Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS65 KBOU 290941
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
341 AM MDT Thu Jun 29 2017

.SHORT TERM...(Today and tonight)
Issued at 340 AM MDT Thu Jun 29 2017

Just a few exiting storms over far eastern Colorado early this 
morning aided by sagging upper jet now across southern portions 
of the state. Showers will have dissipated before sunrise under 
mainly clear skies. 

Today will be characterized with a relatively dry moderate 
westerly flow aloft then an upper trof will approach Northern 
Colorado by later tonight with increasing QG ascent ahead of the 
trof. Jet streak associated with the trof will also be draped 
across northern Colorado helping to aid showers and thunderstorms 
for much of tonight. Building high pressure over the norther high 
plains will keep much of northeast Colorado under shallow upslope 
flow this afternoon and tonight. Temperatures will continue to 
cool by 3-5 degrees this afternoon with behind shallow low level 
cooling tonight. Readings will mainly be in the mid 80s across the
plains. 

High resolution models continue to focus on Elbert and Lincoln 
counties again for thunderstorms and even the chance for a severe 
storm with surface based CAPES ranging from 1000 to 2000j/kg. 
Further north and east, CAPE values are generally under 800j/kg 
and lower dewpoints especially along the Front range with readings
dropping into the 30s/lower 40s. Initially this afternoon, storms
will mainly have gusty winds with only brief rain. With 
increasing upslope flow this evening, low level moisture will 
increase allowing a better chance for showers. Have increased pops
tonight due to the upslope flow and the lift moving in tonight.May
even some stratus developing later tonight so will bump up cloud
cover with mvfr conditions. 

.LONG TERM...(Friday through Wednesday)
Issued at 340 AM MDT Thu Jun 29 2017

Foresee a number of changes in the large scale height/circulation 
pattern over Colorado and the swrn U.S. during the extended period. 
The first of these changes involves a marked cool down on Friday 
with a passing mid-level trough and a moist post-frontal upslope 
flow on the plains. Latest model guidance continues to trend 
temperatures downward for Friday. For instances, 24 hours ago, NAM 
MOS temp guidance gave Denver a high of 77 on Friday. The 29/0000Z 
NAM MOS now indicates a max temp of only 71. On the other hand, the 
last few runs of the GFS, ECMWF and Canadian Gem have not been as 
cool or as wet as the NAM as they do not maintain the moist upslope 
flow east of the mtns as long, and are not as cool aloft. For now, 
will use a blend of these models and stick with mid 70s for highs on 
the plains. However, should the upslope flow be as deep and persist 
as long as the NAM now indicates, high temps may end up being 3-5 
degs F cooler, particularly should a heavy cloud cover linger for 
most of the day up against the foothills. As for precip chances, 
will raise PoPs slightly above those now in the forecast, with the 
higher PoPs on the plains during the morning hours and over and near 
the Front Range during the afternoon with heating and continued 
orographic forcing. Overall, 12-hr QPF should be light, generally 
under a quarter inch. Meanwhile, on the west slope, dry and warm 
conditions will persist with 70s in the valleys and 60s on the high 
slopes. With relatively light winds, the fire danger will ease some. 
By nightfall, precip east of the mtns should quickly decline from 
west to east with subsidence and drying kick in behind the exiting 
trough. 

The next change in our weather will happen this weekend with an 
upper-level ridge building in from the Four Corners region. 
Subsidence and warming aloft on the front side of this ridge should 
pretty much suppress deep convection for all but the higher 
elevations of the Front Range mtns on Saturday. Weakly developed 
updrafts at higher elevations may manage to produce a few, widely 
spaced t-storms. With temps warming aloft, so will sfc temps. Should 
see highs on Saturday 8-12 degs F above those predicted for Friday. 
By Sunday...the 500mb ridge axis and warmest air aloft are predicted 
to be over western Colorado by morning. GFS shows the ridge moving 
east across the state as a weak shortwave trough drops down over 
Utah. Other models show the ridge retrograding resulting in 
northwest flow aloft over the CWA. This appears to generate a dry 
and warmer downslope flow off the nrn fthls and Cheyenne Ridge which 
warms sfc temps another 4-7 deg f on the plains. T-storms on Sunday 
should be isolated in coverage, and mainly over the Front Range and 
Palmer Divide.  

Sunday night and Monday, models differ in the details, but all show 
a weak shortwave trough passing over the fcst area during the 
period. Slightly cooler temperatures, mid-level instability and 
slightly elevated PW should be adequate to go with a slight chance 
of afternoon t-storms mtns and plains. 

Independence Day and Wednesday...should see the large upper ridge 
over the Great Basin returning to Colorado with 700 mb temps warming 
to around 17C. The airmass accompanying the ridge appears quite dry, 
although we may still see a few gusty storms form initially over the 
high terrain and drifting over the nearby plains late in the day. 
Highs both days expected to be in the lower 90s, with very warm 70s 
and lower 80s in the high country. 

One final note, medium range models are now hinting at the formation 
of the North American summer monsoon which they show organizing over 
western Mexico and lower deserts of American southwest. No impact on 
us quite yet, but perhaps by late next week. Stay tune.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 340 AM MDT Thu Jun 29 2017

Mainly clear skies through this morning. Approaching upper trof
and shallow upslope flow will result in isolated high based storms
by later this afternoon, then as moisture increases this evening a
better chance for scattered showers and storms continuing through
the night. Will keep winds at an northeast component through
tonight. 

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

National Weather Service Glossary of Abbreviations