Friday afternoon.