Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 150959

Area Forecast Discussion
National Weather Service Denver/Boulder CO
359 AM MDT Tue May 15 2018

.SHORT TERM...(Today and tonight)
Issued at 359 AM MDT Tue May 15 2018

Convective clouds are moving out of the area early this morning,
though a little pocket of moisture remains over Park county. With
the clearing, there should be some patches of fog/low clouds over
the South Platte valley for a few hours this morning. Probably
just enough drainage wind to keep fog out of the Denver area,
though some thin low clouds could still develop.

The airmass will be a little drier overall today with a little
warming aloft. However, southeast low level winds developing on
the plains will bring some moisture back, especially over the
Palmer Divide region and into Denver, and provide some low level
convergence in this area as well. Again, there will be a weak cap
due to the warming/drying aloft, but convergence over the Palmer
Divide and/or the foothills should be enough to initiate storms
that will be able to survive on the plains for a while. Models
have consistently been generating an organized batch of storms
that moves across Denver in the mid to late afternoon and then ESE
into east central Colorado in the evening. Varying results on
convection further north, where there will be less moisture to
work with, but foothills or Cheyenne Ridge convergence could
overcome the capping there as well.

CAPEs should be lower today, possibly still as high as 1000 J/kg
in the Denver-Limon axis but lower further north. With lighter
winds aloft there will be less shear as well. HRRR starts the
convection as outflow-driven storms, then moves toward a more
discrete structure as the storms move into the inflow and better
moisture toward Lincoln county by early evening. Given the lower
CAPE and shear, the severe hail threat should be lower, though
the cool temperatures could help keep small hail from melting
before it reaches the ground. Slower storm motions may be an issue
today as the steering winds will be lighter SW with SE inflow. If
the low level winds are strong enough, they could cause some
backbuilding. This will probably be a lesser issue near the
mountains with cool outflow to begin with, but storms may slow
down as they get out into the plains winds. Model wind fields
suggest 5-10 knot steering winds, but the real motion will depend
on the balance between outflow and inflow winds. Probably just a
low flash flood threat given the fairly tame moisture availability,
but good potential for minor flooding and some more small hail
accumulation if the backbuilding scenario does develop with some

With continued drying of the airmass, hopefully no fog/low cloud
threat tonight. The clearing will allow low temperatures to still
be similar to this morning despite continued warming aloft.

.LONG TERM...(Wednesday through Monday)
Issued at 359 AM MDT Tue May 15 2018

By Wednesday the upper level low will be moving onshore into
CA as ridging builds over CO. Various models such as the GFS and NAM
show little to no QPF as QG subsidence dominates the synoptic scale
energy picture. However, the EC shows moderate QPF numbers over the
eastern extent of the plains by the late afternoon and evening
Wednesday. This could be in connection with increased moisture and
boundary layer mixing on the east side of the lee trough that the EC
has deeper and misplaced from the other models. From upper air
soundings the best area to possibly receive any convection could be
south of an Elbert to Washington county line. Deep layer shear is
weak with capped areas to the North so if anything does
develop, I don`t expect it to survive very long. Will keep a
slight chance in the grids for now. Temperature-wise, this will
be one of the warmer days of the week with highs in the low 80s
across the plains.

For Thursday, SW flow aloft will increase as the closed low
continues to make its march eastward. A shortwave seen on vorticity
and QG fields embedded in the SW flow will move over the region by
Thursday afternoon helping to be a possible catalyst for storm
initiation into the evening hours. Current model CAPE values are
varied between 2500-5000 with the NAM being the most bullish. Either
way, it is high over the NE corner with an average of 35-40 kts of
deep layer shear. Vertical shear across the NE Plains, largely due
to veering with height, could become sufficient to enhance
convective development and produce supercells with accompanying large
hail. precipitable water values are around 0.8 over Washington county
and North where heavy rain with hail will be possible. Will
increase pops and have the grids reflect this severe possibility.
Highs on Thursday are expected to be the warmest of the week with
values reaching into the lower 80s.

Friday will see a cold front drop south by the afternoon interrupting
prime instability. Daytime heating will just be getting started when
the GFS pulls in the cooler and more stable airmass. There is the
possibility of a convergence boundary as seen on GFS surface wind
and RH fields over SE Colorado but conditions should mostly bring
stratiform rain with upslope and increased QPF along the Palmer
Divide for our area. This will bear watching as any slight delay in
the timing of the frontal passage could mean the difference between
rain and thunderstorms. Saturday will see the after effects of the
cold front with models showing mostly stable conditions and
continued rain over the forecast area. The upper low will have moved
over the state by early Saturday bringing lift ahead of the main
energy. As the trough progresses northeastward across the region
models hint that surface winds could turn more NE that would aid in
increasing upslope along the foothills and prolonging the rain
chances for the region. It is important to note that the GFS is far
deeper on the moisture than the EC currently, with the Canadian more
in line with the GFS on moderate precip possibility rather than
amounts. Temperatures will drop during this period as well with the
cloud cover with highs in the upper 50s to low 60s.

By Sunday into Monday the upper flow will turn SW ahead of another
disturbance over the pacific coast of CA. With lingering moisture
and gradually increasing temperatures showers and thunderstorms will
be possible in the afternoon. Look for mid 70s by Monday.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 359 AM MDT Tue May 15 2018

Some patchy low clouds are expected to form around sunrise,
dissipating by 15z. Any fog is expected to remain north and east
of the Denver area. So localized MVFR/IFR ceilings that should
dissipate by 15z. Scattered thunderstorms are expected 20z-02z
with localized MVFR conditions and variable winds gusting to 30




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