Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 181725
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1125 AM MDT Fri May 18 2018

.UPDATE...
Issued at 1103 AM MDT Fri May 18 2018

There is quite a bit of Stratus over most of the plains currently.
Convection is already developing in the mountains and foothills
right now. The upper low circulation is over northern Utah right
now. There should be some synoptic scale upward motion through
the day, tonight and Saturday for the CWA. Current observations
show pretty uniform lower 50s F dew point readings over the
plains of the CWA. The current ACARS sounds show a decent cap
just above 700 mb. That should be low enough for today`s heating
to get ride of it by mid afternoon. The Stratus is thinning slowly
on the satellite picture. Will keep the mention of severe of
sever going. Will up pops a bit. Went with a flood watch for a
good portion of the CWA from mid afternoon into Saturday evening.
The slow speed of the upper low, upslope and the moisture
expected for the CWA warrants it.

&&

.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 18Z TAFS through 18Z Saturday afternoon)
Issued at 1103 AM MDT Fri May 18 2018

Models keep some direction of upslope low level winds going at
DIA the rest of day, tonight and Saturday. Satellite pictures are
showing the Stratus deck over the airport thinning right now.
This trend looks to continue. Not sure if the current ceiling
will ever totally go away, and the afternoon convection will
likely bring ceilings back in no later than 22Z this afternoon.
Will keep them pretty low.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 3 PM MDT this afternoon through Saturday
evening for COZ035-036-038>040-042>045-048>051.

Winter Storm Watch from 9 PM MDT this evening through Saturday
afternoon for COZ033-034.

&&

$$

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



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