Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 251604
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1004 AM MDT Tue Jul 25 2017

.UPDATE...
Issued at 919 AM MDT Tue Jul 25 2017

Complicated forecast today with several conflicting ingredients in
terms of thunderstorm potential. 12Z GJT sounding is ripe with
moisture, 1.3 PWAT, while the DEN sounding is half that and bone
dry below 500 mb. The good news is that the moisture across W
Colorado is making it`s way eastward into our area throughout the
day today with a short wave trough across S. Utah. That short wave
is impressive on GOES-16 imagery, with deep, moist convection
across SW Utah around the time of sunrise, fairly rare even for
monsoon season. It will take a while for the layer below 500 mb to
moisten up today, and with the overcast skies instability will be
limited. The severe threat today is very low, with the main threat
gusty winds to 40 mph. The heavy rain threat is low, but highest
across the higher terrain where instability and deep layer
moisture will be better. Storms should form around noon across the
mountains then move east.  Any storms that move slowly in the
higher terrain will need to be watched closely for isolated,
localized flash flooding potential. Overall with the cloud cover
and very dry layer below 500 mb, and despite increasing synoptic
scale lift and deep layer moisture, feel the existing chances of
precipitation (20-35%) across the Plains still look good for this
afternoon.

&&

.SHORT TERM...(Today and tonight)
Issued at 330 AM MDT Tue Jul 25 2017

The broad upper high is now centered over the Texas panhandle
which has allowed higher levels of moisture to stream northward
into the mountains and western Colorado from the south and
southwest. A check on integrated precipitable water trends also
show this increase with 24 hours trends up .20" over western
Colorado and now nearing an inch of PW noted at MTJ. Front Range
PW trends are up around a tenth of an inch. Mid and higher level
clouds are also increasing from the southwest which should result
in a bit cooler temperatures this afternoon. Still looks like
lower 90s over the plains but additional cloud cover could even
keep the urban corridor in the mid-upr 80s. Latest high resolution
models showing showers and storms developing in the mountains
this morning and then spreading over the plains late morning and
afternoon hours as also aided by weak QG ascent from the weak
wave.

With the higher moisture levels today the main threat will be
heavy rain with storms. However, forecast soundings over the Front
Range still showing some inverted-v type profiles with drier
conditions at the lower levels. With the stronger flow aloft could
see some gusty winds with storms, upwards of 50 mph with the
stronger outflow over adjacent plains. Surface based CAPES up to
1000j/kg over the far eastern plains so could see a few marginal
severe storms with main threat of winds to 60 mph and hail up to 1
inch in diameter. A weak cold front later tonight will keep the
threat of showers ongoing along with possible lower clouds
developing later in the night.

.LONG TERM...(Wednesday through Monday)
Issued at 330 AM MDT Tue Jul 25 2017

The upper high center will slowly retrograde west, remaining
roughly over the southern state border. High surface pressure
will push down from the northern Great Plains Wednesday morning,
to bring cooler temperatures and an increase in moisture. Some
indication of some stratus around the plains early in the morning
from a push tonight, have increased sky cover for this.
Temperatures should be about 10 degrees cooler than today, mainly
east of the divide, with readings in the upper 70s to lower 80s
over the plains. Scattered to numerous thunderstorms will focus on
upslope areas west of I-25 then isolated activity will spread
east over the plains. NAM has come into better line with other
models with moisture as it was an outlier. PW values of 1.5-1.6
inches will allow for storms to be able to produce brief heavy
rain. These are likely on the high side, as models continue to
overestimate moisture. Storms will continue into the evening with
showers just expected overnight, mainly still concentrated over
upslope favored areas as high pressure will remain over the Great
Plains.

Surface flow will turn more southeasterly Thursday with slightly
drier air expected. Temperatures will continue to be cooler then
normal with little change from Wednesday. Storms will again focus
along upslope areas with slightly lighter rainfall rates expected
from the storms.

On Friday, a shortwave moving over the northern Rockies will down
into the Great Plains for a better chance of storms over the far
eastern plains.

The upper high will then begin to build northwest into the Great
Basin by Monday. Northwesterly flow aloft will shunt the better
moisture south and away from the forecast area and allow for more
sun and warmer temperatures. Still expecting at least isolated
activity over the high terrain and over the Palmer Divide for the
weekend. Past this time frame, models are disagreeing on weak
shortwaves, pushing down in northwest flow. Will keep the
increased chances across both mountains and plains for the
potential for the shortwaves.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 919 AM MDT Tue Jul 25 2017

VFR conditions expected through Wednesday at the TAF sites. Main
forecast issue will be thunderstorm timing. Instability is limited
today with ample cloud cover, thus main impact from thunderstorms
will be gusty winds to 35 kts. High res model guidance and current
GOES-16 trends continue to show mid and high level moisture
streaming into Colorado, so overcast above 14kft looks to occur
through the overnight hours. 21Z still looks like the most likely
time for thunderstorms to impact the TAF sites today. Until the
thunderstorms form and move east, prevailing winds will be out of
the west. The thunderstorms will form to the west of DEN/BJC/APA
around 19Z, possibly impacting the west gates by that time. Not
expecting any hail from these thunderstorms. Thunderstorms are
possible through about 01Z, then winds overnight should be WNW.
Pre-sunrise a cool front should shift the winds to the north
around 08Z, and behind the front ENE winds will occur during the
day with another round of afternoon showers and thunderstorms
possible.


&&

.HYDROLOGY...
Issued at 330 AM MDT Tue Jul 25 2017

Higher moisture levels today spells higher areal coverage of
storms and main impact will be heavy rain with storms as PW values
increase to around an inch or better by later this afternoon.
Storm motions however will be 10-20 kt which will keep the threat
for flash flooding on the lower side.

Precipitable water values are expected to increase Wednesday
behind the cool front will allow storms to produce brief heavy
rain. Storm motions will initially be enough to limit any
flooding potential, however the flow will weaken in the late
afternoon and early evening - increasing the threat.

&&

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

$$

UPDATE...Schlatter
SHORT TERM...Entrekin
LONG TERM...Kriederman
AVIATION...Schlatter
HYDROLOGY...Entrekin/Kriederman


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