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Flood Potential Outlook
Issued by NWS Cheyenne, WY
Issued by NWS Cheyenne, WY
000
FGUS75 KCYS 291703
ESFCYS
NEC007-013-033-045-105-123-157-165-WYC001-007-009-015-021-027-031-
141900-
Probabilistic Hydrologic Outlook
National Weather Service Cheyenne WY
1000AM MST Thu Feb 29 2024
...Spring Flood and Water Resources Outlook Number 2...
...Below-Average Potential for Ice Jam Flooding This Spring...
...Below-Average Potential for Spring Snow melt Runoff Flooding...
This Spring Flood and Water Resources Outlook is the second of three
routine issuances for the Cheyenne Hydrologic Service Area which
comprises seven counties in southeast Wyoming and eight counties in
the Nebraska panhandle. This outlook includes the North Platte River
main stem and its tributaries (the Laramie, Encampment, and Medicine
Bow Rivers), the Niobrara, White and Little Snake Rivers.
.Flood Outlook Summary...
There is a below-average potential for flooding early this spring
due to low-elevation snow melt, ice break up, and ice jams. These
normally occur between February and April. The winter has brought
extended periods of above-normal temperatures along with limited
low-elevation snow pack. This limits the chances for flooding due to
low-elevation snow melt or ice jams during periods of ice break-up
on area lakes and rivers.
The potential for flooding due to snow melt runoff in southeast
Wyoming and the Nebraska Panhandle this spring is below-normal.
At this time, the limited snowpack in the southern Laramie Range
will limit the potential for snow melt flooding. In the Snowy and
Sierra Madre ranges the snowpack has improved over the last two
weeks and is approaching the long-term normal for this date. Near-
normal snow packs in southeastern Wyoming generally translate into
a low probability of flooding. However, a low probability of
flooding is relatively normal for this region. There remains
considerable opportunity for further accumulation of snowpack before
the expected high-elevation snow melt in May and June.
A normal or even low probability of flooding does not mean there is
no chance of flooding. Even a lighter snowpack can generate flooding
if there is a period of extremely warm temperatures and/or
significant rainfall before the ground thaws. Once the ground has
thawed, the flood potential for the rest of the spring and early
summer will remain below normal.
While the snow pack is currently near to below average. The
potential for flooding, early or late in the season, will depend
on future snowfall, temperature patterns, and spring rainfall. All
of these factors affect the total amount of water stored in the snow
pack and the speed with which it is released to the rivers.
.Observed and Forecast Temperatures and Precipitation...
Weather patterns since November have been warmer and dryer than
average across the area. Outlooks by the Climate Prediction Center
(CPC)for the period from 6 to 14 days indicate conditions that favor
near-normal temperatures and above-normal precipitation. The latest
one and 3-month outlooks for March, through May indicates no strong
signal for above or below-normal temperatures or precipitation.
.Snow Cover and Liquid Water Content...
Mountain snow pack across southeast Wyoming is below normal to
normal across most of southeast Wyoming. In southeastern Wyoming
Basin snow water equivalents, measured at the Natural Resource
Conservation Service (NRCS) SNOTEL stations, ranged from 42
percent of median in the South Platte River basin in southeast
Wyoming to 107 percent of median in the Little Snake River basin.
There remains almost 2 months until the median date of peak snow
pack in most basins across the area. While the current outlooks
are not overly optimistic for a robust runoff, March and April can
have a disproportionate affect on the final snow pack accumulation
for the season. Warm windy temperatures or late winter and early
spring storms can cause dramatic, late-season, changes in total
snow pack.
Snow cover across the Nebraska Panhandle counties is extremely light
and inconsistent. It is heavier in the southwestern Panhandle than
in the north. Snow pack there contains relatively small amounts of
water and should not be a major factor in Spring flooding.
.Soil Conditions and Drought...
Soil moisture in the area, as portrayed by the CPC, varies from near
median to above median. Normal or above-normal conditions are
prevalent across southeastern WY and the Nebraska Panhandle.
Moderate to severe drought conditions continue to linger in the
Carbon and Albany counties. Dry conditions have expanded northward
from these areas but remain only abnormally dry and have not yet
developed into drought.
.Lake and River Conditions...
It has been an exceptionally warm winter with only short periods of
extremely cold weather. Despite this, most of the higher elevations
lakes are iced over, although the ice thickness might be less than
in most years. Lower elevations rivers and lakes have less ice than
normal with large areas of open water.
.Climate Outlooks...
The latest outlooks issued by the Climate Prediction Center favor
above-normal precipitation and near-normal temperatures for the
next two weeks. The one and 3-month outlooks for the period from
March through May shows no significant signal favoring above-
normal or below-normal precipitation or temperatures.
In Table 1 below, the current (CS) and historical (HS) or normal
probabilities of exceeding minor...moderate...and major flood stage
are listed for the valid time period.
CS values indicate the probability of reaching a flood category
based on current conditions.
HS values indicate the probability of reaching a flood category
based on historical or normal conditions.
When the value of CS is greater than HS...the probability of
exceeding that level is higher than normal. When the value of CS is
less than HS...the probability of exceeding that level is lower
than normal.
...Table 1--Probabilities for minor...moderate and major flooding...
