Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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803
FXUS65 KBOU 201536
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
936 AM MDT Fri Apr 20 2018

.UPDATE...
Issued at 927 AM MDT Fri Apr 20 2018

WV shows vort center of wrapped up low spinning over southern Utah
with deepening of the dry conveyor belt to the south that will
continue to help pull the system SE. This will bring the def zone
to the NE increasing moisture across the urban corridor and metro
later this afternoon and early evening. At this time radar shows
continued showers along the eastern portions of the state and
through the mountains with enough instability to create
lightning and some graupel with the lowered freezing level. This
is expected to continue on and off through the afternoon with
continued snow over the higher terrain, becoming heavy at times.
Current warnings look good so will maintain with no changes. Main
adjustments to forecast was to increase pops over the east and add
isolated thunderstorms to account for current changing conditions.

&&

.SHORT TERM...(Today and tonight)
Issued at 423 AM MDT Fri Apr 20 2018

Upper low elongated N-S across Utah and Arizona will move eastward
across Colorado through tonight. The configuration of the low will
bring the best lift across the mountains today and the plains
tonight. The cold pool aloft will keep the best instability across
central and southern Colorado, with a broad area of weaker lift
further north. Of note is a dry slot ahead of the main frontal
band which will pivot roughly over Denver during the day today. As
a result, some convection should break out near the eastern border
and be going strong over the mountains by midday, but will
probably wait until later this afternoon to fill in as the low
levels finally moisten. Main change with this package was to put
more of this detail in the forecast, and slow down the whole
eastward progression today by a couple of hours.

Still expecting widespread precipitation tonight, though with the
N-S elongation persisting this will put the cold pool south of us
and the best lift east and north of Denver, without a lot of
upslope flow, so we may have a prolonged period of light
precipitation. With a longer duration and the weak upslope, the
mountains should still do fairly well.

Warm temperatures will moderate the impact. Little change to the
forecast here. The snow level should be 7-8 thousand feet most of
today. It should be cold enough to snow in the middle or later
parts of the night in Denver, but everything is so warm that
accumulations should be limited to grassy areas. Below about 5000
feet, either all rain, or a mixture with no accumulation.

.LONG TERM...(Saturday through Thursday)
Issued at 423 AM MDT Fri Apr 20 2018

On Saturday, a broad upper low will be over eastern CO, NE and KS
by midday. By the afternoon, the mid level cold pool will still be
over eastern CO with a weak northerly flow aloft. Weak mid level
qg ascent will be over the cwa through 00z. Will continue with the
current trends with likely to sct pops in the grids through the
day. By Saturday night, increasing mid level subsidence will
occur as the trough continues to track to the east and a ridge
moves in from the northwest. Sunday and Sunday night, the ridge
aloft will usher in a warmer airmass. NAM12 spatial cross-sections
shows some low level moisture over the plains east of Fort Morgan
through early aftn. The models do show some weak mid level qg
ascent around 00z Monday. That coupled with residual moisture and
afternoon heating will produce isolated to scattered showers over
the mountains in the aftn and early evening. Monday and Monday
night, the ridge axis will shift east with the flow increasing as
the next system moves across Montana/northern WY Monday evening.
Another day with highs in the mid 60s. Weak mid level qg ascent
will be over northern CO as the system passes to the north of the
state around 00z Tuesday. Generally sct pops in the high country,
with isolated to scattered showers over the northeast plains. The
best coverage will be along the northern border. Tuesday and
Tuesday night, increasing mid level subsidence will develop as the
trough axis passes to the northeast of the state. A weak front
will push into the cwa from the north with a fairly strong
northwesterly flow aloft around 00z with an 80 kt jet max
overhead. Still enough moisture coupled with jet aloft to produce
isolated to scattered showers mainly in the mountains. Wednesday
and Thursday there will be a slight chance of afternoon and
evening showers over the cwa. A broad ridge will be over the
region with a weak flow aloft. Temperatures will be back in the
mid 60s. The ridge will continue to be the main influence on
Friday, ahead of a closed upper trough moving onto the central CA
coast.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 927 AM MDT Fri Apr 20 2018

VFR currently over area terminals with winds changing to a more
ESE direction with some light gusting up to 28 mph from increased
southerly flow. APA and DEN are expected to switch to the SE over
the next few hours with precipitation starting around 21-22z. BJC
will also have a 21-22z start time but winds will stay
predominantly to the NNW from 10 to 15. Rain is expected to turn
over to snow this evening after 00z and continue through the
afternoon hours on Saturday. This will be reflected in the next
TAF for all area airports. MVFR conditions are expected with
lowered ceilings between 003 and 006 with visibilities dropping to
below a mile at times with the heavier snowfall possibly between
06 and 10z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until noon MDT Saturday for COZ031-035-
036.

Winter Storm Warning until noon MDT Saturday for COZ033-034.

&&

$$

UPDATE...Bowen
SHORT TERM...Gimmestad
LONG TERM...Cooper
AVIATION...Bowen



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