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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

FXUS65 KBOU 181545

Area Forecast Discussion
National Weather Service Denver/Boulder CO
945 AM MDT Tue Jun 18 2019

Issued at 934 AM MDT Tue Jun 18 2019

|One more cool and stormy day expected across the area 
with the main threat of heavy rain and potential flooding. There is 
also a marginal severe weather threat with hail and damaging winds, 
especially across the Palmer Divide. 

Water vapor imagery showing exiting low pressure over Kansas but 
still a lot of cloudiness across Colorado. Upstream weather feature 
today is a disturbance stretched across Northwest Wyoming and 
northern Utah. This feature will move southeast across Colorado 
later today and tonight with weak synoptic upward ascent. 

As for our flash flood threat today, moisture levels remain high 
today as precipitable water values from the 12z DNR sounding are at 
.91 of an inch, which is significantly above our normal moisture 
level for this time of year. Surface dewpoints are in the 50s across 
all the northeast plains while the mid level flow this morning is 
only around 10kt. However, with the incoming trof from the 
northwest, mid level flow expected to increase to 20-25kt by mid 
afternoon. Instability is more marginal today given the expected 
persistent cloud cover with surface based cape values in the 600-
1000j/kg. Expect storm motions to increase to 10-15kt and so storms 
should be moving just enough to minimize the flash flood threat. 
However, rainfall rates will expected to be 1-2 inches of rain in 
less than hour and could see mainly more advisory type messaging for 
the storms today. If we get hail today, expect more small hail given 
the marginal instability and generally weak flow. SPC has targeted 
the Palmer Divide for a marginal risk and this is ok given elevated 
terrain there.  

Will continue with the high pops today given the moisture levels and 
synoptic scale weak lift expected. Temperatures will remain on 
the cool side with highs only in the 60s/lower 70s with the cloud 
cover this morning and expected storm coverage this afternoon. 


.SHORT TERM...(Today and tonight)
Issued at 244 AM MDT Tue Jun 18 2019

The cool and unsettled weather pattern will continue across the
forecast area for one more day. Areas of fog with a few lingering
rain showers will be possible through 15z then diminish. The high
resolution models indicate another round of showers and
thunderstorms this afternoon and evening. The risk of severe
thunderstorms will be in the marginal category today and is 
expected to be confined to eastern Elbert, Lincoln and southern 
Washington counties. Forecast soundings indicate CAPEs of 
1000-1500 j/kg around Denver. CAPES are lower to the north and 
northeast of Denver, but closer to 2000 j/kg around Limon. 
Precipitable water values remain close to one inch across the 
urban corridor and northeast plains. Main threat with most of the 
storms today will be moderate to heavy rainfall, small hail and 
wind gusts to 35 mph. Storm motions should be a little higher 
today which could help lessen the flash flood threat. Main concern
will be for areas already saturated by heavy rain from 
yesterday's storms primarily southern Washington and northeast 
Lincoln counties. The high res models generate a secondary round 
of showers and thunderstorms this evening, but these storms will 
be less intense than the storms this afternoon. A few showers or 
storms may again linger overnight, but should diminish by late 
tonight as drier northwesterly flow develops behind the exiting
mid level trough.

.LONG TERM...(Wednesday through Monday)
Issued at 244 AM MDT Tue Jun 18 2019

Northwest flow aloft will prevail Wednesday behind the exiting 
short wave trough. The airmass dries some with precipitable water 
values falling a quarter inch. Mix Layer CAPE looks to be 300 to 
800 J/kg. There should be enough moisture and instability for 
isolated to scattered convection. Convection will have to over 
come weak subsidence. Coverage should be less and the storms 
should be weaker. Highs across northeast Colorado are expected to 
climb into the upper 70s to mid 80s.

A closed low will drop south from Canada into the Northern 
Rockies Thursday. This will cause flow aloft to turn westerly. The
is expected to produce a slightly warmer and drier airmass over 
Colorado. Highs across northeast Colorado will climb into the 80s.
Precipitable water values will range from a half to three- 
quarters of an inch. Mixed Layer CAPE will be less than 700 J/kg. 
However, the right entrance region of the jet will be over the 
Central Rockies Thursday. Expect this to help showers and storms 
form. Best coverage looks to be over northern parts of the area 
where the best lift is forecasted to be. A cold front drops south 
during the evening, this may also help trigger convection. Good 
chance showers and storms linger through night due to the synoptic
lift from the jet.

Cooler conditions are expected Friday behind Thursday evening's 
cold front. Highs over northeast Colorado should be in the 70s 
with 50s and 60s for the higher terrain. Showers and perhaps a few
storms will be on going during the morning hours. The right 
entrance region is forecasted to slowly sink south Thursday night 
and Friday. Lift from the jet and a slight increase in moisture 
will result in a better chance for showers and a few 

Slightly cooler temperatures are expected for Saturday as a short
wave trough rotates around the low to the north and across the 
Central Rockies. This will bring another round of showers. A few 
storms will also be possible, but instability will be weak. This 
system will be chilly with snow possible above timber line Friday 
night and Saturday. 

Models show a westerly downslope flow for Sunday. This should 
result in drier conditions with slight warming. The warming and 
drying trend is expected to continue into next week as an upper 
level ridge forms over the Central Rockies. 


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 934 AM MDT Tue Jun 18 2019

MVFR ceilings expected to dissipate by later this morning but
still a mid level deck of clouds covering much of the area. Expect
several rounds of storms today, the initial one from 19-23z, and
another wave this evening with the disturbance from Wyoming. 
Could see ILS approaches with the storm passages along with gusty 
and erratic winds and heavy rain.


Issued at 934 AM MDT Tue Jun 18 2019

All area rivers and streams continue to behave themselves and all
are under flood and action stages today. Cool conditions will also
favor this very slow melt off today. Main impact today will be
heavy rain with storms as our precipitable water values approach
an inch with storm motions 10-15kt. Will not issue a flash flood
watch today due to expected increasing in storm motions this
afternoon and marginal instability. More information available in
the discussion above.