Hydrologic Outlook
National Weather Service Riverton WY
206 PM MDT Thu Apr 5 2018


...Increased snowmelt runoff below 8000 feet expected
Friday afternoon through Saturday afternoon...

...Minor flooding of low lying areas possible over
Star Valley and Jackson Hole Saturday afternoon through
early Sunday...

There is still 10 to 20 inches of snow on the ground in
several areas along Star Valley and Jackson Hole. The low
elevation snowpack has become very ripe or water laden in
the last few weeks.

Afternoon valley temperatures are expected to warm into the
mid 40s to around 50 through Saturday across far western Wyoming. 
Rainfall, of one quarter to one half in is expected from later
this afternoon through tonight. Another half inch to three quarters of
inch of rain is expected for Saturday. Snow levels are expected to
fluctuate around 7500 to 8000 feet through Friday and around 9000
feet on Saturday.

Expect noticeable increases in snowmelt runoff below 8000 feet
Friday afternoon through Saturday afternoon. A combination of
rainfall and snowmelt runoff over Star Valley and Jackson
Hole will cause noticeable sharp rises along small creeks and
streams Saturday night through early Sunday. Minor flooding
is possible along low lying areas.

Residents living along small creeks and streams in Jackson Hole
and in Star Valley should be vigilant for possible significant
rises in water levels Saturday afternoon through late Sunday.
Road crossings along low lying areas may become hazardous due
to rising waters.

Snowmelt runoff is also expected to increase Friday afternoon
through Saturday afternoon across the Greys River Basin to include
the Porcupine Slide area.

Flood Watch
National Weather Service Pocatello ID
322 AM MDT Fri Apr 6 2018

...Heavy rains Saturday could lead to flooding problems...

.A moisture laden warm front, bringing very mild air up from the
central Pacific, will move into southern Idaho late tonight or
Saturday morning. The warm and moist air will have two effects on
the ground underneath. Heavy rainfall will mean a large percentage
turning into surface run-off, and the warm rain will melt existing
snow pack, to an elevation near 9000 feet for almost all of
central and eastern Idaho. The snow melt and rainfall will start
overland, also known as sheet or areal, flooding. The areas
outlined indicate the most likely areas where large amounts of
snowmelt are expected, since it will be the main driver of where
flooding occurs. As the water seeks the lowest level, it will fill
creeks and main-stem rivers, and you should expect rapid rises on
both. Reservoir operators may have to increase releases from dams,
further adding to the risk of being near the streams and rivers of
central and eastern Idaho. While skies may partially clear
Sunday, river stages may continue to surge as the water continues
its downhill movement.


Upper Snake Highlands-Caribou Highlands-
Cache Valley/Idaho Portion-Wasatch Mountains/Idaho Portion-
Including the cities of St. Anthony, Ashton, Island Park, Driggs,
Victor, Henry, Soda Springs, Downey, Lava Hot Springs, Preston,
and Montpelier
322 AM MDT Fri Apr 6 2018

...FLOOD WATCH REMAINS IN EFFECT THROUGH LATE SATURDAY NIGHT...

The Flood Watch continues for

* Portions of eastern Idaho and southeast Idaho, including the
  following areas, in eastern Idaho, Upper Snake Highlands. In
  southeast Idaho, Cache Valley/Idaho Portion, Caribou
  Highlands, and Wasatch Mountains/Idaho Portion.

* Through late Saturday night

* Heavy rain is expected Saturday, with rain as high up as 9000
  feet elevation. This will melt much of the snow already on the
  ground from this winter, causing large areas of surface run-
  off.

* This overland, or areal, flooding can inundate homes and other
  buildings, damage road surfaces, and make roads impassible.
  Storm water drains can be overwhelmed, especially if blocked
  by debris such as branches and leaves.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Flood Watch means there is a potential for flooding based on
current forecasts. No flooding is occurring at this time.

You should monitor later forecasts and be alert for possible
Flood Warnings. Those living in areas prone to flooding should be
prepared to take action should flooding develop.