Cooler and Somewhat Wetter End to the Week

Followers of already know about the fall-like change that has arrived. We’ve discussed in previous posts the cooler and wetter than average pattern that we are currently in, and this update to discuss if we break it, or keep on keepin on.

Monsoonal moisture, with some help from the tropical system Norbert, has been streaming over Arizona directly into Colorado. That’s one piece of this pattern.

Another piece is the cold front that pushed through Thursday morning. No mistaking that cold front, as the Mile High City went from near-record heat in the 90s to the mid 70s in just 24 hours. That cold front is brought to us by the increasingly persistent ridge over the Gulf of Alaska...I’ll come back to this in a moment.

For Denver, the rain chance is climbing. Some rain showers will move through overnight, and Friday. The chance for rain isn't all that high, about 40% south of I-70 and 20% north of I-70.

Rainfall amounts will be somewhat light -- less than 1/2" for the metro area. The highest rainfall totals will be over southern Colorado, and that could amount to several inches over the next couple of days. This is great news, as the drought is most extreme in that corner of the state. That area of heaviest rainfall is where the cold front will stall, and will be fed a steady supply of the monsoonal moisture.

Here is the total precipitation expected to fall through Sunday evening. Bullseyes of INCHES of rainfall over southern Colorado, New Mexico, and Mexico is the “heart” of the monsoonal flow.

In addition to cool rains, snow will fall for some of the mountains. This cooler air has hit them too. As overnight lows fall into the 20s and 30s in the high country, snow will fall; indeed, we’ll see a lot more use of the term ‘snow’ in the coming weeks. The current snow zone will be those higher peaks of the Continental Divide along I-70 above 12,000 feet.

Okay, enough about snow for now… back to that front which will stall over southern Colorado as Denver remains in the colder air. On Friday we’ll be well below normal, and only in the 60s for highs, with a mix of rain and cloud cover throughout the day. The last time Denver recorded a high temperature in the 60s was on July 30th with the heavy rain event that moved through; that event is actually somewhat similar to our current setup, with a cool front pushing in from the north and plenty of monsoonal moisture.

Like that July system, this one will move out pretty quickly too. The frontal boundary will “wash-out” over Colorado and a rebound in temperatures will ensue for the weekend. Saturday will warm back into the 70s, with scattered afternoon storms. Then Sunday, the highlight day of the weekend, will be back into the 80s with only a 10% storm chance.

Those 80s will last a couple of days into the early work week. There will be some storms here and there, but overall these three days will be drier. Now, let’s get back to that ridge near Alaska…

We recently pointed out that ocean atmosphere oscillations will combine in such a way that a persistent pattern of colder and wetter than average conditions will probably stay with us throughout almost the entire month. The ridge near Alaska is part of that teleconnection; this will cause a northwesterly flow aloft, bringing more frequent cold fronts down into the region, and into the state. The next cold front will arrive by the middle of next week, with a temperature drop similar to this one we are currently in.

As for moisture, little changes in that regard. The Alaskan ridging affects the flow of cold air into the state. We will see a persistent monsoonal flow from the Gulf of Mexico, over Mexico, toward the Pacific and then into Arizona and into the state. With that steady supply of humidity, any cold front that should arrive will have increased rain chances. The only thing that may disrupt the monsoonal flow through September would be a strong tropical system. Currently, Norbert is in the Pacific and is enhancing the flow. A strong tropical system would need to develop in the Gulf to choke off the monsoonal flow. Such a system would act like a sponge, sucking in the moisture surrounding it and temporarily drying the monsoonal flow from Mexico into the US.

For a variety of reasons worthy of a full post (I’ll get to work on it), the Atlantic/Gulf season has been very quiet, so the odds of disrupting the monsoon remain incredibly slim.

See how this all ties together? Complexity is vast in all the teleconnections that affect Colorado, but indications are that September will remain generally cooler and wetter than normal overall. Enjoy Sunday, Monday, and Tuesday’s warmer temperatures as they will be fewer and farther between as our fall pattern is in place and ready to bring us autumn.