Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 211556

Area Forecast Discussion
National Weather Service Denver/Boulder CO
956 AM MDT Fri Jul 21 2017

Issued at 942 AM MDT Fri Jul 21 2017

Cloud cover does not appear like it will increase any sooner than
yesterday, which will again impact the high temperatures this
aftn. For this reason, have increased the max temperatures today
by a couple degrees. Timing wise, expected most of the showers and
tstms to develop over the higher terrain around 18z, then move
off the foothills and into the urban corridor in the 21-22z time
frame. Precipitable water values this afternoon will be close to
an inch. Storm motions will be slow, around 10 kts. Locally heavy
rainfall will be the main concern along with frequent lightning
with the stronger storms.


.SHORT TERM...(Today and tonight)
Issued at 400 AM MDT Fri Jul 21 2017

Southwest flow aloft around an upper level high over the southern
plains will continue to bring monsoon moisture to the area today.
Models indicate moisture will increase a little with precipitable
water values climbing over an inch across the Front Range and
across the eastern plains. Northeast surface winds will push into
the foothills by late morning behind a weak cold front. This
combined with the increase in moisture is expected produce
additional storms today. The airmass below 600MB will remain dry,
so the initial storms that form over the higher terrain should
produce good outflow boundaries. Storm development will spread
eastward along the outflow boundaries this afternoon and evening.
CAPE values are expected to be 300-800 J/kg. This along with weak
shear, will keep the severe thunderstorm threat low.

Main concern will be the heavy rainfall with the thunderstorms.
Storm movement is expected to be a little slower today at around
10 knots. Below 600mb the airmass will remain dry through the
evening, this will limit the heavy rain. Winds this evening from
750MB to 500MB turn northeasterly along the Front Range. Below
this, westerly outflow winds are predicted to prevail. Additional
showers and thunderstorms will be possible this evening and
overnight due to the upslope flow. However, models keep this layer
dry, thus no precipitation. Will keep pops in the forecast in
case more moisture prevails than is forecasted by the models. |

.LONG TERM...(Saturday through Thursday)
Issued at 400 AM MDT Fri Jul 21 2017

With the central Great Plains upper ridge breaking down, flow aloft
over WY and northern CO will gradually turn northwesterly during the
day Saturday. A weak shortwave trough embedded in this flow is
projected to pass over n-central CO late tonight and eventually over
nern CO during the day Saturday which causes the monsoon moisture
plume, which is beginning to curve easterly, to gradually translate
southward away from the fcst area. Model QG omega and height
tendency fields indicate some strengthening/amplification of this
wave as it swings out over the plains during the morning hours with
both the NAM and GFS going so far as to indicate the formation of a
weak closed low at 500mbs just east of Denver around 15Z tomorrow.
Model mean layer PW values east of the mtns during the day range
anywhere from 0.95 to 1.45 inch with the higher values in the
vicinity of the aforementioned cyclone. The NAM, GFS, EC and
Canadian models all indicate light QPF scattered across nern CO
during the day, with the greater precip amounts in the vicinity of
the cyclone. Later in the day this showery precip expands westward
along the srn foothills and western portion of the Palmer Divide
apparently the product of moist outflow from convection on the
plains. Present forecast has 30-40 pct PoPs at lower elevations and
30-60 pct PoPs in the high country on Saturday. Overall, PoPs still
look reasonable, although PoPs may need to be raised some in areas
east of the I-25 corridor with passage of the shortwave trough. The
threat of heavy rainfall appears low, although with light winds
aloft with the passing shortwave, could see a couple of slow moving
storms deposit a half inch of rain in under 30 mins. By Saturday
evening, models show this wave nearing the CO/KS border. In its wake
a drier nwly flow should put an end to most of the shower/t-storm
activity both in the high country and on the plains by around
midnight. Temperatures look cooler on Saturday due in part to
cooling aloft and potentially greater cloud coverage with highs
running anywhere from 5-10 degs f below average for the date.

Looking ahead to Sunday and the upcoming week, models show the upper
ridge taking up residence over the Great Basin again for a few days.
Clockwise circulation around the upper high sitting in this location
should divert the main stream of the subtropical moisture westward
away from the Front Range which should reduce t-storm activity
across the region. Less cloud cover and warming aloft will allow
temperatures to return to near average if not slightly above

For Wednesday and Thursday...medium range models show and a fairly
robust shortwave trough swinging onshore which displaces the upper
ridge east of Colorado again. This sets the stage for a return to
the monsoon moisture plume which models show moves up over Colorado
during the period. The high country will more likely to see more t-
storm coverage as a result, yet the plains should also see a
modest uptick in storm chances each day.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 942 AM MDT Fri Jul 21 2017

Southerly wind component this morning may become variable by 17z
then northeasterly by 18Z and remain in that direction until
outflow boundaries from convection reach the Denver airports. Good
chance for thunderstorms across the Denver area today. Will have
a tempo -tsra and wind gusts to 35 knots for the thunderstorm
threat today in the TAFs from 22Z to 02Z. The storms are expected
to push east of the Denver area by 04Z, but there will be a slight
chance for storms through the night.


Issued at 400 AM MDT Fri Jul 21 2017

Heavy rain from slowing moving thunderstorms will increase the
flooding threat today. The main concern will the higher terrain
and urban areas that are more susceptible to flooding. Dry low
levels and a lack of large scale forcing is expected to limit the
flooding threat.




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