SHARKS IN THE DISTANT WATER...
For the 4th consecutive day, the weather was of the highest level around the central Midwest. With highs around 80, coats were nowhere to be found. Take a look at 3:00 PM temperatures. How sweet it was.
Many parts of the Midwest and Great Lakes were 25-30 degrees above normal
Notice the blue and green departures out west, not to be negative but that is is a sign of sharks in the water. They won't be swimming here right way but all the signals suggest a much cooler pattern is on the way as we push towards the middle of April.
The change all starts with upper level blocking at higher latitudes in Canada and Greenland. One of the first signs I look for when it comes to colder weather is the AO (Arctic Oscillation) entering a negative phase. That trend is now most pronounced now on the GFS.
The GFS extended ensembles are even more bullish indicating a significant negative plunge in the AO that peaks around April 19th-20th
The -AO correlates strongly to the high latitude blocking on the modeling at 500mb that looks like this. The higher heights at 500mb forces the colder air to vacate Canada and relocate in the mid-latitudes where we reside. The reddish colors in Greenland back to Hudson Bay and SE Canada represents the block. It's forcing energy and cooler air into the Midwest (represented by the blue colors).
Another important driver in this whole set-up is the EPO (Eastern Pacific Oscillation). When it's in a negative phase it points to ridging over the west coast and Gulf of Alaska (that can also be seen above). The ridging opens the door for cool air to seed the pattern here in the mid-latitudes. The GFS ensemble EPO really goes into the tank the same time the AO reached its greatest negative value.
Then there is the NAO (North Atlantic Oscillation). This is a significant teleconnection at this time of year and when its negative you expect high 500mb heights over Greenland. That in turn creates a block with the downstream effect of troughing over the eastern 2/3rds of the nation. As was the case with the AO and EPO, the extended ensemble of the GFS NAO goes strongly negative during the period we've been discussing limiting warm air.
With three major teleconnections all in sync, that leads to high confidence in a cooler than normal pattern developing next week.
The icing on the cake though comes with the MJO (Madden Julien Oscillation). A prolonged period of convection in a specific region of the tropical Pacific will push the index into phase 7. The EURO extended MJO forecast clearly depicts the movement from 6 (the warm phase we are in now) to phase 7. Once it enters 7 it shows the progression stalling in that phase into early May. You can see in the phase analogs what a chilly pattern that is for the Midwest and central U.S. in April.
This also correlates well with the temperature departures the EURO ensemble shows for the period April 14-21st.
There's always the chance models back off in coming days but I have high confidence that's not likely to happen. I think this chill is coming and spring is going to experience a set back for at least a couple weeks. The good news is the fact we are entering mid-April as opposed to January or February where the cold would have a much higher impact. As it is, there are going to be some fresh days ahead and I would not be shocked if at some point we see some snow flakes. I don't see anything that would indicate that now but temperatures aloft will be cold enough to support it at times if we get the right forcing and instability. That's a wild card and far less certain. Sorry!
Circling back to Wednesday's sensible weather it finally looks as though most areas will get into rain chances. Storms that formed Tuesday night out west will try and catch the SW half of my area (especially in Iowa) Wednesday morning. However, these will be moving away from the best forcing and should weaken and dissipate about the time they get into my area.
Later in the day, instability will be higher with dew points 58-60. An occluded front will swing into the region late in the afternoon and evening kicking up scattered showers and storms. In the SW there is a chance of a strong storm into evening thanks to a high shear low CAPE environment. These are going to be fast movers that are followed quickly by the dry slot so rain totals will be variable and banded where thunderstorms track. Rain totals could range quite a bit over small distances from an inch to as little as 0.25 of an inch. It's the luck of the draw. Here&