Flash Flood Warning 74, NWS Denver/Boulder, CO

323 
WGUS75 KBOU 240120
FFSBOU

Flash Flood Statement
National Weather Service Denver CO
720 PM MDT Wed Aug 23 2023

COC069-240130-
/O.CAN.KBOU.FF.W.0074.000000T0000Z-230824T0130Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Larimer CO-
720 PM MDT Wed Aug 23 2023

...FLASH FLOOD WARNING IS CANCELLED FOR CENTRAL LARIMER COUNTY FOR 
THE CAMERON PEAK BURN AREA...

Flood waters have receded. Flooding is no longer expected to pose a 
threat.

LAT...LON 4080 10558 4079 10568 4069 10573 4058 10572
      4053 10563 4055 10553 4064 10549 4078 10552

$$

AD

Area Forecast Discussion
National Weather Service Denver/Boulder CO
350 PM MDT Wed Aug 23 2023

.SHORT TERM...(This evening through Thursday)
Issued at 202 PM MDT Wed Aug 23 2023

Upper level ridge over the Central/Southern Plains will produce a
southerly flow aloft across Colorado. This southerly flow will
continue to transport subtropical moisture into the region.
Precipitable water values will climb to around an inch across
northeast Colorado and 0.75(in) over the higher terrain by this
evening. Still a fair amount of warm air aloft which is limiting
instability over eastern Colorado. However over the mountains, the
slightly cooler air aloft is leading to surface based CAPE of up
to 600 J/kg. Expect scattered showers and storms through mid
evening over the mountains. Isolated convection will be possible
eastward to about I-25. The convection along the I-25 Corridor
will be weaker with gusty outflow winds and light/moderate
rainfall under the showers/storms.

For Thursday, similar weather conditions will prevail. Airmass
cools just a little (up to 3 degrees). Highs should be able to
reach the lower 90s over most of northeast Colorado. Moisture and
instability looks to be similar as today with only a slight
increase over the mountains. This should lead to scattered to
numerous thunderstorms over the higher terrain. These storms will
track north-northeast. If convection is to take place, it will
need to form over plains, which may be difficult due to the
limited instability.

.LONG TERM...(Thursday night through Wednesday)
Issued at 300 PM MDT Wed Aug 23 2023

Friday and Saturday will be a very active stretch of weather.
Remnants of Tropical Storm Harold will move around a strong ridge
centered over the Midwest and end up over northern Colorado on
Friday and Saturday. There will be significant moisture in place
with forecast precipitable water values around 1.4-1.5" across
the I-25 corridor and plains. There are only 3 recorded soundings
in Denver that have had a precipitable water value above 1.5" so
if values were to be that high, it would be a remarkable event. A
shortwave trough associated with this system will provide forcing
along with upslope low level flow. This upslope flow will form
behind a cold front that will move through Friday morning. Highs
will be on the cooler side as a result as they will stay in the
upper 70s to low 80s across the plains. Model soundings indicate
decent instability with mixed layer CAPE values up to 2,000 j/kg
across the far eastern plains. This may allow for a couple severe
storms in Elbert, Lincoln, and Washington Counties Friday
afternoon with large hail and gusty winds the primary threats.
Further west along the I-25 corridor and foothills, sounding show
more of a long, skinny CAPE profile which is conducive for
efficient rain producing storms. The combination of near record
high moisture, good forcing, and instability, will lead to
widespread coverage of showers and storms Friday afternoon through
Friday night. The majority of the our CWA is forecast to receive
over an inch of rainfall from Friday morning through Sunday
morning. While plenty of locations will receive a welcome dose of
rain, the fear is that there will be areas of much heavier
rainfall that could lead to flooding. Ensemble guidance from the
ECMWF has a mean QPF of 1.5" at DIA and 2" at Fort Collins (FNL).
Some members exceed 3" of rain at both locations so the ensemble
guidance does indicate the potential for heavy rainfall and flooding.

Of particular note is the ECMWF Extreme Forecast Index which
shows a large portion of our forecast area having values of around
0.8 for QPF which indicates a high likelihood that a portion of
our forecast area will see near record daily rainfall values
during this event. The shift of tails shows a value of 2 which
means it is possible the rainfall records are broken by a wide
margin. This, unfortunately, all seems reasonable given the
threat for warm rain processes from the tropical moisture. As a
result, we will need a Flood Watch for portions of our forecast
area for this event. The reason that watches were not issued at
this time was due to the low confidence in the exact location of
the heavy rainfall. Models don't have much agreement for the
location of the heavy rainfall with some keeping the heavy rain
near Cheyenne and others keeping it as far south as El Paso
County.

