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Area Forecast Discussion (AFD)

Issued by Denver/Boulder, CO (BOU)

FXUS65 KBOU 191547

Area Forecast Discussion
National Weather Service Denver/Boulder CO
947 AM MDT Wed Jun 19 2019

Issued at 945 AM MDT Wed Jun 19 2019

Adjusted sky grids to reflect the clearing from west to east, as
well as refined the PoP grids to reflect the consensus of the
high-resolution models of a better chance to see the first storms
of the day popping over northern Weld County then pushing east, 
as well as adding a slight chance of showers/storms over southern 
Lincoln County later this afternoon. No other adjustments needed 
at this time. 


.SHORT TERM...(Today and tonight)
Issued at 301 AM MDT Wed Jun 19 2019

Another wave of showers and thunderstorms is slipping across the 
Wyoming border at this time. Will keep the mention of this in the 
grids until around 12z, then these showers/thunderstorms are 
expected to dissipate as the short wave exits the cwa. Patchy fog
will persist as well until around 15z mainly north of Denver and 
east of I-25. For today, the mid and upper levels will becoming 
drier and more subsident by this afternoon. Temperatures today 
will climb back to near normal. Thunderstorm coverage will be 
isolated across most of the forecast area. Fairly healthy CAPES 
still exist however over the far northeast plains. SPC still 
keeps a marginal risk of severe thunderstorms along the 
Nebraska/Kansas border this afternoon, with a better chance of 
thunderstorms east of Sterling and Akron line. CAPES still around 
1500-1800 j/kg there this afternoon with the stronger storms 
capable of producing moderate to heavy rainfall, hail up to nickel
size and wind gusts in the 50-60 mph range. A stronger 
northwesterly flow aloft will negate any flooding threat. In the 
mountains, it will be windy especially above timberline. Late 
tonight, NAM12 spatial cross-sections show some mountain wave 
amplification so stronger winds expected to occur in the Front 
Range mountains and higher foothills of Boulder and Larimer 
counties. Gusts around 50 mph range may develop in those areas 
after 06z. 

.LONG TERM...(Thursday through Tuesday)
Issued at 301 AM MDT Wed Jun 19 2019

On Thursday, a large upper level low will be over the Northern 
Rockies with several short wave troughs rotating around it. Over 
Colorado, westerly flow aloft will prevail. At the surface, a low 
pressure trough will be east of the foothills. West of the trough,
gusty westerly downslope winds will prevail. East of the surface 
trough, southerly flow will transport moisture northward. Surface 
based CAPE east of the trough will be up to 500-700 J/kg, so a few
strong storms will be possible. Precipitable water values are 
expected to range from a half inch over the higher terrain to 
three- quarters of an inch across the plains, somewhat higher than
expected given the westerly flow. Temperatures will warm to near 
normal with highs in the 80s across northeast Colorado. A jet at 
base of the low is expected to provide lift over northern parts of
the area. This is where the most showers and storms are expected 
to be. 

The main upper level low tracks east across Montana Thursday 
night and Friday. One piece of energy rotates south across the 
Great Basin Friday and strengthens into a deep trough or closed 
low over Nevada. Flow aloft turns southwesterly across Colorado. 
Models show a cold front pushing south through eastern Colorado 
sometime Friday. The timing is still unclear. Upslope flow behind 
the front and lift from the jet, combined with precipitable water 
values nearing an inch along the Front Range should result in 
numerous showers and thunderstorms. May be a marginal severe 
threat as well Friday. Biggest challenge for this...will it be 
unstable enough for severe storms. Plenty of shear will prevail 
with the strong flow aloft. 

Models still trying to resolve what happens to the upper level 
low/trough over the Great Basin. There is a good amount of 
uncertainty on the strength and timing of this system as it tracks
east across the Central Rockies. On Saturday, there will be lift 
from the jet and approaching upper level low/trough. Good moisture
will remain in place with precipitable water values near an inch 
across northeast Colorado. A cooler airmass and cloudy skies is 
expected to result in a chilly day. Highs may struggle to reach 
the 70s. If there is enough clearing for temperatures to climb 
into the 70s, there should be enough instability for a chance of 
severe thunderstorms. 

Another thing to watch is the cold air over the mountains with 
this system. There is enough cold air to drop snow levels below 
10,000 feet MSL Saturday night on the ECMWF. The NAM is quicker 
with the snow levels falling below 10,000 feet MSL by Friday 
night. Most models indicate the potential for snow over the north 
central mountain this weekend. If the cold air arrives sooner, a 
good amount of snow could fall above 10,000 feet. At this time, 
best precipitation is expected to occur before the cold air 

Sunday's forecast is still unclear. If the low/trough is moving 
over Colorado or still to the west, would expected another round 
of numerous showers and thunderstorms. If the low/trough is east 
of the area, subsidence behind it should limit convection to 
scattered at most. Either scenario should produce another cool day
with highs in the 70s. 

For Monday and Tuesday, an upper level ridge starts to build over
the Southern and Central Rockies. This will start a warming trend
with highs reaching the 80s by Tuesday. Models indicate moisture 
will decrease, but enough should be around for at least isolated 


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 945 AM MDT Wed Jun 19 2019

Generally expected VFR conditions over the next 24 hrs. Drainage 
winds are beginning to transition to west/northwesterly late this
morning and afternoon with speeds in the 8-12 kt range. Only 
isolated thunderstorm coverage today mainly near the higher 
terrain and not expected to impact the terminals at this time.
Winds will return to drainage overnight, with southeast-east winds
tomorrow as a Denver Cyclone develops due to a surface low forming
over the southern Foothills.