Area forecast discussion 
National Weather Service Denver/Boulder Colorado 
301 PM MDT sun Sep 21 2014 

Short term...(this evening through monday) 
issued at 301 PM MDT sun Sep 21 2014 

Upper level low over Nevada will slowly move northeast tonight and 
Monday. Scattered showers and isolated showers are expected to 
spread from the high country onto the front late this afternoon and 
early this evening. Precipitable water values are around an inch 
along the Front Range. This combined with slow moving showers and 
storms could produce locally heavy rain...mainly over the higher 
terrain where the greatest instability will be. Showers will 
decrease overnight...but due to the moisture and weak qg 
lift...still could see a few showers. Will keep low probability of precipitation in the 
forecast through the night and into Monday morning. 

Qg lift will remain over Colorado Monday as the upper level low 
moves across Wyoming. Convective available potential energy will be up to 1000 j/kg and precipitable 
water values will linger around an inch. The qg lift combined with 
the instability and moisture is expected to produce scattered 
showers and thunderstorms Monday afternoon. Could see a couple of 
storms reach severe limits...with large hail and strong winds. 
Temperatures will be slightly warmer Monday with highs in the middle to 
upper 70s. 

Long term...(monday night through sunday) 
issued at 301 PM MDT sun Sep 21 2014 

Anomalous height falls are prognosticated over the eastern Pacific 
through midweek...resulting in ridge building over The Heart of 
the country. Before the ridge builds over Colorado...the closed 
upper low spinning over the central West Coast at this time is forecast to 
open up and track east-northeastward across eastern Wyoming by 00z/Tuesday. 
Qg vertical velocity fields indicate weak deep layer ascent over 
eastern Colorado during the evening hours on Monday...coincident with 
a north-south band of 750-1500 j/kg of surface based cape along a 
rather sharp boundary layer wind shear axis. NAM...GFS...ECMWF and 
sref all show this axis of lift and instability shifting slowly 
ewrd across the northeast plains during the evening hours. 
Scattered probability of precipitation on the plains and isolated to scattered probability of precipitation in the 
high country are called for Monday evening. Isolated storms on the 
far northeast plains may produce brief heavy rain...gusty winds 
and possibly some small hail. Meanwhile...things dry out and 
stabilize over mountains and high valleys with west-northwesterly flow 
in the wake of the trough. 

On Tuesday...trough continues to slowly move away...but not before a 
bundle of jet energy...moisture and middle-level cold advection sweeps 
down its back side. All this should produce a fair amount of cloud 
cover and at least a few showers/T-storms mainly east of the mountains 
during the late morning and afternoon hours. Any precipitation should be 
light and spotty with the boundary layer continuing to dry out. Do 
not see much if any cooling at low levels on Tuesday with model temperature 
guidance indicating low/middle 80s on the plains and upper 50s to lower 
70s in the high country. For your information...average high for Denver is now 76. 

With the upper ridge building over the Rocky Mountain region midweek... 
dry conditions should prevail across Colorado as temperatures 
continue to creep up. Models split between Wednesday and Thursday as 
being the warmest day of the week. Either way...we/re looking at 
highs in the middle to upper 80s at lower elevations and a comparable 
warmup for the the high country both days. Should not see much wind 
beneath the upper ridge to blow away the beautiful autumn foliage in 
the high country. 

By Friday and Saturday...medium range models show the 589 upper 
ridge moving over the Great Lakes and upper Mississippi River valley 
as a high amplitude longwave trough plows into the West Coast. Deep 
southwesterly flow ahead of the trough is forecast to advect moist 
subtropical air up over Desert Southwest Thursday night into 
Friday...and eventually up over Colorado late Friday through 
Saturday. Models also show a weak wave accompanying this northward 
surge of moisture. Assuming this holds true...should see precipitation 
chances increasing during the weekend with the approaching upper 
trough. However a southwesterly 700-400 mb flow would favor higher precipitation 
chances west of the Continental Divide during this period. Until 
we get a better handle on this next Pacific storm system...will 
go with a chance of precipitation in the high country and a slight 
chance of showers/T-storms east of The Divide late Friday through 


Aviation...(for the tafs through 00z Monday late afternoon) 
issued at 301 PM MDT sun Sep 21 2014 

Scattered showers and thunderstorms are expected through 03z. A 
slight chance for showers and storms will continue overnight and 
into Monday morning. Another round of scattered showers and 
thunderstorms is expected Monday afternoon. South to southeast winds 
are expected to prevail though 12z with gusts of 20 to 25 knots at 
times at kden and kapa. However...a Denver cyclone will be near kden 
and may cause variable winds at times. Winds will become light 
Monday morning. Ceilings may fall into the 3000 to 6000 foot range 
under the heavier showers and storms. 


Bou watches/warnings/advisories... 


Short term...meier 
long term...Baker 

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