After an up and down, La Nina driven winter (cold mainly from late January through February) with slightly below normal snowfall with nearly 50 inches (our snowfall prediction was 55-65"), the question is: Will the same La Nina drive this summer's pattern?
First the April pattern recap:
High pressure was dominant over the northern Pacific and the north Atlantic. This drove temperatures well above normal during the first half of April. We had snow on April 1 in northern Ohio. 6-7 days late we reached 80 degrees!
Since the middle of the month, the pattern flipped. The northern Pacific ridge shifted over Alaska. The ridge over the north Atlantic shifted north over Greenland. Both ridges linked together over the North Pole. The trough developed underneath and across the eastern 2/3rd of the US with widespread below normal temperatures. We had snowfall on April 20th in northern Ohio, the 3rd most this late in the season ever (2005 was the most).
Much of this was influenced by the high amplitude MJO in April rotating through phases 5 through 8.
April 2021 high temperatures vs normal in Cleveland.
Late this winter and into early April the cooler water along the equator (La Nina) has been steady but slowly warming.
The depth of cool water is also becoming more shallow.
The
MEI is another great measure of the ENSO state. It takes into account multiple oceanic and atmospheric variables (not just the ocean water temperatures) to produce an
overall number either positive or negative. The current MEI is at 0.8. The MEI site's description here:
"The new version of the MEI (MEI.v2) has been created that uses 5 variables (sea level pressure (SLP), sea surface temperature (SST), surface zonal winds (U), surface meridional winds (V), and Outgoing Longwave Radiation (OLR)) to produce a time series of ENSO conditions from 1979 to present"
The evolution of La Nina in years which are similar to the current event's development.
The model projections show weakening with some intensification of the La Nina late this summer/early fall. Since it take time (months) for the ocean/atmospheric state to completely transition out of its current state, I think this will have only small effects on the overall summer pattern.
Here is a great tutorial on La Nina from NOAA:
What do some of the similar past La Nina summer look like?
These are overlapping months (May through September) showing the upper level pattern.
 |
| 2018 |
 |
| 2012 |
 |
| 1999 |
 |
| 1996 |
Now summer temperatures (June, July and August) vs normal
What are we expecting this summer?
Based on top analogs looking at past La Nina summers along with other variables, we expect an above number of 90 degree days this summer especially in late July and August. A few 90s are highly possible into early September.
The ground moisture is running slightly below normal as of this writing (May 4). For comparison, last year we were running a rain surplus.
Over the last 3 weeks, the rainfall deficit has dropped across the state.
Crop moisture is only slightly below normal but trending to near average
...same with the longer term Palmer Drought Severity Index (PDSI)
* Wet ground/above normal rainfall in May/early June will limit the expansion of heat and ease of reaching 90 degrees for multiple days. Drier ground over multiple states will enhance ridges of heat in summer making 90+ easier to attain deep into summer.
What are we watching?
* The position and strength of the Sonoran Ridge (high pressure) and the Bermuda high will dictate the position of the summer storm systems.
* Our analysis shows that these ridges especially the Bermuda High will develop by mid June and pulsate north frequently. The relaxation of the ridge periodically (flattening along the northern periphery) will allow storms to drift over the ridge and into the Ohio Valley especially in late June and July.
* We expect a more active July with storms.
