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May Outlook 2012 | Above average, but not without a fight
After a 13 month streak of above average departures in New York City, many meteorologists and meteorology enthusiasts alike are left wondering:
when will this pattern finally buckle? Sure, we’ve seen our share of below-normal departures. We detailed in a blog post on the main site a few days ago how April finished with 9 straight days of below-normal departures. But the fact of the matter is that the streaks of above normal departures have been more intense, more focused near our area, and of longer duration. So, really, it’s no surprise that we’ve experienced the streak that we have.
May 2012 has started with a bit of an up and down feel. The first day began chilly and cold — but ended with temperatures soaring into the 70’s by late afternoon after a warm front passed. But shortly thereafter, a backdoor cold front moved through and kept May 2nd cool, dreary, and well below normal. Such will be the trend over the first two weeks of April. The main culprit will be high latitude blocking which is forecast by most global ensembles to develop by around the first weekend of May. The advertised blocking over Greenland and the Davis Straight hasn’t been a staple in the pattern since last May (not surprisingly, we had a period of below-normal departures then as well).
With the GFS Ensemble mean image at 500mb to the right, (departures in the 500mb height field) we can see the pattern pretty clearly. There is a well defined area of above normal departures situated from the Davis Straight into Central Canada and the ridge axis extends towards Greenland. The -NAO in place extending that far west usually means a West Coast ridge will be involved as well — in turn developing an east coast trough. So, as long as the medium range guidance isn’t awful, it looks like we will be veering away from the much-above-normal departures at least through Mid-May.
All of that being said, there is still lots to question in such a pattern. The first would be the source of the cold air. With a block over Gerenland and the Davis Straight extending into Canada, the cold air source will be moderate at best. Much of the NAO area has been consumed in above normal temperature departures over the past few months, anyway. Compounding the issue, the wavelengths by this time of the calendar year don’t support the same severity of a -NAO block as they would in, say, mid winter. Additionally, the duration of the cold and troughiness will be in serious question.
The MJO is forecast to spiral towards Phase 8 by the second week of May by the GFS and ECMWF forecasting schemes. Such an occurrence, albeit modeled as weak and near the COD, supports above normal temperature anomalies in the Northeast and Eastern United States historically. The presence of the Southeast ridge seems fairly strong on these historical analogs as well.
The general theme being advertised on most of the 500mb and surface analogs for May is a ridge over the Davis Straight and Central Canada, and a ridge on the West Coast, with troughiness over the Central US. The bulk of the “below normal” temperature anomalies may very well be centered over the Central and Southern Plains, extending towards the Arlatex and Southeast US. But here in the Northeast, despite the presence of such a block for a period of time, virtually every analog suggests temperatures should still average +1 to +2 for the month. Precipitation analogs seem to average near normal, possibly a hint above normal..which would come as a major relief after such a dry period.
Our official May Forecast for the NYC Area (all 5 reporting stations, with some obvious variance) is below:
Temperature Anomaly: +1 to +3 (High Confidence)
Precipitation Anomaly: -1 to +1 (Moderate Confidence)
500mb Geopotential Hieght Average Anomaly: 0 to -10 dm (Moderate Confidence)