Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 281704

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1104 AM MDT Fri Apr 28 2017

Issued at 1104 AM MDT Fri Apr 28 2017

Larger-scale models are fairly consistent in showing some slowing
of the storm as it moves across New Mexico on Saturday. This looks
reasonable given the digging of the jet streak into Arizona today.
This means a longer period of light to moderate snow for Denver.
As a result, we will issue a Winter Storm Warning for Boulder and
Denver as well as the Limon area.

Some question about the more serious snow wrapping around the low
Saturday into Sunday morning. This should stay east of Denver but
may impact east central Colorado. Slightly stronger/slower motions
would lead to even higher snow amounts from about I-76 south. As
it currently looks, we don`t think there will be much snow in
Denver after about noon Saturday, but we will keep the warning
going longer as the current model trends are slower. We could wind
up with a lot more out around Limon if the storm continues to
slow. Main period of heavy snow will be overnight tonight and
could produce some slush on the roads, but main concern for Denver
is accumulation on trees and possible power outages. Road impact
should be small on Saturday with lighter snow rates and warmer

Convection-allowing models continue to produce some very heavy
snow amounts, but they have unreasonable precipitation rates in
convection for a very long time, even into Sunday morning. Even if
they wind up being right in the duration, the rates are way too

UPDATE Issued at 646 AM MDT Fri Apr 28 2017

It`s also worth noting the variation in the models, with the ECMWF
now trending away from the heavier amounts, the GFS and Canadian
holding fairly consistent, but the NAM is now trending much
heavier. SREF plumes are extremely high with amounts of 12-18
inches across Denver, but they seem to have it snowing far too
long for the expected storm track, and the high biased ARW cores
are also averaged in. That said, even the NMM have amounts closer
to a foot, but again, think they`re hanging onto the snow far too
long. Needless to say, with the convection early on and models
playing games, our confidence in the snow forecast is moderate at
best at this time. Stay tuned for further updates.


.SHORT TERM...(Today and tonight)
Issued at 435 AM MDT Fri Apr 28 2017

...Spring storm system to affect the area with accumulating

Complex weather pattern in place to say the least. Latest
satellite loop shows an elongated trough to our north producing
widespread precip, with one short wave rotating in southwest
Nebraska and another in south central Wyoming. Obviously this lead
short wave has disrupted the entire storm system, and this has
been previously discussed in days past so no surprise here.

Another piece of energy will drive down the back side of the
elongated upper trough today, as seen in the Q-G fields. This
will reinvigorate an upper level low developing near the Four
Corners late this afternoon. At the same time, the pressure
pattern over northeast Colorado collapses, ushering in much colder
air on gusty northeast winds. This will set the stage for
convective showers and perhaps a couple thunderstorms given a
couple hundred J/Kg of CAPE under the cold pool aloft. The
convective showers are likely to organize in and near the Front
Range Foothills by late afternoon and evening. In the foothills,
this will translate to heavy snow with snowfall rates of 1-2
inches per hour likely under the heavier showers. On the plains,
some snow already mixing in near the northeast corner (per
Ogallala, NE observation), so the cold air is there. That should
allow some snow to mix in across all of the plains with the
showers by late afternoon and temperatures aloft/wet bulb zeroes
showing mostly snow across the plains by 6-9 PM. Given the
expected convective nature early on, this is when our heaviest
snow should occur. After the late afternoon/evening precipitation,
snow may very likely decrease in intensity and coverage as the
atmosphere quickly stabilizes. This is seen nicely in the model
lapse rates and forecast soundings with a sharp inversion near
8000-9000 ft MSL. That should also translate to more of the
precipitation focusing in the foothills where forcing is driven by
upslope. Given the above mentioned factors, increased snow
accumulations during the late afternoon/early evening hours and
then decreased the forecast later tonight. Also refined a bit
sharper gradient focusing toward the higher terrain.

We are upgrading the Winter Storm Watch to a Winter Storm Warning
for the Foothills & Palmer Divide through Saturday with amounts of
8-15 inches expected. Also started it a bit earlier at 4 PM for
the convection this afternoon. Surrounding that, we`ll have a
Winter Weather Advisory for the Denver metro around out through
Deer Trail and Limon with 3-7 inch amounts expected at this time,
but confidence is only medium at best. Could see impacts to trees
with broken tree limbs should sufficient snow accumulate. Front
Range mountains to Park county will also have advisories as the
eastern side of the Front Range mountains should fair quite well
with regard to upslope snow.

.LONG TERM...(Saturday through Thursday)
Issued at 435 AM MDT Fri Apr 28 2017

Snow will be falling across most of the forecast area Saturday
morning as the upper low is forecast to be tracking across
northern New Mexico. Eastern and northeastern Colorado will be
left under the diffluent region on the north side of the upper
low. There will be plenty of dynamic forcing moving across the
state, producing a widespread shield of precipitation over the
state. The strongest dynamics and lift will be closer to the upper
low, over southeast Colorado, but the lift over northeast Colorado
will combine with low level northerly flow to produce light to
moderate snowfall over northeast Colorado as well. Each of the
mid-range forecast models show decent amounts of QPF over the
forecast area as this Spring storm system moves over the region.
By Saturday afternoon and evening precipitation will be winding
down as the upper low moves out over northern Texas and drier air
begins moving into Colorado from Wyoming. Back to Saturday
morning, the Palmer Divide and southern foothills may still be
receiving snow at rates up to one inch per hour. But as the day
progresses, snow will begin tapering off and accumulated snow on
the ground will begin melting. By Saturday night and Sunday, only
the eastern fringes of the forecast area might be receiving any
snow, as the upper low progresses northeastward to southern

The first half of next week will see a continuation of cool
temperatures and a chance of showers each day as the flow aloft
remains out of the northwest. Temperatures aloft will only
gradually warm up as cool air moves down from the northern Rocky
Mountains. The moist northwesterly flow pattern will be unsettled
with embedded short waves generating rounds of afternoon and
evening showers over the mountains and out onto the plains.

A stronger trough is forecast to move over Colorado Tuesday night
as the flat ridge over the southwest U.S. begins to amplify. This
will lead to warming and drying for the latter half of the week.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1104 AM MDT Fri Apr 28 2017

Still expecting showers to develop this afternoon, with MVFR
conditions developing by 00z, and periods of IFR in snow tonight
into Saturday morning. Conditions should be improving to MVFR by
18z Saturday. Expected snow accumulations are 4-8 inches at KDEN
and KBJC with 8-12 inches at KAPA.


Winter Weather Advisory from 4 PM this afternoon to 6 PM MDT
Saturday for COZ033-034-037.

Winter Storm Warning from 4 PM this afternoon to 6 PM MDT
Saturday for COZ035-036-041.

Winter Storm Warning from 6 PM this evening to 6 PM MDT Saturday
for COZ039-040.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Saturday for COZ045>047.



SHORT TERM...Barjenbruch
LONG TERM...Dankers
AVIATION...Gimmestad is the U.S. government's official web portal to all federal, state and local government web resources and services.