Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 181732
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1132 AM MDT Thu May 18 2017

.UPDATE...
Issued at 1120 AM MDT Thu May 18 2017

Main highlights...Winter storm watch for the I-25 corridor has
been upgraded a warning. Next, a winter weather advisory has been
issued for the upper Colorado River zone 32 and South Park zone
37. Lastly, no change to the winter storm warning for the Front
Range mtns and foothills, although storm total snow amounts have
been increased by 4-8 inches as a rule, the greatest amounts over
the higher terrain in Boulder and Larimer counties.

Snowfall now enhancing on radar along the Front Range and Palmer
Divide will likely continue to do so over the next couple of hours
as this latest moderate to heavy snow band migrates slowly
northward along the face of the foothills. If snowfall continues
to intensify could see 1-2 inch per hour snowfall rates in lower
areas along and west of I-25...and 2-3 1/2 inch per hour rates in
the foothills from central Jefferson County northward. The Front
Range mtns from Gilpin County northward could see local snowfall
rates of 2-4 inches per hours today through this evening. Could
see another 1 -2 feet of heavy wet snow on the east face of the
mtns and another 3-7 inches in the urban corridor generally west
of I-25 (higher amounts possible on the Palmer Divide) and 1-3
inches east of I-25. Farther out on the plains...precip still
mainly rain where temps still in the upper 30s/lower 40s. Through
the day as temps cool with the strong upper low moving closer to
the area. Lastly heavy rainfall not expected to be as significant
or as wide spread as earlier though with precip likely to stay
snow most if not all day in and near the mtns.


UPDATE Issued at 659 AM MDT Thu May 18 2017

Snow levels continue to lower with heavier precipitation and cold
air damming in place. Have increased snow accumulations for the
Denver area including the western and southern suburbs this
morning where very heavy wet snow was falling. Will make a
decision on tonight`s Winter Storm Watch later today, but cold air
already in place does not bode well for potential tree damage.

&&

.SHORT TERM...(Today and tonight)
Issued at 337 AM MDT Thu May 18 2017

...Strong and slow moving spring storm with significant rain and
snow through Friday...

The spring storm is well underway with already 6 inches of snow
reported in the foothills and averaging close to an inch of rain
along the I-25 Corridor. Snow levels have dropped more than
expected early this morning and already down to 6000 feet or a
little lower per latest observations. Appears cold air damming is
occurring with northwest winds along the I-25 corridor in the
heavier precipitation, and warmer easterly winds across a good
chunk of the plains. Rainfall rates will be increasing this
morning and some snow will even mix in at times over portions of
Denver, Douglas county, and near the Wyoming border. However,
there should not be much of an accumulation threat except over the
crest of the Palmer Divide where surface wet bulb temps are near
0C.

Before we get to tonight, we`ll discuss the heavy rain and heavy snow
issues today. Increasing Q-G lift, a neutral to slightly unstable
airmass, and stronger orographics across the forecast area will
result in high precipitation efficiency. We should see rainfall
rates increase to between one quarter and one third of an inch per
hour in the heavier rain, with snowfall rates increasing to 2"
per hour and locally heavier rates in the Front Range mountains
and foothills. With the southeasterly flow, the heaviest
precipitation is expected to occur across Larimer and Weld
counties. Park county is also favored in this regime today, so
added a Winter Weather Advisory there for periods of heavy
snowfall through at least early afternoon. It should be noted that
the cold air has driven farther southward early this morning as
evidenced by the latest surface observations and snow levels. That
may end up creating better lift farther south and a more easterly
component through the day, thus keeping heavier precipitation
closer to Denver and Boulder county than the Wyoming border. This
will also keep snow levels lower than most model guidance, closer
to 6500 feet through the afternoon. Winter Storm Warnings are in
good shape for the Front Range Mountains and Foothills...see storm
total details in Long Term Discussion.

For tonight, there is still considerable variance in model
output, but GFS and Canadian are siding toward a deeper/farther
south low with stronger Q-G dynamics holding in place. Meanwhile,
EC and NAM are taking a little farther northern track, and also
may be handling the Central/Southern Plains severe weather
outbreak better. If the GFS and Canadian verify, then strong
TROWAL will develop across eastern Colorado with heavier
precipitation rates continuing. On the other hand, if the
convection and latent heat release on the Central/Southern Plains
interferes with our storm, the some energy/moisture may shift
further east. There will still be plenty of latest heat release
centered over Colorado, so hard to say which is exactly right. A
compromise seems reasonable. Finally, snow levels are still all
over the place in the models ranging from near 4500 feet (NAM) to
as high as 8000 feet (GFS). EC is a compromise but toward the
lower end and around 5500 feet. In any case, based on trends with
the first cold air surge we would lean toward colder guidance at
this point. There`s still considerable uncertainty however with
QPF and also snow level so will keep the I-25 Corridor in a Winter
Storm Watch for tonight and Friday. If the colder air arrives as
expected tonight, could see 2-5" on vegetated surfaces down to
around 5000 feet, with additional snowfall Friday morning.

