Wildfire Burn Scars are a Flood Risk Infographic

Wildfire Burn Scars are a Flood Risk

Heavy rain can produce flash floods, mudslides & debris flows over burned areas from wildfires.  Water repellent soils are formed when organic material such as trees, scrubs, plants and litter burn at high intensity (high temperatures), causing water repellent compounds to become vaporized which then condense on cooler soil layers below the surface, which prevents the soil from absorbing water after a fire. During heavy rains, water cannot penetrate water repellent soil layers, so it runs off like pavement which causes dangerous flash flooding, debris flows and mudslides.

August 2016 Climate Stats at Boise

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August 2016 was dry and seasonably warm, with the average temperature within one degree of normal. There were no record high or lows set.  Only traces of rain fell at the airport. That’s not unusual, because August is generally the driest month at Boise.  Historically at Boise, 25% of Augusts had no measurable rainfall and 50% had no more than a 0.10″.

Weather patterns were typical of this time of year. Pacific systems were relatively weak, mainly moving inland north of our area with precipitation confined to northern and central Idaho and adjacent sections of eastern Washington, northern Oregon, and western Montana.  But even those areas were drier than normal, and little if any rain fell on southwest Idaho.

Incursions of monsoon moisture were usually deflected to the east ahead of trailing pacific cold fronts, so measurable rain from that source came no closer than the Nevada border and eastern Idaho.

At the Boise Airport, where records go back to 1940, June-August 2016 ranked among the driest 15% of summers.

July 2016 Weather Stats at Boise

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July 2016 was the first month with below normal temperatures since November 2015. Although July’s average temperature and total precipitation were close to normal in Boise, it was an unusual month.  The average temperature of 75.4°F was the lowest July average since 2001.  The low temperature of 47°F on July 11th was the coolest July temperature in Boise since 2000.  The high temperature of 69°F on July 10th was the coolest July daily high temperature since 2001.  The hottest day of July 2016, 101°F on the 27th, was the lowest monthly July maximum since 1997. There were no record daily high or low temperatures set in July 2016.  A daily precipitation record was set on July 10th, when the total rainfall of the month fell (0.27in) in one day. Even heavier amounts were measured elsewhere in the Boise area, including 0.76in at Eagle and 0.61in at Meridian and over an inch was measured in surrounding areas.

Drought Level since 2000 across Idaho and Oregon

Drought levels across Idaho and Oregon have improved since the recent drought from 2014-2015.  Here a look at the amount of area affected by drought in Idaho and Oregon. The graphs depict the state percentage of area affected by drought. Data is courtesy of the United States Drought Monitor.

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June 2016 Weather Stats at Boise

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June 2016 was the fourth warmest at the Boise Airport, and seventh warmest all-time, but still around 4 degrees cooler than June 2015, the warmest on record.  Temperatures reached 90°F on 13 days in June 2016 (June average is 5 days).  It was a dry month, ranking among the driest 20 percent of Junes on record.  New record daily highs were set on the 5th (97°F) and the 8th (101°F). Record high daily minimum temperatures were set on the 6th (68°F) and the 8th (69°F).

May 2016 Climate Stats at Boise

May2016May was unsettled, with showery periods interspersed with warm dry weather.  June-like temperatures alternated with cool blustery days more typical of March. There were no record highs or lows.

During the first week, temperatures averaged as much as 15 degrees above normal, thanks to an upper level ridge and southwest flow aloft.  Highs reached 81°F on the 3rd and 85°F on the 4th.  On the 5th and 6th, a low pressure system which soaked up plenty of moisture off the southern California coast, moved inland over the Desert Southwest. Some of this moisture reached southern Idaho, which was under an unusual easterly flow on the northern periphery of the low. Showers and thunderstorms crossed the Boise area on the 6th, dumping heavy rain on a few spots. One observer in southwest Boise measured 1.36 inches of rain in 20 minutes, resulting in local flooding across the Boise Metro area. Another observer in the foothills north of Boise reported 0.64 inches in 30 minutes!

The low was kicked east out of the intermountain region on the 7th and 8th by a cold upper level trough which moved over western Canada from the Gulf of Alaska. Cooler drier air associated with the trough drifted into the Treasure Valley and kept temperatures a few degrees below normal from the 9th through the 11th.

Temperatures soon rebounded, with highs from 80°F to 85°F from the 12th through the 14th under a temporary upper level ridge.  A weak trough followed the ridge, with cooler air and showers on the 15th.

A ridge building inland from the northwest coast brought a warming trend from the 16th through the 18th. It was abruptly ended by another Gulf of Alaska trough, which arrived over the Pacific Northwest on the 19th and covered most of the Intermountain West by the 21st.

Unseasonably cool, moist and unstable air associated with the trough generated scattered showers and isolated thunderstorms each day from the 19th through the 22nd.  The high of 55°F at Boise on the 20th was the coolest day since April 14.

Although the trough remained over the western U.S., temperatures moderated to near normal by the 23rd due mainly to long days and widespread sunshine.

The next trough in the series had less effect on our area. Initially centered over southeast Alaska and the British Columbia coast, it sent a dry cold front across the Boise area on the 27th, lowering high temperatures by around 5 degrees.

