National Weather Service Raw Text Product

309 
FXUS65 KBOU 310222
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
822 PM MDT Fri Jul 30 2021

.UPDATE...
Issued at 822 PM MDT Fri Jul 30 2021

...FLASH FLOOD THREAT REMAINS OVERNIGHT AND THROUGH SATURDAY...

Unfortunately, it looks like we're just getting started with this
heavy rain event. We have to get through the next 24 hours before
things may improve with regard to flash flooding. One large batch
of convection with heavy rainfall already spread across the 
Cameron Peak Burn area and then work southward across portions of 
the I-25 Corridor. That area has generally weakened over the last 
hour or so, but now more strong storms with heavy rain are moving 
slowly north from the Palmer Divide area. There are various 
solutions to this convective complex, and some are not good. The 
atmosphere continues to juice up this evening with likely a 
substantial increase in Precipitable Water. 00Z Denver sounding 
was up to 1.09", and that was before the surge of deeper low level
moisture. We're probably closer to 1.25-1.3" now, or near the 99th
percentile from historical perspectives. 

Thus, with all the favorable ingredients for heavy rain in place
including a deepening warm cloud depth and more efficient/warm
rain process, we could see flash flooding remain a problem along 
the Front Range overnight. That could even be enhanced by a 
mesoscale convective vortex which is hinted at by some of the Hi 
Res models like HRRR, while NAMNest produces some rather amazing 
rainfall totals of several inches along the Front Range. This 
gives us strong concerns for additional flooding. The one thing 
that's not as favorable for heavy rain is the current low level 
northwest wind off the foothills. We still can't be sure what 
evolves overnight, but certainly there's potential we're getting 
into a long-lived heavy rain event and dangerous flash flood 
threat for the Front Range. Flash Flood Watches will remain in 
effect for the Mountains, Foothills, and I-25 Front Range Urban
Corridor through the night, and all the way through Saturday 
evening.

&&

.SHORT TERM...(This evening through Saturday)
Issued at 300 PM MDT Fri Jul 30 2021

Main concern through tonight and all of tomorrow will be threat of
flash flooding. We're seeing storms with heavy rainfall
unfortunately developing in the burn areas this afternoon and that
will continue into this evening. We're currently loading up the
precipitable water across the mountains and Front Range, and that
will only increase through the evening with a surge of moist
north/northeast flow from a combination of convective outflows and
an embedded front pushing across the plains this evening. There is
still considerable uncertainty as to how this all unfolds and what
this means for heavy precipitation and flash flood threat.  

Convective allowing models (CAMs) have been all over the place and
understandably so. There is not much forcing and CAMs really
struggle with these scenarios. Thus, an ingredients based approach
to this forecast is optimal. Regarding those ingredients, we'll
have everything in place for locally heavy rainfall and a flash
flood threat, including; Precipitable water values reaching close
to the 99th percentile (around 1.30 inches for Denver), very slow
storm motions given lack of steering winds, warm cloud depth 
growing to around 6000 feet (but could be more if we fully 
saturate), and a push of weak but deepening upslope winds 
tonight. Stronger storms are already putting down 1-2 inches of 
rain in an hour, and intensities could get even higher with the
environment only becoming more favorable through late tonight and
tomorrow. One of the biggest questions surrounds the exact
evolution and when/if we get stronger convection on the adjacent
plains. Right now, we're still capped from the upper level ridge
and drying/subsidence noted on water vapor imagery, but this could
very well change by late evening with a north/easterly low level
push. If that's enough to spark convection, then the threat for
flash flooding would occur overnight in/near the Front Range.
Latest HRRR is trying to come around to this while Canadian has
been very consistent. Even the NAM/NAMNest are showing hints of
this potential.

If we don't get heavier rainfall tonight, then we may end up more
likely to get heavy rain and local flash flooding Saturday as we
destabilize slightly. If we do get more rain tonight, then we
could very well stay too stable to fire things up again (at least
across the plains) on Saturday. From above, ingredients are even
better tomorrow, depending on tonight's evolution. As a result,
the long duration Flash Flood Watch for this event from tonight
through Saturday evening seems appropriate. 

