407
FXUS65 KBOU 060553
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1153 PM MDT Sat Sep 5 2026

.KEY MESSAGES...

- Locally heavy rainfall possible with a few strong thunderstorms through
  early Sunday morning for the far northeast plains.

- Slight increase in afternoon shower and thunderstorm coverage is
  favored Sunday and Monday for most areas.

- Temperatures will remain very warm, with only slightly cooler
  conditions starting Tuesday.

&&

.UPDATE...
Issued at 525 PM MDT Sat Sep 5 2026

Given ongoing convection across Logan County as well as the
latest CAMs, we have issued a Flood Watch through 3am Sunday for
far Northeast Colorado (Logan, Phillips, and Sedgwick counties). A
Severe Thunderstorm Watch is also in effect for these locations
through 9pm this evening. Storms are capable of producing gusty
winds and large hail (up to about 2 inches in diameter), with a
report of 1.75" hail in Sterling in the last 30 minutes.

The bigger concern through this evening and overnight will be the
flooding threat. Dew points in Logan, Sedgwick, and Phillips
counties are currently in the upper 50s to low 60s. Moisture is a
bit more impressive than originally modeled. As a result, the
environment has been overperforming so far this afternoon with
rainfall totals already as high as 1.30 inches in two hours in
Logan County. Mean cloud layer winds are expected to slow this
evening with multiple rounds of showers and thunderstorms
possible- increasing the flooding risk. The majority of CAMs are
now showing that isolated locations in the Flood Watch could
receive 3 to 5 inches of rainfall this evening.

&&

.DISCUSSION /Through Saturday/...
Issued at 1153 AM MDT Sat Sep 5 2026

Midday cumulus development is more expansive today, suggestive of
the modest increase in mid-level moisture regionwide. Near surface
moisture however is much more variable, with dewpoint fields
exhibiting a considerable zonal gradient between the I-25
corridor (~40-45F) and our eastern plains (~58-63F). Given
limited forcing, meager instability and drier low-levels, areas
from the urban corridor westward will be limited to largely
isolated high-based shower activity late this afternoon and
evening, producing brief localized gusts of 25-35 mph. Over the
eastern plains, the warm and moist advection regime will hold
steady through tonight, leading to an increasingly buoyant
environment with MLCAPE values of 1,500-2,500 J/Kg present across
our northeasternmost counties by late afternoon. Thunderstorms
look to develop come this evening, with the bulk of the activity
favoring the late evening period coinciding with a moderate LLJ
setup over the plains. A few will be strong to perhaps marginally
severe, with an initial wind and hail threat that will
nonetheless be tapered by the lack of shear, limiting overall
storm organization. Given the clustering potential and abundant
moisture (precipitable water values approach 1.5" this evening),
locally heavy rain can be expected with any storms.

We`ll gain a touch more moisture still on Sunday as moist
southerly flow continues and the trough to our west inches
slightly closer. The urban corridor also looks a little better
positioned to tap into higher surface dewpoints, supporting
modestly unstable conditions for the afternoon. Thus, anticipate
an increase in afternoon shower activity as well as potential for
actual convection. There will be minimal change as far as
temperatures are concerned though, with most of the lower
elevations breaking into the lower 90`s once more.

A slight waning in the high pressure on Monday will allow for the
intrusion of stronger northwest flow at upper levels and thus some
actual forcing, which should support a slight uptick in coverage
of showers and thunderstorms, and likely the best precipitation
chances of the next few days. Unsurprisingly, temperatures will
continue to hold mostly steady.

At risk of sounding like a broken record, ensemble guidance still
indicates a flattening of the ridge starting Tuesday (though more
pronounced Wednesday), followed by a westward shift of the ridge
axis over Colorado. Slightly cooler temperatures - albeit still
above - should result from the initial flattening and transition
to zonal flow, but this trend appears increasingly transient in
nature, with ensemble guidance favoring a return to hotter weather
for the second half of the week. As previous discussions have
alluded to, confidence in precipitation potential remains lower,
although we still look to retain at least some moisture beneath
the ridge and thus a few opportunities for afternoon showers and
thunderstorms.

&&

.AVIATION /06Z TAFS through 06Z Monday/...
Issued at 1150 PM MDT Sat Sep 5 2026

As of 0545z, radar shows a rather small thunderstorm located over
KAPA and drifting toward the vicinity of KDEN. It has produced
wind gusts of 20-25 kts. Have kept a PROB30 until 7z at KBJC for
light showers and at KDEN for the aforementioned thunderstorm
located near KAPA.

Winds tonight are a bit uncertain, but it appears winds will
generally shift from NW to SW through the overnight. Wind speeds
should be under 12 kts after any showers/thunderstorms have ended
over the next 1-2 hours.

For Sunday, winds turn clockwise to ESE by the afternoon. With
better instability and moisture than today, there is a 30 percent
chance for thunderstorms late Sunday afternoon into Sunday
evening after 22Z (21z at KAPA and KBJC). These thunderstorms
could produce wind gusts of around 30 kts. Depending on any
residual outflow boundaries, winds likely turn to drainage late
Sunday evening.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 3 AM MDT Sunday for COZ048-050-051.

&&

$$

UPDATE...MV
DISCUSSION...BRQ
AVIATION...MV

NWS BOU Office Area Forecast Discussion