WHERE IT RAINS, IT WILL POUR...

September 20, 2019

Saw this cartoon earlier today and got a chuckle out of it. As messed up as my life's been lately at least I'm not using chocolate to change the channels yet. Wonder if that old duffer is watching T. Swails...not!

Well, what do you say we talk some weather? I'm all about that after the distractions of the past few days. The first thing I'm going to lay on you is the SOI (southern oscillation index). Thursday's reading was a robust -35, derived by measuring changes in surface pressure between Darwin, Australia and Tahiti in the tropical Pacific. 

This downward turn is part of a long term trend that has taken the value from 0 August 24 to -35 today. There is no doubt in my mind that this plunge is responsible for the late surge of summery weather we've been seeing and are likely to continue experiencing into early October.

One of the things you should find interesting is that extended periods of negative SOI correspond with El Nino events, characterized by warm SSTs in the eastern and central tropical Pacific. The last 90 days the SOI average is -8.78. The past 30 days it's -13.56. Sustained negative SOI values below −7 generally indicate El Niño episodes.  Based on the criteria we should be in El Nino but there's a problem. Sea surface temperatures would look similar to those in the boxed in area of 2015 below. However, despite the prolonged period of negative SOI readings, the current sea surface temperatures today (right box) are dramatically reveresed. Instead of warm water, cool water dominates the central and eastern Pacific resembling La Nina. See the contrast, we are way out of sync here! 

No one knows for sure where this is going but it could have important implications for winter. If El Nino strengthens more than anticipated, meaning the eastern tropical Pacific warms, all bets are off for a tough winter. Important to note though, most models do not show El Nino developing. The latest forecast from the Climate Prediction Center has neutral conditions through spring of 2020...see the gray bars below compared to the red and blue ones.

The U.S. based climate model the CFSv2 also has a neutral outlook.

Now I'm going to get funky on you and throw in the QBO here (Quasi-Biennial Oscillation). What this refers to are alternating wind fields in the tropical Pacific near the stratosphere. These winds regularly flip in intervals of 28 to 29 months between west to east.  This winter is expected to be a westerly phase. You combine that with neutral enso conditions and a solar minimum (low sunspot numbers) and it teleconnects to a cold winter. Winter temperatures should look something like the panel I circled below.

I'm mystified why the tropical Pacific is so cool in the central and east. It has to do with up-welling within the sea water but that's as far as I'll go with that. This is an issue to watch going forward, especially since we are still in the tropical season of hurricanes and typhoons. These large powerful storms have the potential to alter sea surface temperatures and that can be a game changer for winter. Thanks for hanging with me.

 

The last thing I will touch on (briefly) is the potential for locally heavy rain in some part of the central Midwest this weekend. All models show an unusually moist air mass with persistent forcing expected over the central Midwest through next week. We can expect regular rainfall chances with the potential for significant rain in spots. There is a tropical connection in place with Imedla's remnants generating PWAT values pushing 2 inches. That's 99 percent of normal and any storms that form in that type of environment are going to be big rainers. The question is where?

 

Flash flood or just plain old flood watches may eventually be issued. for parts of my area. As it stands now the heaviest rains are expected over the SE half of the region over the weekend, especially south and east of I-80. This graphic from the NWS in the Quad Cities shows some of the potential. A few models actually have amounts that are even higher in localized spots.

The Weather Prediction Center has this for rain through Sunday. 

Over 7 days, WPC amounts look like this.

So while it won't rain all the time, it's a pretty good bet that at some point (or points) your weekend will be interrupted by showers and storms.  The severe threat is low but the rain amounts may be high, especially over the southeast. It will be muggy and temperatures will be mild despite any rain and associated clouds. I'll keep an eye on it and have an update late Friday. Roll weather...TS

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© 2019 Terry Swails