Does a Warm May offer Summer Hints for the Northeast?

[donsutherland1]

That's an issue that arose at another weather message board. As it is a good question, I am posting my reply here, as well.

Using New York City's records (1869-2003), the answer is not necessarily.

The records show 51 years in which the temperature in May averaged above normal. In 24 of those years, the summer saw above normal temperatures. In 1 the summer averaged out at exactly normal. In 26 others, the summer was cooler than normal.

The following has occurred during those 51 years:

Summer averaged 1° or more above normal: 14 (27%)
Summer averaged within less than 1° of normal: 23 (45%)
Summer averaged 1° or more below normal: 14 (27%)

Moreover, the same pattern holds up with somewhat of a bias toward the warm side, if one takes the warmest ever months of May.

In the years in which May averaged 3° or more above normal (1880, 1896, 1944, 1959, 1965, 1975, 1980, 1986, 1991, 1993), 5 saw warmer than normal summers and 5 saw cooler than normal summers.

Breaking these years down by the 1° deviation, the following held true:

Summer averaged 1° or more above normal: 4 (40%)
Summer averaged within less than 1° of normal: 5 (50%)
Summer averaged 1° or more below normal: 1 (10%)

Finally in years that ENSO data is available (1950-present), there was little difference between whether the ENSO was cool, neutral, or warm. No warmer summer saw a moderate or stronger La Nina.

<b>For warmer than normal summers:</b>
Weak La Nina: 20.0%
Neutral ENSO: 40.0%
Weak El Nino: 13.3%
Moderate El Nino: 20.0%
Strong El Nino: 6.7%

<b>For cooler than normal summers:</b>
Moderate La Nina: 9.1%
Weak La Nina: 18.2%
Neutral ENSO: 36.4%
Weak El Nino: 9.1%
Moderate El Nino: 27.3%

<b>Note:</b> There is a rounding error of 0.1% in the last set.

When May temperature and April-May precipitation patterns are taken into consideration, there is one combination that does quite well in hinting at the coming temperatures for the summer (June-August): a cool, dry May typically leads to a cool summer. In this case, I define a "dry" April-May as seeing precipitation of 10% or more below normal.

In the 46 years in which such a pattern occurred in NYC, 38 (83%) of summers saw below normal readings and 25 (54%) saw readings average 1° or more below normal.

If one changes the definition for a "dry" April-May to include just below normal precipitation, this pattern still holds up well: 80% of summers saw below normal readings and 54% saw readings average 1° below normal.

However, if one changes the definition of a "wet" April-May to include all periods where precipitation averaged above normal, one also finds that such periods typically result in cooler than normal summers: 12 (67%) saw summers with below normal readings and 9 (50%) saw summer readings average 1° or more below normal.

Here are the breakdowns:

<b>May Temperature 1° or more above normal; April-May precipitation 10% or more below normal:</b>
Summers with above normal readings: 13
Summers with below normal readings: 15

<b>May Temperature 1° or more above normal; April-May precipitation 10% or more above normal:</b>
Summers with above normal readings: 2
Summers with below normal readings: 1

<b>May Temperature 1° or more below normal; April-May precipitation 10% or more below normal:</b>
Summers with above normal readings: 8
Summers with below normal readings: 38

<b>May Temperature 1° or more below normal; April-May precipitation 10% or more above normal:</b>
Summers with above normal readings: 5
Summers with below normal readings: 7

If one relaxes the constraint for precipitation anomalies of 10% or more and just categorizes data by drier or wetter than normal April-May periods, the results change little.

<b>May Temperature 1° or more above normal; drier than normal April-May:</b>
Summers with above normal readings: 13
Summers with below normal readings: 16

<b>May Temperature 1° or more above normal; wetter than normal April-May:</b>
Summers with above normal readings: 3
Summers with below normal readings: 2

<b>May Temperature 1° or more below normal; drier than normal April-May:</b>
Summers with above normal readings: 10
Summers with below normal readings: 42

<b>May Temperature 1° or more below normal; wetter than normal April-May:</b>
Summers with above normal readings: 6
Summers with below normal readings: 12

Having said this, summers that averaged warmer than normal generally saw less than normal precipitation during April and May.

• 72% of summers that averaged 1° or more above normal saw below normal precipitation during April-May.

