Extended Streamflow Guidance
Issued by NWS West Gulf RFC

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SPRING FLOOD OUTLOOK
NATIONAL WEATHER SERVICE
WEST GULF RIVER FORECAST CENTER, FORT WORTH, TX
4 PM CST THU MARCH 4, 2015

The spring flood potential for the West Gulf River Forecast Center
drainage area covering portions of southern Colorado, New Mexico, and
Texas that flows into the Gulf of Mexico, is below average based upon
current antecedent conditions.  For the southern Rocky Mountains,
which includes the San Juans and Sangre de Cristos, snowmelt runoff
draining into the upper Rio Grande and Pecos river basins to produce
spring flooding is unlikely and not expected based on the current
snowpack conditions.

This outlook considers antecedent rainfall, snowpack, soil moisture,
streamflow, and watersupply conditions compared to climatology
combined with longer-term 90-day forecasts for temperatures and
precipitation across the forecast area.  The most significant factors
for spring flooding are amount of water in the snowpack and the timing
of the melt.

The primary factor in development of significant river flooding over
most of the region is the occurrence of excessive rainfall in
relatively short periods of time, even for areas where drought
conditions persist or have developed.  In addition, the mountainous
area in the northwest portion of the  forecast area can be affected by
rapid snowmelt, usually occurring in the spring months.  These
mountainous areas of southern Colorado and northern New Mexico can
receive significant winter snowpack accumulation.  However, normal
weather regimes producing rapid snowmelt and/or heavy rain on snow
events, statistically these have a low probability of occurrence in
this area.  Above average snowpack is currently not observed or
forecast within the southern Colorado and northern New Mexico
mountains.

The general outlook for specific basin areas in the region follow:


...Upper Rio Grande and Pecos basin, Colorado and New Mexico

The current general basin wide snow conditions within the upper Rio
Grande in Colorado and New Mexico are still generally tracking below
average, but improving with a regular pattern of storm systems over
the past few week. In contrast, the Pecos River basin received some
unusually heavy rainfall amounts last fall along with additional
snow/rain events boosting the precipitation totals over the basin to
values ranging from 150 to 400-plus percent above average over the
past 60 days.

Recent storms have also dropped much needed snow over the mountainous
regions throughout south central Colorado and northern New Mexico,
increasing seasonal forecast streamflows, but still generally below
normal; the exception being near normal streamflow forecasts along the
middle/upper Sangres.  However, the potential for spring flooding
remains low.

Looking at snowpack, the range varies widely, however, the upper Rio
Grande basin in Colorado is generally 65 to 90 percent of normal for
snow water equivalent basin-wide with localized above and below
outliers.  Across New Mexico in the Rio Grande and Pecos basins, snow
water equivalents range from 60 to near 90 percent of normal.  Along
the northern and middle Sangres in Colorado and New Mexico, the
snowpack improves with a general range from 90 to 110-plus percent

Historically, mountainous snowpack, particularly in the higher
elevations, increases into April with additional storms.  The timing
on storms in the region is tracking near normal but the magnitude
continues to be genrally below normal amounts.  As a result, peak
snowpacks are expected to continue below normal levels with melt
likely occurring earlier than normal this season. However, any
enhanced precipitation and cooler temperatures through March would
provide protection despite the typical dust-on-snow and wind events
that greatly reduce the snowpack. Based on current snowpack
conditions, below average streamflow is expected this spring and
throughout the seasonal runoff period.  The potential for significant
flooding from springtime snowmelt in the upper Rio Grande and Pecos
River basins remains significantly reduced for the mainstem and
associated tributaries.  Current basin streamflows along the mainstem
and tributaries are below normal in Colorado and New Mexico, as well
as, reservoir storage.

The U.S. Drought Monitor indicates abnormally dry to moderate drought
conditions for the upper Rio Grande basin in Colorado and New Mexico
extending to severe drought conditions across the upper Pecos basin.
The 90 day U.S. Seasonal Drought Outlook indicates persistence or
slight improvement of drought conditions in the entire Rio Grande
basin in Colorado and New Mexico.

