Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 210250
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
850 PM MDT Sat Apr 20 2019

.UPDATE...
Issued at 840 PM MDT Sat Apr 20 2019

Satellite water vapor shows the backend of the upper trough as it
pushes through the state with drying over SW Colorado. We will see
continued cloudiness with a slight chance of light rain over the
higher terrain and virga over the plains into the evening hours.
Periods of gusty winds with microbursts from the high based
convection will continue over the next few hours bringing gusts up
to 35 mph at times over the urban corridor and eastern plains.
Made only slight adjustments to tonight`s forecast with lows
expected in the 40s across the plains and 30s in the high country.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 319 PM MDT Sat Apr 20 2019

Lead shortwave will lift north from Utah and western Colorado
toward the Nebraska panhandle tonight and Sunday. This will bring
some weak lift in an unstable but fairly dry airmass in the
mountains this evening. Moderate southwest winds aloft are mixing
to the ground fairly well over western Colorado, and this plus
some weak convective outflow should bring westerly winds off the
foothills this evening as expected. We beefed this up a bit, but
in general, expecting a period of 15-30 mph winds in the evening,
then less. There could be some localized higher gusts due to
virga, but we`re not expecting much of that.

It looks like most of the area will get a break Sunday morning as
this wave develops to our northeast. By morning the QG lift is all
along and north of the state line, then the moisture starts to
wrap southward again behind the low. There is only slight cooling
aloft, but this will help with destabilization while cooler air
moves south across the plains during the afternoon and provides
low level convergence. With the morning break, it may be slightly
warmer and there should be a small area with better instability,
around 1000 J/kg, ahead of the front. Hard to tell exactly where
and when this will be released, but sometime in the afternoon and
probably somewhere between I-25 and Akron. There may be a small
risk of severe hail given the amount of CAPE and relatively cool
temperatures, but expect that this would probably just be with the
initial batch of storms, so a small area for a couple of hours.

There were concerns about heavy QPF earlier, but with the lift
mainly north of us and the likely southward propagation of the
frontal boundary, storms may be slow moving but not persist in the
same area for more than a couple of hours. Also with the cool
temps the available moisture is limited to something like 0.6 inch
of precipitable water. Meanwhile the snow level in the mountains
will likely stay up around 9000 feet through the day as there`s
not much cooling aloft. We generally kept the same pattern we had
for PoPs, but delayed the increase by about 3 hours, resulting in
most of the showers being in the afternoon and a pretty good
chance of at least a dry Easter morning, and possibly even a nice
one, in Denver.

.LONG TERM...(Sunday night through Saturday)
Issued at 319 PM MDT Sat Apr 20 2019

Southern stream 500 mb low settles into the four corners region
for the first half of the extended period, while a northern
stream shortwave trough continues east across southern Canada. The
closed low opens up and begins to move east across New Mexico &
Texas by mid week, with variable upper winds becoming northeast
Monday night and Tuesday in response to the circulation`s
movement to the south and southeast. Northerly low level flow on
the plains behind the cold front Sunday night and Monday, with
cooling temperatures on the plains and running up against the
northern front range & foothills.

Biggest forecast challenge is Sunday night`s placement of
precipitation and height of snow level. Overall scenario Sunday
night of upslope north to northeast flow, with rain on the Palmer
Ridge and eastern slopes of the northern foothills, changing to
snow with elevation. Snow level to lower throughout the night in
the northern Front Range as well. Orographic and synoptic forcing
do not quite line up, with better QG ascent further south across
southern Colorado nearer to the upper low. 00z-06z QPF has a strip
of up to 0.5 inch in upslope areas before moisture declines from
its Sunday`s highs.

For Monday precip chances shift to the mountains in better
instability driven by axis of cooler temperatures extending
northeast from 500 mb closed low over Arizona to the departing
northern stream trough. Steepening lapse rates with daytime surface
heating and cooler temperatures aloft will support afternoon
showers, and thunderstorms are possible in the afternoon in CAPES
of 100-400 j/kg. Summits will see some snow, but overall snow
levels in the mountains remain high as this region missed out on
low level cooling from the Plains front on Sunday.

From mid week onward models agree in movement of upper low along the
Gulf states, but disagree on the degree of rising heights in the
west. GFS wants to build a ridge over the western US with rising
heights, while ECMWF depicts flatter flow. End result is 10 degree
difference in max temps on Friday, and for that far out right now a
blend is the way to go until the pattern settles out.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 840 PM MDT Sat Apr 20 2019

VFR conditions through the overnight hours with periods of
gusty winds provided by microbursts from scattered high-based
convection. Wind gusts up to 25 mph will be possible over the next
hour with wind direction generally from the WNW through 5z when
winds will decrease. After 17z tomorrow winds are expected to
turn more NNE with the cold frontal push ahead of scattered
rainshowers after 20z Sunday. ILS may be possible after 00z Monday
with increased clouds.

&&

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

$$

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


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