Jun 112014

The Arizona Daily Sun published this article over the weekend: Forecasters: Wet Summer and Winter likely in Flagstaff. This article hints at El Nino as the cause for both extra rainfall this summer and more snow next winter. About a month ago I started to look into El Nino’s effects on Flagstaff’s summer precipitation. My hypothesis was that El Nino actually lowered rainfall amounts in the summer.

Digging into the data since 1950, there really isn’t much difference between the over all normal for 1950-2010 and any of the El Nino averages based on the strength of the El Nino. There are huge swings in the summer (June-September) rainfall amounts. Just to make the point, The El Ninos of 1957 and 1982 are both strong. They produce 4.81 and 13.42 inches of rainfall. The El Ninos of 1986 and 1991 are both moderate strength. They produced 20.64 and 3.53 inches of rainfall. The weak ones are all around 7-8 inches of rain. It looks like anything goes with El Nino Summers.

There is more than just El Nino driving the events for the year. The Climate Prediction Center still has Flagstaff in the above average rainfall zone for the summer. They should have an updated model later this week.

On a side note, I noticed last night that AccuWeather.com had a thunderstorm in the long-range outlook for Flagstaff on June 24. It’s gone this morning.

June July August September Total
El Nino average 0.41 2.33 3.02 2.11 7.87
1950-2010 average 0.43 2.4 2.89 2.12 7.84
Strong El Nino Average 0.86 1.78 2.37 3.22 8.22
Moderate El Nino Average 0.28 2.59 3.38 1.72 7.97
Weak El Nino Average 0.15 2.42 3.05 1.61 7.23
1950-2010 average 7.7
 Posted by at 7:01 am
Sep 262013

A recent question I have heard frequently is “Since the monsoon season was so wet, does that mean we will have a snowy winter.” A few years ago, the National Weather Service in Flagstaff included a slide in their winter outlook presentation that showed no clear link between summer and winter precipitation. In general, given the mechanisms for out summer and winter weather patterns, they shouldn’t be linked.

I cover the summer monsoon pattern in detail on this link: Summer Monsoon Mechanics. In this season, a thermal low develops over the deep southwestern United States. Also, a high pressure system forms over the middle of the country. The anticyclonic and cyclonic flows around these systems drive flow from the south and southeast. This flow draws up moisture from the Gulf of Mexico and the west coast of Mexico. Since the jet stream remains far to the north, these systems can be fairly stable. The big disruption of the monsoon pattern usually comes from hurricanes in the Gulf of Mexico. Interestingly, this year’s hurricane season has been anemic with only 2 hurricanes forming in the Atlantic. The other very wet year, 1986, saw a season total of 4 hurricanes.

Summer Monsoon Season Pattern

Summer Monsoon Season Pattern

In our winter pattern, the jet stream drops farther to the south and drives strong storms from west to east. In winters with a El Nino in the equatorial Pacific, the jet stream can split in two. One jet remains to the south and carries storms across the southern tier of states. The moisture reservoirs for the winter pattern end up being different from the summer. One supply area is from a large area near the Hawaiian Islands. When this water is warmer than normal it can be a powerful moisture supply. This happened in December of 2010. The other moisture supply can be from the equatorial Pacific in El Nino years. Occasionally, in very cold years, the Pacific Ocean off the west coast of Mexico can be warmer than normal and provide limited moisture.