Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 242038
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
238 PM MDT Mon Jul 24 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 147 PM MDT Mon Jul 24 2017

Center of circulation is currently over AZ/NM with a short wave
progged to move over the region late Tuesday. At this time high
pressure dominates the region with dryer conditions over the CWA.
There is enough moisture however for convection over the higher
terrain with weak storm motion from the NW. This will help to
carry some storms over the foothills and adjacent plains through
the late afternoon and evening. There is a surge of moisture over
the western portions of the state as the ridge dips south and
allows for increased flow. This will help to sustain storms into
the overnight hours...mainly over the southern portions of the
state but some areas of the southern mountains and Park county
could see some late precipitation. The main hazards for the storms
will be gusty winds and brief heavy rain. Current PW values are
hovering around 0.6 so less then the previous week but enough to
drop a quick inch with slower moving storms. Overnight
temperatures will be mild with lows in the 60s on the plains.

For Tuesday a shortwave will move around the ridge into Western CO
by the morning hours then into the northern mountains and NE
plains by the afternoon. Model PW values increase to over 1 inch
with CAPE values on the plains from 500 to 700 j/kg. Decent QG
ascent will help to increase storm chances by the afternoon hours.
Frontal passage will bring increased NW flow on the surface that
will converge with southerly flow over the plains and help to fire
off convection along a line from Sterling south to Akron. Expect
main hazards to be brief heavy rain and gusty winds. With
increased moisture and instability storms could produce 1 to 1.5
inches in a short period of time. Temperatures will be slightly
cooler than today but still expect highs in the low 90s.

.LONG TERM...(Tuesday night through Monday)
Issued at 147 PM MDT Mon Jul 24 2017

On Wednesday the sub-tropical ridge is just south of our area and
elongated west to east extending from New Mexico to southwest
Missouri. This will continue to advect the monsoonal moisture
field into Colorado from the southwest. A subtle short wave trough
looks to be embedded in the westerly flow aloft, and it should
aid in synoptic vertical motion midday Wednesday as the short wave
trough moves across the CO-WY border. With monsoonal moisture in
place and upper support from a trough, there is a good chance of
afternoon thunderstorms over the mountains with 0.7 to 0.8 inch
PWATs. Steering flow out of the WSW looks to be about 15-20 kts
which would keep storms moving, but certainly will be watching for
the potential for localized heavy rain which could lead to flash
flooding. Across the Plains, PWATs are forecast to be between
1.2-1.4", which would be about the 95th percentile for Denver in
late July. Models have been overestimating PWAT over the past week
or two so confidence in moisture amounts that high is low. One
ingredient that is new for Wednesday is a frontal passage midday
across the Plains. Models differ on the low-level moisture behind
the front but one would think that it will certainly be a lot more
moist than it was today with Td the 30s, regardless of preferred
model output (i.e. subscribing to the notion that anything would
help our rain chances at this point). The low-level moisture
increase looks to be the result of westerly flow across the
mountains inducing a lee trough late Tuesday into Wednesday, and
this trough will migrate southeast during the day Wednesday. At
the same time high pressure behind a Plains cool front will move
south, providing ENE upslope flow across the Plains of Colorado by
mid afternoon. ML dewpoints should range from the low 60s far
eastern Plains to mid 50s across the I-25 urban corridor. A caveat
however is that the NAM is indicating the cool front will have
much drier air in the low- levels than the GFS or ECMWF. Best
chances for thunderstorms east of the mountains will be across the
Palmer Divide and west of I-25. Storm coverage east of I-25 will
be isolated at best. Thin CAPE profiles and deep warm cloud layers
(6-7 kft deep) mean heavy rain would be the biggest impact from
any storms that form east of the mountains. Even if most areas do
not receive rain across the Plains, cloud cover and cooler
temperatures will begin Wednesday afternoon with highs barely
topping 80 degrees most areas.

The cooler weather continues Thursday and Friday with temperatures
and cloud cover keeping our area below normal. The 500 mb ridge
retrogrades to the west during this period placing our area under
weak NW flow. There is still enough moisture in the low-levels
with southeast flow that thunderstorms will be possible each
afternoon.  Precipitation rates and storm coverage on Friday and
Saturday will be lower than Tuesday/Wednesday with less
instability given the cloud cover behind the front with northwest
flow aloft, but with that much moisture hanging around we can`t
rule out thunderstorms pretty much area-wide, especially the
mountains and Palmer Divide.

Saturday through Monday the ridge builds over the Great Basin,
suppressing the monsoon flow of moisture well south of Colorado.
Overall precipitation chances area wide drop significantly, with
just slight chances of diurnal convection over the mountains.
Plains look to remain dry during this time frame. Temperatures
should be close to seasonal norms with weak NW flow aloft,
generally at or just below 90 across the Plains with 70s to low
80s in the high country.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 147 PM MDT Mon Jul 24 2017

VFR conditions are expected through the period. Wind speeds will
increase from the NNE from 5 to 10 mph before veering to the S by
later this afternoon. Increased speeds with storms over the higher
terrain will be sustained from 10 to 15 kts with gusts to 25 from
01 to 06z. Winds will return to drainage by 06z. Storms will stay
outside the vicinity of BJC and DEN but some could get close to
APA around 22z.

&&

.HYDROLOGY...
Issued at 147 PM MDT Mon Jul 24 2017

Moisture values will increase tonight through Tuesday with PW
values from 1.0 to 1.3. Storms that will form over the plains by
the afternoon will be able to produce over an inch of rain in a
short period time.

On Wednesday afternoon the Palmer Divide and Front Range
Mountains west of I-25 could see storms capable of generating
brief heavy rainfall, similar to Tuesday afternoon. Storms should
be moving at 15-20 kts so any one spot shouldn`t see the highest
rain rates for more than 20-30 minutes, but will watch for
localized areas of slower storm movement or if any storms impact
burn scars across our area.

&&

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

$$

SHORT TERM...Bowen
LONG TERM...Schlatter
AVIATION...Bowen
HYDROLOGY...Bowen/Schlatter



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