Johnstown, Colorado 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

176
FXUS65 KBOU 161720
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1120 AM MDT Wed Jul 16 2025

.KEY MESSAGES...

- Conditional severe storm threat across the I-25 corridor and
  plains on this afternoon and evening, but confidence remains low.

- Active pattern continues into the weekend, with scattered
  afternoon showers and storms each day. Best chance of rain
  across the Front Range/Foothills and South Park most afternoons.

- Trending warmer and drier next week.

&&

.UPDATE...
Issued at 216 AM MDT Wed Jul 16 2025

Still not really sure what to make of today`s severe weather
threat across the I-25 corridor and adjacent plains. Surface
moisture has increased substantially over the past several hours,
with low/mid 60s Td observed in the post-frontal airmass.
Mesoanalysis reveals >1000 J/kg of MUCAPE in this early morning
hour, but so far convection has remained over Wyoming and most
guidance keeps that trend through the rest of the morning hours.

Meanwhile, stratus has developed quickly across the plains
overnight... but mainly north of US-34 so far. Guidance remains
relatively consistent with stratus filling in across the rest of
the plains towards daybreak this morning, then lingering through
the mid/late morning hours.

The evolution of the stratus deck will likely have considerable
impacts on the overall severe threat today. Mid/high cloud cover
will quickly advance into the forecast area by late morning or
early afternoon, leaving a limited window for surface
heating/destabilization. It will be difficult to get sufficient
destabilization across the entirety of the I-25 corridor... but
fairly impressive shear profiles would support a few organized
multicells/supercells with a hail/wind threat if the more unstable
solutions pan out. My thoughts haven`t changed much since
yesterday... and the severe threat remains highly conditional.
Regardless, the moist upslope flow and passing shortwave should
still generate widespread showers and thunderstorms this afternoon
and evening, and should linger well into the evening/overnight
hours.

For Thursday and beyond, little else was changed from the
previous/NBM forecasts. The cooler post-frontal airmass Thursday
will lead to less coverage of showers and storms (generally
confined to the higher elevations). A better plume of moisture is
expected to return by Friday and Saturday. A warmer/drier trend
looks likely for next week.

&&

.DISCUSSION /Through Tuesday/...
Issued at 350 PM MDT Tue Jul 15 2025

Current satellite and radar imagery show a few different things
going on over our area. First, the upper level shortwave is
trekking over southern Wyoming, which has generated some mountain
convection over the Cheyenne Ridge and northern Front Range
mountains. However, as these showers/storms have spilled onto the
northern plains, they have quickly dissipated. This is due to the
cold front arriving a few hours early, bringing in a more stable
environment. In addition, mountain convection has initiated over
the southern Front Range mountains, producing brief gusty winds
at the surface. Modeled Skew-T soundings have steep lapse rates of
~9 C/Km and high LCLs over the higher elevation, so main threat
will continue to be gusty winds up to 40 mph this afternoon.

With the early arrival of the cold front, the overall severe
weather threat has decreased for the rest of the afternoon.
SPC mesoanalysis page shows 500-1000 J/kg of MLCAPE over the
northern/northeastern plains, with instability increasing in the
next few hours. However, with decent CIN settled over the area and
without the cold front to provide enhanced lift, storms that do
generate over the northeastern plains are expected to be sub-
severe at best, with the possibility of small hail and gusty
winds.

Wednesday looks to be more interesting severe weather wise as
another shortwave trough crosses the region- although it will be
conditional on how the morning evolves. With increased moisture
advection, a thick stratus deck is expected by Wednesday morning.
Deterministic models have dew points in the high 50s to low 60s,
so there is a chance for localized foggy conditions, especially
along the foothills. Guidance is in general consensus of 1000-2000
J/kg of MLCAPE along with 40-45 kts. of effective bulk shear,
which would favor supercell and multicell development with all
threats possible. However, the low clouds in the morning could
inhibit surface heating/instability, thus hindering the severe
weather threat. Another factor to keep in mind is the smoke from
the fires out in the west. Even if clouds clear out in the
morning, smoke in the atmosphere could keep the air slightly
cooler than their convective temperatures. Lastly, it is
important to note that if storms are able to develop, there will
be localized heavy rain, which could bring a threat of flooding.

Scattered light showers are expected Thursday, with temperatures
hovering around the mid 80s for most of the plains. With overall
lack of shear and instability, the severe weather threat is low.
By Friday, an upper level ridge will amplify over the southern
United States, bringing warmer and drier air back into our area.
Ensemble guidance is in good agreement of multiple shortwaves
traversing the area over the weekend, with increased chances of
scattered afternoon showers and storms in the afternoon hours.

&&

.AVIATION /18Z TAFS through 18Z Thursday/...
Issued at 1116 AM MDT Wed Jul 16 2025

Stronger tstms this aftn will depend on heating and it`s really
a 50/50 probability if that occurs or not. Have left tempo
between 21z and 01Z for tstms. Lower have scattered out wit just
a mid level deck now.  Winds will stay E or ESE thru the aftn.

Overnight, there will be a weak Denver Cyclone but location isn`t
certain at this point. If winds go W or NW at DIA late tonight
can`t completely rule out some stratus in the 11z to 15Z time
period. Winds tonight will become more southerly and then go
more variable in direction by 14Z Thu.

&&

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

&&

$$

UPDATE...Hiris
DISCUSSION...Ideker
AVIATION...RPK