Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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111
FXUS65 KBOU 221659
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1059 AM MDT Fri Sep 22 2017

.UPDATE...
Issued at 1059 AM MDT Fri Sep 22 2017

Cold front has moved south to about I-70 this morning. Latest
radar trends show this front retreating back north, as the models
indicated will happen. May lower temperatures a few degrees over
northern areas where it is expected to remain north of the
retreating cold front.

Dew points are running a little higher at this time than expected
over the eastern plains. This may keep relative humidities above
Red Flag criteria, 15%. It will be a close call, so will keep the
Red Flag Warning in effect, especially with the gusty winds still
expected. This increase in moisture may bring a better chance for
storms to the eastern plains. Will watch and see if moisture
continues to increase. If so, may need to increase pops for later
today.

&&

.SHORT TERM...(Today and tonight)
Issued at 448 AM MDT Fri Sep 22 2017

Lots of things going on today as the upper trough to our west inches
closer. Shallow cold front trying to ram into the southerly flow
is moving into Denver at this hour. Models are in good agreement
in eroding this with better mixing this afternoon. Most switch the
winds around to southerly across our whole area, though there is
still some temperature gradient north of Denver. HRRR has a pretty
good depiction of this and is not too different from the larger
scale models, so the forecast was nudged toward this idea. One
thing the models may be a bit off with is the amount of low level
moisture. It does still look like this will increase as the day
goes on, but it is much drier in the dry air and the upstream
moisture is not that great and will be arriving at the time of
peak mixing, so it will be hard to drive the dew points up too
much. NAM MOS and HRRR do seem to capture what`s going on
reasonably well, the other guidance seems too moist. This means
lowering dew points a little in our forecast. So for both the
winds and moisture trended toward the drier and slightly windier
HRRR/NAM solutions for today. The much better moisture over Kansas
and Nebraska should stay there, so not much hope of diurnal
convection much west of the state line, and anything that does
form in the drier air should be weak. If there are storms, they
could accelerate the already strong winds.

Lead shortwave kicking out of the trough now across western
Wyoming and Eastern Utah will inch over the mountains this
evening. Even though it is a pretty sheared feature, it does have
some lift with it and should help pull the colder air down on the
plains in its wake late tonight. Not much moisture to work with
but the idea of at least scattered showers over the mountains and
a slight chance on the plains looks good. With little cooling from
this feature the snow level should stay above 10,000 feet.

.LONG TERM...(Saturday through Thursday)
Issued at 448 AM MDT Fri Sep 22 2017

Strong southerly flow aloft is expected Saturday and Saturday
night. The flow aloft decreases a bit and is south-southwesterly
on Sunday and Sunday night as the slow moving upper trough creeps
eastward. At 12Z Monday morning, there is an upper closed low
centered over the northwestern corner of Colorado with the upper
trough axis down into the desert southwestern United States. The
synoptic scale energy for the forecast area is all upward motion
Saturday through Sunday night. It is not as strong on these
latest runs compared to previous model runs. The lower level
pressure and wind fields are showing different things these latest
runs as well. Most of the models show southeast and southerly low
level winds for Saturday now, the NAM being the outlier out with
northerly upslope flow. This represents a significant difference
in temperatures on Saturday, Sunday too actually. The MOS
guidance`s show it too. At least all the models agree on Sunday`s
boundary layer winds, upslope northerlies, but not very strong.
Stronger upslope northerly winds are progged Sunday night.
Concerning moisture, it too is somewhat convoluted now compared to
previous runs. Precipitable water progs still show 0.50 to 1.10
inches from west to east on Saturday and Saturday evening. Values
dry out a bit on Sunday and Sunday night. The QPF fields continue
to show considerable measurable precipitation for all the CWA
Saturday through Sunday night. Will stay the course this package
and leave the "pretty high" pops going along with the amounts of
QPF. The snow level in the mountains will be around 8,500/9,000
feet Sunday night into Monday morning. The synoptic scale energy
on the latest models is not great and the upslope is a bit sketchy
for the plains and foothills, at least on Saturday into Saturday
night. We`ll see. The temperature forecast is a bit tricky. The
previous shift bought off on a warmer Saturday, which looks good
to me. Will go more with the GFS and ECMWF compared to the NAM for
Sunday`s highs. They will be 3-6 C cooler than Saturday`s.

For the later days, Monday through Thursday, models have the
upper closed low over Wyoming on Monday, then over North Dakota
early Tuesday. A piece of the upper trough moves across Colorado
later Monday, but the main trough continues to stay just west of,
or right over, western Colorado Tuesday and Wednesday. The GFS
brings the upper trough across Colorado Thursday and Thursday
night, the ECMWF does not. It actually shows upper ridging on
Thursday with a weak upper closed low and trough over the desert
southwest and southern Great basin. So basically out first change-
of-season-trough is a mess. One thing that seems reasonable, is
that our temperatures will stay below seasonal normals from
Saturday through Thursday. That is a significant change
considering there was a new record high at Denver on Thursday,
September 21, 2017 of 92 degrees.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1059 AM MDT Fri Sep 22 2017

Weak northerly winds will become southerly 18-19Z at KDEN as a
cold front retreats to the north. Southerly wind gusts to 30
knots will be possible at KDEN and KAPA through 00Z. At KBJC,
light northerly winds are expected through the afternoon. A cold
front will drop south across the area around 01Z and bring
northerly winds. VFR conditions will prevail through tonight. Mid
and low clouds are expected to move into the area Saturday morning
or early afternoon.

&&

.FIRE WEATHER...
Issued at 448 AM MDT Fri Sep 22 2017

One more day of hot/dry/windy conditions today. Humidities should
be a little higher today, but will still reach around 15% over the
plains south of Interstate 76. Winds may be a little stronger than
yesterday, and spread a bit further north into northeastern
Colorado than they did yesterday. A Red Flag Warning will be in
effect for the plains south of Interstate 76. Further north there
will be a bit less wind and higher humidities. It will be windy in
the mountains, especially south of Interstate 70 and over the
higher ridges, but the humidities will be a bit higher in these
areas as well. Much cooler and wetter weather starts Saturday
morning.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 7 PM MDT this evening for COZ241-244>247-
249.

&&

$$

UPDATE...Meier
SHORT TERM...Gimmestad
LONG TERM...RJK
AVIATION...Meier
FIRE WEATHER...Gimmestad



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