We're about halfway through May and so far...not so good for temperatures. Here in Cedar Rapids we are running about 6 degrees below normal per day making this the coolest start to the the month since 1989 (30 years ago).
Up to this point we've had no 80 degree days which is a bit unusual. However, in 1990, 1973, and 1935 we were skunked the entire month. On the other hand, in 1934 20 days were 80 or higher.
Examining the 1934 data for Cedar Rapids is eye opening. In the monthly summary below aside from the 20 or more 80 degree days, were 10 highs of 90 or more. The 104 on the 31st is the highest May temperature ever recorded. One contributing factor to the heat was the overall dryness and low humidity levels. Rainfall for the month was 4.15" below normal (.64"). The last 18 days saw less than 1/10th of an inch (.09").
In the weather world things eventually change and it does look like we are going to break out of the cool pattern for at least a few days later this week. Just look what happens to highs Wednesday-Friday. The projected 84 Thursday would tie for the warmest reading of the year so far. Humidity levels are also going up with dew points pushing 70. It should feel summery!
You can see why the change is coming. Here's the 500mb jet stream flow Monday. (Chilly)
Now look at the flip that comes Saturday.
If the GEFS ensembles are correct the general warmth would last through the end of the month. The is the mean 500mb flow May 28th. Ridging is shown over the SC U.S. which would allow for the warmth.
This is the 5 day temperature departure at 5,000 ft ending Saturday.
Now the 5-10 day departures.
And finally days 10-15.
With the return of warm humid air comes the potential for thunderstorms when fronts and forcing are in place to kick them off (this is the heart of the severe weather season). The first opportunity comes Thursday night along a slowly sagging front. How far south the boundary gets is still a bit nebulous but most models are generating showers and storms in Minnesota and Wisconsin Thursday evening. They would have the ability of drifting into my area from the north before fizzling early Friday. After that the position of the wavering front though the weekend will determine additional storm chances and temperatures. It currently looks like Saturday night or Sunday has the greatest potential for coverage depending on timing. We'll know much more on the rain threat in the next couple of days. Most likely there will be some spots that see some locally heavy downpours. A strong storm is possible too. Roll weather...TS