Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 221546
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
946 AM MDT Tue Aug 22 2017

.UPDATE...
Issued at 945 AM MDT Tue Aug 22 2017

Only minor changes made to today`s forecast. Feel the HRRR, RAP,
NAMnest are a tad overdone with convection east of I-25. PWATs are
very low currently and the low-level moisture should mix out in
the next few hours so have trimmed PoPs east of the foothills.
There is also a fair amount of synoptic scale subsidence across
eastern Colorado which would strengthen the cap and inhibit
convective potential. Did not touch the temperatures because
those look on track for low to mid 80s across the Plains and upper
60s to near 80 in the mountains. Convection across Park County
and the East slope and southwest foothills continues to look like
a safe bet but do not expect the storms to survive east of those
areas with weak PWATs and mixing out low- level moisture. Impacts
from the storms would be brief heavy rain, cloud-to-ground
lightning, and gusty winds to 35 mph.

The stratus across Larimer, Weld, and east of metro Denver will
continue to slowly erode this morning. Expect skies to scatter out
in those areas by noon or 1 PM.

&&

.SHORT TERM...(Today and tonight)
Issued at 350 AM MDT Tue Aug 22 2017

Latest GOES-16 imagery shows areas of stratus becoming more
widespread early this morning in light but moist easterly flow.
These should stick around through at least mid morning although
some breaks will also be noted. Stratus should then break up late
this morning through the noon hour with daytime heating and
mixing. With regard to fog, closest T/Td spreads are over the
plains roughly Elbert/Lincoln counties to just east of KDEN and
then to near Fort Morgan and Akron. Will continue to mention areas
of fog in those locations through early morning, but overall
visibilities should hold up in most areas as stratus cloud deck
is becoming more widespread, limiting surface cooling.

Attention this afternoon will then turn toward potential for
thunderstorms. The highest coverage will develop this afternoon
along the interface between the low level moisture and best
warming (e.g. The Front Range and Park county). However, there is
also a threat of a couple stronger thunderstorms developing
Denver convergence zone. Where that zone develops all depends on
extent of morning stratus and differential heating through the
afternoon. At this time, it is most likely to set up across the
southern portions of the Denver metro area eastward toward DIA, so
have opted to push the thunderstorm chance a bit farther eastward
to account for this late in the afternoon and evening. There is
also weak Q-G lift noted which may help weaken the cap and help
convection develop onto the nearby adjacent plains. Farther east
it is still too stable to support any storms.  CAPES are
advertised to reach 700-1400 J/kg with T/Td of mid 80s/~50, so
stronger storms would be capable of producing heavy rain, gusty
winds, and some hail. Shear profiles are too weak to support any
organized severe threat. Only partial clearing expected overnight
with mid/upper level moisture plume pushing northeast across the
forecast area.

.LONG TERM...(Wednesday through Monday)
Issued at 350 AM MDT Tue Aug 22 2017

On Wed there appears to be a weak system moving across the area in
the aftn and evening hours.  Thus should see a better chc of showers
and storms.  As for highs readings will be in the 85 to 90 degree
range over nern CO.

By Thu the flow aloft will be mainly zonal with some subtropical
moisture affecting the area.  The ECMWF has one weak disturbance
moving across ern WY while another feature tracks across srn CO.  As
a result nrn CO is in between these two features.  However there
still should be a chc of aftn and evening storms.  As for highs
readings will remain in the mid to upper 80s over nern CO.

On Fri the flow aloft will become more wnw as an upper level high
begins to build fm the swrn US into the Great Basin.  Cross-sections
show decreasing moisture over the rgn so expect less tstm activity.
Aftn highs will be in the 85 to 90 degree range across nern CO.

For the weekend the flow aloft will become more nwly on Sat.  The
ECMWF shows a weak feature embedded in the flow along with an
increase in moisture thus may see a better chc of storms across the
plains in the aftn with wdly sct activity over the higher terrain.
By Sun and Mon the flow aloft will become more nly as an upper
level high strengthens over the Great Basin. Thus will see less
moisture and lower chances for tstm activity. Temps will be in the
mid to upper 80s over nern CO each day.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 945 AM MDT Tue Aug 22 2017

Convective potential is looking less likely across the terminals
this afternoon, and will keep a close eye on the dewpoints to see
if the trend continues. We expect them to mix out into the upper
30s to near 40 over the next few hours and if that happens, there
would be a much reduced chance of convection across the terminals.
If on the other hand the low 50s dewpoints at the surface do not
mix out and a Denver Convergence zone sets up just west of KDEN,
then convection is possible with gusty winds near Denver after 21Z.
For now, leaning towards no TS at DEN but will keep a close eye
for signs that the less likely more moist scenario unfolds early
this afternoon. Flow at DEN and APA should be south and southeast
today 10-15 kts the remainder of the day. BJC could be east of
the Denver Convergence zone and thus would see north or northeast
and light flow this afternoon.

In other news the stratus deck continues to be east and north of
DEN and this should continue to slowly erode in those areas. It
will not impact the airfield, just approaches from the north and
east into DEN. No ceiling/VIS issues expected today otherwise at
any of the TAF sites. Stratus development tonight looks unlikely
and drainage winds should occur overnight. Much better chance for
storms Wednesday afternoon.


&&

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

$$

UPDATE...Schlatter
SHORT TERM...Barjenbruch
LONG TERM...RPK
AVIATION...Schlatter


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