LEADING THE WAY TO WETNESS...

June 25, 2020

The Madden Julien Oscillation (MJO) has done a very good job of depicting potential periods of wet weather around the central Midwest the past 30 days, well in advance of the rain itself. What I look for from the MJO ishow it handles the cycle or progression of convective clusters in the tropical Pacific. By observing which of the 8 pases we'll be in at any specific time, I can then correlate the precipitation analogs with that phase to see if it has a tendency to be wet or dry. It's a fascinating way of capturing trends, often before the models even see the.pattern unfolding

 

The last couple of days I've been pushing the idea that my area was going into a 2 week period where precipitation could be significantly above normal. Much of my initial belief in a potentially wet pattern was centered on the MJO cycle. Other factors were considered that added confidence but the big driver was the MJO which both the GFS and EURO indicate stuck in phase 1 through the last week of June and first week of July. The dotted green lines are showing the MJO progression June 24 through July 8th. The dots are all clustered with some degree of amplitude in phase 1.

 As you can clearly see, the precipitation analogs for both June adn July are quite wet centered on Iowa and Illinois during phase 1.

To get an idea of whether or not the operational models are in sync with the MJO I looked at the MJO precipitation anomalies for phase 1 over the tropical Pacific basin. The area in brown shows extensive dryness over the western Pacific near the equator and north of Australia that is typical of phase 1 at this point in the summer.

I then went to the GFS model which shows these precipitation departures for the 16 day period ending July 10th. It is spot on with what phase 1 should look like over the Pacific basin showing the dry signature north of Australia that's to be expected. That gives me confidence that the model is on the right track and that its output should accurately reflect trends.

Of course the next step is showing you what the GFS depicts for precipitation departures around the central Midwest. Feast you eyes on this. Much of my area is 1 to 3" above normal over the 10 day period.

The total 10 day precipitations numbers look like this. There's a heavy corridor from SE Minnesota and NE Iowa to SW Wisconsin and NW Illinois where 3-5" amounts are widespread.

That is not welcome news with the NW half of that corridor drenched by 30 day rainfall totals such as this.

 From eastern Iowa to central Wisconsin precipitation has been 150 to 300 percent greater than the mean during June.

Despite the skill the MJO has for seeing trends, it and the models are not advanced enough to precisely nail the location and amounts.For that reason, all I can do right now is broad brush the central Midwest to point out the concern that a significant precipitation threat exists the next 10 days.

 

While Thursday will be a dry day for all of my area, rain chances kick in again for Friday and Friday night before leaving all but my far southern counties (SE Iowa and WC Illinois) Saturday. The set-up Friday involves deep moisture with available water vapor of more than 2.00 inches...well above normal. With that in place and a slow moving front the potential is there for torrential downpours in the stronger updrafts. Hopefully, a convectively enhanced cold pool keeps the storms progressive enough that amounts stay in the 1-2 inch range as the storms sink southeast.

The GFS and EURO have this for total precipitation Friday through Saturday morning.

 

The GFS

 The EURO

Additionally, there should be enough CAPE (convective available potential energy up to 2,000 j/kg) for some strong storms capable of high winds and perhaps some hail in the more robust anvils. As of Tuesday evening the Storm Prediction Center has a slight risk of severe storms for all of my area Friday.

Again, most spots are likely to get a break Saturday and parts of Sunday depending in how far south the front travels. It is expected to return northward Sunday and linger into next week bringing renewed rounds of showers and thunderstorms that again will have the ability to dump more in the way of heavy rain. We'll have to take things day by day as the pattern evolves but it certainly does look active as of today. That's all I have for now. Roll weather...TS

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© 2019 Terry Swails