(Tropical Tidbits)

Cold flip increasing likely, but longevity in question

A good Wednesday afternoon to you! While the temperature remains largely above average in the Central and Eastern United States today and over the next few days, we have been discussing for quite some time the idea that a noticeable change is forthcoming. This change is still very much on schedule, and in fact confidence in its occurrence has increased over the past 24 to 48 hours. Forecast model guidance has come into better agreement on the movement of large-scale features throughout the hemisphere.

One thing we continue to discuss internally, however, is the longevity of this flip towards cooler air. Often times, these pattern changes are discussed at length — and with good reason, as they are significant — but there is not enough detail paid to the exact intensity and duration of the change. In this instance, ensemble guidance is already offering some hints as to how the atmosphere will behave after the flip to a cooler pattern in the Central and Eastern USA Week 2.

As discussed for several days now, the flip to a cooler pattern over the Central and Eastern United States is happening in large part due to wholesale changes in the Pacific Ocean. A retrograde of key hemispheric features in the atmospheres mid and upper levels is enhancing the likelihood that ridging will develop along the USA’s West Coast by the middle of Week 2, which will inevitably support the development of a trough from the Central United States into the Eastern United States.

European model ensembles show colder air invading the Central and Eastern USA in late October.

European model ensembles show colder air invading the Central and Eastern USA in late October.

With this trough will come a noticeable shot of cooler than normal air, which will be made to feel even more significant due to the antecedent airmass that has featured above normal temperatures for what seems like an eternity. The flip to cooler than normal air east of the Mississippi will be particularly notable as markets will almost certainly react (and some already are reacting) to the abrupt change.

GEFS guidance valid for around October 26th shows a large trough in the East and a storm signal, but downstream high latitude ridging near Greenland does not turn into a block (Tropical Tidbits).

GEFS guidance valid for around October 26th shows a large trough in the East and a storm signal, but downstream high latitude ridging near Greenland does not turn into a block (Tropical Tidbits).

The main question that remains is: How long will this cold last? While this is still pretty far out there in the medium to long range of ensemble data, models are already offering some hints as to how the atmosphere will behave. One of the key pieces in duration and intensity of colder than normal air in the United States actually is on the Atlantic side, and in the higher latitudes, where we often see blocking ridges of high pressure development.

While changes can often be initiated in the Pacific, these blocking ridges on the Atlantic side from Greenland into Canada are the important pieces that keep the cold air sustained – or in this case, don’t. Forecast model and ensemble guidance is currently in good agreement that ridging in the aforementioned areas will be quite progressive. This will, in turn, allow troughing over the Central and Eastern United States to remain progressive as well.

GFS ensemble guidance valid for the end of October and beginning of November show the +PNA collapsing with moderating temperatures returning (Tropical Tidbits).

GFS ensemble guidance valid for the end of October and beginning of November show the +PNA collapsing with moderating temperatures returning (Tropical Tidbits).

The result? The height pattern over the United States should remain relatively progressive as well. Ridging along the USA’s west coast should collapse eastward into the Central and Eastern states, allowing warmer, Pacific air to again overtake the United States as a whole. This will result in a volatile “roller coaster” of temperatures in a 5 to 7 day period, and while anomalies will remain cool, there will be a moderating trend over time.

The volatility appears likely to continue as we move into November, with forecast model and ensemble guidance as well as tropical forcing and MJO indicators suggesting that additional shots of colder than normal air are likely. The roller coaster ride is just beginning, so to speak, so stay tuned for information over the coming days as we try to pin down where the rises and falls of temperatures will lie exactly.

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