A Look at Record Precipitation in July and the Upcoming Seasonal Outlook

With cold temperatures on the higher summits at the end of July, many people were surprised to see the change in conditions so early in the year.

Statistically speaking, however, cooler temperatures to welcome the month of August are not atypical. According to our climate record, in August, average daily temperatures begin their downward tendency for the rest of the calendar year.
August also marks the last month of meteorological summer, which may be disappointing to some considering this season has been characterized by excessive rainfall, high humidity, and unhealthy air quality for New England. Whether it was the threat of thunderstorms, flooding, or wildfire smoke, poor weather conditions have made it challenging to pursue outdoor activities throughout the season. All of this has people wondering, “when will this pattern end,” and, “what does all of this precipitation mean for ski season”? Before I address these questions, let’s take a closer look at some statistics from the summer thus far.

To begin meteorological summer, June on Mount Washington had a chilly start. Below-normal temperatures were recorded for eight days in a row from June 3 through June 10. This, along with another cold spell in the middle of the month, led to an average monthly temperature of 45°F, which is 1°F cooler than normal.

Cool temperatures also came with snow. A quasi-stationary low sat over New England for much of the first half of the month, delivering 8.4 inches of snow. This made June 2023 the snowiest June on record at Mount Washington Observatory.

This pattern ultimately set the tone for the rest of the summer as blocking high pressure developed over the North Atlantic and a broad area of high pressure developed over the western portion of the United States. This left New England “stuck in the middle” with upper-level troughing overhead.
Due to cyclonic flow aloft, a deep supply of moisture from the Atlantic Ocean, and a persistent generation of surface low-pressure systems in the Northeast, New England endured 24 days of rain out of the available 30 days in the month of June. Mount Washington received 17.30 inches of liquid equivalent precipitation, which is 8.71 inches higher than normal and makes June 2023 the second wettest June on record (our precipitation records began in 1932).

With increased available moisture in the atmosphere, relative humidity was also proportionally high throughout the month, coming in at an average of 80% (average humidity calculation excludes periods of fog in which relative humidity is 100%). The lack of sunshine for the month is also evident with the summit achieving just 23% of the available sunshine minutes.

July followed suit with high precipitation totals, high humidity, and dense cloud coverage. The jet stream held its persistent troughing pattern over New England while sea surface temperatures in the North Atlantic rose and the El Niño Southern Oscillation continued its transition from La Niña to El Niño. Warm sea surface temperatures in the North Atlantic, the equatorial Pacific, and the Gulf of Mexico all made contributions to increasing the available moisture in the atmosphere.

In July, low-pressure systems forming beneath cyclonic flow aloft tended to move west of New Hampshire, frequently giving the summit southerly winds that brought increased temperatures, moisture, and higher dew points from the southern region of the United States. This was further emphasized with the onset of El Niño, which commonly brings warmer temperatures and increased precipitation to the Southeast and along the Eastern Seaboard.

All of this resulted in rain falling on the summit in 22 out of the 31 days in July. In total, 17.08 inches of rain was measured, which is 8.15 inches higher than normal and makes July 2023 the wettest July in Observatory history. The average relative humidity of 86% was fairly high, contributing to periodically high heat index values at the lower elevations. On the summit, the average temperature for July was 53°F, which is 3°F warmer than normal. Another month with a lack of sunshine also occurred in July with only 25% of the available sunshine minutes achieved.

While it is hard to say when the wet, humid pattern will change or what it means for ski season, the Climate Prediction Center (CPC) has posted seasonal outlooks through the end of 2023, shown above. Some long-range models are also suggesting that high pressure at the surface will be more apt to move in every few days through the middle August, although troughing in the upper levels will still be prevalent. What that means is that we might be able to expect a few more sunny, dry days than we have had over the last couple of months, but the seemingly daily chance for showers and thunderstorms will likely continue. The CPC is projecting normal precipitation for the rest of this calendar year and temperatures that are warmer than normal.

Measuring weather in this extreme environment requires durable, advanced instruments. We are asking for donations of $17.08 to fund the purchase of a new temperature sensor (HMP -155A), which will help us continue the Observatory’s long-term climate record. Your contribution will make an impact!

Alexandra Branton, Weather Observer & Education Specialist

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