Area forecast discussion...updated 
National Weather Service Denver/Boulder Colorado 
940 PM MDT Friday may 22 2015 

issued at 937 PM MDT Friday may 22 2015 

Weak qg ascent over northestern Colorado at the present immediately 
ahead of a low amplitude wave in southwesterly middle-level flow. Have 
witnessed a modest reduction in T-storm intensity and coverage 
across southeastern portions of the County Warning Area...while showers have grown in 
number along the Front Range and western portions of the Palmer 
Divide. In addition...pockets of dense fog has begun to form on 
the plains...specifically in the Akron area where the prevailing 
visibility has dropped to a quarter mile. Over the next couple of 
hours...should see shower coverage expanding northward over northern 
portions of the I-25 urban corridor...the Denver metropolitan area and 
points to the east-northeast of the metropolitan area. Whereas T-storms 
presently concentrated in the Lincoln County area should shrink in 
coverage as they track east-northeast out of the County Warning Area. Believe 
these storms will still have the potential to produce small 
hail...frequent cloud-to-ground lightning and locally heavy rainfall. Elsewhere 
rainfall amounts should generally remain under a quarter inch... 
though embedded convection could deposit a quick half inch 
resulting in minor street and Gully flooding. 06z and beyond cloud 
cover should gradually thin once the shortwave passes allowing 
cooling to produce areas of some areas dense fog reducing 
visibilities down to a quarter mile. Should see fog form in the Denver 
metropolitan area not long after showers move out as temperature/dewpt spreads 
are already 1 to 2 degree f. Updated the 1st period forecast grids to 
better reflect current conditions and for greater fog coverage. 


Aviation...update (for the tafs through 00z Saturday late 
afternoon) issued at 937 PM MDT Friday may 22 2015 

Showers and isolated thunderstorms drifting northeast across the 
Denver metropolitan area next 1-3 hours may produce MVFR ceilings and visibilities 
due to light to moderate rainfall and fog. Once these showers move 
out of the metropolitan area...sometime between 11 PM and 12 am MDT 
tonight...look for areas of fog to form or advect in from the 
plains. Visibilities may lower to below 3 miles in the fog after 1 
PM and possibly as low as 1/4 to 1/8 mile after 3 am at Denver 
area terminals. Should see this fog gradually recede from the 
foothills and Palmer Divide not too look after sunrise as drier 
air drains down off the higher terrain. 


Short term...(this evening through saturday) 
issued at 340 PM MDT Friday may 22 2015 

Areas of rain from this mornings upper disturbance have just about 
cleared the northern parts of the forecast area while an area 
showers and thunderstorms has developed over the Palmer Divide. 
Convection that has been forming in the warm air on the south side 
of the Palmer Divide has been weakening as it progresses into the 
cooler air present over the northern side of The Divide. 
Additional showers should continue developing for a few more hours 
with thunder still possible due to elevated instability. Showers 
are moving fast enough that flash flooding rains are not a threat. 
Showers should end this evening as the airmass cools further and 

Overnight...light easterly flow and the moist boundary layer 
should allow areas of fog and stratus to develop. For 
tomorrow...the main upper level low which is currently over 
Arizona will be ejecting over western Colorado tomorrow afternoon. 
Strong dynamics associated with this feature will combine with 
abundant low level moisture and a deepening surface low to set the 
stage for an outbreak of severe thunderstorms across the plains. 
Have increased probability of precipitation to the likely category and also mentioned that 
some storms may be severe. Heavy rain from these storms may cause 
localized flooding problems. Stay tuned for the latest forecast 

Long term...(saturday night through friday) 
issued at 340 PM MDT Friday may 22 2015 

Showers and thunderstorms continue across north central and 
northeastern Colorado Saturday night as the upper level low moves 
across the state. The activity will be most numerous north of 
Interstate 70...with a threat of severe storms mainly north and east 
of Denver during the evening. There will be the potential for 
additional flooding as locally heavy rainfall will be possible. The 
upper low will lift north into northeastern Wyoming and western 
South Dakota by Sunday evening. Could see some low cloud...drizzle 
and fog Sunday morning on the plains...with scattered showers and 
thunderstorms by afternoon. Less shear and instability should keep 
storms below severe levels. 

In the mountains...rain and snow will be likely as moisture on the 
back side of the low wraps into north central Colorado. The snow 
level will remain above 8500 feet with a few inches of snow possible 
above 10 thousand feet. 

On Monday the atmosphere dries out somewhat as the flow becomes more 
zonal. However...there should be enough residual moisture combined 
with daytime heating to produce isolated to scattered showers and 
thunderstorms...most numerous over the mountains. Warmer and drier 
weather is expected on an upper level ridge of high 
pressure builds over the Rocky Mountain region. Low level moisture 
and shear looks to be limited with the models showing inverted v 
soundings. This would result in gusty thunderstorms with light to 
moderate rainfall...however a few stronger storms can not be ruled 
out. Showers and storms will increase somewhat by Wednesday as an 
upper trough approaches Colorado from the west. GFS sounding show 
convective available potential energy across far northeast plains above 2000 j/kg and some shear. 
Therefore...wouldn't be surprised to see some severe storms over 
portions of the plains. 

Slightly cooler temperatures and scattered showers and thunderstorms 
are expected on Thursday as the weak upper trough moves across 
Colorado. On Friday...the upper low moves into the Central Plains 
states...with a northwest flow aloft setting up over Colorado. 
Cooler unsettled weather is expected to return by Friday a cold front moves into northeastern Colorado. The 
GFS is stronger with the front and has more precipitation along the 
Front Range urban corridor than the European model (ecmwf). Have decided to raise 
probability of precipitation somewhat during this period. 


Aviation...(for the tafs through 00z Saturday late afternoon) 
issued at 340 PM MDT Friday may 22 2015 

The threat of rain showers one or two thunderstorms will continue 
in the early evening. MVFR conditions will be possible as showers 
move over. Overnight...light easterly flow and a very moist 
airmass may lead to the development of areas of stratus and fog. 
This may drop ceilings and visibilities to low IFR for a few 
hours before and after sunrise. Tomorrow afternoon will feature 
scattered thunderstorms with heavy rain and MVFR conditions at 


issued at 340 PM MDT Friday may 22 2015 

Rainfall amounts earlier today were light enough that only small 
rises have been registered on local rivers. Spotty shower activity 
this evening should not be of too much additional consequence. 
Soil moisture conditions will be given an 18-24 hour period to 
recover through tomorrow afternoon. But when thunderstorms develop 
Saturday afternoon...runoff will be fast to begin. Flash flooding 
may be a possibility tomorrow afternoon and evening...but it is 
hard to tell at this time where the focus of heavy precipitation 
is going to be. 

The Cache la poudre and South Platte rivers remain above flood 
stage in Larimer and Weld counties...and will be that way for a 
couple more days. 


Bou watches/warnings/advisories... 


short term...dankers 
long term...Kalina 

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