Valid Period: 02/17/2024 - 09/30/2024
: Current and Historical
: Chances of Exceeding
: Flood Categories
: as a Percentage (%)
Categorical :
Flood Stages (FT) : Minor Moderate Major
Location Minor Mod Major : CS HS CS HS CS HS
-------- ----- ----- ----- : --- --- --- --- --- ---
:Encampment River
Encampment 6.5 7.5 8.5 : <5 <5 <5 <5 <5 <5
:North Platte River
Saratoga 8.5 9.5 10.5 : 32 40 12 17 <5 <5
Sinclair 9.0 10.0 11.0 : 28 31 12 17 5 6
:Medicine Box River
Hanna 6.0 7.0 8.0 : <5 <5 <5 <5 <5 <5
:North Platte River
Glenrock 7.0 8.0 9.0 : <5 <5 <5 <5 <5 <5
Orin 8.0 9.0 10.0 : <5 <5 <5 <5 <5 <5
:Laramie River
Woods Landing 6.0 6.5 7.0 : <5 5 <5 <5 <5 <5
Laramie 9.0 10.0 11.0 : 10 22 <5 6 <5 <5
Bosler 7.5 8.5 9.5 : <5 8 <5 <5 <5 <5
Fort Laramie 7.0 8.0 9.0 : <5 7 <5 <5 <5 <5
:North Platte River
Henry 5.5 6.5 7.5 : <5 <5 <5 <5 <5 <5
Mitchell 7.5 8.5 9.5 : 5 14 <5 8 <5 5
Minatare 7.5 8.5 9.5 : <5 6 <5 <5 <5 <5
Bridgeport 9.5 10.0 10.5 : <5 <5 <5 <5 <5 <5
Legend
CS = Conditional Simulation (Current Outlook)
HS = Historical Simulation
FT = Feet
In Table 2 below...the 95 through 5 percent columns indicate the
probability of exceeding the listed stage levels (FT) for the valid
time period.
...Table 2--Exceedance Probabilities...
Chance of Exceeding Stages
at Specific Locations
Valid Period: 02/17/2024 - 09/30/2024
Location 95% 90% 75% 50% 25% 10% 5%
-------- ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment 4.1 4.3 4.6 4.9 5.3 5.5 6.1
:North Platte River
Saratoga 6.2 6.6 7.4 8.1 8.9 9.6 10.0
Sinclair 5.4 5.9 6.9 7.9 9.1 10.2 10.8
:Medicine Box River
Hanna 3.2 3.2 3.6 4.2 4.4 4.8 5.2
:North Platte River
Glenrock 2.7 2.8 2.9 3.0 3.2 3.4 4.3
Orin 4.7 4.8 4.9 5.0 5.1 5.6 5.9
:Laramie River
Woods Landing 2.0 2.0 2.3 2.8 3.4 4.0 4.5
Laramie 4.8 5.1 6.0 7.4 8.2 9.0 9.3
Bosler 1.3 1.3 1.7 3.0 5.4 6.2 6.6
Fort Laramie 3.0 3.2 3.4 3.5 4.2 5.6 6.1
:North Platte River
Henry 3.5 3.6 3.7 3.8 3.9 4.1 4.4
Mitchell 3.0 3.1 3.4 3.8 4.6 6.5 7.4
Minatare 3.6 3.7 3.9 4.4 4.8 6.1 6.6
Bridgeport 5.0 5.0 5.2 5.6 6.0 6.8 7.2
In Table 3 below...the 95 through 5 percent columns indicate the
probability of falling below the listed stage levels (FT) for
the valid time period.
...Table 3--Nonexceedance Probabilities...
Chance of Falling Below Stages
at Specific Locations
Valid Period: 02/17/2024 - 09/30/2024
LOCATION 95% 90% 75% 50% 25% 10% 5%
-------- ------ ------ ------ ------ ------ ------ ------
:Encampment River
Encampment 1.8 1.8 1.7 1.7 1.6 1.5 1.4
:North Platte River
Saratoga 3.6 3.6 3.6 3.6 3.5 3.5 3.3
Sinclair 3.2 3.2 3.2 3.2 3.1 3.1 2.7
:Medicine Box River
Hanna 2.1 2.1 2.1 2.0 2.0 2.0 1.9
:North Platte River
Glenrock 0.9 0.9 0.9 0.9 0.8 0.8 0.8
Orin 3.5 3.5 3.5 3.5 3.5 3.5 3.5
:Laramie River
Woods Landing 1.2 1.2 1.0 0.3 0.3 0.3 0.3
Laramie 3.8 3.7 3.7 3.6 3.6 3.5 3.5
Bosler 0.9 0.8 0.7 0.7 0.7 0.7 0.7
Fort Laramie 2.4 2.4 2.4 2.4 2.4 2.4 2.4
:North Platte River
Henry 2.0 2.0 2.0 2.0 2.0 2.0 2.0
Mitchell 0.8 0.6 0.3 0.1 0.1 0.1 0.1
Minatare 1.7 1.7 1.6 1.6 1.6 1.6 1.5
Bridgeport 3.9 3.9 3.8 3.8 3.8 3.8 3.8
These long-range probabilistic outlooks contain forecast values that
are calculated using multiple season scenarios from 30 or more years
of climatological data...including current conditions of the
river...soil moisture...snow cover and 30 to 90 day long-range
outlooks of temperature and precipitation. By providing a range
of probabilities...the level of risk associated with long-range
planning decisions can be determined. These probabilistic
forecasts are part of the National Weather Service`s Advanced
Hydrologic Prediction Service.
Visit our web site weather.gov/cys for more weather and water
information.
The next outlook will be issued on or before March 15.
$$
AJA
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- Page last modified: Sep 19th, 2023 19:10 UTC