312 
FXUS65 KBOU 120044
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
644 PM MDT Mon Sep 11 2023

.UPDATE...
Issued at 636 PM MDT Mon Sep 11 2023

Drying and the loss of diurnal heating has brought most of the
showers to an end with the convective cloudiness starting to
decay. There are still a few light showers around Park county, and
the residual clouds from there out towards Limon will take longer
to decay, but there will be a lot of clearing from Denver north
over the next couple of hours.

Not much change in the thinking on fog in the morning. There will
probably be some on the north side of a developing Denver cyclone.
Weld county most likely, but it could develop in surrounding areas
as well, and then try to advect into Denver as it lifts during the
morning. It's not clear if we'll cool enough for the fog to be
widespread though, or how long it will take for the moisture to
mix out.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 208 PM MDT Mon Sep 11 2023

Drier air will continue to move into the area from the north with
skies becoming clear this evening. Prior to this, there is enough
instability (CAPE 200-500 J/kg) for scattered showers to continue
through the afternoon. There's even been a few weak storms over 
the higher terrain. As temperatures begin to cool early this 
evening, the airmass will stabilize and bring the showers to an 
end around sunset. Mostly clear skies, moist boundary layer, and 
weak winds are expected to lead to areas of fog, mostly in the low
lying areas to the north of Denver.

For Tuesday, weak ridging in the westerly flow aloft will track 
across the region. Mild and dry conditions will return under this 
pattern. Highs will climb into the mid to upper 70s across 
northeast Colorado. 

.LONG TERM...(Tuesday night through Monday)
Issued at 208 PM MDT Mon Sep 11 2023

Cooler temperatures are expected all week into the weekend, with 
another batch of widespread rainfall Thursday and Friday.

Models are showing upper ridging over the CWA Tuesday night.  The 
upper ridge flattens out Wednesday into early Thursday.  An upper 
trough moves into western Colorado on Thursday morning and moves 
slowly east across the CWA well into the later days. There is upward 
vertical velocity in the synoptic scale Tuesday night through 
Thursday night and even on into the later period.  The low level 
winds should adhere to normal diurnal patterns into Thursday 
evening. Models show a cold front move into the CWA late Thursday 
night.
 
Looking at moisture, precipitable water values are in the 0.50 to 
0.80 inch range Tuesday night into Thursday morning.  Moisture 
increases with the next upper trough, with the PWs increasing a
bit into the 0.50 to 1.10 inch range Thursday and Thursday night.
There is limited MLCAPE during the late day periods each day, but
nothing substantial. I pared back some pops for Tuesday evening. 
Kept in the "likely" pops in for the alpine areas late day 
Wednesday. Left the high pops going in most areas Thursday 
afternoon and night with the incoming trough.

For temperatures, Wendesday's highs are close to Tuesday's values. 
Thursday's highs are 2-5 C cooler than Wednesday.

For the later days, Friday through Monday, models have the slow 
moving upper trough over the CWA Friday well into Saturday. Upper 
ridging moves in for Sunday and Monday.  Will keep the high pops 
going into Friday evening with scattered late day alpine pops on 
Saturday. Little to no pops Sunday and Monday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 636 PM MDT Mon Sep 11 2023

Areas of fog will develop north of Denver after 09z, with 
IFR/LIFR ceilings and visibilities through about 15z, dissipating 
by 18z. There is a chance that low clouds could move into the 
Denver area from the north, producing a few hours of MVFR or IFR 
ceilings at KDEN. The main risk of this is between about 14z and 
17z. There is a slight chance, about 20%, that north winds will 
develop earlier and there could be LIFR fog at KDEN between 11z 
and 14z. VFR conditions are expected outside of this period of fog
development, with only isolated showers/storms over the mountains
Tuesday afternoon.

&&

.HYDROLOGY...
Issued at 208 PM MDT Mon Sep 11 2023

No hydrology issues through Tuesday. There will be scattered 
showers with a few weak storms over the higher terrain through 
this afternoon. Brief moderate rain is not expected to produce any
flooding issues. 

There will be a limited threat of flash flooding over the burn
areas Wednesday through Friday, mainly during the afternoon and 
evening time frames. 

&&

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

$$

UPDATE...Gimmestad
SHORT TERM...Meier
LONG TERM....RJK
AVIATION...Gimmestad
HYDROLOGY....Meier/RJK