Extended 90-day climate outlooks indicate equal chances of above,
below, or near normal temperatures and a 40 percent chance of above
average precipitation. These conditions may result in mitigating early
snowmelt.  Overall, the potential for spring time flooding as a result
of snowmelt runoff is very low.


...West and south Texas generally west of Wichita Falls to San
   Antonio to Brownsville line, including portions of the upper
   Brazos, upper Colorado, upper Guadalupe, Nueces, San Antonio
   and lower Rio Grande basins

The potential for significant spring flooding over these basin areas
in Texas is  below average at this time.

In the past 60 days, most of these basins have received near average
rainfall ranging from 75 to 125 percent of normal.  Current basin
streamflows are generally below seasonal baseflow.  Reservoir storage
in the region is also  very low ranging from nearly empty to 30
percent of conservation.  The  U.S. Drought Monitor indicates
improvement in drought conditions across most of western Texas with
some smaller extreme to exceptional drought areas defined along the
middle to upper portions of the Brazos  river in Texas and headwater
areas of the Guadalupe, San Antonio, and Nueces river systems around
the Edwards Plateau.

The U.S. Seasonal drought outlook for the next 90 days indicates
drought persistence likely across all river basins in these  areas.
Extended 90-day climate outlooks indicate a 33 to 40 percent chance of
above average precipitation and 33 to 40 percent chance of below
average temperature.  The potential for spring flooding remains well
below average at this time.


...The eastern portions of Texas including the Neches, Sabine,
   Trinity, lower brazos, lower Colorado, lower Guadalupe, and
   San Jacinto river basins.

Potential for spring flooding in this area is near average at this
time with slightly higher potentials across the far northeastern Texas
basins and along the upper Texas Gulf Coast.

Most of the basins in this region have received near average rainfall
for this time of year ranging from 75 to 125 percent of normal, with
near to slightly above normal rainfall experienced along the Gulf
Coast to extreme south Texas.  Streamflow conditions across this
region are near to slightly below normal. Recent rain events have lead
to increased soil moisture content over eastern Texas, primarily in
the Sabine and Neches river basins.  Most area reservoirs are below
conservation levels ranging from 70 to 100 percent of conservation
storage percent capacity.

For the area in focus, the U.S. Drought Monitor indicates recovery
from drought conditions all across eastern Texas with some abnormally
dry conditions in northeastern Texas persisting, but improving.  No
additional drought development is expected over eastern Texas.
Elevated soil moisture over basins in eastern Texas may result in
substantial runoff from any heavy, localized convective spring rains
over this area.  Extended 90-day climate outlooks indicate a 33
percent chance for above average precipitation with a 40 percent
chance of below average temperature. Typically, river flooding is
increasingly probable during the spring months in this region.
Convective storms are generally the flood producers during this
period. Slow vegetation growth combined with the possibility of near
normal precipitation means the potential for spring flooding across
this region is enhanced.


REFERENCES/LINKS:

Precipitation Analysis:
http://water.weather.gov/precip/

Snowpack Conditions:
http://www.wcc.nrcs.usda.gov/reports/SelectUpdateReport.html
ftp://ftp.wcc.nrcs.usda.gov/data/water/wcs/gis/maps/
west_swepctnormal_update.pdf
ftp://ftp.wcc.nrcs.usda.gov/data/water/wcs/gis/maps/WestwideSWEPercent
.pdf

Streamflow Conditions:
http://waterdata.usgs.gov/nwis/rt
http://www.dwr.state.co.us/Surfacewater/default.aspx

Reservoir Summaries:
http://www.wcc.nrcs.usda.gov/basin.html
http://www.waterdatafortexas.org/reservoirs/statewide

Soil Moisture Conditions:
http://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/US/Soilmst/
Soilmst.shtml

Us Drought Monitor And Outlook:
http://droughtmonitor.unl.edu/
http://www.cpc.noaa.gov/products/expert_assessment/season_drought.png

Tx Drought Information:
http://waterdatafortexas.org/drought/

Climate Graphics:
http://www.cpc.noaa.gov/
http://www.cpc.noaa.gov/products/predictions/long_range/lead01/
off01_temp.gif
http://www.cpc.noaa.gov/products/predictions/long_range/lead01/
off01_prcp.gif

WGRFC




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