Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 212122

Area Forecast Discussion
National Weather Service Denver/Boulder CO
322 PM MDT Wed Sep 21 2016

.SHORT TERM...(This evening through Thursday)
Issued at 322 PM MDT Wed Sep 21 2016

The strong mid and upper level trough we have been watching for
the past several days is currently located across the Pacific
Northwest. It will dig south and west over the next 36 hours and
by Thursday evening it will be located over the northern half of
Nevada. The trough continues to advect mid level moisture into
Colorado while the low levels remain dry until overnight tonight.
The moist southwest flow will provide a slight chance of
thunderstorms in the mountains and across parts of the Plains the
remainder of the day. Early this evening the HRRR and NAM generate
showers over the mountains and and the HRRR has been consistent in
sustaining the showers across the urban corridor and then further
east across Morgan, Lincoln, and Elbert Counties. Both models did
the same thing yesterday under similar atmospheric conditions yet
nothing materialized. Will introduce 10-20 percent PoPs in those
areas but not expecting much if anything to reach the ground.

Tonight a cool front brings mid 50s dewpoint air in through the
backdoor across northeast Colorado after 3 AM. Stratus is possible
if not likely across northeast Colorado. Less confidence in the
stratus making it to metro Denver but it could come close to
Denver International Airport.

On Thursday strong southwest flow aloft continues and the
atmopshere is fairly moist with PWATs east of the Rockies in the
0.75-1.00 inch range. The good news is that surface dewpoints east
of the Rockies will be the highest they have been in many days.
The bad news is that the moisture is shallow, short lived, and
there is too little instability available to generate convection
of any significance. As the trough approaches Colorado the low to
mid level flow turns more south-southwest and will act to dry out
the lower levels from west to east starting Thursday evening. Best
chances for afternoon thunderstorms will be across the mountains.
Temperatures should be in the mid 80s most areas though if stratus
hangs on as long as expected, the north and northeast corner of
Colorado may stay in the 70s.

.LONG TERM...(Thursday night through Wednesday)
Issued at 322 PM MDT Wed Sep 21 2016

For Thursday night, we could see a couple high based showers/
storms drift northeast out of the mountains onto portions of the
adjacent plains, but overall airmass on the plains is still rather
dry and stable so only low PoPs warranted if at all.

By Friday, the unseasonably deep upper level low in the Great
Basin is expected to lift northeast across Utah. Upper level
support will increase but latest runs indicate some splitting of
this energy, and a couple models including the GFS are very
aggressive with the dry slot moving across the plains. At this
time it`s looking like the drier airmass should prevail as a very
strong and deep southerly flow develops across much of the
forecast area. Winds will increase with lowering humidities
increasing fire danger...see Fire Weather discussion below. There
is still enough lift for a few high based afternoon showers and
thunderstorms. Again, the best chance would be over the mountains
and far northern sections of the forecast area where a little more
moisture should hold in place.

Friday night, the upper level trough or at least a piece of it and
the cold front will move across the state. This will usher in
cooler temperatures and also bring some snow to the mountains. At
this time, looks like mainly light snow but some accumulation
would be possible mainly over the far northern mountains from
Rabbit Ears to Rocky Mountain National Park where deeper moisture
and stronger orographics exist. That same general weather pattern
will remain in place through Saturday with highs mainly in the 60s
on the plains and 40s/lower 50s in the mountains.

By Sunday, a few of the models are now indicating a second wave of
energy now dropping across the area in Northwest flow aloft. That
is in contrast to the shearing trough noted in yesterday`s model
suite. In any case, will maintain the low PoPs but trend temps a
bit cooler. If ECMWF verifies then we could see snow down through
most of the foothills.

For Monday and Tuesday, again in contrast to yestereday`s runs,
models now show the deeper upper level low developing into the
Midwest. Not sure exactly how this will eventually play out but
will keep temperatures cooler than normal with only a slight
chance of showers returning by Tuesday only if another wave drops
into the area on northwest flow. Wednesday should see a return to
near normal temperatures as long as upper trough moves to the


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 322 PM MDT Wed Sep 21 2016

|There is a slight chance of showers across the terminals this
evening that would temporarily cause shifting and gusty winds.
Around 12Z a backdoor front approaches KDEN. Winds behind the
front will be light but with high low-level moisture in place,
stratus is a concern. Not confident the stratus will make it to
KDEN but put in a TEMPO group to provide a heads up for the
potential. Aside from the low but nonzero chance of IFR ceilings
between 12 and 15Z, VFR conditions should prevail with diurnal
winds under 10 kts until late Friday afternoon when winds will
pick up from the south.


Issued at 322 PM MDT Wed Sep 21 2016

Fire danger will increase in all areas Friday as strong southerly
flow results in gusty winds. Anywhere from the Mountains east
across the Denver/Palmer Divide area onto the plains could see
gusts around 40 mph due to the strength of low level flow and
pressure gradient. Humidity is still marginal but if latest GFS
trends continue we`ll have to upgrade to a Fire Weather Watch.




SHORT TERM...Schlatter
LONG TERM...Barjenbruch
FIRE WEATHER...Barjenbruch is the U.S. government's official web portal to all federal, state and local government web resources and services.