For a couple of days now I've been transferring some of my focus to the western Pacific. The SOI (Southern Oscillation Index) has begun to fall after reaching a maximum reading of 19.25 Monday.
Today the reading is back down to 10.96. The rapid fall is tied to a rise in surface pressures near Darwin, Australia. The rise is expected to continue and by April 22 a massive 1034mb high is situated just south of Australia as you can see below.
With pressures high near Darwin and much lower around Tahiti to the east, the SOI should continue to fall reaching negative territory in the next couple of days. This type of crash usually forces a downstream reaction or pattern change. It generally impacts the U.S. in the 6-10 day time frame.
By Monday April 17th, I'm already noticing a huge 1040mb high over NW Canada. The expansive high pressure in that part of North America signals a build up of cold air.
Below you can see in the April 17 temperature departure how cold air is flooding much of Canada.
It's looking more and more likely that some of that cold will work its way into the Midwest in the long range period. The GEFS ensembles are certainly catching the trend. Today though the 500mb flow is a mild one.
With that type of flow aloft temperatures the next 5 days will remain mild over the central Midwest.
However, come April 26th the central U.S. ridge of today is flattened and replaced by an east coast trough.
That northwest flow will set spring back a bit as above normal temperatures are replaced by below normal readings. It's starting to happen in the 5-10 day departures.
It's really evident in the 10-15 day departures.
In fact, the operational GFS actually shows highs in the 40s April 23rd. Normals should be pushing 65. Hopefully later models will back off on the depth of the cold air intrusion.