The trough moved east along the Canadian border over the Memorial Day Weekend, with little effect on Boise other than breezy northwest winds

A warming trend began on the 31st as an upper level ridge built offshore.

April 2016 Climate Stats at Boise

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April 2016 was the warmest April ever recorded at the airport and it tied for 3rd warmest since temperature recording began in the Boise area in 1864.  It was the 5th consecutive warmer than normal month.

A warm upper level high pressure ridge, which dominated the Inter-mountain West during the first three weeks of the month, was responsible for the record warmth.  Relatively weak Pacific weather systems interrupted the ridge a couple of times, on the 4th and 5th and the 14th and 15th, briefly lowering temperatures to a few degrees below normal.  Northwest winds accompanied the cooler air, gusting to 36 mph on the 4th and 37 mph on the 15th.  Only light amounts of rain fell.

A strong and wet upper level trough plowed into the ridge on the 22nd and passed directly over Boise on the 23rd, dumping nearly half an inch of rain at the airport, and up to twice that amount at a few Treasure Valley locations west of Boise.

This system ushered in a pattern change, with westerly flow aloft splitting over the Northwest coast, and a cool upper level trough expanding over the Western United States, lowering temperatures in our area to near normal.  There was no measurable rain at Boise after the 23rd, as most precipitation was either deflected south to California and the Desert Southwest, or confined north and east of Boise.

Following a cold front on the 25th, Northwest winds gusted to 45 mph at the airport.  The 28th through  the 30th were not quite that windy, but gusts exceeded 30 mph each day.

March 2016 Climate Stats at Boise

Mar2016March 2016 was the 7th warmest at the airport, where records go back to 1940.  It was a month of changeable and sometimes extreme weather typical of early spring.

From the 1st through the 13th the temperature averaged 8 degrees above normal. This was primarily due to persistent relatively warm southwest flow aloft.  Disturbances embedded in the flow weakened as they moved inland, but they carried enough moisture to bring showers to Boise nearly every day.

The pattern changed at mid-month. Northwest flow aloft followed the passage of an upper level trough on the 14th, keeping temperatures below normal through the 18th.  The low of 28°F on the 18th was the first freezing temperature since February 25th.

On the 19th an upper level ridge made a brief sojourn over the Intermountain region and temperatures were again above normal.

After the ridge departed to the east, southwest flow aloft ahead of a Gulf of Alaska trough kept temperatures above normal through the 21st. The 70°F on the 20th, The First Day of Spring, was the high for the month and the first 70°F since October 25th.

As the trough drew closer on the evening of the 21st, strong thunderstorms developed along the cold front, dumping over a third of an inch of rain at the Boise Airport.  Two to three times that fell at other locations around town, along with hail up to grape size which covered the ground in some places.

That trough paved the way for two more Gulf of Alaska troughs.

A second trough arrived on the 25th, ushered in by chilly northwest winds gusting to 43 mph at the airport.  In contrast to the trough, four days earlier, there was only a trace of rain.  The winds subsided that evening and skies cleared, allowing the temperature to fall to 27°F after a week of above-freezing lows.

The third and deepest trough arrived on the 28th. The coldest air was already over Boise early that morning, reflected by a low of 28°F.  By evening the upper level low center and coldest air had settled over the Great Basin and California.

By the morning of the 29th a deep surface low had formed, centered near the Utah-Colorado border.  By afternoon it had developed into a major spring snowstorm for the Magic Valley and southeast Idaho.

Boise, being too far north and west to see any precipitation, experienced only dry northwest winds, with gusts in the 20 to 30 mph range at the airport.  In contrast to the cold wet weather farther east, the temperature at the airport reached 60°F that afternoon.

Under mostly clear skies temperatures continued to warm on the 30th and 31st in response to a high pressure ridge which extended from the California coast all the way to Alaska.

February 2016 Climate Stats at Boise

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February 2016 ranked among the warmest 15% and the driest 35% of February’s at the Boise airport.

Temperatures on most days were more typical of March and early April.  Highs reached 60°F or above on four days.  The 65°F on the 26th was the warmest day since the 70°F on October 25 2015.  Lows were above freezing on eleven nights, including every night from the 14th through the 21st.  Despite the unseasonably warm weather, no records were equaled or broken.

Only four days averaged below normal, including the 1st through the 3rd, due to a deep cold upper level trough, and the 23rd, when clear skies and dry air allowed the temperature to drop to 24°F in the pre-dawn hours.

Measurable precipitation fell on only five days.  The only measurable snowfall was 0.7 inch on the 4th. An average February has 2.9 inches of snow.

The pattern responsible for the relatively warm and dry February was a persistent warm upper level high pressure ridge, which established itself following the departure of the upper level trough.  There was enough flow through the ridge to allow Pacific weather systems to cross our area, but they dropped most of their rain and snow on the mountains of northern and central Idaho.

Thunderstorms accompanied a cold front which crossed the Boise area around 5 am on the 18th. Some locations even reported small hail. Early morning thunderstorms, and winter thunderstorms, are rare in
southwest Idaho.

The front was followed by winds which gusted to 41 mph at the Airport.

Another cold front crossed the Boise area around 5 pm on the 19th, bringing brief heavy rain showers and a wind gust of 47 mph.

Fog and low clouds were observed on eleven days, including dense fog on four of those days. The fog usually dissipated in the afternoons.