Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 191842
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1242 PM MDT Wed Apr 19 2017

.UPDATE...
Issued at 955 AM MDT Wed Apr 19 2017

Shift to gusty west/northwest winds has been delayed a couple
hours by surface anticyclonic flow that developed early this
morning. Also, GOES 16 data shows a secondary short wave in
northwest Colorado which is slowing the onset of stronger
subsidence and thus winds. We still expect winds to increase and
become gusty this afternoon as subsidence and stronger mixing
occurs, so overall forecast was just nudged to slow the onset. The
winds may also end up with more of a northerly component based on
latest surface analysis and pressure trends. In the mountains and
high valleys, rain and snow showers (snow level near 8500-9000
feet) are ongoing and will continue into the afternoon with
sufficient instability and orographic forcing. Finally, later
moisture departure on the plains may allow some rain to reach the
ground so added some low PoPs in the northeast corner of the state
for the afternoon.

&&

.SHORT TERM...(Today and tonight)
Issued at 240 AM MDT Wed Apr 19 2017

Strong short wave trough currently moving across Wyoming will
track northeast into the Northern Plains later today. Subsidence
on the back side of this system will bring dry conditions to the
Front Range and eastern plains. Surface low pressure trough over
the eastern plains will move east of the state this morning.
Behind it, gusty west to northwest winds will develop. Steep
lapse rates will lead to good mixing and allow stronger winds
aloft to mix down. Expect winds gust in the 35 to 45 mph range
late this morning and afternoon. The mixing is also expected to
produce cumulus clouds over the eastern plains this afternoon.
Because of the dry airmass in place, if any of these produce rain
it is expected to evaporate before reaching the ground. Will not
mention any showers for the eastern plains, though a few sprinkles
could reach the ground under these high based showers. Cold air
advection will result in cooler temperatures today with highs
around 70 across northeast Colorado. Appears there will be enough
orographic lift and moisture for rain and snow showers over the
mountains today. A few spots could pick up 2-3 inches of snow, but
most locations are expected to see a dusting at most. The showers
are expected to end this evening as drier air moves in and the
airmass stabilizes. The cooler and dry airmass should lead to
overnight lows in the mid 30s to lower 40s over northeast
colorado.

.LONG TERM...(Thursday through Tuesday)
Issued at 240 AM MDT Wed Apr 19 2017

Thursday will see westerly flow under a weak ridge prior to the
onset of the next trough centered over CA by 12z Thursday. At the
surface low pressure will increase over the SW portions of CO with
increasing pressure to the NE. This increasing pressure gradient
will bring SE winds over eastern CO. Winds will increase into the
late afternoon hours with models suggesting a Denver cyclone set-
up which is not uncommon with SE winds over the divide. This
combined with increased de-stabilization from orographic lift
across the foothills will bring a slight chance of storms for
Thursday late afternoon into the evening hours. Current model
soundings show 0 to 6 km shear values between 40 and 50 with CAPE
between 400 and 600 j/kg. This would be enough to support more
severe storm formation. If conditions persist, main threats would
be showers with gusty winds and small hail. Temperatures for
Thursday will stay slightly cooler then previous days with highs
in the mid 60s.

The upper level trough that was over CA on Thursday will have
transitioned to more of an elongated feature spreading over
Colorado with the low center further North into SW Wyoming. As
the trough spins to the NE and continues to deepen into Central
Colorado showers will increase over NE portions of the state. The
surface low will deepen and move over Denver keeping strong SE
flow over the area. Behind the surface low dryer SW winds will
move in that could create enough clearing to de-stabilize the area
for a slight chance of thunderstorms by Friday afternoon over
portions of the plains south of I-70. Elsewhere conditions will be
more stable with rain showers expected. Current model trends show
upwards of 1 inch of rain for many areas on the plains by late
Friday. In the mountains snow levels will be around 8500ft by
midnight Friday then drop to 7000 ft by the late morning. Amounts
will vary with time as rain changes over to snow but 6 to 12
inches will be possible by late friday with the higher amounts
over eastern slopes of the higher mountains. Temperatures will
drop with highs on Friday in the 50s on the plains and 20s to 30s
in the mountains.

Precipitation will linger in the mountains into early Saturday but
subsidence will increase behind the exiting trough as upper
ridging builds. Kept a slight chance over the mountain zones to
account for this. Temperatures for Saturday will stay mild in the
50s with a big jump back to warm with 70s and sunny expected for
Sunday.

The start of the week could bring another round of precipitation
for the region with the hint of an upper level disturbance moving
over the state. Models currently show this event to be more for
the mountains and weak at that with strong NW flow. Kept a slight
chance for the mountains with temperatures in the 70s through
Tuesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1240 PM MDT Wed Apr 19 2017

Strong west/northwest winds are spreading across the Front Range
airports and will continue through the afternoon. Peak gusts 30-35
knots expected at KDEN and KAPA with near 40 knots at KBJC. A
more north/northeasterly transition is anticipated 23Z-02Z with
another frontal push expected then, but speeds should gradually
subside. After that, look for a lighter east/southeast flow to
develop and remain in place through early Thursday AM.

&&

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

$$

UPDATE...Barjenbruch
SHORT TERM...Meier
LONG TERM...Bowen
AVIATION...Barjenbruch



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