The rain will last most of the night on Friday night before coming
to an end Saturday morning. If clouds break and instability is
able to build in, more showers and storms will form on Saturday.
There is some uncertainty, however, as to whether it will be too
stable for storms Saturday afternoon. Highs on Saturday will be
below normal with the mid 70s expected for a high in the Denver
metro.

A shortwave trough will move across our forecast area on Sunday
within northwesterly flow aloft. Meanwhile, a surface cold front
will push through the area providing low level lift. With plenty
of moisture still in place, scattered showers and storms will form
Sunday afternoon.

A ridge will develop over the Intermountain West on Monday and
will push eastward throughout the rest of the week. Conditions in
Colorado will dry out and warm up throughout the week. The 90s
may return to the plains by the end of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1134 AM MDT Wed Aug 23 2023

VFR to prevail through tonight and Thursday. Scattered showers and
storms will form west of Denver after 18Z and for the most part
stay west of the area through the evening. A stray shower (and
perhaps a storm) may produce an outflow causing a wind shift.
Thus, kept the tempo for variable winds in the TAF. Convection
dies off mid evening. As far as winds go, east to southeast winds
are expected to prevail through late evening. Wind speeds should
pick up after 21Z with gusts to around 20 knots. A wind shift to
the northwest is expected in the 04-07Z window tonight.

&&

.HYDROLOGY...
Issued at 300 PM MDT Wed Aug 23 2023

For the rest of today and Thursday, best moisture will be west of
I- 25 and more so in the mountains. Precipitable water values
will be around an inch across the plains, which is near normal.
For the mountains, the 0.75(in) and a little more is 130-170
percent of normal. Add in better instability as well for the
higher terrain and this will lead to the higher terrain seeing the
bulk of showers and storms. Some of the storms will produce heavy
rain. Storm motion will be a decent speed, so heavy rainfall
should be short lived. Chances for flash flooding over the burn
areas will be low/limited today and Thursday.

Significant moisture from a remnant tropical system will move into
Colorado on Friday and Saturday. As a result, storms could produce
very heavy rainfall as warm rain processes could develop. Rainfall
amounts exceeding 3 inches may occur which could lead to flash
flooding. This event will need a Flood Watch for Friday afternoon
through Friday night but confidence in the location of the
heaviest rainfall means that a watch will not be issued until
tonight or tomorrow. The recent burn areas, especially Cameron
Peak, will see a significant flood threat. Flood prone areas in
the foothills and eastern plains will also see a flood threat.
Please monitor the forecast for updates on the flash flood risk
and rain amounts for Friday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

SHORT TERM...Meier
LONG TERM...Danielson
AVIATION...Meier
HYDROLOGY...Meier/Danielson

Hazardous Weather Outlook
National Weather Service Denver/Boulder CO
1044 AM MDT Wed Aug 23 2023

COZ030>051-241645-
Jackson County Below 9000 Feet-
West Jackson and West Grand Counties Above 9000 Feet-
Grand and Summit Counties Below 9000 Feet-
South and East Jackson/Larimer/North and Northeast Grand/
Northwest Boulder Counties Above 9000 Feet-
South and Southeast Grand/West Central and Southwest Boulder/
Gilpin/Clear Creek/Summit/North and West Park Counties Above
9000 Feet-Larimer and Boulder Counties Between 6000 and 9000 Feet-
Jefferson and West Douglas Counties Above 6000 Feet/Gilpin/Clear
Creek/Northeast Park Counties Below 9000 Feet-
Central and Southeast Park County-
Larimer County Below 6000 Feet/Northwest Weld County-
Boulder And Jefferson Counties Below 6000 Feet/West Broomfield
County-
North Douglas County Below 6000 Feet/Denver/West Adams and
Arapahoe Counties/East Broomfield County-
Elbert/Central and East Douglas Counties Above 6000 Feet-
Northeast Weld County-Central and South Weld County-Morgan County-
Central and East Adams and Arapahoe Counties-
North and Northeast Elbert County Below 6000 Feet/North Lincoln
County-
Southeast Elbert County Below 6000 Feet/South Lincoln County-
Logan County-Washington County-Sedgwick County-Phillips County-
1044 AM MDT Wed Aug 23 2023

This hazardous weather outlook is for northeast and north central
Colorado.

.DAY ONE...Today and Tonight

Scattered showers and thunderstorms will develop over the
mountains this afternoon, with a few moving off the foothills in
the late afternoon and evening. Most of the showers will only
produce light rain, but wind gusts up to 45 mph are possible.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Scattered storms will develop over the mountains on Thursday while
the plains remain hot and dry. Moisture increases substantially on
Friday with widespread showers and thunderstorms expected to
develop Friday afternoon and evening. There will be a threat of
flash flooding Friday and Friday night especially over the burn
areas as well as flood prone areas on the plains. Saturday and
Sunday will see scattered afternoon storms. Conditions dry out and
warm up next work week.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today or tonight.