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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

FXUS65 KBOU 240306

Area Forecast Discussion
National Weather Service Denver/Boulder CO
906 PM MDT Fri Aug 23 2019

Issued at 906 PM MDT Fri Aug 23 2019

Latest water vapor and IR satellite imagery shows strong upper
level diffluence over northeast Colorado. This is occurring on the
nose of a 60+ knot upper level jet diving into Central Colorado.
Stronger storms that have tapped into the moisture and instability
have produced large hail earlier this evening, but looks like
we'll be shifting more toward a locally heavy rain threat as
storms are encountering a richer but slightly more stable airmass
over the northeast corner of the state. Storm propagation has been
relatively slow to the east, so we'll have to watch for better
organization through the late evening/early morning hours before 
the action finally shifts east of our forecast area. Behind that, 
there are still isolated weak showers over the northern mountains 
and along the I-25 Corridor where favorable QG lift is occurring, 
but most of this should remain weak and eventually dissipate with 
the loss of already weak instability later this evening. 

We'll add some patchy fog on the northeastern plains where T/Td
spreads become quite small overnight. 

Unfortunately, main concern shifts toward fire weather this
weekend. Winds come up tomorrow and we'll be approaching Red Flag
conditions in some of the mountain valleys already. The highest 
threat day is certainly shaping up to be Sunday when winds 
increase and the hottest/driest conditions occur. See Fire Weather
section for more discussion. 


.SHORT TERM...(This evening through Saturday)
Issued at 212 PM MDT Fri Aug 23 2019

So far very little convection has developed this afternoon due to
the swath of mid/high level moisture which has been over the 
areas for much of the day. PW values are under a half inch over 
the Front Range while dewpoints are generally in the 40s. As 
expected any severe threat will be east of the urban corridor 
given the low level dryness and only expect gusty winds with the
weaker showers. Further east, first hints of stronger convection 
beginning to fire up along a boundary across Central Elbert county
and deeper moisture ahead of it and stretching across much of far
northeast Colorado. LAPS/ACARS data indicating ML CAPE values 
from 1500-2500j/kg and decent southeast low level flow. Overall 
target area for severe has not changed since this morning and will
continue to target the northeast plains through this evening.
Severe thunderstorm watch just getting ready to be issued for the
plains through this evening. 

The upper trof will sweep across northern Colorado tonight and be 
replaced with a more subsident and drier airmass on Saturday. Most 
of the county warning area will be thunderstorm free with the 
exception of the far eastern border area with Kansas with some 
residual low level moisture hanging on. This might even be a 
stretch but will leaving in low pops. Main concern will be 
elevated fire conditions (see discussion below).

.LONG TERM...(Saturday night through Friday)
Issued at 212 PM MDT Fri Aug 23 2019

The long term period will trend from hot to cooler as northwest 
flow aloft and lower heights dominate the central Rockies and a 
series of cool surface surges from Canada move into northeast 
Colorado. Highest risk for critical fire weather conditions is 
still expected on Sunday over the western portion for the 
Cheyenne ridge and far northern mtns and foothills and high parks.
Overall, no significant changes from the previous forecasts in 
the extended, mainly just dialing in timing of a series of mostly 
dry cool fronts.

By Saturday evening, strong synoptic scale subsidence and dryer 
air behind an exiting shortwave trough over the central US plains 
will begin to take the moisture content over the entire column 
significantly lower than what we've seen the past few days and 
weeks. Low-end ML CAPES Saturday evening should exist over the 
far eastern plains and eastern Palmer, so will expect some storm 
coverage near the CO/KS border, but ending before midnight.

Forecast soundings by Sunday morning indicate precipitable water 
content in the 0.45 to 0.55 inch range with fairly strong mid-
level stability all day. As a lee trough develops over the 
eastern plains by midday Sunday, 700mb level winds increase to 
the 25-35 mph range during the afternoon, which would bring some 
localized gusts up to 25-35 mph range over the higher Medicine 
Bow range, High Parks and along the western portion of the 
Cheyenne ridge. With 6-8 hours of relative humidities in the mid-
teens Sunday afternoon along with increased winds speeds and 
favorable fuel status, will issue a Fire Weather Watch at this 
time for these elevated locations.

