- Oct 23, 2019

THE TREND IS YOUR FRIEND, UNLESS YOU DON'T LIKE COLD AIR...

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WEDNESDAY'S FEATURED POST

All right, we're coming out of the gate hot in this post talking about the EPO (Eastern Pacific Oscillation) The EPO is a variation in the atmospheric flow pattern across the eastern Pacific, as well as Alaska. When the EPO is in a positive phase, mild Pacific air flows straight into the West Coast of North America. When the EPO is in a negative phase, a ridge in the upper levels of the atmosphere is present over the eastern Pacific/West Coast providing a pathway for cold air to enter the Midwest.

This time of year the EPO becomes a big teleconnection (or indicator) as to what type of temperatures to expect, especially when other signals are weak. Right now the enso thresholds for El Nino and La Nina are generally neutral and that makes the EPO a strong player. Here's what the EURO is showing for the EPO through November 5th. A very pronounced negative phase, especially around Halloween.

Here's what the negative phase looks like. That ridge over Alaska and the west coast brings the cold air southward.

Here's another perspective of what the negative EPO can do in the heart of winter.

Conversely, the positive EPO phase is a mild one as it allows modified Pacific air to flood the country. When you see the EPO go positive in winter not only is it mild, it tends to be dry with a zonal/westerly component to the jet.

Unfortunately for you warm weather fanatics we've got the negative phase to deal with. As I showed you at the very top of the post, there's a very pronounced dip in the EPO just before Halloween. You can see why that would happen in this forecast of the 500 mb jet on the EURO ensembles October 31st. Clearly you can see the ridge over the Gulf of Alaska and the strong northerly component to the heights over the central U.S.

These are the associated temperature departures. In Fahrenheit these are 20-30 below normal. No treat for Halloween if this verifies!

As I also showed you the negative phase of the EPO is still going strong into early November which would indicate a prolonged period of below normal temperatures. Even the GFS ensembles are in agreement. The departures for days 5-10.

Days 10-15

Not to beat a dead horse but the Climate Prediction Center is picking up on the signals. Here's the 6-10 day temperature outlook October 28-November 1st.

The 8-14 day temperature outlook Oct. 30-November 5

CPC has a moderate to high risk of hazardous temperatures October 30-November 5th

I've not focused much on precipitation here but as mentioned in a previous post Tuesday there is the chance a significant storm could accompany the cold around October 30th. That is accounted for with above normal precipitation chances October 28-November 1st.

I'll certainly have more to say on potential storminess in the transition to cold later Wednesday.

One last thing to address and that is a clipper that dives SE across Iowa during the day. The trends in the high-res models are for a little more forcing and a narrow band of precipitation up in my northern counties later in the day. Below you can see the precip shield around 4pm with the low just north of the Quad Cities.

Now the tricky thing about this situation is whether or not temperatures are cold enough to support snow. Not all models agree on this but the 3K NAM does show some accumulations in far northern Iowa. This is one of those marginal situations where now casting is the way to go. In other words, it's not a sure thing up north that snow will fall but it's not out of the question. I will monitor the trends early Wednesday and have more on the potential in a late morning update. The 3K NAM snow forecast.