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Area forecast discussion
National Weather Service Denver/Boulder Colorado
836 PM MDT Thu Sep 19 2019

Update...
issued at 827 PM MDT Thu Sep 19 2019

Although a brief thunderstorm did develop in the high country this
evening, most of the activity has been in the form of showers.
Have updated to primarily showers in the high country the rest of
tonight. Strong thunderstorms east of Colorado have enhanced
southeast winds across the northeast and east Central Plains this
evening. Will keep a slight chance of thunderstorms over the far
northeast corner until 09z but nothing elsewhere. Main issue will
be breezy condtions across the plains especially this evening.
Adjusted the trends in the pops based off radar, otherwise the
grids look good.

&&

Short term...(this evening through friday)
issued at 250 PM MDT Thu Sep 19 2019

An upper low over the Great Basin will be slowly lifting
northeast tonight and across the northern rockies by Friday
evening. As it moves closer, southwesterly flow aloft will be
increasing, while a surface low remains over the northeastern
plains of Colorado. A ribbon of moisture evident on satellite is
currently pushing in over the mountains, helping to spark off some
showers and thunderstorms. Cape values of 400-600 j/kg and precipitable water of
0.5-0.6 will allow for scattered activity the rest of the
afternoon with gusty winds and potential for brief moderate rain.
An inversion present around 550mb on recent acars should limit
convection over the plains for most of the day. However, with a
surface cyclone south of Denver creating easterly upslope flow to
the north, and qg lift increasing due to the upper system getting
closer, we should see some convection activity mainly north of the
Denver Metro area up to the Wyoming boarder late this afternoon
into the evening. Further east, better moisture exists but little
shear, and current high res model runs have all activity east of
the forecast area. Will keep just a slight chance of storms over
the northeast corner this evening, with a threat of brief heavy
rain along with strong winds.

Overnight, the qg lift will continue to increase, keeping a threat
of showers and thunderstorms over the mountains. Southerly surface
winds over the eastern plains will remain breezy as well. This
will contribute to mild overnight lows.

On Friday, drier and cooler air will push over the mountains,
so should see less thunderstorm activity as well as cooler
temperatures. Over the plains, a dryline will set up from the
surface low pulling in the drier air from the west and moisture
up from the south. Shear increases as the southwesterly jet moves
overhead. Therefore will see a chance for severe strength storms
over the far northeastern corner, with large hail, strong winds
and possibly a tornado.

Long term...(friday night through thursday)
issued at 250 PM MDT Thu Sep 19 2019

A dryline will shift east across the far eastern plains through
the course of Friday evening, taking the threat of isolated severe
storms east into Nebraska and Kansas by late evening. A cold front
will then sweep south across the plains, ushering in a cooler and
drier airmass. Large scale subsidence will bring clearing skies as
well.

On Saturday, the main upper level trough is expected to shift east
across the area. There is good agreement with a shot of q-g
support during the afternoon and early evening. However, the main
question is how much moisture can return across the plains. At
this time, the return flow does not look great, but the large
scale support would support a decent chance of showers and
isolated thunderstorms. The best opportunity for light rain
continues to support areas just east and southeast of Denver
toward Limon. Have cooled off temperatures a couple degrees to
account for the shallow but cool upslope across the plains.

Sunday will feature dry and warmer conditions as the upper level
ridge builds overhead. The dry and warmer weather will likely hold
through the middle of next week, despite the passage of an upper
level trough toward Tuesday. The reason for the dry weather
scenario is that there is pretty strong agreement the upper
trough and all the upper level support cuts off into the Desert
Southwest into Arizona or New Mexico. Temperatures will have
potential to be considerably cooler on any given day Tuesday to
Thursday, but very difficult to pinpoint which day would be cooler
given the pattern and dispersion in the ensemble data. We'll keep
forecast temperatures closer to normal for now.

&&

Aviation...(for the 00z tafs through 00z Friday evening)
issued at 827 PM MDT Thu Sep 19 2019

VFR overnight and Friday. Strong southeast winds this evening at
kden a product of drainage and strong outflow from thunderstorms
well to the east over Kansas. Will continue with some gusts to
25-35 kts through 05z then should see the stronger gusts settle
down overnight.

&&

Fire weather...
issued at 250 PM MDT Thu Sep 19 2019

Strong south-southwesterly winds are expected tomorrow over the
plains, with gusts in the 25 to 35 mph range. However humidity
will be hovering in the mid to upper teens, with lowest values
found on the northern fringes of the Palmer Divide with help of a
downsloping southerly flow. Seems like relative humidity will stay just above
criteria of 15 percent, but gusty winds will still help to
increase fire weather concerns. May see a couple 15 percent
readings for an hour, but not the 3 hour criteria, so will not
issue a highlight.

Saturday will feature dry but breezy conditions over the
mountains and high valleys. Despite cooler temperatures,
humidities will be approaching critical 15% threshold in Grand
County, the valleys of Summit County, and Park County. Winds will
be marginal for any highlights, with speeds generally at or under
15 mph with gusts up to 25 mph at times. Fire danger will still be
elevated, but we should stay just shy of red flag criteria.

&&

Bou watches/warnings/advisories...
none.
&&

$$

Update...Cooper

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