Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 270226

Area Forecast Discussion
National Weather Service Denver/Boulder CO
826 PM MDT Wed Jul 26 2017

Issued at 826 PM MDT Wed Jul 26 2017

A very moist airmass is in place with precipitable water value of
1.34 inches from the 00Z sounding at DNR. This is the highest
ever recorded for July 27th. Very heavy rain from the
thunderstorms is occurring with rainfall rates of up to 1 inch in
15 minutes with the strongest thunderstorms. The heavy rain and
flash flooding threat will continue through the evening as the
thunderstorms progress eastward.


.SHORT TERM...(This evening through Thursday)
Issued at 400 PM MDT Wed Jul 26 2017

An abnormally moist and marginally unstable atmosphere covering the
forecast area today will continue to fuel shower and thunderstorm
formation over and along the mountains. Storm motions this afternoon
have been quite slow, generally in the 10 to 20 mph range.
Instability is generally lower across northern sections of the CWA
as is evident by the poor organization and relatively short duration
of storms drifting off the foothills in Larimer and Boulder
Counties. However, a few of these storms before moving off the
foothills managed to deposit 0.25 to 0.50 inch of rain in under 45
minutes. The HRRR and RAP mesoscale models have done a pretty decent
job predicting the formation and slow progression of these storms,
although storm QPF has been over done. Will keep that in mind going
forward with storms downstream on the adjacent plains is evening.

Turning to areas farther south, such as along and south of
Interstate 70. Over the next few hours, should see storm coverage
expand eastward off the foothills as low-level moisture is fed into
the area by a deepening northeast flow. Should see the convection
drifting off the high terrain tapping into this moist and relatively
unstable environment resulting in scattered to numerous showers and
embedded t-storms. Slow easterly storm motions and mean layer PW
values in the 1.1 to 1.5 inch range should have little problem
producing storms capable of locally heavy rainfall. High res models
show storm coverage expanding eastward along the Palmer Divide
during the early evening hours with rainfall of 0.50 to 1.5 inch per
hour possible. Through the evening, high res models show this
activity moving across Elbert and Lincoln counties where higher
theta-e and PW could result in local rain totals in excess of 2
inches. This area will bear watching this evening. No flash flood
watch at this time, but local flash flooding is a fair bet. Further
more could see scattered storms in this area linger beyond midnight.
Late tonight, showers and storm activity will drop off and with
partial clearing could see patchy dense fog form in eastern
sections toward morning.

On Thursday...the moist atmosphere is expected to shift west of the
Front Range with the upper high pressure ridge migrating westward
over Utah and western Colorado. Should see few storm tomorrow with
PW values on the decline. And with winds on the plains turning
southerly, upslope will play less of a role in producing storms
along the Front Range. Still, the high country should see scattered
showers and t-storms form after the noon hour with locally heavy
rainfall still a possibility during the afternoon west of the Cont
Divide. Temperatures in the next 24 hours should be little

.LONG TERM...(Thursday night through Wednesday)
Issued at 400 PM MDT Wed Jul 26 2017

The upper ridge remains over Colorado Thursday night into
Saturday. Models still show it to move westward the rest of
Saturday and Saturday night. There is very weak upward vertical
motion on the QG Omega fields much of the time. Models hang on to
southeasterly low level flow Friday and Saturday. There is
substantial moisture to remain over the CWA through Saturday
night. Precipitable water values stay in the 0.75 to 1.50 inch
range Thursday night through Saturday night. The surface dew point
progs have 60s F readings over the plains all five periods. The
mountains and foothills have mid 40s F to upper 50s F through
Saturday night. There is pretty high CAPE values over much of the
CWA late day Friday and a little less late day Saturday. The QPF
fields show some fairly decent measurable rainfall, mostly over
the foothills Thursday evening. There are higher amounts progged
for much of the CWA later friday afternoon and much of the night.
There is a bit less coverage and amounts late day Saturday, but
still plenty. It appears the higher amounts of moisture will be
sticking around for a while. Will keep decent pops going, and
"likely"s in the high country. For temperatures, Friday`s highs
warm up 1-4 C from Thursday`s highs. Saturday`s highs will be
0-1.5 C cooler than Friday`s. For the later days, Sunday through
Wednesday, models have the upper ridge center well west of
Colorado through Wednesday. The moisture the models show is still
significant through Tuesday, then there is some drying Wednesday.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 826 PM MDT Wed Jul 26 2017

Thunderstorms are expected to continue across the Denver area
through about 05Z. Airmass will remain moist with low clouds
likely after 08Z through Thursday morning, 18Z. Ceilings are
expected to be drop into the 2000 to 4000 foot range. There is a
slight chance for lower ceilings and reduced visibility due to fog
around 12Z. There will be a chance for thunderstorms again
Thursday afternoon, after 21Z.




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