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fxus65 kbou 242042 
afdbou

Area forecast discussion
National Weather Service Denver/Boulder Colorado
242 PM MDT Wed may 24 2017

Short term...(this evening through thursday)
issued at 242 PM MDT Wed may 24 2017

A ridge of high pressure aloft combined with an impressive 700 mb
thermal ridge has allowed temperatures to warm nicely into the
low 80s across the plains of Colorado this afternoon with 60s in
the mountains. The warm and dry conditions will change by tomorrow
as a storm system across the Pacific northwest will flatten the
ridge over Colorado tonight and ushering in lower heights
throughout the day Thursday. This will result in cooler
temperatures area wide and a decent chance of thunderstorms.

Tonight skies will remain mostly clear and the gusty west winds
occurring now will diminish later this evening as the atmosphere
decouples with the loss of solar heating and a lack of a
significant surface pressure gradient. Given how warm it is this
afternoon overnight lows will only drop to the mid 50s across the
plains and I-25 urban corridor, with upper 30s to 40s in the
mountains.

The changes in the weather should reach the mountains first
beginning late morning Thursday with the arrival of cold advection
at 700 mb, driven by a 700 mb low moving across Wyoming. 700 mb
temps cool 5-8 degc across the northern 1/4 of Colorado from
overnight into midday Thursday. Cooling also occurs in the mid
levels as the 500 mb low moves from Idaho into Wyoming by midday
Thursday. All this cooling aloft occurring during Max surface
heating will generate a fair amount of convective instability
across our area by early afternoon. Early Thursday there should
also be a weak surface cool front that moves from NE Colorado
towards the foothills. Behind (east of) the front slightly cooler
temperatures and east-northeast winds will exist, as well as much
more moist low-level air, with dewpoints in the upper 40s to near
50. How far west the boundary makes it will play a role on where
thunderstorms will form. They could form along the surface
boundary itself during peak heating because of good convergence
along the boundary with drier, west winds ahead of the front.
Storms should also form early afternoon across the higher terrain
as the pool of cold air aloft is advection into northern Colorado.
Those storms would move east across the plains given the west-
southwest storm-level flow. Feel that across Weld and Larimer
counties, closest to the cold air aloft, the surface front should
initiate convection there early afternoon and move southeast
along the frontal boundary. Elsewhere, storms that move off the
higher terrain may have trouble sustaining themselves if they
encounter the dry air west of the cold front if the cold front
does not make it to the foothills by mid afternoon. In the moist
air behind the front MLCAPE should exceed 1000 j/kg, and any
convection that develops along and east of the front could be
capable of small hail and gusty winds. Deep layer wind shear would
also support organized storms. Storm coverage across the plains
depends on the location of the front but chances of thunderstorms
are in the forecast for much of the area north of I-70, highest
across Larimer/Weld counties where the best instability and
convective forcing should be. The mountains, especially along and
north of I-70, have a good chance of diurnal convection Thursday
afternoon. Snow levels will remain above 11.5kft tomorrow during
the convective precipitation so am not expecting any impacts from
the snow. With convection and cool outflow, increased cloud cover,
and the weak cool front...Thursday will be a cooler day across
the area with 70s on the plains and 50s to mid 60s expected in the
mountains.

Long term...(thursday night through wednesday)
issued at 242 PM MDT Wed may 24 2017

The threat for showers and thunderstorms will decrease Thursday
evening as the airmass stabilizes. May be enough moisture and
orographic lift for showers to linger overnight. An upper level
trough dropping south over the northern Great Basin and a jet near
by will help provide lift to trigger afternoon thunderstorms
Friday. Temperatures in the upper 60s to lower 70s and dew points
in the mid to upper 40s yield surface based cape of 700 to 1500
j/kg Friday afternoon. This combined with good shear should result
in a few severe thunderstorms Friday afternoon. Models in fairly
good agreement that convection should be able to break through the
cap during the afternoon. Will continue to carry high scattered
pops for Friday afternoon/evening.

The upper level trough continues to sag southeast and will pass
over Colorado Saturday. Showers will be likely for the much of the
area. Cloud cover will make it a cool day with temperatures
struggling to make it into the lower 60s. If temperatures make it
into the lower 60s this should make it unstable enough for
isolated thunderstorms.

It dries Saturday night as a north-northwest flow aloft sets up
behind the exiting trough. This pattern will continue into Sunday
and Monday. It warms a little Sunday and Monday because of some
clearing, but expect temperatures to remain below normal. Still
may be enough moisture and instability for showers and weak
thunderstorms Sunday and Monday, but coverage will be less than
Saturday.

Weak northwest flow aloft will prevail Tuesday and Wednesday as
an upper level ridge forms over the Great Basin. Temperatures will
warm, but stay slightly below normal. Models indicate there
should be enough moisture and instability for late day convection
each day, so will keep low pops in the forecast.

&&

Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 242 PM MDT Wed may 24 2017

VFR conditions through Thursday afternoon. West winds at the taf
sites will continue through early evening with wind speeds in the
10-15 kft range, but occasional gusts to 25 kt should occur.
Den's gusts to 25kt are fewer and far between this afternoon,
which should continue through early evening thus have handled the
possible gusts to 25 kts with a tempo group. Apa and bjc will see
prevailing gusts 20-25 kt into this evening. Winds will reduce to
10 kt or so after 01z/02z, and then go to drainage through early
morning. On Thursday a cool front looks to shift winds at den to
the east-northeast 19z or so, and thunderstorms are possible at
all three taf sites after 20z. Handling for now with a prob30 to
be consistent with the current forecast messaging for the Metro
area. Any thunderstorms that impact the terminals will contain
gusty winds to 35 mph or so and possibly small hail.



&&

Bou watches/warnings/advisories...
none.
&&

$$

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