Flood Potential Outlook
Issued by NWS Portland, OR

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Hydrologic Outlook
National Weather Service Portland OR
213 PM PST Sat Jan 14 2017

...HEAVY RAIN AND SNOWMELT WILL HEIGHTEN FLOOD POTENTIAL ACROSS
SOUTHWEST WASHINGTON AND NORTHWEST OREGON TUESDAY AND WEDNESDAY...

A wet and mild series of storms will move into the region next
week. The signal has been consistent for a heavy rain event
somewhere in the region in the Tuesday to Wednesday timeframe.
Wherever the heaviest rain axis sets up, well over 5 inches of
rain can be expected on the coast and coastal mountains and 2 to 4
inches in the interior lowlands. Rainfall amounts in the Cascades
are expected between 3 and 5 inches with over 5 inches possible in
the south Washington Cascades. Details remain unclear at this
time, but more specific forecast information will be available
Sunday and Monday as time gets closer to the start of the event.

In addition, snow levels will be rise rapidly Monday night,
to near 8000 feet by Tuesday morning. This warmer air, combined
with the rain, will lead to snowmelt. Expect significant amounts
of snow in the Coast Range to melt Tuesday and Wednesday which
will add runoff in addition to the heavy rain. Current snow water
equivalent estimates place between 0.25" and 1.0" below 1000 feet.
1.0 to 2.5" between 1000 and 2000 feet, and over 3.0" in the
limited areas above 2000 feet in the Coast Range.

The snowmelt component to the runoff is more complicated in the
Cascades. Snow water equivalent is generally 120% to 200% of
normal, but with much above normal snow depths in some areas and
the lack of a thawing cycle yet this season, snowpack will likely
be able to absorb a lot of rainfall, especially above 3500 feet.
This uncertainty about the runoff contribution from the snowpack
leads to highly variable flooding potential for rivers draining
the Cascades.

While snowmelt is a contributing factor to the flooding
potential, the amount of heavy rain will remain most significant
to flooding magnitude. The details of rainfall totals and expected
flooding will be refined as these storms approach the Pacific
Northwest, however confidence is increasing for flooding on area
rivers, possibly approaching major flooding in some locations.

$$

Bentley


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