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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

                            
000
FXUS65 KBOU 201640
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1040 AM MDT Tue Aug 20 2019

.UPDATE...
Issued at 1029 AM MDT Tue Aug 20 2019

Northeast CO will be under the peak of a subtle ridge axis 
through tonight ahead of a trend toward lower heights starting 
with the passage of a cool front late tonight and into early 
Wednesday. Latest 12z KDNR and ACARS soundings from this morning 
still showing a pretty strong suppressive lid just above 500mb, 
so it may take some time to erode that feature with no significant
temp advection in the mid-levels expected thru this afternoon. 
Temps still expected to reach the mid to upper 90s across the 
lower elevations today and the elevated fire danger will certainly
exist due to the lower humidities especially over the high 
terrain.

Latest short term models indicating slightly less thunderstorm 
coverage this afternoon along the Front Range and backs off from 
stronger storm initiation over the northern Urban Corridor. At 
this time, will expect isolated gusty weak to moderate storms to 
fire off the higher terrain of the mtns and foothills later this 
afternoon with a better chance for scattered stronger storms into 
the evening hours out on the NE plains. These storms should have 
the potential to become severe while containing large hail, heavy 
rain and strong outflow mainly for locations east of the I-25 
corridor and along the Cheyenne Ridge area north of the Platte 
River valley. By early evening, would also expect an another 
array of near random outflow boundaries, so can't rule out storm 
initiation elsewhere across the plains and I-25 Corridor later 
this evening. Would give a bias toward late evening storms 
rolling past midnight mainly east of an Akron to Limon line as ML
capes are still indicated in the 2500-3500 J/kg range while in an
environment of decent vertical shear over the far northeast 
plains.

Cooler temps are still on track for Wednesday with the 
possibility of some low clouds during the morning hours, followed
by scattered coverage of afternoon showers and storms.

&&

.SHORT TERM...(Today and tonight)
Issued at 307 AM MDT Tue Aug 20 2019

An upper level ridge will continue over the area with weak 
southwest mid level flow.  At the sfc, low pres will extend from the 
front range into sern CO. This will allow for the low level flow 
to be more easterly by aftn, however, main cold front won't move 
into nern CO until this evening. Thus expect another hot day over 
the plains with highs in the 95 to 100 range. Record high at 
Denver is 99 so it could be tied.

Meanwhile, as for tstm potential, there isn't much agreement 
among the models as to what may transpire over the plains. The 
ECMWF keeps most activity over the far nern plains while the GFS 
has activity closer to the foothills. The hi res data is just as 
confusing with varying solutions as well. Overall, will keep pops
in the chance category, except closer to the WY-NE border for 
tonight, where this some agreement for better tstm coverage. Over 
the higher terrain there will be some widely sct high based 
activity this aftn into the early evening hours. 

As for severe potential MLCAPE will be above 2500 j/kg over the
far nern plains late this aftn into this evening. Mid level flow 
is only around 15 kts but with decent cape can't rule out isold 
severe storms. Furthermore with hi theta-e values in the 850-700 
mb layer and weak flow aloft certainly could see very heavy 
rainfall with the storms as well this evening closer to the WY-NE 
border.

.LONG TERM...(Wednesday through Monday)
Issued at 430 AM MDT Tue Aug 20 2019

Models show upper ridging over the forecast area Wednesday 
through Thursday night. There is a weak upper trough over the 
Great Basin through much of that time. There is weak upward 
synoptic scale energy noted on the QG Omega fields Wednesday and 
Wednesday night, with neutral energy progged Thursday and Thursday
night. The low level wind and pressure fields have easterly winds
on Wednesday and Wednesday evening then weak normal drainage 
patterns overnight. Southeasterlies are expected on Thursday, with
more drainage Thursday night. Concerning moisture, it increases 
on Wednesday and Wednesday night with precipitable water values in
the 0.75 to 1.25 inch range for the plains and lower foothills. 
There is only slightly drying on Thursday, but more drying 
Thursday night. CAPE values are pretty decent Wednesday and 
Wednesday evening over the CWA east of the divide. On Thursday, 
CAPE is even higher for the plains, but then values decrease 
significantly by Thursday evening and overnight. Lapse rates on 
the cross sections show a pretty decent low level cap in place 
over the plains for much of the Wednesday with the new airmass in 
place. However, above 700 mb, lapse rates are pretty steep. The 
QPF fields have fairly decent amounts of measurable rainfall for 
the CWA Wednesday afternoon through the evening. Same goes for 
late day Thursday, with just a tad less than the previous day. 
Will go, or leave in 30-60% pops late day Wednesday and 20-50% 
pops late day Tuesday. For temperatures, Wednesday's highs are 
3-7 C cooler than today's expected highs behind a cold front that 
is progged for the CWA later this evening. Thursday's readings 
come back up 1-4 C for much of the CWA. For the later days, Friday
through Monday, the upper ridge flattens on Thursday and Friday, 
with the upper ridge building back into Colorado from the due west
over the weekend. Models show it to be pretty dry all four days.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1029 AM MDT Tue Aug 20 2019

Will expect the normal diurnal turning of surface winds into the 
afternoon ahead of thunderstorms development in the 22z Tue to 
00z Wed timeframe. At this time, still expect main impacts from 
the storms to be strong gusty outflow this afternoon. Later this 
evening, the likelihood for outflow boundaries with an easterly 
component should be the rule for the mid to late evening. A 
surface cold front, behind exiting convection to the east, is 
expected to move into all terminals in the 05z to 07z Wednesday 
timeframe. Will expected a persistent north to northeasterly 
component to the winds to continue into early Wednesday along with
a lower clouds deck. At this time still expect VFR conditions 
through Wednesday morning.

&&

.FIRE WEATHER...
Issued at 307 AM MDT Tue Aug 20 2019

Elevated fire danger will continue over the higher terrain and
along the front range today due to low humidities and hot 
temperatures. However winds will remain generally light.

&&

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

$$

UPDATE...Fredin
SHORT TERM...RPK
LONG TERM....RJK
AVIATION...Fredin
FIRE WEATHER...RPK