661 
FXUS65 KBOU 171822
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1222 PM MDT Thu Sep 17 2026

.KEY MESSAGES...

- Active weather pattern through Saturday. Locally heavy rainfall
  will be the greatest storm threat, although a couple severe
  storms possible Saturday. 

- Near or just above normal temperatures settling below normal for
  Sunday and Monday. 

- Trend toward drier weather by early next week.

&&

.DISCUSSION /Through Wednesday/...
Issued at 311 AM MDT Thu Sep 17 2026

Satellite shows a large plume of monsoonal moisture lifting
northeast from the Desert Southwest across much of Utah, Colorado,
and Wyoming this morning. A couple shortwaves were noted in the
flow, with one lifting northeast through southwest Utah, another
in east central Arizona, and yet another in west central New
Mexico. Rain and thunderstorms were fairly widespread ahead of
these features, although much of it continues to dry as it moves
across the numerous mountain ranges on it's way to the Colorado
Front Range and plains. That said, today is shaping up to be a 
more active weather day as precipitable water values climb to 
record values for this late in the season, and close to 225% of 
normal. That means locally heavy rainfall will be possible with 
showers and storms today. The main question is how widespread 
they will be, and that depends mostly on two factors; 1) the 
extent of mid level clouds and 2) the timing of best lift 
associated with the passing shortwaves. More on that in just a 
bit. Right now, it also appears the best lift may now stay to our
west and eventually north in the proximity of the upper level 
speed max, and thus we're unsure of shower/storm coverage over the
plains.

We do think all of the mountains will see measurable rainfall 
being closer to the lift and without the stronger downslope 
effects, so periods of light to moderate rain can be expected 
there through early afternoon. On the plains, a few light showers 
linger in these pre-dawn hours, but can't rule out an isolated 
storm near the Wyoming border in the area of best mid level 
moisture/theta-e advection. Then it appears we're in for a break 
and possible slight break-up of some of the mid level clouds by 
mid morning. Then those thicken again as the above mentioned 
shortwave(s) and deeper moisture move in. Most models have some 
light rain attempting to work into the lower elevations as this 
deepest moisture arrives very late this morning into mid 
afternoon, but that would also limit instability. It could very
well end up taking some late day breaks or deeper convection 
initiation over the mountains to start any storms with heavier 
rain that could potentially impact the I-25 Corridor. East of the 
I-25 Corridor, we do try to destabilize with potential for a few 
more peaks of sun, so the higher chance of storms end up there 
late this afternoon and evening. Also of note, there will be 
stronger low level moisture convergence along a warm 
front/boundary extending roughly along/north of a line from 
Greeley to Akron, so that would favor areas from there to the 
north and east for convective initiation. 

All in all, it appears the highest threat for locally heavy rain
today would be in the mountains and higher foothills, and then
farther northeast across the plains. However, it should be noted
that given the favorable parameters (record PW for this time of
year and warm cloud depth 3500-4500 feet), locally heavy rainfall
will occur with any storms. They will be fast moving (25-30 mph on
average) so the threat of any significant flash flooding is on the
lower end. We do think local minor flooding and a couple mountain
rockslides would still be possible. WPC Excessive Rainfall 
Outlook highlights this rather well with the greater (Slight Risk)
over southwest and west central Colorado and also farther north 
toward southeast Wyoming into the Nebraska Panhandle. 

Friday could end up being more active along the I-25 Corridor
versus today. PW values drop slightly, but will still be 160-220%
of normal. Also, instability should have a chance to grow slightly
with a little more sunshine. Therefore, we expect another round of
scattered afternoon and evening thunderstorms this time
originating over the mountains and pushing east onto the I-25
Corridor and adjacent plains in the afternoon and evening.  

Saturday could end up being the most active weather day. While PW
is forecast to drop off, it will still be above normal and
instability is forecast to grow further with more sunshine and 
some cooling aloft and steepening lapse rates. MLCAPE may surpass 
1500 J/kg on the eastern plains, and plenty of shear is forecast 
with rather strong mid level flow. Thus, a couple severe storms
will be possible. In addition, we'll have some synoptic scale 
lift as an upper level trough approaches from the northern Great 
Basin and northern Rockies. 

While Sunday is a transition day with the expected trough 
passage, depending on it's speed we will have a chance at more 
showers and thunderstorms. Temperatures will be turning cooler 
with cool advection (most likely highs in the lower 70s for the 
plains). But, if front is a little earlier then we could see just 
upper 60s. 

As mentioned in yesterday's discussion, Monday into Tuesday will
be turning drier. However, we still expect isolated weaker
convection. Models remain consistent showing another surge of 
monsoonal moisture by late Tuesday or Wednesday, which would 
result in an uptick of shower/storm coverage. 

&&

.AVIATION /18Z TAFS through 18Z Friday/...
Issued at 1221 PM MDT Thu Sep 17 2026

Showers have started popping up over the foothills this morning, 
with several skirting over KBJC. Throughout the afternoon and 
evening, we expect the greatest threat for showers/heavy rain to be 
over the foothills and northeastern plains, though we can't rule out 
some scattered showers or storms over the metro area. While there's 
not much instability, plenty of moisture has been brought in from 
the south, and so, should any showers or storms move over the TAF 
sites, locally heavy rainfall would be the main concern. Still, 
confidence regarding storms over the TAF sites is lower than it was 
yesterday, so we've dropped the previous TEMPO group down to a 
PROB30 for this afternoon and evening. Most models keep 
precipitation in the area through at least 02Z, with rainfall 
lingering a little longer in the vicinity of KBJC. As showers leave 
the area, winds should be light and variable before settling into 
drainage overnight.

The late afternoon/evening tomorrow will be our next best chance for 
storms/showers. As such, we've introduced a TEMPO starting at 22Z 
and lasting through the end of the TAF period; that being said, 
model guidance currently suggests the bulk of convection to be later 
(23Z-03Z), so we'll likely be extending the TEMPO group at the 00Z 
TAF update this evening. 

&&

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

&&

$$

DISCUSSION...20
AVIATION...AGS