Jefferson City, Montana

1 7 1991 and may 14 2013
than winds from thunderstorms... the weekend in may 2011 probably
produced the strongest easterly-component winds of record at Cut
Bank... Bozeman... Havre. Lewistown has seen stronger east winds this
year.

Occurrences of last time of strong east winds...

Cut Bank... on may 14 2011 winds were sustained at 29 mph. There
are no records indicating east winds as strong as those recorded on
the evening of may 22 2013... 32 mph.

Bozeman... on October 3 2009 winds were east at 27 with gusts to 33
mph. Other than thunderstorms... there is no higher sustained
easterly winds on record other than on may 15 2011. There have been
eleven hours of sustained easterly winds of more than 25 mph since
1999. On may 22 2013 the strongest sustained east wind was 22 mph.

Havre... on April 29 1948 winds were sustained from the east at
36 mph. There are no records indicating east winds of the strength
recorded on may 15 2011... 37 mph. From 1970 through 2012 there were
only 13 hours of sustained easterly winds of more than 25 mph. On
may 22 and 23 2013 there were nine hours of sustained easterly winds
of 30 mph or greater.

Lewistown... on April 2 2003 winds were from the east at 32 with
gusts to 38 mph. On may 15 2011... winds were sustained at 22 mph.
There are no records indicating east winds as strong as those
recorded on may 23 2013... 36 mph. In may 2011 there were eight hours
sustained easterly winds of 30 mph or greater. This year on may 22
and 23 there have been seven hours.

To further highlight the rarity of strong east winds... historical
percentages of occurrence illustrate this. The following table shows
the percentage of time that easterly and westerly-component
sustained winds of 25 mph or greater... and any speed... occur.

           East east-component west west-component
        >25mph any spd >25mph any spd >25mph any spd >25mph any spd
Bozeman 0 pct 11 pct <0.1 21 pct 0.1 15 pct 0.1 23 pct
Cut Bank 0 pct 7 pct <0.1 11 pct 0 45 pct 8 53 pct
grt falls 0 9 pct <0.1 14 pct 2 24 pct 4 49 pct
Havre 0 21 pct 0.1 25 pct 1 31 pct 1 51 pct
Lewistown 0.1 20 pct 0.1 25 pct 1 30 pct 2 46 pct

Key:
east = east-northeast through east-southeast directions
west = west-southwest through west-northwest directions
east component = northeast through southeast directions
west component = southwest through northwest directions

This table shows that at Great Falls... sustained east winds of 25
mph or greater occur less than 0.1 percent of the hours in the
year... sustained 25 mph northeast through southeast winds occur 2
percent of the year... and west winds occur 49 percent of the hours
in the year.