911 
FXUS65 KBOU 292014
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
214 PM MDT Tue Jul 29 2025

.KEY MESSAGES... 

- Heavy rain and flash flood threat remains over the Palmer Divide
  area into this evening.

- Cooler and more active weather pattern with scattered
  thunderstorms and locally heavy rain through Friday, 
  particularly east of the Continental Divide.

- Drier and warmer weather pattern begins this weekend and lasts
  through early next week. 

&&

.DISCUSSION /Through Tuesday/...
Issued at 213 PM MDT Tue Jul 29 2025

Satellite shows clearing of the stratus deck in most areas, but a
pretty healthy mid/high level cloud deck holding strong. The
greatest surface heating was over the northeast and east central
plains, where CIN was decreasing and MLCAPE ranged anywhere from
1000-3000 J/kg. The highest MLCAPE values were along/north of 
I-76, but that's also where the cap is strongest. ACARS still 
shows a pretty healthy cap as of 20Z, but it is weakening. We 
still think the highest risk of strong/severe storms would be over
Lincoln County where the weakest cap exists. The Palmer Divide 
and points east across the plains would be most favored for heavy 
rainfall - see Hydro Discussion below. Overall, we'll keep 
scattered thunderstorms in the forecast through this evening, with
a gradual decrease from west to east across the forecast area. 
Just a slight chance that a few showers/storms linger past 
midnight over the northeast plains. That would be further 
supported by passage of another push/weak cold front late tonight.

On Wednesday, the main concern will again be strong thunderstorms
with locally heavy rain and a flash flood threat. We will have a
frontal push in the pre-dawn hours, and potentially another
reinforcing surge during the morning. Thus, the highest instability
could very well be pushed up against the Front Range Foothills, 
while at the same time a stronger cap develops over the eastern 
plains. At this time, the greatest risk for heavy rainfall and 
flash flooding appears to be setting up for the foothills, Palmer 
Divide, and portions of the I-25 Corridor where the greatest 
instability/weakest cap is forecast. Again, more flash flood
threat details in the Hydro discussion below. Temperatures will 
be cooler than today with further cool advection and expected 
areas of stratus in the morning.

For Thursday, we could be looking at a repeat scenario with
scattered thunderstorms and locally heavy rainfall, as very little
changes synoptically. The upper level ridge holds overhead, with
weak shear but a moisture laden airmass. For Friday, the ridge
starts to break down with a little more westerly flow aloft. While
moisture values decrease slightly, we'll likely be unstable enough
to support scattered afternoon and early evening storms once
again.  

We'll see a stronger and more zonal flow aloft develop this
weekend as shortwave moves from the Great Basin through the
Central Rockies. This likely means drier weather and warmer
temperatures. While an embedded shortwave could bring a day of
thunderstorms Saturday, the additional drying/warming means
they'll be higher based and more conducive to gusty winds than
meaningful rainfall. Highs on the plains should rebound back to
90F or higher. Prognostications for early next week show flat
ridging according to the ensembles, so we'll have a drier 
forecast with temperatures at/above normal. 

&&

.AVIATION /18Z TAFS through 18Z Wednesday/...
Issued at 1149 AM MDT Tue Jul 29 2025

Stratus has been persistent due to continued low level moisture
advection and mid level cloud deck on top. There are more breaks
now showing up in surface observations, so we think a dissipation
or lift to SCT stratocu deck is expected through 19Z. Main chance
for thunderstorms is delayed slightly due to a stronger cap in 
place thanks to the morning stratus. At this point, the highest 
risk of thunderstorms still exists to the south and southeast, 
roughly KAPA and points S/SE, but still a PROB30 warranted for 
KDEN as storms will still be possible. Any stronger storms will be
capable of producing heavy rainfall and brief visibility 
restrictions. 

For tonight, we expect most of the storm threat to be over by
02Z-03Z, and then VFR conditions for awhile. However, another
surge is expected to arrive late tonight 09Z-12Z (slightly ahead
of average model guidance) which is expected to bring in more 
stratus and MVFR ceilings (~70% confidence) through much of 
Wednesday morning. Storm chances increase Wednesday afternoon 
again as the airmass destabilizes. 

&&

.HYDROLOGY...
Issued at 213 PM MDT Tue Jul 29 2025

Heavy rain threat exists through mid evening due to enhanced
precipitable water (PW) values at 120-140% of normal. Warm cloud 
depths are forecast to be near 7000 feet - a rather impressive 
value for these parts, even in the middle of summer. Storm motion
will hold under 15 mph in most cases, and sufficient MLCAPE where
stronger storms develop will be favorable for very heavy rainfall
(1-2 inches in 30 minutes). The main question for the rest of
today surrounds the amount of instability that can be tapped, with
a fairly well defined cap noted Denver northward. Given the above,  
we have the highest confidence of stronger storms over the Palmer 
Divide, so we'll maintain the current Flood Watch for flash 
flooding over Douglas, Elbert, and Lincoln Counties until 9 pm. 
There's still a threat that stronger storms with very heavy rain 
can make it into Denver and points north/east due to the weakening
cap/outflow interaction, so something to still watch closely 
through the mid evening hours. 

For Wednesday, the ingredients for flash flooding remain about 
the same (PW 120-150% of normal and 7000 feet warm cloud depth), 
although a stronger post-frontal upslope could anchor storm 
development and limit cold pool spread in/near the foothills. 
Thus, the overall threat of flash flooding Wednesday afternoon and
evening is expected to be greater in/near the foothills, Palmer 
Divide, and I-25 Corridor. We already have one Flood Watch for 
portions of this area going into this evening, so we won't issue 
another on top of that for now. However, Wednesday's threat looks 
to be greater so unless the late tonight/Wednesday morning surge 
is too strong to stabilize, we may very well issue another Flood 
Watch for flash flooding in coming updates. 

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 9 PM MDT this evening for COZ041-046-047.

&&

$$

DISCUSSION...Barjenbruch
AVIATION...Barjenbruch
HYDROLOGY...Barjenbruch