Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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119
FXUS65 KBOU 181012
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
412 AM MDT Fri May 18 2018

.SHORT TERM...(Today and tonight)
Issued at 412 AM MDT Fri May 18 2018

...Severe thunderstorms expected over parts of northeast
Colorado this afternoon and evening...

...A chance for very heavy rainfall and flooding over parts of
northeast Colorado tonight and Saturday...

Outflow from thunderstorms over southwest Nebraska and far
northeast Colorado sent a strong outflow boundary southwest
through the area late last evening. Models did a poor job
capturing this feature, so the airmass near the surface is already
more moist than the models indicate. The fine details in the
models are all over the place for the next 24-36 hours and have
little confidence in the mesoscale features through tonight.

Broad upper level trough over the western U.S. and a closed
circulation over Utah will weaken and shift east today. Lowering
heights and lift with the approaching trough with trigger
thunderstorms this afternoon and evening. Mixed layer CAPE reaches
2000-3000 J/kg over northeast Colorado today. Shear is marginal
for supercells, so expect storm mode to be a mix of multicellular
storms and a few supercells. Golf ball size hail, damaging winds,
and an isolated tornado will be possible. Severe storm potential
could be limited along the Front Range if low clouds move in this
morning. Some models do not show low clouds, but low clouds are
just starting to form. Wouldn`t be surprised if we end up cooler
with more clouds than the models indicate. Lowered highs a little
for this. Even with the cooler conditions, still expect storms to
form due to lift ahead of the trough.

Focus tonight shifts from severe weather to heavy rain and
possibly flooding. Models show a new closed 500 mb low, likely
a mesoscale convective vortex (MCV), forming somewhere over
northeast Colorado or southwest Nebraska. Little agreement among
the models where this will form, and thus where the heaviest
rainfall will occur. General trends in the models has been
stronger and farther southwest with the formation of this MCV. The
00Z GFS goes bonkers with an area of 5-8 inches of rainfall from
Denver north to Fort Collins and extending west into the foothills
for this afternoon through Saturday afternoon. The NAM and GEM
show 3-6 inches of rainfall on the backside of the MCV in the
northern foothills and southeast Wyoming. The ECMWF is lighter and
only shows about 2 inches of rain. The heavier rainfall amounts
appear due to a stronger low east of the foothills with 50 knot
upslope flow. The ECMWF is weaker and farther east with the
circulation, so there is no upslope component to produce the
heavier rainfall amounts. The stronger trend and farther
southwest development with the models is worrisome.

Snow level will fall overnight and end up close to 9000 feet MSL
by 12Z Saturday in the northern mountains. Will issue a Winter
Storm Watch for the Front Range mountains, north of I-70.


.LONG TERM...(Saturday through Thursday)
Issued at 412 AM MDT Fri May 18 2018

The threat for heavy snow in the mountains of Larimer and Boulder
counties will continue through the day Saturday. The upper low at
this time remains over northeast Colorado through 00z Sunday. This
will also produce a prolonged period of moderate to heavy rainfall
for foothills of Larimer and Boulder counties. Consequently, the
threat for flooding and flash flooding will be elevated.
Uncertainty in the track of the different model solutions keep the
potential rainfall amounts in question as well so will need to
keep a close eye on the hydro conditions through Saturday. The
system is expect to slowly transition to the northeast Saturday
night. Will keep the trend of decrease pops Saturday night. By
Sunday, some ridging and subsidence at least through Sunday
morning. Could still see some lingering light rain showers early
along with patchy fog. After a cool, wet Saturday with highs
around 50, the temperatures are expected to moderate back into the
low 60s. The best chance of showers and thunderstorms Sunday will
over the elevated terrain Sunday afternoon. Some of this will
spread eastward across the I-25 corridor late in the day and in
the evening. No changes regarding the extended forecast with
warmer temperatures and a chance of mainly afternoon and evening
thunderstorms each day.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 412 AM MDT Fri May 18 2018

Thunderstorms over southwest Nebraska has pushed an outflow
boundary through the Denver area bringing northerly winds this
morning. Moisture will continue to increase and low clouds may
form around or just after 12Z. Models are not showing much for low
clouds in the Denver area, however it wouldn`t be the first time
the models miss the low clouds with this kind of set up. With the
moist airmass in place thunderstorms will form and move across the
Denver area beginning around 20-21Z. Storms are expected to exit
the Denver area early evening 01-03Z. Low clouds and rain will be
possible Friday night and into Saturday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from this evening through Saturday afternoon
for COZ033-034.

&&

$$

SHORT TERM...Meier
LONG TERM...Cooper
AVIATION...Meier



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