National Weather Service Forecast Discussion

435
FXUS65 KBOU 170115
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
715 PM MDT Sun May 16 2021

.UPDATE...
Issued at 656 PM MDT Sun May 16 2021

Thunderstorms on the plains, mainly Lincoln County, beginning to
wind down as instability decreases. After peaking at over 2500
j/kg this evening values have begun to fall, along with storm
intensity. Still expect storms to persist, but have lost updraft
strength and ability to produce hail and landspouts. Have blended
in the latest available 23z HRRR pops to bring down precip
chances. Still some lingering rain/snow showers moving through the
mountains, and continued nonzero pops there through the remainder
of the evening. N Central plains of Weld, northern Morgan & Logan
counties are in relative min in pops, centered under surface high
and solid cap. Relative minimum in pop chances late tonight are
fleeting, with cooling temperatures aloft, some QG ascent, and
return of daytime heating tomorrow morning POPs increase again.
For temps, overnight lows look good but blended in some observed
to even things out.

&&

.SHORT TERM...(This evening through Monday)
Issued at 256 PM MDT Sun May 16 2021

Scattered showers and storms are developing over the high terrain
and should start moving out over the plains later this afternoon.
Daytime heating was significantly limited in the foothills and
adjacent plains by the persistent stratus since this morning,
generally in the area around and north of I-76. A recent sounding
from near Fort Collins showed a large capping inversion with
about 130 J/kg of CIN. There may be enough lift from outflow from
nearby convection for some isolated storms to initiate in this
area but they would be weak and precipitation may also struggle to
reach the ground. Models didn`t pick up on the early stratus well
and aren`t picking up on the cap very well either. I have reduced
temperatures, PoP, and QPF in this area through the evening to
account for this, and it also reinforces the idea that severe
weather will be limited to the area southeast of I-76 today where
there is better instability.

Scattered showers and storms will persist through the night with
best coverage and intensity gradually shifting east with a
potential MCV. Models are still showing an MCV developing over
east-central Colorado tonight and moving northeast towards the
Kansas border through Monday morning. The main impact with this
system would be moderate to heavy rain on the eastern plains. More
showers and storms are likely Monday afternoon and evening with
increased vorticity advection aloft ahead of the cutoff low to our
southwest as it advances east. The potential for severe storms
still seems limited by a lack of instability and weak flow aloft
however.

.LONG TERM...(Monday night through Sunday)
Issued at 256 PM MDT Sun May 16 2021

For Monday night and Tuesday, the broad upper low will meander
slowly across southern Colorado but with only weak upward QG
ascent with the low. Airmass will be mostly saturated with
increasing/deepening northeast upslope flow with approaching low
on Monday night. Expect the highest chances of showers in this
Monday night and Tuesday morning time period. By Tuesday afternoon
the low level flow shifts more southeast and upper low center is
over south central Colorado. Still expect likely showers during
the afternoon/evening, but instability will again be limited with
more isolated thunderstorms.

The weakening upper low will move across Eastern Colorado and into
the central plains later Wednesday and Wednesday night with some
drying and weakly subsident airmass. Chance of showers on Wednesday
will see less areal coverage of storms but may be a bit more
instability with possible better heating. Temperatures will be
warmer on Wednesday with readings back around seasonal normals.

For Thursday and Friday, a new strong upper low will move onshore
from the Pacific and move into the northern Rocky Mountain region
for Friday and Saturday. This will increase the southwest flow
pattern aloft, somewhat drier air so expect more isolated coverage
of late day storms. There could be better strength of storms across
the far eastern plains of Colorado with deeper low level moisture
expected to reside.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 656 PM MDT Sun May 16 2021

Not an easy forecast overnight. KDEN briefly went into VFR and
near ILS mins, but have since dropped back to MVFR. Despite
decrease in precipitation coverage low level moisture to remain in
place overnight tonight, although challenge is in picking a
ceiling height. Model soundings show ceilings at KDEN generally
dropping to MVFR, with rain chances tapering off by midnight and
renewing again late Monday morning. KAPA and KBJC stand a better
chance for IFR ceilings overnight with a little elevation
advantage. Wind forecast to shift around west to southwest
overnight, southeast Monday morning, then by mid day an easterly
upslope flow to continue low clouds.

&&

.HYDROLOGY...
Issued at 256 PM MDT Sun May 16 2021

There is still some risk of slowing moving storms producing
moderate precipitation over the burn areas in the foothills
through this evening. There is potential for development of an MCV
and persistent showers on the eastern plains overnight through
Monday morning. Moderate to heavy rainfall is possible under the
MCV with a low flooding risk.

The flash flood risk will be low on Monday night and Tuesday with
more light to moderate rain with only weak instability. Expect
lower areal coverage of storms on Wednesday but better heating may
result in stronger storms for Wednesday afternoon. A drier
airmass will be over the area Thursday and Friday with only
isolated late day storms possible.

&&

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

$$

UPDATE...Hanson
SHORT TERM...EJD
LONG TERM...Entrekin
AVIATION...Hanson
HYDROLOGY...Entrekin/EJD

NWS BOU Office Area Forecast Discussion