See the hydrology section below for expected liquid precipitation
amounts and flooding concerns.

.LONG TERM...(Friday through Wednesday)
Issued at 337 AM MDT Thu May 18 2017

Confidence in the Friday forecast is less than average as differences
in the upper low track and speed exist already at 12z between the
models. Feel as though the latest 06z NAM is completely off as it
places the low far south into northwestern Arizona. Will follow
the GFS ensemble mean and a blend of the other models - which
would take the the upper low from south- central Colorado slowly
over northeast Colorado through the day. Good upslope flow over
the northern mountains and foothills will continue to produce
moderate to heavy snowfall Friday morning. Snowfall levels
continue to be tough to pinpoint. Expecting it to be around 5200
early in the morning for a wet slushy snow for most of the urban
corridor. Best dynamics and upslope should begin to decrease
Friday afternoon for a decrease in precipitation rates, with a
sharper decrease in the evening to shut rain and snow off from
northwest to southeast. By Friday evening, storm total snow
amounts are expected to be 1 to 3 feet over the mountains and
foothills, with the highest values expected in Larimer and
northern Boulder counties. Over the plains, the rain/snow line and
subsequent snow amounts are still difficult to forecast at this
point due the model differences. For now will expect 2 to 6 inches
over the urban corridor with upwards of 8 to 12 inches near the
foothills and Palmer Divide possible with a colder solution. As
precipitation decreases Friday evening and should shut off
overnight, snow will continue in the mountains as flow aloft turns
northwesterly for good orographics.

Saturday will be mostly dry and warmer, except for a few showers
in the mountains in the northwest flow aloft. Sunday will see an
increase in moisture but still warmer temperatures as a shortwave
drops down in the northwest flow aloft. A stronger shortwave on
Monday will bring keep temperatures the same and bring more
widespread showers. Northwesterly flow aloft will continue through
midweek to keep cooler than normal temperatures and showery
weather due to several weak shortwaves.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1120 AM MDT Thu May 18 2017

Expanding band of moderate to heavy snowfall lifting north over
the Denver Metro Area at this time already significantly
impacting KAPA and KBJC. Only moments always from hitting KDEN.
Expect this heavy snowfall to produce vsbys from 1/4 to 3/4 mile.
Snowfall rates up an inch per hour will likely cover roads and
runways with slush and/or snow. Looks like snowfall will diminish
by around 1 pm this afternoon, but then increase again late this
afternoon with deeper upslope flow. Should see additional snow
accumulation overnight. As for winds, expected highly variable
winds with the passing snow bands. But as a general rule, plan on
south-southeast winds of 8-16 kts on the east side of the metro
area today and north-northwest winds of 5-12 kts on the west side.


&&

.HYDROLOGY...
Issued at 337 AM MDT Thu May 18 2017

Overall forecast of 2-4 inch rainfall amounts is on track for the
I-25 Corridor eastward toward Fort Morgan and Deer Trail, and see
little need to adjust. Locally heavier amounts up to 5 inches or
so would still be possible in Larimer/Weld counties especially if
stronger and deeper GFS/Canadian verify. We coordinated with the
River Forecast Center last evening and ran contingency models on
the rivers for the higher end rainfall of 4 inches over 36 hours,
and a few spots on the South Platte could reach or just exceed
flood stage. There will not be much runoff from the foothills
given the already low snow levels there. At this time, confidence
is still pretty high that most hydrology issues for the rainfall
will initially be minor flooding which would be handled by
advisories today and tonight. We`ll continue to diagnose the short
term rainfall and runoff, and also the main stem rivers including
the South Platte and Cache La Poudre as this event unfolds.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 PM MDT Friday for COZ033>036.

Winter Storm Warning until 3 PM MDT Friday for COZ038>041.

Winter Weather Advisory until 6 AM MDT Friday for COZ032-037.

&&

$$

UPDATE...Baker
SHORT TERM...Barjenbruch
LONG TERM...Kriederman
AVIATION...Baker
HYDROLOGY...Barjenbruch



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