• 69% of summers that saw warmer than normal temperatures received below normal precipitation during April-May.

<b>Summary:</b>
• A cool May, particularly in years in which April-May see below normal precipitation, hints at a cooler than normal summer.
• A wetter than normal April-May hints at diminished prospects for a warmer than normal summer.
• A warm May offers little meaningful information on the likely summer temperatures.

[stormraiser]

Has this been a warm May?

Thanks for the post Don. I always enjoy reading your posts. I was just wondering if May has been warm elsewhere, as it has been quite chilly here.

[donsutherland1]

Stormraiser,

Even with the sharp cool spell last week, May 1-7 saw readings average above normal in much of the eastern United State:

Boston: +5.6°
New York City: +1.6°
Washington, DC: +1.4°

Analogs combined with the latest computer guidance/ensembles continue to indicate that at least in the DCA-BOS area, May should see above normal temperatures. The current week could see some much above normal readings if everything comes together.

[donsutherland1]

I've taken some of the data for Boston this evening just to see how things look using that city's data, which goes back to 1872.

Here are the breakdowns:

<b>May Temperature 1° or more above normal; April-May precipitation 10% or more below normal:</B>
Summers with above normal readings: 10
Summers with below normal readings: 13

<b>May Temperature 1° or more above normal; April-May precipitation 10% or more above normal:</b>
Summers with above normal readings: 4
Summers with below normal readings: 9

<b>May Temperature 1° or more below normal; April-May precipitation 10% or more below normal:</b>
Summers with above normal readings: 3
Summers with below normal readings: 14

<b>May Temperature 1° or more below normal; April-May precipitation 10% or more above normal:</b>
Summers with above normal readings: 2
Summers with below normal readings: 25

<b>Summary:</b>
• In years in which the combined precipitation for April and May came to 10% or more above normal, 34/40 or 85% of summers saw temperatures average below normal. 23/40 or 58% of such summers averaged 1° or more below normal.
• In years in which the mean temperature for May averaged 1° or more below normal, 89% of summers (39/44) saw below normal readings with 59% (26/44) seeing readings of 1° or more below normal.
• The combination of a cool May (1° or more below normal) and dry April-May (precipitation 10% or more below normal) resulted in cooler than normal summers 82% of the time. 59% of such summers saw readings average 1° or more below normal.
• The combination of a cool May (1° or more below normal) and wet April-May (precipitation 10% or more above normal) resulted in cooler than normal summers 93% of the time. 59% of such summers saw readings average 1° or more below normal.
• The combination of a warm May (1° or more above normal) and wet April-May (precipitation 10% or more above normal) had a tendency to result in cooler than normal summers 69% of the time, while a warm May-dry April-May was indecisive.

Overall, there was consistency with what the data revealed, particularly that wet April-May periods indicated diminished prospects for a warmer than normal summer and cool Mays typically indicated cooler than normal summers.

With respect to Summer 2004, given the record monthly precipitation in April, Boston may be headed for a summer with below normal readings except if May's temperature averages 1° or more above average. Then, there will still remain a fairly good chance of a warmer than normal summer.

I believe May will see readings average 1° or more above normal in Boston, so my overall ideas of a warmer than normal but not extreme summer for the DCA-BOS area still does not appear unreasonable, especially the farther south one goes.

[Guest]

Great reading as always Don. This very much pertains to me. I for one am not looking forward to all the 90+ days again.

[Stephanie]

I'd say that last year was a great example of a cooler than normal summer following BOTH a wetter and cooler than normal April - May.

So far Don, your prognosis for May 2004 seems to be right on at least for my area. Last week, while we did have some days with the temps in the 70's, we did have one or two in the 60's. This coming week we're supposed to see temps in the 80's everyday. Your forecast had an above normal temp trend for the second week of May.

[donsutherland1]

KOW,

I believe that the coming summer will likely see above normal readings, though the averages probably won't be extreme. So far, things continue to evolve in that fashion.

[donsutherland1]

The first week of May saw readings somewhat above average in spite of the cool spell. This week seems on course. Overall, I believe the month should average somewhat warmer than normal and I see little reason based on the latest computer guidance/ensembles to believe otherwise.

Enjoy the warmth!