472 
FXUS65 KBOU 190231
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
831 PM MDT Sat May 18 2019

.UPDATE...
Issued at 830 PM MDT Sat May 18 2019

IR and water vapor satellite imagery is showing that subsidence is
beginning to take hold as cloudtops are warming and the upper airmass
is beginning to dry out. Lower, observations are shoing the colder
airmass sliding south out of Wyoming with pressure rises, cooling
temps, lowering ceilings and gusty northerly winds. Expect this to
continue to advance south, though it may mean some resistance from
the southerly drainage winds setting up. Still expecting the low
stratus to move in after midnight, with the stratus possibly
causing visibility problems into tomorrow morning at the level where
it pushes against the foothills, around 8000 feet MSL. 

Will keep the Frost Advisory going, and it still looks best for
the eastern plains, though could have patchy frost further west
along and west of I25. 

&&

.SHORT TERM...(This evening through Sunday)
Issued at 236 PM MDT Sat May 18 2019

Water vapor satellite imagery showing the center of a disturbance
over southeast Colorado. This system will move east of the state 
by this evening. Scattered showers and isolated thunderstorms will
slowly decrease late this afternoon and end early this evening as
the airmass stabilizes and subsidence moves in behind the exiting
system. Expect the convection to be done by mid evening. A weak 
cold front will surge south across the area tonight. This will 
cause winds to shift to the northeast. Low clouds are expected to 
form. Airmass is quite cool for this time of year. Lows are 
expected to drop into the mid 30s. Over the eastern plains, low 
clouds are not as likely, so have temperatures in the lower to mid
30s. Since temperatures will be close to 32F degrees, will issue 
a frost advisory (criteria is 32-35 degrees). Best chance for 
frost will be over the eastern plains. 

Large upper level low will move across central California Sunday.
Over Colorado, a short wave ridge will slide east of the state 
during the afternoon. Behind this, southwest flow and moisture 
will increase. The airmass becomes unstable over the mountains 
with CAPE climbing to 500 J/kg. Over the plains, the cooler 
airmass at the surface and warming aloft is expected to keep it 
stable. Diffluent flow aloft along with the instability over the 
mountains is expected to produce scattered showers and 
thunderstorms over the mountains. Don't think storms will survive 
off the higher terrain where there is no CAPE, but there should be
enough synoptic lift for rain to survive late in the afternoon 
and into the evening. 

.LONG TERM...(Sunday night through Saturday)
Issued at 236 PM MDT Sat May 18 2019

Cool and unsettled with be the ongoing theme through next week.
Sunday night, a broad closed low will be over Nevada with
increasing moisture spreading into CO. The low is forecast to
move to the UT/AZ line by 12z Monday. The models generally 
indicate weak to moderate mid and upper level QG omega over the 
area as a 100 kt upper jetstream spreads across southern and 
eastern CO by 12z Monday. Showers will increase in coverage across
the region Sunday night, painting the highest qpf over Larimer, 
Boulder and Weld Counties as low level southeasterly winds develop
over the plains. Monday afternoon through Tuesday morning, the 
mid level trough lifts further to the north and east and becomes 
negatively tilted as it shifts into southern and eastern CO by 12z
Tuesday. The models start to differ in the location of the closed
low by 12z Tuesday, with the GFS further north than the ECMWF and
NAM12. In any event, they all indicate moderate to strong mid and
upper level QG omega over the cwa at that time. Timing wise the 
heaviest precipitation is expected to occur Monday night through 
Tuesday morning. From 00z Monday to 00z Wed, it appears total 
liquid precipitation will range from 1 to 2.5 inches along the 
northern half of the forecast area, with anywhere from 0.25 to 1.5
inches across the southern half. More snow in the mountains 
although the models, especially the NAM12, are likely not 
handling the expected qpf well due to embedded instability 
associated with thunderstorms. A winter weather highlight for the 
mountain may be necessary at some point, but this is still in the 
5th and 6th periods, so nothing at this time, but storm totals of 
8 to 16 inches not out of the question above 10 thousand feet. 
Snow level could lower to around 6500 feet by 12z Tuesday. The 
NAM12 is the coldest but also the outlier at this point, but it 
rain/snow mix a possibly Tuesday morning. Some clearing Tuesday 
night could result in frost issues again for Wednesday morning. 
Nothing is very clear by mid week with a broad trough still 
stretched across the Great Basin/central and northern Rockies, 
with impulses circulating around the trough and across the state. 
Will broad brush the rest of the period with cooler than normal 
temperatures with a chance of showers and thunderstorms each day.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 830 PM MDT Sat May 18 2019

A few showers remain around the terminals but aren't lasting long.
Still waiting for the weak cool front to push in, all
northeasterly winds over Weld County and into Morgan County are
battling the southerly drainage winds setting up. But these ENE
winds should still push in over the next couple hours. Then
another northeast push later tonight after midnight will bring low
clouds with ceilings falling to 1000 to 2000 feet after 09Z. The 
low clouds are expected to lift and scatter late morning, 17Z to 
19Z. 


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Frost Advisory from midnight tonight to 8 AM MDT Sunday for 
COZ038>046-048>051.

&&

$$

UPDATE...Kriederman
SHORT TERM...Meier
LONG TERM...Cooper
AVIATION...Kriederman