.LONG TERM...(Saturday night through Friday)
Issued at 159 PM MDT Fri Jul 30 2021

Precipitation should be on the downswing Saturday night, with 
just a few lingering showers expected during the overnight hours. 
Our moisture does decrease a little bit during the day on Sunday, 
with cooler and more stable air also expected. Still, PWATs are 
still near or above an inch, and there's just enough instability 
across the high country for another day of slow moving 
thunderstorms. Storm coverage should be a little lower than 
today/tomorrow, especially across Larimer county.

Monday could see another round of potentially heavy rainfall 
across most of our forecast area, with precipitable water amounts 
well above normal. There is some question as to how much 
instability will be available in the area, but heavy rain will be 
a distinct possibility across the Foothills and mountains. 

Tuesday appears to be our last chance for heavy rainfall before a
warming and drying trend begin. Mixing ratios are still 7g/kg and
PWATs above an inch, with models also suggesting better 
instability in the afternoon. Storm motions may be a touch faster 
overall, but there is certainly potential for additional heavy 
rains.

Warmer temperatures are expected for Wednesday and beyond. At 
least a little moisture is projected to remain trapped under the 
upper level ridge. Expect temperatures to slowly make it into the 
90s by next weekend, with slight chances or chances of afternoon 
storms each afternoon over the higher terrain.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 822 PM MDT Fri Jul 30 2021

Main concern will be showers and thunderstorms with locally heavy
rain, variable gusty winds, and reduced visibility and lower 
ceilings. Main threat of that would be from now through 06Z,
although some showers/isolated storms possible after that. If
showers persist, then lower ceilings will develop as well with a 
moistening of the boundary layer and possible MVFR conditions. 
Northwest winds should mostly prevail overnight before turning 
more northeasterly Saturday.

&&

.HYDROLOGY...
Issued at 822 PM MDT Fri Jul 30 2021

The threat of flash flooding will continue overnight and through
much of Saturday. Storm coverage will remain relatively high with
increasing moisture, and intensity of storms will be strong with
increasing rainfall efficiency due to moistening low levels and
warm cloud depths (up to 6000 feet, but possibly up to 8000 feet
depending on low level saturation). Stronger storms are already
producing 1-2 inches of rain in an hour, and thus a considerable 
flash flood threat. We may very well see locally heavy rain 
continue overnight along the Front Range, focusing in/near the 
foothills and I-25 Corridor. A very efficient warm rain process 
is expected as showers and storms are persisting. This coincides
with precipitable water values increase to the 99th percentile 
and a deep warm cloud depth/efficient rainfall process. We'll 
also develop a weak but deepening upslope overnight into Saturday 
morning which would help precipitation production and anchoring to
the Front Range. More widespread flooding potential would develop
if a mesoscale convective vortex develops which is indeed possible
given the existing amount of convection. 

That heavy rain and flash flood threat may very well last through
the day Saturday, as ingredients are even better tomorrow, 
depending on tonight's evolution. As a result, the long duration 
Flash Flood Watch for this event from tonight through Saturday 
evening seems appropriate. 

The flash flood threat should remain limited for most of our burn
areas on Sunday, with less moisture and more stable air overhead.
Nonetheless, PWATs will still be above normal and there should be
enough instability around in the high country to see isolated to 
scattered thunderstorms. Mean flow is still very weak, and slow 
moving thunderstorms still could produce briefly heavy rain across
the burn areas through the afternoon.

Moisture increases again on Monday, with more robust thunderstorm
development possible during the day. Storm chances continue 
Tuesday, with moisture decreasing only slightly. We'll have to 
watch both of these days closely as well, with mean winds still 
fairly slow. As noted in previous discussions, the repeated heavy
rainfall over several days could lead to much more sensitive soils.

A warmer and drier pattern begins to take over by the
middle of the week, which should decrease the flash flooding 
threat substantially.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Saturday evening for COZ030>041-043.

&&

$$

UPDATE...Barjenbruch
SHORT TERM...Barjenbruch
LONG TERM...Hiris
AVIATION...Barjenbruch
HYDROLOGY...Barjenbruch/Hiris