Extended Streamflow Guidance
Issued by NWS

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National Weather Service
Colorado Basin River Forecast Center
Salt Lake City, Utah
March 6, 2017

                 INTERNAL NWS PRODUCT FOR GUIDANCE PURPOSES ONLY

                        SPRING FLOOD POTENTIAL OUTLOOK UTAH
                                 AS OF MARCH 1, 2017

The 2017 spring runoff flood potential due to snowmelt is high for Weber, Provo,
Duchesne and Bear River basins of Utah at this time. This potential is due to
the much above median snow conditions in all of these areas. It should also be
emphasized that snow typically accumulates into mid April and even May and that
the threat of spring flooding will largely be determined by hydrometeorologic events
that occur during the next several months. Conditions in the Virgin and Sevier River
basin are above normal. Conditions in the Six Creeks basin near Salt Lake City are also
above average and conditions in Weber, Bear or Duchesne and Green
River Basins are much above average.

Specific forecast procedures and flood flow levels do not exist for all streams,
however the CBRFC hydrologic model is forecasting above average peak flows across
north central Utah during the snowmelt runoff period. 90% exceedance forecasts for
many sites are projected to be above defined bankfull levels, with 50% and 10%
exceedance forecasts above the defined flood flow at quite a few of those
locations.

February precipitation was much above average at betwee 200-300 percent of normal
in central and northern Utah. The snow water equivalent in the Weber, Provo,
Duchesne, Green and Bear River basins are much above normal with many of our snow gaging
locations already exceeding the annual peak snowpack which typically occur in mid
April or early May. The current snow water equivalent is 163% of median in the Weber
River drainage, 191% of median in the Provo River drainage, 187% of median in the
Duchesne and 171% of median in the Bear and 148% of median in the Green. Many of Utah`s
SNOTEL`s are currently ranked in the top 3 highest years for the period of record.
Another sign of the exceptional year is that the December-February precipitation totals
across the board for the majority of SNOTEL stations ranks number 1 for the period
of record.

Current volumetric forecasts for the April through July runoff period are much
above average for all of the above mentioned basins with the exception of those
in south central and southern Utah.

Although spring temperatures affect the pattern of snowmelt runoff and consequently
the magnitude of peak flows, peak flows may roughly correspond to volumetric flows
in their magnitude. It is also important to keep in mind that an extended period of
much above normal temperatures or heavy rainfall during the melt period can cause or
exacerbate flooding problems during any year.

The spring runoff flood potential will be re-evaluated in late March and an updated
product will be issued the first week of April.


CBRFC/B.Bernard, A.Nielson, T. Cox

NNNN
$$




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