Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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939
FXUS65 KBOU 271518
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
918 AM MDT Thu Apr 27 2017

.UPDATE...
Issued at 846 AM MDT Thu Apr 27 2017

Forecast looks on track for today. GOES-16 water vapor imagery shows
a deformation zone across western Colorado moving quickly east.
This feature is enhancing precipitation rates as it moves across
the state per radar trends. Expect snow showers in the mountains
to be briefly enhanced through midday, including and especially
the I-70 corridor. As the short wave trough and deformation zone
reach the Plains, a fairly well organized line of convective rain
showers should impact the foothills and I-25 corridor midday, and
then push east into increasingly (but still limited) unstable air
across the eastern Plains. Thunder a possibility east of the
Denver metro area this afternoon as the line of showers moves
east. The precipitation should end across the I-25 urban corridor
by 2 or 3 PM, and the rest of the eastern Plains by late evening.

Current forecast has these trends well in hand, and the Winter
Weather Advisory will continue through 6 PM for another 1-4
inches across the area through 6 PM.


&&

.SHORT TERM...(Today and tonight)
Issued at 442 AM MDT Thu Apr 27 2017

Water vapor and upper air analysis shows the sharp upper level jet
streak pushing toward the Four Corners area, with broad upper
level divergence noted over Colorado. One band of lighter
precipitation was reaching the mountains, but another stronger and
more convective band associated with strong cold advection will
reach the mountains shortly after daybreak. This band is well
defined on the satellite imagery over northeast Utah at 3 AM. As
orographics and instability increase with the onset of cold
advection, snowfall rates may briefly hit 1-2 inches per hour
across the northern mountains between 7 and 10 AM, with conditions
expected to improve by the noon hour.

The chance of showers will also increase on the plains this
morning with the increase of Q-G lift and the convective nature
of this next band. Have increased PoPs into the likely category
for the most part. Showers may tend to organize as they spread
east across the plains, aided by low and mid level moisture
convergence. Despite the cool temperatures today, enough
instability with the strong cold advection aloft and 500 mb cold
pool of -25C approaching to support a few thunderstorms. The
higher probabilities will be on the eastern plains as heart of
lift arrives later in the morning/early afternoon there, during
peak heating.

For tonight, snow showers will likely continue in the mountains
although for the most part expect them to decrease in intensity
with lower precipitable water and specific humidity. With the jet
axis still to our south, can`t rule out another couple inches and
some travel impacts into the night. Won`t adjust the Winter
Weather Advisory any later at this point given the lighter
intensities expected. On the plains, best chance of rain will
shift to the far northeast corner with downslope drier air pushing
across the rest of the plains and I-25 corridor.

.LONG TERM...(Friday through Wednesday)
Issued at 442 AM MDT Thu Apr 27 2017

Friday night through Saturday will be an active period as a potent
late season storm affects the entire state. Each of the mid-range
models are in decent agreement in swinging an upper low from
eastern Utah, over the Four Corners Region and then across
northern New Mexico. At low levels, a strong cold front will drop
out of Wyoming and across eastern Colorado Friday evening. Strong
and deep upslope flow will develop behind the front, and all the
ingredients should be in place for a night time of snow along the
Front Range and down to the Palmer Divide. 700 mb temperatures are
forecast to drop to about -10C, which will be cold enough for snow
levels to drop down to 5000 feet. Model QPF fields show between
and half inch and one inch of liquid equivalent, which should
deliver a decent shot of moisture to the region. As the snow falls
Friday night, roads shouldn`t be impacted too much, but snow
sticking to recently leafed out vegetation may lead to breaking
tree branches. The two areas that will see more significant
accumulations will be the foothills and over the Palmer Divide.
Snow accumulations of 4 to 10 inches will be possible. Will issue
a Winter Storm Watch for those areas, since there is still some
uncertainty on the the exact track of the upper low. If the low
dives farther south than forecast, then the areas with heaviest
precipitation would shift further south, to southeast Colorado
and northeast New Mexico.

The effects of the storm will be felt strongest Friday night and
Saturday morning, with the storm moving out onto the southern
plains. Colorado will remain under cool and unsettled conditions,
leading to some afternoon and evening shower activity due to
daytime heating. Sunday could see another batch of diurnal shower
activity as cool northwesterly flow aloft continues. Cyclonic
northwesterly flow is actually forecast through about Wednesday.
This will keep the chance of afternoon shower activity in for
forecast through then. For the latter half of the week, ridging
finally builds over the southwestern U.S. with drier and warmer
conditions eventually spreading across Colorado.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 846 AM MDT Thu Apr 27 2017

Mostly VFR conditions expected through Friday morning.  Winds at
DEN go from West to NW by in the next hour or so.. Meanwhile, a
weak short wave trough will kick off convective rain showers that
will impact the terminals late morning through about 21Z. Handling
the showers with a tempo group given convective nature and brief
lower ceilings as the line(s) of showers move west to east. Behind
the convective showers gusty northwest and then north winds should
prevail through 01Z or so. After 21Z precipitation chances are too
low to mention in the TAF and improving CIGs should occur. VFR and SW
drainage winds tonight at DEN and APA, WSW at BJC, 10 kt or less.
By Friday late morning northerly flow at the surface should
occur at the three terminals.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT this evening for COZ031-
033-034.

Winter Storm Watch from Friday evening through Saturday
afternoon for COZ035-036-041.

&&

$$

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



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