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Typhoon - how about laymens terms, huh? What in the heck are you saying? Should the USA expect anymore tropical weather this season or are we in the clear till June??
Hi...
..You're never in the clear my friend... Some years, the odds are essentially 0 in January, other years....maybe not. But, you are never absolutely 0...
However, for the sake of discussion...I would say that we are going to have very slim chances of the U.S. being affected by a tropical cyclone again before the seasonal return, late next spring, while at the same time (and rather ironically) we will have above average chances of renogade systems developing after the planned December 1st termination date.
Positive departed SSTs working in conjunction with a very powerfully negative Quasi-Biennial Oscillation, means that conditions will be unusually fertile for the generation of tropical cyclones....(here's the catch!) at lower latitudes. The reason that is emphasized is because, not unlike every year, the planned winter time incursion of westerlies into the subtropical latitudes, to where they can subtend an influence to points farther S (in more intense scenarios..), is well underway; with one caveat: more intense than normal closer to the U.S. mainland. This latter intimation is because we have a Multi-Decadal Oscillation that favors severe winters underway - which is by no means an "always" scenario, but one that means we should be weary of any tendencies at all, to drive the +PNA along... This is because when the MDO is positive, there is a teleconnection to positive phases of the North Atlantic Oscillation..
Just so you are familiar with what these are:
North Atlantic Oscillation:
1) Positive Phase: +SD heights anywhere from N of Ireland to Davis Straights just W of Greenland, and associated +SD surface high pressure... This effectively blocks the cold from the polar Canadian districts from punching E into the N Atlantic, complimented by the fact that the jet stream in such conditions "buckles" in the "means" ("means" meaning, not all the time, but the average state of the main westerly core positioning) and sends these cold air masses, instead, S into the U.S. more frequently.
2) Negative Phase: -SD "........................." and associated -SD "............................", and the opposite means there after. This implies warm regimes for the contiguous U.S. if you are following me.
3) Neutral Phase: No discernable departure from tropospheric mass balance in the N. Atlantic Basin and adjacent areas; no discernable height bias either... Caries little influence either way on the weather in the contiguous and basically is a big boring waste of time.
Multi-Decadal Oscillation:
...About ever 20-30 years or so, the Atlantic Ocean decides to cool off and then heat back up again...by a 1 or 2C average across...pretty much the broad expanse of the ocean - of particularly current concern, N of the tropics/sub-tropics.. It's like our own ENSO in this hemisphere, but one that has a much longer periodicity, than that which takes place in the Equitorial Pacific. (Which by the way, since the ENSO signal is essentially neutral, we are not thinking a huge influence on the Pacific Jet, but that's a digression). Studies have shown that warm SSTs in various locations in the N Atlantic tends to correlate with the existence of blocking ridge tendencies, subsequently the positive phase of the NAO is accepted. If you look at the current SST charts along with the anomalies, you will see that the positive phase of the MDO is definitely present, having substantial warm departures from normal existing over the waters of the N Atlantic.
Since these indices/teleconnector existences, in the longer term, favor the positive phases of the NAO, it stands to reason there is a stronger than normal likelihood for higher amplitude flow. That is why the GFS going bonkers with a sub-polar vortex that looks like it belongs in that terrible movie "The Day After Tomorrow", over SE Canada. There is likely back-ground physics that are insisting a winter to remember over the continent of N America....
Point being, higher amplitude flows will tend to shunt activity away from the U.S. proper. That is what it means in your layman's terms, which really cut down the probability that a wayward tropical disturbance can find its way on board in a latter Novembers, particularly when you have amplified flow regimes Globally statistically supported to exist - therein we derive our confidences.
That doesn't mean it "can't" happen of course, and usually, that's when people die -s trangely enough... Also, it would seem reasonable to assume much of this has less ability to apply to S. Florida its self, which stick out there like a sore thumb at any time of the year.
Lastly, the reason this is interesting for the potential sub-tropical feature 1000 naut miles NNE of the Leewards is because if you understand the basics of atmospheric circulation, a high parked in the N Atlantic tends to drive a deep layer easterly flow, which tends to lower shear in that particular geography of the Atlantic... When you look at the IR imagery, you can see a near E Coast baroclinic zone lingering there...That is the shear axis...Anything E of there is "theoretically" in a favorable U/A arena...Anything along and W of that? Winter time and nadda...
Edited by typhoon_tip (Thu Nov 17 2005 02:37 PM)
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