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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

FXUS65 KBOU 150351

Area Forecast Discussion
National Weather Service Denver/Boulder CO
951 PM MDT Tue Aug 14 2018

Issued at 945 PM MDT Tue Aug 14 2018

The late afternoon and evening strong to severe storm activity 
along the Front Range has diminished considerably with the last 
remaining t-storms continuing to track east-southeast away from
the Denver metro area into increasingly stable boundary layer air.
Eruption of today's hail and heavy rain storms coincided with the
passage of a shortwave trough which GOES IR/Water Vapor imagery 
now show over far eastern Colorado/Nebraska Panhandle. Clearing, 
albeit gradual, is now underway, although mid-level moisture 
visible on 6.19 and 10.35um GOES sat loops streaming up from the 
Desert Southwest may keep at least high cloud cover around 
through Wednesday morning. In addition, low-level moisture 
feeding up into southeast sections of the CWA may once again 
result in patchy to areas of fog towards dawn. Do not believe the
fog tonight will extend as far west into portions of the I-25 
corridor as it did last night. Other changes to tonight's 
forecast include only minor adjustments to wind and temperature 


.SHORT TERM...(This evening through Wednesday)
Issued at 222 PM MDT Tue Aug 14 2018

Thunderstorms will continue to form over the mountains and 
foothills this afternoon. Once the Urban Corridor and near by 
plains destabilize more, the storms will be able to progress 
eastward. Latest RAP and HRRR showing CAPE of 800-1400 J/kg in the
foothills and across the Urban Corridor late this afternoon and 
evening. This combined with precipitable water values around an 
inch and lift from a trough over Montana and Wyoming is expected 
to produce scattered thunderstorms over the mountains, foothills, 
and Urban Corridor. The airmass becomes more capped and more 
stable across the eastern plains. Expect storms to die off as they
move east of the Urban Corridor this evening. 

For Wednesday, a boundary will be over eastern Colorado. To the 
west of it, models indicate there will be drier air. Also, most of
the models keep convection south of I-70 Wednesday. However, 
there should be enough convergence along the boundary, moisture, 
and instability to produce a few storms. Flow aloft will increase 
and produce better shear. This along with CAPE of 1000-1500 J/kg 
could produce a few severe storms Wednesday afternoon over the 
eastern plains. 

.LONG TERM...(Wednesday night through Tuesday)
Issued at 222 PM MDT Tue Aug 14 2018

By Wednesday night the upper ridge will be over New Mexico and 
extend east over the Gulf coast. Specific humidity values will 
increase with PW values reaching to above an inch by late Wednesday. 
For Wednesday night storms will mainly be focused over the higher 
terrain of the Palmer divide with some cells being able to move over 
the plains with the breakdown and northern movement of the surface 
low. These storms will mainly be capable of gusty winds and small 

For Thursday and Friday the upper ridge will retrograde back to the 
West and North in Colorado. This will help to bring increased NW 
flow over the state. A cold front will push through over the NE 
plains Thursday afternoon that could produce enough CIN around peak 
heating time to inhibit much convective potential outside of the 
higher elevations of the mountains and foothills. Temperatures 
Thursday will be hovering just around normal for this time of year 
with highs in the mid to upper 80s. For Friday, some fog could be 
possible over the plains during the early morning hours as moisture 
will be high and temperatures slightly cooler in the post frontal 
airmass. By the late morning hours a disturbance embedded in the NW 
flow aloft will move over helping to introduce lift for storm 
development across the plains by the late afternoon hours. 
Instability is better over the far eastern plains after convective 
initiation with CAPE in the 1000-1500 j/kg with just around 500 over 
the Urban Corridor according to the GFS. PW values remain over an 
inch with steering flow only 10-15 kts. This indicates that any 
storms that do form will be slower moving and could produce a quick 
half inch in less than 30 minutes under the stronger cells. Friday 
will be slightly warmer than Thursday with highs in the upper 80s. 

For the weekend and into the start of next week the upper ridge will 
fully transition to over the western CONUS leaving Colorado in 
increased NW flow. This will allow for the influx of upper waves and 
fronts to bring cooler air and increased chances of showers and 
thunderstorms to the region. The first shortwave will move through 
Saturday into Sunday bringing storms Saturday late afternoon and 
evening capable of brief heavy rain and small hail. As the shortwave 
moved over forcing could continue storms overnight into Sunday. A 
cold front is set to move out of WY into CO Monday ushering in 
continued cooler temperatures with highs possibly in the 70s by 


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 945 PM MDT Tue Aug 14 2018

Storms for the most part have moved east of the Denver metro area
and ceilings have begun to rise with the development of a drying 
south-southwest drainage wind of 5-12kts. Can't rule out a stray 
rain shower at any one of the Denver area terminals before 06z 
tonight, then gradual clearing thereafter. Restrictions due to fog
are not expected tonight.