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National Weather Service Tucson Arizona
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2009
Monsoon outlook for
southeast Arizona |
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updated on July 17th |
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The official
National Weather Service forecast for the second half of the monsoon
has not changed significantly since last month. It calls for equal
chances that rainfall will be above or below average in August,
and in the August through October period. This continues to highlight
the great degree of uncertainty brought on by a rapidly developing
El
Niño over the tropical Pacific Ocean. There remains
an elevated chance that temperatures will be above average for the
rest of the monsoon season, which runs until September 30th.
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| August |
August
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forecast |
precipitation forecast |
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The
monsoon circulation pattern developed over southern Arizona and New
Mexico in mid June as the subtropical high moved north into Texas,
and upper level lows along the west coast helped to draw mid level
moisture north a couple of weeks earlier that usual. However, lower
level moisture did not spread into southern Arizona until the last
week of June, and into northern Arizona until around July 1st. That
was about a week earlier than average. Through July 12th, rainfall
was generally near or above average across southeast Arizona, with
some notable exceptions over western Pima and southeast Cochise Counties.
Farther north across Arizona, precipitation was generally below average. |
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The
monsoon high has been a little stronger than usual, which usually
translates to above average rainfall. However, instead of setting
up over New Mexico as it usually does, the high has been mainly over
southern Texas since mid June where severe drought persists. The jet
stream across the northern U.S. has also been stronger than usual,
which may also be helping to suppress the ridge. |
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The
atmosphere in the tropics has begun to respond to the El
Niño. Since early July, more thunderstorms and tropical
cyclones have been noted over the eastern Pacific, while fewer thunderstorms
have been noted over the western Pacific. Monitoring tools we use
to track how the El
Niño is interacting with the atmosphere have not indicated
that the jet stream is responding as yet, but it will only be a matter
of time. |
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Through
the end of July, the tendency will be for the monsoon high to meander
into its more typical position in New Mexico or the Four Corners region,
which will increase the chance of significant rainfall again in Arizona.
However as we head into August, particularly the second half of August,
the high is likely to relocate in Mexico earlier than usual as the
jet stream dips down the West Coast more frequently and forms more
persistent troughs. |
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The
official forecast purposely does not call for a dry second half to
the monsoon. That is because in previous years when an El
Niño developed this early, either moisture from tropical
cyclones was pulled north by the West Coast trough and recharged the
monsoon, or a tropical depression or tropical storm moved directly
into southern Arizona, bringing both heavy rain and high winds. Atmospheric
conditions have to be just right for this tropical interaction to
occur, but this rapidly developing El
Niño increases the chance for these atmospheric conditions
to set up sometime between late August and early October. |
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Webmaster
US Dept of Commerce
National Oceanic and Atmospheric Administration
National Weather Service
Tucson Weather Forecast Office
520 North Park Ave, Suite 304
Tucson, AZ 85719
Tel: (520) 670-6526
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