September 2019 Climate Statistics

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Unseasonably cool weather at the end of the month compensated for hot weather at the beginning and middle of the month, so the average temperature was exactly normal. Total precipitation was a quarter inch above normal.

Summer weather persisted through the first week of September, with a strong upper level high pressure ridge anchored over the western half of the nation.

The 99 degrees on the 5th was the culmination of a streak of 9 consecutive days with highs over 90.

Airflow from the Desert Southwest brought enough monsoon moisture for isolated thunderstorms on the 5th and 6th. On the 6th, 0.15 inch of rain fell, exceeding our total summer rainfall by a hundredth of an inch.

A Pacific cold front moving through the northern edge of the ridge generated a third of an inch of rain on the 8th. The upper level trough following the front kept temperatures a few degrees below normal through the 11th.

After the trough exited on the 12th, warm dry air south of the jet stream raised temperatures above normal. The last 90-degree highs of the summer were recorded on the 14th and 15th.

On the 16th an upper level trough from the Gulf of Alaska, followed by a second trough on the 19th, maintained cool showery weather through the 21st.

After the trough departed, temperatures were slightly above normal from the 22nd through the 27th.

During the early morning hours of the 28th, a strong cold front imported much cooler air from Alaska as a major upper level trough deepened over the northwest U.S. Temperatures averaged nearly 15 degrees below normal for the final three cays of the month.

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September 2016 Climate Stats at Boise

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September’s average temperature was slightly below normal due to cool upper-level troughs which dominated the inter-mountain region for most of the month.

Highs were only the 60s on six days.  The high of 63°F on the 14th was the coolest day since June 15.

A very warm upper-level ridge built over the region on the 25th and 26th, bringing Boise the highest temperatures of the month.  The 93°F on the 28th was only two degrees shy of the record for that date.

As the ridge moved east on the 29th and 30th, southwest flow aloft on its western flank, combined with southeast surface winds, continued the warm weather.

Precipitation was one third of normal. For most of the month, Boise was on the western margin of moist air moving up from the south, so upper-level troughs interacting with this moisture dumped most of their rain on south-central and southeast Idaho.

Locations east of Boise received substantial monthly precipitation totals, ranging from just under an inch at Mountain Home to over two inches at Twin Falls and Pocatello, much of that from the 21st through the 23rd.