National Weather Service

Watches, Warnings & Advisories
NWS Homepage

Hydrologic Outlook

Hydrologic Outlook
Hydrologic Outlook
National Weather Service Riverton WY
1006 AM MDT Tue May 22 2018

...Accelerated snowmelt at the 9,000 to 10,000 foot elevation
Friday through Sunday...

...Significant rises in creeks and streams during the
Memorial Day Weekend...

The snowpack at the 9,000 to 10,000 foot elevation has ripened
considerably in the last week to 10 days due to mild temperatures
and recent rainfall.  Rainfall today and Wednesday will only
further ripen or increase the water content of the current snowpack.

Afternoon mountain temperatures at the 9,000 foot elevation are
expected to warm to near 60 for Thursday--and into the lower to
middle 60s Friday through Sunday.  Morning low temperatures are
forecasted to be at or above the frost temperature of 37 degrees
Thursday through Sunday.

Increased snowmelt at the 9,000 to 10,000 foot elevation Friday
through Sunday will cause significant rises in creeks and streams
during the Memorial Day Weekend.

Rain showers are in the forecast for Sunday afternoon through and Memorial
Day. The combination of rain and snowmelt runoff will greatly increase
river levels.

The Wind River and the South Fork of the Shoshone River should see flows
high enough to cause flooding by Monday and Tuesday of next week.
Tributary streams that feed into the Big Horn and Snake Rivers may
see the highest flows of the season early next week.

During active snowmelt runoff, mountain creeks and streams are
at their highest levels during the late afternoon/early evening
hours.  Crossing creeks and streams during that time is highly

Graphical forecasts are available at:
Visit and select the rivers & lakes tab for
graphical forecasts and additional information.


Hydrologic Outlook

Hydrologic Outlook
National Weather Service Great Falls MT
313 AM MDT Tue May 22 2018

...Flooding concerns to increase this week due to mountain
snowmelt and runoff from showers...

River and creek levels will likely rise once again throughout this
week, causing an increased concern for flooding across portions of
north central and southwest Montana adjacent to mountains.

Temperatures will remain well above normal for this time of year,
which will increase melting of the still substantial snowpack at
higher elevations of the mountains. The weather pattern will also
be quite active, and the potential for brief heavy rain showers
from thunderstorms could create periods of rapid runoff.

This additional water going into area rivers and creeks will keep
them running at least high, fast, and very cold, especially near
mountains. Water could flow out of their banks and cause flooding
of fields and low-lying areas.

Some river gauge points have already reached flood stage, but
additional locations may approach or exceed flood stage
throughout this week. These additional locations include, but are
not limited to:

In southwest Montana:
Jefferson River at Twin Bridges...
Gallatin River at Logan...
Missouri River at Toston...
Tenmile Creek near Helena...

In north central Montana:
Sun River near both Simms and Vaughn...
Badger Creek near Browning...

Please continue to monitor future flood statements for updates on
this ever-changing situation. Also, please have a plan-of-action
for long-term rises on rivers and streams through the remainder
of May, and into June as well.



1238 PM MDT Mon May 21 2018

...Precipitation and warm temperatures push rivers and streams
higher this week...

With more precipitation and warmer temperatures expected, river
forecasts show rising water levels through the week into the
holiday weekend. This will keep the Musselshell River near to
above flood stage upstream of Roundup, and may push other rivers
and streams near or above bankfull by mid to late week. Those
living along waterways should pay close attention to the forecast.
Now is the time to make final preparations for runoff and move
any equipment and livestock to higher ground.

Even at levels below flood stage, bank erosion can be significant
and there is danger from undercut river banks collapsing, so stay
away from the waters edge and keep a close eye on children when
near flowing water.

The weather pattern is shaping up to bring a good chance for
showers and thunderstorms this week, especially Tuesday night
through Wednesday night. The potential for locally heavy rainfall
will exist with any thunderstorms through the week, and Tuesday
night through Wednesday night could see significant widespread
precipitation. In addition to the precipitation potential,
temperatures through the week are expected to be warm. Highs in
the 70s Monday through Wednesday could reach the mid 80s for
Thursday into next weekend. These warmer temperatures are expected
to accelerate snowmelt later in the week.

If flooding is noticed please report it to local law enforcement.


U.S. Dept. of Commerce
NOAA National Weather Service
1325 East West Highway
Silver Spring, MD 20910
Page last modified: June 2, 2009
NATIONAL WEATHER SERVICE: for Safety, for Work, for Fun - FOR LIFE