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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

                            
000
FXUS65 KBOU 210323
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
923 PM MDT Tue Aug 20 2019

.UPDATE...
Issued at 839 PM MDT Tue Aug 20 2019

Minor updates were made to POPs to reflect the current convection.
Right now, it appears the majority of the storms will be sub- 
severe this evening. However, better instability exists over the
I-76 corridor northeast of DIA up to the Cheyenne Ridge as the 
RAP mesoanalysis has over 2000 j/kg of mixed layer CAPE with up 
to 3000 j/kg near the Nebraska border. This is enough to support
the potential for a severe storm or two with large hail the
primary threat. The Severe Thunderstorm Watch goes until 11pm for
Weld and Morgan Counties which should be sufficient to encompass
the threat. 

Storms may continue into the early morning hours across the far
northeast corner. Stratus and fog may develop but there is low
confidence on the fog. 

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 220 PM MDT Tue Aug 20 2019

Fair weather CU over the higher terrain and east-west ridges this
afternoon will eventually give way to convection by this late 
afternoon. By 21z, will expect to see some isolated gusty 
convection fire off the mountains, foothills and ridges and expect
these to migrate off onto the east plains while strengthening 
into the evening hours as they move into much better moisture. 
Latest NAM12 run has MLCAPES out on the far northeast plains in 
the 3000-3800 J/kg range on the southern slope of a theta-e ridge 
extending from southeast Wyoming eastward into western and 
southern Nebraska. Rich surface boundary layer moisture of 8-10 
g/kg will reside over the far NE plains lending to the potential 
for strong to severe storms this evening along with the potential 
for heavy rain and large hail. Storm motion for this evening is on
the weak side as 500-300mb layer winds are only in the 15 to 25kt
range. A few of the stronger, slower or back-built storms could 
present some local hydro issues later this evening out along the 
I-76 corridor mainly northeast of Denver. Behind tonight's batch 
of convection will be the passage of much cooler air behind an 
approaching cold front currently near the Sheridan WY / Black 
Hills SD vicinity as a decent 1022mb surface high spreads 
southward across north plains states tonight. Will look for the 
main wind shift arrival in the 05-07z Wednesday timeframe. This 
will likely keep a north to northeasterly component to the surface
winds overnight and into Wednesday morning along with areas of 
low clouds towards dawn.

For Wednesday, with the initial cool moist, stable air, will look
for light easterlies through the morning along with the potential
for low clouds and patchy fog over the far eastern plains. Highs 
along the urban corridor and plains should only reach into the 
lower to mid 80s. Forecast soundings indicate the profile 
remaining fairly stable in the lower levels through late 
afternoon, so with persistent easterlies through the day, the 
best MLCAPES and boundary layer specific humidities will build up
against the Urban Front Range and foothills. Will keep the 
scattered pops across the area, and at this time will expect the 
main impact to be heavy rain associated with the storms along with
potential local hydro issues. Storm motion from 500-300mb winds 
aloft is still fairly weak and with surface easterlies and 
disorganized flow in the low to mid-levels, a few storms may have 
difficulty exiting locations which may be a problem for flood 
prone area such as an urban areas and burn scars. 

With cooler temperatures along with higher daytime dew points and
relative humidity, the fire danger should relax for Wednesday.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 220 PM MDT Tue Aug 20 2019

Scattered thunderstorms will continue into the evening hours 
along the Front Range and across the northeast plains Wednesday. A
few severe storms with large hail will be possible due CAPE 
climbing to 1500-2500 J/kg and 0-6km Bulk Shear climbing to 30-40 
kts.

A weak westerly flow aloft will again be over the state Thursday.
Surface low pressure over central Colorado and Wyoming will 
produce southeast winds across eastern Colorado Thursday. This 
will help keep moisture over the much of the area. Isolated to 
scattered storms should form over the Front Range and eastern 
plains Thursday afternoon and evening. Temperatures warm a little 
over Wednesday's highs with readings in the mid to upper 80s.

On Friday, moisture is starting to get pushed to the east by the 
westerly flow aloft. May have enough moisture for a few storms. 
The best chance will be across the eastern plains. With the 
drying, expect temperatures to warm back around 90 degrees. 

For the weekend and early next week, the upper level ridge will 
shift south as westerly flow aloft increase. This will dry out the
airmass and warm it up. Dry conditions with highs in the 90s is 
expected for the weekend. A weak cold front may back into the area
Sunday night and early Monday. This will bring cooler air with 
highs expected to reach the upper 80s for Monday and Tuesday. 
Could see enough moisture return to the area for storms on 
Tuesday. 

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 839 PM MDT Tue Aug 20 2019

A thunderstorm or two may briefly pass over the terminals this
evening with minor, if any, impacts. Outflow boundaries may continue
to develop and push through the terminals tonight shifting winds
temporarily. The next concern shifts to the possible development
of stratus during the early morning hours. With plenty of low
level moisture in place, it is better than a 50 percent chance
that stratus develops so I included a from group for it at DEN. 
There is some uncertainty, though, as most models have downslope 
winds for much of the night so it is not a sure bet. 

Strong thunderstorms will be possible Wednesday afternoon with
gusty winds and lower ceilings the primary threats. 

&&

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

$$

UPDATE...Danielson
SHORT TERM...Fredin
LONG TERM...Meier
AVIATION...Danielson