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    Joe Lundberg

    No Room for Sustained Excessive Heat in Much of the Nation

    6/05/2014, 8:33:07 AM

    The summer solstice is now 16 days away, and the days are about as long as they are going to get in the Northern Hemisphere. Yet it seems like we're struggling to see a 'typical' summer pattern unfolding across much of the nation. And when I say that, I'm thinking of areas of sustained and excessive heat and, in many cases, humidity as well. To date, there just hasn't been any of that in most of the nation, and there isn't any on the horizon. Oh, there have been a couple of days here and there than have been fairly toasty and uncomfortable, but they have been then exception to the rule.

    It begs an honest question. Why?

    To answer that, let me take a look at some of the global teleconnections to see if we might gain some insight. First up is a look at the North Atlantic Oscillation (NAO):

    Since the middle of April, it has been almost exclusively neutral to negative. When the NAO is negative, it is an indication of blocking in the higher latitudes of the Northern Hemisphere, one in which the jet stream tends to have a lot more north and south movement. This allows cooler-than-average air to get pulled farther south that would ordinarily be seen in a more zonal (west to east) flow.

    In looking at the Pacific-North America (PNA) pattern since the middle of April, it, too, has been largely neutral to negative, especially since early May:

    While a negative PNA tends to indicate troughing in the West, it can still come up negative if a trough is nearby, but not necessarily over the West. That has been the case over the past few weeks, with enough ridging either poking into the Northwest, or some strong upper-level low occasional diving in through the West and the Rockies to force it negative while the overall pattern for the month of May came out warm.

    What the two of these combined tells us is that the jet stream has been farther south than usual on a consistent basis. At times, it's been the main jet stream, and when that has happened, it's been quite chilly. Sometimes, though, it's been a result of a more a split in the jet stream, where the main branch has been pretty far to the north, and a weaker southern split has developed that has just not allowed heat to build in a normal fashion from the Southern states on north. Of late, this has been the preferred mode of the atmosphere if you will.

    Look at the 0z June 5 GFS ensemble 500mb heights and anomalies for tomorrow morning:

    The main jet, it can be argued, is flowing in through British Columbia, skirting the northern Rockies, then arcing up into northeast Canada. Look at the actual 500mb forecast for tomorrow morning:

    The weaker southern branch will come in through California, then flow across the Plains and end up passing underneath the upper-level low over New England. It is this later flow zone that contains much of the active weather. It really represents the battleground between the heat over the Southwest and the heat and humidity in the South. It may not be excessive from the I-35 corridor eastward to Georgia and Florida, but it is considerably different than the air mass to the north of this boundary, an air mass that is relatively cool and certainly much drier, as measured by the dew point temperatures that are generally in the 50s or lower.

    The northern branch continues to bring a series of disturbances along that bring cold fronts through the flow. One is morphing into the upper-level low over the Northeast. Another bypassed Washington state overnight and will move across Montana into North Dakota tonight, then into the Great Lakes later Saturday and Saturday night as it runs into a slower flow aloft and one more parallel to its orientation. There will be a weaker front approaching the Upper Midwest late Sunday, while a strong one moves into the Northwest on Monday.

    The teleconnection indices don't change much going forward:

    This implies the overall lack of sustainable and excessive heat, especially from the northern and eastern Rockies through the central and northern Plains to the mid-Atlantic and New England, will continue into the middle of June. Yes, some locations will sneak in a day here or there with temperatures getting to or even a tick beyond 90, and there will be more where the humidity gets pretty uncomfortable. The latter looks to be true in the mid-Atlantic later next week. Still, it's not a pattern where energy usage would be considered excessive, or where there will be a run on air conditioners and fans. That will inevitably come, just not in the next couple of weeks in most of the nation.

    I'm out again for the weekend, this time for a more relaxing time in the Finger Lakes area of New York State to participate in the Great Finger Lakes Bicycle Tour. It'll be two-and-a-half days of cycling, camping and eating, and the weather there looks about as nice as it can get! It won't be hot, it won't be humid, and it won't rain!

    The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

    Joe Lundberg