Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 131006

Area Forecast Discussion
National Weather Service Denver/Boulder CO
406 AM MDT Sun Aug 13 2017

.SHORT TERM...(Today and tonight)
Issued at 400 AM MDT Sun Aug 13 2017

Front left exit region of an 80+ knot jet over western and
central Colorado is bringing mid and high clouds to the state. The
high clouds will limit radiational cooling and is expected to
keep low clouds and patchy fog over far eastern Colorado. The lift
from the jet is will progress eastward today and is expected to
be over eastern Colorado and Kansas, perhaps east of the forecast
area by mid afternoon.

A lee-side surface trough is forecasted to set up over eastern
Colorado. West of it, the airmass will be somewhat dry with
precipitable water values around three-quarters of an inch. CAPE
values will mainly be less than 700 J/kg. With the main lift
shifting east of the area expect convection to be mainly isolated
to widely scattered. These storms are expected to produce brief
heavy rain and wind gusts to 40 mph. Small hail will be possible
with the strongest of these storms.

East of the lee side trough, the airmass contains more moisture
with precipitable water climbing above an inch. Higher dew points
will lead to surface based CAPEs of up to 2500 J/kg. This combined
with good shear is expected to produce severe storms this
afternoon and evening. The chance for severe storms will be east
of a line from Sterling to Akron to Limon. There is a chance
severe weather sets up just east of the forecast area.

.LONG TERM...(Monday through Saturday)
Issued at 400 AM MDT Sun Aug 13 2017

The large scale wave-trough pattern across the CONUS will undergo an
adjustment during the first half of the upcoming week. Operational
models show a 100 plus kt jet diving southeastward over the Pacific
Northwest where it then carves out an upper trough on Monday. Height
and pressure falls ahead of this eastward advancing system appear to
drive an energized surge of subtropical moisture northward over
Colorado during the day Monday into Tuesday. Best PW values/specific
humidity are progged to be over the southeast corner of the forecast
area on Monday and over nearly all of northeast Colorado on Tuesday.
Pressure falls to our north on Monday generates a 15-25kt s-sely low-
level flow on the plains which aids in advecting moisture up over
eastern portions of the CWA. The NAM, GFS and ECMWF also show a late
day southerly push of slightly cooler and drier air from Wyoming.
The collision of these two airmasses appears to create a fairly
sharp convergence zone from Elbert County northward to Logan County
late in the day. Models show storms firing within this zone
producing hourly rainfall rates approaching an inch per hour during
the evening, esply at its southern extent where the low-level inflow
will be strongest. However, it`s up across the northeast corner of
the state where models indicate the greatest shear and instability.
NAM indicates sfc based CAPE values from 2200 to 3400 j/kg from 00Z
to 03Z. Latest convective outlook from SPC does not highlight this
area for svr t-storms at this time. But, this bares watching should
later models run continue to indicate these conditions. On Tuesday,
could see a somewhat similar setup for heavy rain producing t-storms
mainly on the plains, and also the potential for severe weather. The
upper trough axis is progged to pass over nern Colorado late in the
day, about the time a cold front races south out of Wyoming and
Nebraska. Could see a line of storms moving off the Front Range
around mid-afternoon and interacting with this frontal boundary
somewhere east of a Greeley-to-Elbert line. East of this line, the
NAM indicates CAPES in excess of 2500 joules and mean layer PW
values in excess of 1.2 inches. Multicell storm development appears
likely, particularly across the northeast corner of the CWA with
large hail and damaging winds a possibility. A favorable vertical
shear profile could result in a brief tornado or two somewhere in
this area. After midnight, lift and instability drop off with the
trough exiting the state and with gusty northerly winds driving
drier air down from Wyoming.

By Wednesday...models differ on how much cooling and drying will
take place. The GFS indicates a relatively cool and moist
easterly/upslope flow east of the mtns, while the ECMWF and CMC
indicate the development of a drier and slightly warmer west-
southwesterly flow. For now, will use a blend of the models and
trend slightly drier but hang onto the cooler than average
temperatures expected for Tuesday.

For Thursday and Friday, models generally agree in moving the strong
upper ridge off the West Coast inland over the Great Basin by
Friday. However, models continue to offer differ ideas as to much
much drying will occur over Colorado. Some go so far as to return
subtropical moisture to the state by the weekend, thereby causing
lower temperatures and increasing the chance for t-storms. Because
of low confidence in the models, will refrain from introducing any
big changes to the fcst beyond Friday.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 400 AM MDT Sun Aug 13 2017

VFR conditions will continue through tonight. Patchy fog and low
clouds are expected over the eastern plains around 12Z. High clouds
moving over Colorado will limit radiational cooling and is expected
to prevent the low clouds and fog from forming in the Denver area.
Normal south-southwest drainage winds should prevail through mid
morning and then become light. Thunderstorm activity across the
Denver area is expected to be weak and isolated due to slight
drying. Will not mention thunderstorms in the TAFs at this time due
to the low chances thunderstorms impact the Denver airports. Main
threat will be outflow winds causing a wind shift.




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