Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 240955
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
355 AM MDT Tue Apr 24 2018

.SHORT TERM...(Today and tonight)
Issued at 355 AM MDT Tue Apr 24 2018

A 500 mb trough north of Colorado has a pool of cold air aloft
with temps around -23 degC. The trough axis is currently nearly
oriented west to east across the northern half of Wyoming.
GOES-16 water vapor imagery shows the trough nicely with strong
drying/subsidence on the backside of the trough occurring in a
dry conveyor belt extending from Oregon and Idaho southeast into
northern Wyoming. The deformation zone ahead of and just south of
the dry conveyor belt marks the back edge of the moisture and
lift, and it`s slowly sagging south into northwest Colorado this
morning. A 70 kt westerly jet is south of the deformation zone
across northern Colorado. The left exit region of the jet is
combining with weak synoptic lift and frontogenesis in the
800-600 mb layer to generate banded precipitation across the
plains of Colorado this morning. The bands are slow moving
southeast this morning, impacting metro Denver and areas to the
northeast presently. The band should exit metro Denver before
sunrise and the rest of our area by mid-morning. Elsewhere lighter
precipitation is occurring and should continue through at least
mid morning with plenty of moisture and weak lift around until the
dry air from the northwest works it`s way into our area. Web bulb
temperatures across the plains are between 31-34 degF. Webcams
and dual- pol radar are suggesting mostly snow at this time
associated with the main precipitation band(s). But with the warm
wet bulb temperatures and still warm road surfaces, roads will
remain wet through the morning commute. Snow in the mountains will
be limited with up to three inches accumulation expected because
of weak moisture west of the Divide and poor orographics given
northerly flow at 700 mb. Highs in the mountains today will be
much cooler than yesterday with highs in the 30s to upper 40s.
Across the plains, we expect ample cloud cover much of today and
continued cold advection, bottoming out this afternoon around -7
degC at 700 mb. Combined with north winds 10-20 mph, this means a
raw and dreary day is in store across the plains of Colorado with
high temps only reaching the low to mid 40s with wind chills 30s.
Interestingly the HRRR breaks out diurnal showers this afternoon
above the cold air trapped near at the surface across the Plains,
primarily across the central mountains and Park County. Will
retain low PoPs for this possibility, and and precipitation should
be light. The showers would extend across the Palmer Divide and
into Lincoln County with a snow level around 6500 feet MSL.
Elsewhere east of I-25 and the rest of the northeast Plains should
remain dry today.

Tonight the 500 mb trough slowly moves into the Central Plains
northeast of Colorado with continued westerly flow aloft.
Increasing subsidence and much drier air from the west will lead
to clearing skies overnight. Surface high pressure settles in as
well resulting in much weaker winds and nice radiational cooling
conditions area-wide. Expect cold temperatures and a hard freeze
across nearly all of our area, though metro Denver`s urban heat
island may keep temperatures in the 30-32 deg range.

.LONG TERM...(Wednesday through Monday)
Issued at 355 AM MDT Tue Apr 24 2018

An upper level ridge will build over the area Wednesday. There
may be lingering low clouds and possibly fog Wednesday morning,
mainly over the eastern plains before the ridge builds in. The
airmass will be quite dry Wednesday so once any low clouds or fog
burn off, sunny skies will prevail. Temperatures will be near to
slightly below normal with highs reaching the upper 50s to lower
60s.

On Thursday, an upper level trough embedded in the northwest flow
aloft will race across northeast Colorado early Thursday. The
cold front associated with this system will drop south across
eastern Colorado, early Thursday morning before sunrise. Low level
moisture will increase behind the front and low clouds should
spread across eastern Colorado. Not much moisture associated with
this system. So any precipitation should be light. Will have the
highest pops over the foothills where northeast flow behind the
front will provide low level lift. The cold air and moisture is
shallow with this system and is not expected to make it over the
Front Range mountains. Mostly cloudy skies will keep temperatures
cool with readings topping out in the lower to mid 50s. Skies
clear Thursday night. This will result in chilly overnight lows.
Can`t rule out patchy fog, mainly in low lying areas.

Upper level ridge rebuilds over the Central Rockies Friday with
dry and mild conditions prevailing. This ridge shifts east of the
state Saturday and a low moves onto the west coast. Flow aloft
over Colorado will become southwesterly. This will usher in warmer
conditions for the weekend. Highs are expected to top out in the
70s both days across northeast Colorado. Models show a short wave
trough embedded in the southwest flow aloft which produces
scattered/isolated afternoon showers and thunderstorms. The GFS
shows this happening Saturday while the ECMWF and Canadian models
have this activity occurring Sunday. Will have low pops in the
forecast for this.

For Monday, models generally agree with the large scale pattern
which is troughiness over the west and southwest flow aloft over
Colorado. This should bring above normal temperatures. The amount
of moisture and if there will be a short wave trough is still
uncertain. For now, will keep the mention of showers and
thunderstorms out of the forecast, except for the mountains.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 355 AM MDT Tue Apr 24 2018

Light snow across the metro area terminals will move southeast by
12Z. From now until then reduced visibility/CIGs in the IFR
category can be expected. By 11Z conditions at BJC and DEN should
improve quickly, especially in regards to VIS once the snow moves
out of the area. It will take longer to improve at APA with
upslope flow but should be better after 12Z. A slight chance of
snow accumulation on grassy surfaces but concrete surfaces will
remain wet this morning.  After 12Z conditions should improve to
MVFR and expected to stay that way today with a stratus deck
holding strong. Winds have come down significantly and will remain
light today with north and northeast around 12 kts or less.

&&

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

$$

SHORT TERM...Schlatter
LONG TERM...Meier
AVIATION...Schlatter



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