Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 132113

Area Forecast Discussion
National Weather Service Denver/Boulder CO
313 PM MDT Sun May 13 2018

.SHORT TERM...(This evening through Monday)
Issued at 313 PM MDT Sun May 13 2018

Moisture and fingers of mid-level energy pinwheeling
around the deep closed upper low now centered over northern
Nevada will gradually shift eastward during the next 24 hours. In
the lower levels, an east-northeast flow on the plains will
provide additional moisture up against the Front Range. For that
reason, the extensive low cloud cover has been reluctant to burn
off today, and only in the past couple of hours clouds have
cleared over and just north of the Palmer Divide where convective
temperatures have been reached. Furthermore, an elongated sfc
trough extending northeast from a sfc cyclone in wrn Elbert County
will likely act as a focus for strong updraft formation. Already
seeing some of this development on radar and satellite. Believe
the stronger t-storms later this afternoon and early evening will
form with this Denver Vorticity Convergence Zone before darting
north-northeastward at 25-30 kts. Hail up to golfball size hail
and quarter inch of rain in under 20 mins will be possible with
these storms. Favorable vertical shear along the boundary may also
be sufficient to spill up an isolated tornado or two. Closer to
the Front Range, cooler temperatures and a low broken to overcast
will hinder storm formation. However, should a convective outflow
move down off the Front Range, could see one or two strong
t-storms blow over portions of the I-25 corridor, but the risk of
severe wx with these cells looks low. A weak shortwave swinging up
from southern Colorado may be all we need to set off t-storms out
across the plains late afternoon/evening evening.

Then, severe risk is expected to shift northeast over eastern
portions Weld, Adams and Arapahoe Counties, and eventually over
Morgan and Washington Counties through the evening where low-level
moisture convergence will be greatest, potentially resulting in
locally heavy rainfall. Could see upwards of an inch of rain in
less than 30 mins. A few storms in this area may also produce
large hail, possibly up a quarter in size. The severe weather
threat is expected to diminish after 04z this evening, but the
change for light to moderate rainfall for parts of the plains
could linger well beyond midnight. After midnight, should again
see fog form, and possibly become dense in areas toward dawn. This
includes the greater Denver Metro Area.

On Monday...temperatures are expected to be slightly cooler, but
that should not reduce the chance of t-storms or the risk for
severe weather. With the upper trough moving closer, the vertical
sheer profile looks even more favorable for supercells on the
plains. A moist easterly upslope flow will also continue to
moisten and destabilize the boundary layer. Thunderstorms may
begin forming a few hours earlier tomorrow, first over the high
terrain, then over the nearby plains. SPC now has most of
northeast Colorado in a marginal risk for severe weather tomorrow.
Can`t argue with that, but I believe the better threat for severe
will fall west of a Fort Morgan-to-Elbert line. Large hail and
damaging winds main threats, but again cannot rule out isolated
tornadoes during the afternoon with increasing low/mid-level

.LONG TERM...(Monday night through Sunday)
Issued at 313 PM MDT Sun May 13 2018

Upper level closed low over the Northern Great Basin will
continue to slowly weaken Monday night and Tuesday as it remains
nearly stationary. Thunderstorms are expected to be progressing
eastward across the area Monday evening. A severe storm or two
will remain possible, but the better chance looks to be in the
afternoon hours. Low clouds and patchy fog are expected to form
again late Monday night and Tuesday. However, it won`t be as thick
and widespread as previous nights. Expect the clouds to clear out
sooner on Tuesday with warmer temperatures expected. Airmass
becomes unstable again with CAPE topping around 1000 J/kg. Surface
to 6km shear reaches around 30 knots, so a few storms could just
reach severe limits.

Eventually the closed low is ejected north into the Northern
Rockies Wednesday as the next closed low moves onto the California
coast. Over Colorado, upper level ridging will take place. This
will result in warmer temperatures and a drier airmass. Highs are
expected to climb well into the 70s across northeast Colorado.
Models are not showing much for thunderstorms on Wednesday. Not
surprising given the warmer air aloft and drier low levels. Still
can`t rule out a few weak showers and storms, mainly over the
higher terrain.

The upper level ridge over the state will shift east of the area
Thursday as the upper level low moves into the Great Basin.
Southwest flow aloft will prevail over the state. At the lower
levels, southeast winds will increase low level moisture. Appears
there will be enough moisture and instability for afternoon and
evening storms. If the shear and instability in GFS model are
close to correct, some the storms will be severe over eastern

The upper level low will weaken as it nears Colorado Friday and
moves across the state Saturday. This will keep the chance for
showers and thunderstorms in the forecast. Expect a cold front to
drop across the state in this pattern sometime Friday or early
Saturday. Friday may remain mild if the cold front arrives later
in the day or that night. Saturday should be a cooler day with a
good chance for showers and thunderstorms. It is expected to warm
up Sunday once this system shifts east of the state. If convection
forms Sunday, the best chance will over the elevated terrain.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 313 PM MDT Sun May 13 2018

LFR Ceilings will prevail for remainder of the afternoon, and
throughout the night with a moist north-northeast upslope flow.
Could see areas of ILFR CIGS and VSBYS overnight with scattered
thunderstorms in the evening, and scattered showers and areas of
fog after midnight. VSBYS may lower to less than 1/2 mile in fog
late tonight. On Monday, areas of fog may lower VSBYS in the 1/4
to 1 1/2 mile range through 15z. Should then see foothill showers
moving out over the Metro Area by around midday, then a good
chance of t-storms during the afternoon. Hail, gusty winds and
locally heavy rainfall all possible with these storms with cloud
bases generally in the 3000-4500 ft agl range.




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