731 
FXUS65 KBOU 280232
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
832 PM MDT Fri Sep 27 2019

.UPDATE...
Issued at 832 PM MDT Fri Sep 27 2019

Light rain showers have been on the increase the past few hours as
northerly low level winds combined with vertical forcing aloft
associated with an upper jet over Nebraska. Lightning has been
absent so far, but some has been detected over central and west
central Colorado, so the slight chance of thunder will be left in
the forecast for now. The previous forecast had indicated that
most shower activity would be north of Denver, so no changes to
the forecast are necessary at this time. Showers should continue
for a few more hours before decreasing after midnight. 

&&

.SHORT TERM...(This evening through Saturday)
Issued at 240 PM MDT Fri Sep 27 2019

The forecast for tonight's showers and isolated thunderstorms
looks good. We're still seeing the highest coverage area north of
I-76, with the highest PoPs generally from the northern mountains
through the Fort Collins and Greeley area toward the Nebraska
border. Those showers will be forced by the right entrance region
of the departing upper level speed max to our north, as well as
weak mid level frontogenesis. There's smaller chance from Denver
southward as the airmass is drier and forcing is weaker in those 
locations.

A few showers may still linger overnight, and would not be
surprised to see a light dusting of snow come daybreak over a few
of the mountain peaks. Low level moisture flows in behind the
cool surge late this afternoon and evening, so expect some stratus
development overnight, with a slow erosion late tonight through
Saturday morning from the Palmer Divide area to points north and
east. Temperatures on Saturday may be held back a few degrees with
lack of lee troughing and morning stratus, but highs should still
end up near normal. Winds will be increasing from the south across
the Palmer Divide and plains east of I-25 where gusts around 35
mph can be expected. Mountain areas will also see gusty winds.
Fortunately, humidities will stay up.

.LONG TERM...(Saturday night through Friday)
Issued at 240 PM MDT Fri Sep 27 2019

An upper low over Oregon will deepen and become negatively tilted
Saturday night. This will further increase flow aloft and become
more southerly. Warm advection in this pattern will flow into the
area for a warm Sunday. A surface low will move northeast into
Wyoming, helping to pull stronger winds along the plains during
Sunday. Winds over the mountains will likely gust 45 to 55 mph as
the upper jet moves into northwestern Colorado, with gusts up to
45 mph over the eastern plains. This will likely bring critical 
fire weather conditions to most of the area, see discussion below.
A few showers may brush the northern mountains, but little if any
precipitation is expected. Temperatures will likely be about 10 
degrees warmer than Saturday. 

Models continue the slower trend of the evolution of the system
advertised earlier, so for Monday, should continue to see warm 
and dry conditions over the southern half of the forecast area and
another critical fire weather day. Over the northern half 
however, the surface low will be over eastern Colorado allowing 
for some cooler and more moist air to push in from the north. 

The upper low will have opened up to a trough stretching from
California up into Saskatchewan by Tuesday, lifting northeast as
it pushes against the strong upper high over the southeastern US.
High surface pressure pushing down from Montana will push in much
cooler air beginning Monday night and lasting into Wednesday, 
with a few showers expected over the eastern plains. Tuesday's and
Wednesday's highs will be limited to the 50s or 60s over both the
plains and mountains. Wednesday may see 

An upper ridge will be building over the area Thursday and 
Friday, allowing for warmer and drier conditions.  

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 832 PM MDT Fri Sep 27 2019

Evening shower activity has missed KDEN so far, but have come 
close to KAPA. In the past hour KBJC has begun to report rain.
Radar shows that almost all of the showers have been north of KBJC
up to the Wyoming border and then out across Weld County. Shower
activity should persist for a few more hours as it is being
produced by the right entrance region of an upper jet located over
Nebraska. Even if rain showers stay away from KDEN and KAPA, the
airmass will be getting cooler and wetter due to the shower
activity up to the north. This should bring ceilings down
overnight, possibly down to 3000-4000 feet AGL afte 10 PM to
midnight. These clouds should presist through sunrise, but then
diminish tomorrow morning as dry air surges back northward.  

Winds will be northerly overnight as cool air moves into the
region from the northern plains. Tomorrow will see winds veer
around to southerly and then increase through the afternoon. 

&&

.FIRE WEATHER...
Issued at 240 PM MDT Fri Sep 27 2019

Very strong southwesterly winds will develop on Sunday with gusts
up to 45 mph over the plains and up to 55 mph in the mountains.
Meanwhile, relative humidities will plummet to as low as 10 
percent over the plains, southern foothills and mountain valleys.
This will create elevated to critical fire danger with red flag 
criteria looking to be reached. A Fire Weather Watch will be
issued to almost all of the CWA. Northeastern Boulder and eastern Larimer
Counties as well as western Weld may not see these conditions due
to a likely development of a weak Denver Cyclone just to the 
north of DIA that will weaken the pressure gradient and wind 
speeds there. Also, humidities will be slightly higher and winds
slightly lower over the far northern mountains and plains and 
northeastern corner of the state.

There will likely be elevated fire weather conditions again on 
Monday, mainly over the eastern plains southeast of I-76, Palmer
Divide, southern foothills and high mountain valleys. Winds will 
gust to 35 mph with relative humidities dropping to as low as 10 
percent.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch Sunday afternoon for COZ212>214-216-239>241-
243>247-249.

Fire Weather Watch Monday afternoon for COZ213-214-216-241-
245>247-249.

&&

$$

UPDATE...Dankers
SHORT TERM...Barjenbruch
LONG TERM...Kriederman
AVIATION...Dankers
FIRE WEATHER...Kriederman