Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 220303

Area Forecast Discussion
National Weather Service Denver/Boulder CO
903 PM MDT Fri Jul 21 2017

Issued at 903 PM MDT Fri Jul 21 2017

Have updated grids to reflect ongoing deep moist convection
developing behind a strong gust front pushing east across the
Plains of Colorado. With weak bulk shear values the convection
has formed a broken line, and the gust front continues to outpace
the developing convection. Given this scenario we do not expect
any severe winds or hail, but underneath the strongest individual
cells an inch of rain could fall in 30-60 minutes. Outside of
those cores amounts will be light. Expect the convection to move
east along the gust front and out of our area by midnight.
Lingering stratiform showers will likely continue overnight.

Latest GOES-16 and radar trends shows a nice push from the NW
moving across Larimer County ATTM, most likely a short wave trough.
It appears to be responsible for light to moderate rain showers
that have been generating and could generate 0.1-0.3 inches of
rain as it moves south. Not sure how far south the short wave
trough/NW surge will get, but it should be the focus for
continuing rain shower development as it moves south with moist
conditions across the I-25 corridor in place. Fort
Collins/Loveland/Greeley should see a good shot of rain over the
next 2 hours. At it`s current pace the stratiform rain would reach
Boulder County and north metro Denver around 1030 PM. Latest
forecast products keep a slight chance of rain showers into the
overnight hours given this latest trend.


.SHORT TERM...(This evening through Saturday)
Issued at 238 PM MDT Fri Jul 21 2017

A weak west to southwesterly flow aloft will be over the region
this evening. Slow moving thunderstorms developing over the higher
terrain this aftn, with the stronger storm producing one half to
three quarters of an inch of rainfall in 30 minutes. So far these
storms have been fairly localized with lesser rainfall with the
storms elsewhere. Some small hail also reported. Thunderstorm
coverage is still expected to increase over the mountains then
gradually spread eastward across the northeast plains overnight.
An upper level jet max will be passing across northern WY in the
vicinity of the right entrance region of the jet. As a result,
there will be some weak mid level QG ascent along the northern
border which could allow for some showers to persist over the
far northeast plains late tonight.

On Saturday, a drier and still weak westerly flow aloft will be
over the northern and western part of the cwa. The best chance of
thunderstorms will be south of I-70 so best coverage will be
there. Temperatures on the plains for Saturday will not as warm as
today but still dependent on how much precipitation falls
tonight. If the storms under perform tonight, then drier soils
may allow for warmer temperatures than the max temperatures
currently in the grids.

.LONG TERM...(Saturday night through Friday)
Issued at 238 PM MDT Fri Jul 21 2017

Drier air and weak subsidence will continue to advance southward
across the forecast area Saturday night. This occurs as ridging
begins to pop back up across the Great Basin. This drying will
bring an end to showers and storms during the evening.

For Sunday through Tuesday, the ridge axis amplifies a bit more
over the Great Basin and inches eastward toward Colorado. This
will bring mainly dry conditions to the plains, while the
mountain areas should still see isolated to scattered storms each
day. With the building ridge and somewhat drier airmass,
temperatures will undergo a warming trend with highs pushing back
into the 90s across the plains by Monday and Tuesday.

For Wednesday through Friday, the ridge axis is forecast to shift
slightly eastward. This may allow for weak passing short waves and
potential influx of a mid/upper level monsoonal moisture plume.
However, low level moisture may remain limited, so main threats
would be gusty outflow winds, lightning, and only brief heavy
rain from storms. We should see slight cooling during this period,
but temperatures will likely remain above normal.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 903 PM MDT Fri Jul 21 2017

Thunderstorms that impacted APA and DEN have shifted east this
evening. The storm generated a strong gust front that continues to
plow east across the Plains of Colorado, initiating new convection
behind the gust front. It`s not a solid line of convection but
it`s strong enough that it will likely continue to impact
approaches to Denver from the east. In terms of the terminals VFR
conditions will prevail through Saturday evening. Plenty of
uncertainty around wind direction since we are not expecting
typical drainage winds given the environment across the metro
area is completely worked over by convection, and high-res models
have not been helpful with winds because they have continued to
generate additional convection well past 07Z with attendant
outflow. Additional deep convection this evening and overnight is
not going to happen across the terminals, far too stable. But
rain showers (shallow, no lightning) will continue through 06Z or
so which will impact wind direction for any that reach the
terminals. Believe winds will gradually diminish between now and
04Z as the strong convection continues to move east. Expect light
winds through mid morning at the TAF sites. On Saturday high res
models are showing drying across the area and model soundings
showing stable conditions. Thus, chances of TSRA are much lower
than previous days, have removed mention of VCTS from the TAFs for
Saturday afternoon. Expect N and NE winds Saturday
afternoon/evening with speeds around 10 kts.


Issued at 238 PM MDT Fri Jul 21 2017

Heavy rain from slowing moving thunderstorms will be the primary
concern for tonight and Saturday. The main concern will the
higher terrain and urban areas that are more susceptible to
flooding. Elsewhere the dry low levels and a lack of large scale
forcing is expected to limit the flooding threat.




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