Midweek Weather & The AO...(12-26-11)
Winter weather is going to try again to return to central New York over the next few days. The system responsible for this is now taking shape along the Gulf Coast . A look at the computer models show this storm strengthening and taking a track (based on the latest NAM) almost right over us Tuesday evening.There is no real chilly air preceding the storm but we still may manage to squeeze out a bit of snow at the start Tuesday afternoon. Here’s what’s happening. The air in the lower atmosphere will be fairly dry at first so evaporation of the leading edge of the precipitation will cool things enough that there could be a brief shot of snow at the beginning before we go to all rain. We should go back over to snow once the low tracks to our north and cold air starts to return to central New York. By that time, however, the bulk of the deep moisture will be lifting to our north so the chances of a significant accumulation looks low. Based on what I’ve seen on the models over the last 24 hours I would figure Syracuse is probably in a Tr-2” range for Tuesday night. Over higher elevations of central New York, where the changeover to snow will occur sooner, I could see some higher snow totals. Here is a look at some raw data from the Monday morning NAM run using the BUFKIT program. It goes forward in time as you read from right to left on the graphic:
Interestingly, it downplays the potential for some snow at the start. If you were to take it take it at face value we would have rain from late afternoon into the evening then a change to snow overnight. The peak in snow for Syracuse from the vantage point of the NAM would be during the Wednesday morning commute. Even if the amount of snow in Syracuse is not great, be prepared for a wintry day on Wednesday. There is going to be quite a bit of wind (gusts to 40 mph), low wind chills (between 5 and 15 above) and more snow. It will be one of those rare days so far this winter season that will look and feel like winter. In spite of the winter weather in the forecast for Tuesday night and Wednesday it still doesn’t look like we are throwing the switch just yet for a more prolonged period of true winter weather in central New York. We are still awaiting signs of atmospheric blocking taking hold in the higher latitudes so we can get some really chilly air down over us for more than a day or two. One of the tools we use to see if that is going to happen is the AO (Arctic Oscillation) Index. Here are a couple of charts showing what has happened over the last few months and also what the forecast is two weeks out in time. This is based solely on the GFS ensembles.
Courtesy: Climate Prediction CenterThe index has been strongly positive for the last month but if you look at the bottom frame the forecast is for the index to trend a bit more neutral toward the end of the first week in January For winter lovers this is a better sign that maybe, just maybe, we are getting close to a pattern change. When I looked this morning at the European model out that far in time there were also signs from that model that some cold air was getting ready to plunge south out of Canada. I do have to caution that I am just speculating based on a couple of models runs but it is a trend we’ll want to watch for over the next few days.Also, I thought I would include the latest on Lake Ontario. Even though we haven’t had much lake effect snow yet we are constantly checking to see what the lake temperature is to get a feel for the potential. As of yesterday (Christmas Day) the average lake temperature was still above the long term average.
Courtesy: NOAA Great Lakes WatchThe lake temperature is a bit more than 5c (or 41 F) which means when you look at charts showing projected temperatures at 5,000 ft we need at least -8c air aloft coming over the lake to get the lake effect started. Again, the chart above only shows the potential. As I have said many times you still need the cold air coming over the warm lake to cause the lake effect and we just haven’t had that much in the way of true arctic yet this winter season. One trend I’ve noticed is we’ve are now below what the lake temperature was at this point in the winter of 2006-07. That was the winter with the week long lake effect snowstorm in Oswego county.
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