Little Humidity. Little Rainfall. The Complete May 2023 Recap

Lots of colorful  👇   charts here so be prepared!  

We were coming off of an extremely wet April. Temperatures were slightly above normal.  7 of the last 10 days had measurable rainfall.  Back in late April the changes in the tropical Pacific (SOI drop), northern Atlantic (negative NAO) and Bering Sea Low pressure were strongly pointing to a cooler than normal start to May.

Early May Outlook posted back on April 21

The upper level pattern in 7 day increments:  500mB (18,000 feet on left)  Jet stream on the right featured an active west-to-east storm track left over from late April.

May 1 to May 7

By the second week of April, the ridge over central Canada shifts west and south.

May 8 to May 14

Notice the split flow which emerged in the middle of the month. Storm track along the southern border and west to east through central Canada and into New England.

May 15 to May 21

By the final week of May, two pronounced jet stream tracks kept storm systems away from the central keeping the mid-west and corn belt very dry.

Overall temperatures west of the Mississippi and in the upper mid-west were above normal.

The center of the heat was in western Canada.

The split flow kept a large portion of the central US, deep south and Great Lakes extremely dry.

In northern Ohio, we had rain to start the month which skewed the overall totals. 25 of the 31 days were dry.

Temperatures in northern Ohio were slightly below average.  We had more days below than above.  Days in grey were a degree to either side of average.

By far the biggest element was the lack of humidity.  

In fact, April had more humid periods than May. 

We average more than 105 total hours of high humidity in May.  We ONLY had ONE HOUR!  Only 1966, 67, 68 and 1971 compare.

April and May high humidity periods (total hours) compared to past years.

The average northern Ohio humidity in May was the 3rd lowest since the last 1940s.

Severe storm warnings were non-existent in Ohio and surrounding states compared to April.

What about June?  On May 22nd, the tropical Pacific indicated a HUGE drop in the SOI.  Using the historical analogs, the "less humid & dry" pattern looks to continue through the middle of the month.  The projections on June 2 were latching onto this idea bigtime.

What we will watch during the second half of June/early July is how much the severe dry conditions will persist.  If they do, the dry ground could feed the central US ridge further enhancing the heat. More on this in the weeks ahead.

Why The Abrupt Cooler Pattern in the Northeast?

After the warmest April on record in places like Cleveland, Ohio and across much of the Ohio Valley and much of the eastern US, the pattern has not only reverted back to spring, it took several steps back resembling elements of winter. Throughout April, this high pressure ridge stayed east which allowed the ridge across the eastern US to build allowing above normal temperatures. Many were fooled into thinking that summer was here and we were in the clear to plant gardens. Not so fast!

Look at April temperatures versus the 30 year average:

Now look at the first week or so of May per the American model (GFS).

Why the sudden flip?  In this case, we have to look over Greenland for the answer.

Remember that across the mid-latitudes, troughs and ridges (valleys and peaks in the upper levels of the atmosphere) travel like a fluid through the atmosphere. When one location is lucky enough to be blanketed by a ridge, fair weather typically is the result. Once the succession of trough and ridges is changed by an interruption in the wave pattern, the weather will change at the surface. These changes (too much to go into here. See Southern Oscillation Index) often originate over the Pacific Ocean and propagate east. Currently, these changes are reflected over the northern Atlantic and Greenland.

Notice the red (high pressure) at the top inching west blocking/stalling the blue (low pressure systems) over the NE

We can quantify the strength of this ridge using the NAO of the North Atlantic Oscillation.  The more negative the index goes, the stronger the Greenland ridge. The NAO has been forecasted to drop close to -300. Historically this has only happened once in the month of May since record keeping began.  That was back in 1993.

NAO drops to -300 soon

But be warned, historically, this pattern over Greenland in May doesn't always produce a cold and rainy May. Look at the high temperatures in Cleveland during these years of record low May NAO:

Look at the resultant upper level pattern across the US in each of these years. All but 1948 and 2008 show dominant warmth.

May 1948 - DOMINANT COLD OVER GREAT LAKES/NEW ENGLAND

MAY 1951 - WIDESPREAD WARMTH ACROSS US. COLDER SOUTHWEST

MAY 1962 -  WIDESPREAD WARMTH ACROSS US

MAY 1980 - WIDESPREAD WARMTH ACROSS US

MAY 1993 - MORE US WARMTH. COLDER DEEP SOUTH

MAY 2008 - MUCH OF US BELOW NORMAL

The NAO this low (or any other index) isn't always a great predictor of temperature and the overall pattern. 

Unfortunately, unlike any of the previous instances of strongly negative NAO in May, this cold is locked in for at least another week.  So get ready for rain, scattered frost and perhaps some light snow!