Lurking to the north late Sunday evening will be the first of 
three cool fronts expected for the northeast CO this upcoming 
week. This first surge will enter the plains near 12z Monday and 
is expected to be mainly a cooler-temperature wind shift. The 
NAM12 indicates meager CAPES along the Palmer and southern Urban 
Corridor by Monday afternoon, however with dry subsident flow 
aloft and limited surface moisture, will expect only gusty 
isolated afternoon storm coverage. Main thing this surge will do 
is bring temps down 10-15 degF from Sunday as highs only peak in 
the lower to mid 80s for much of the lower elevations. Can't rule 
out some low level clouds overnight Monday into early Tuesday as 
soundings indicate some low level moisture trapped in the boundary
layer. With a bit more moisture indicated in the lower levels 
Tuesday will keep isolated afternoon shower and storm coverage in 
place through the evening. Synoptically, not much change takes 
place from Monday into Tuesday as dry moderate to strong northwest
flow aloft persists. Will expect a slight increase in high temps 
for Tuesday under mostly sunny to partly cloudy skies as temps top
out near seasonal averages.

As Tropical Storm Ivo moves northwestward out the Baja region and
out into the E Pacific, this eventually allows for better 
organization of a stagnant southern US/subtropical ridge to begin
re-development late Tuesday into Wednesday. This ridge should 
remain somewhat established into next weekend over the southwest 
US. By Tuesday evening into Wednesday, due to the persistent 
strong and dry northwesterly flow aloft over the northern US 
Rockies and lower mid-level heights, near seasonal temps are 
expected to persist. A weak disturbance in this northwest flow 
over the northern plains states will send southward the second in 
a series of weak cool surges on to the high plains. This surge 
will enter onto the NE CO plains midday Wednesday. This should 
bring temps a few degrees lower again with highs in the lower to 
mid 80s. As the flow aloft weakens due to the influence of the 
building subtropical ridge, a bit deeper moisture will be drawn 
northward into the central Rockies by Wednesday afternoon. With 
cooler surface temps and while under an environment of synoptic 
subsidence, only isolated PoPs are expected. 

For Thursday and Friday, the upper ridge remains established over
the desert southwest and northern Mexico with weak northwesterly 
flow aloft over the CO. The GFS and Canadian hint at the third 
weak surface surge moving into onto eastern plains late Thursday 
into early Friday. With a slight increase in mid and upper level 
moisture over the Rockies, will expect scattered afternoon shower 
and storm coverage to return both afternoon with temps holding 
near to slightly below seasonal average.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 906 PM MDT Fri Aug 23 2019

Last of the boundaries appears to be pushing through KDEN as of
03Z. Isolated light showers should be dissipating over the next
hour or two. VFR conditions will persist, with winds returning to
normal diurnal wind patterns by/shortly after 06Z and continuing
through Saturday. No chance for storms Saturday as a much drier
airmass moves in. 


Issued at 212 PM MDT Fri Aug 23 2019

Elevated fire concerns for Saturday afternoon with drying airmass 
and some gusty winds. Overall threat is marginal as wind gusts 
generally in the 10-20 mph range over mountain valleys and not 
expecting many gusts beyond 25 mph for an extended period of time. 
Will not have any fire hilites for Saturday and leading the way for 
a higher threat of fire danger on Sunday. 

Critical fire weather conditions are expected to develop for 
Sunday afternoon. A ridge of dry and stable high pressure will 
develop over the central Rockies this weekend along with a 
tighter pressure gradient expected to develop in the lower and mid
levels of the atmosphere Sunday and a lee trough across the
eastern plains. This will allow for the combination of warmer 
temperatures, low relative humidities and gusty northwest winds 
over the favored elevated locations during the afternoon hours 
Sunday. The main areas set for elevated fire conditions will be 
the far northern mountains, high parks and Cheyenne Ridge. A Fire 
Weather Watch has been issued for these locations Sunday 
afternoon from 17z to 02z.


Fire Weather Watch from Sunday morning through Sunday evening 
for COZ211-213>215-238-242.



SHORT TERM...Entrekin
LONG TERM...Fredin