Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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717
FXUS65 KBOU 290257
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
857 PM MDT Fri Apr 28 2017

.UPDATE...
Issued at 850 PM MDT Fri Apr 28 2017

Ingredients continue to come together for what should result in a
significant snow event along the Colorado Front Range during the
next 12-18 hours. After the burst of moderate to heavy snowfall
across southern and western portions of the Denver metro area
earlier this evening, KFTG radar has indicated a general reduction
in snowfall intensity across the area in the past hour. Whereas
snowfall coverage continues to increase, notably over and the
along the foothills in Boulder and Larimer Counties. This is in
response to a northeasterly/upslope flow of 15-20 kts up through
600 mbs/16000 ft ASL as per the KFTG VAD wind profile. Over the
next several hours, this northeast wind is projected to become
more east-southeasterly as the upper low spinning over the
4-Corners attm tracks over northwest New Mexico. As a consequence,
should see a resurgence in snowfall intensity esply over/along
the Palmer Divide, and up against the foothills in the Larimer
County. Mesoscale models lend support to this happening. Later
tonight as the 700-500 mb low continue to track east across north
central New Mexico, the 850-500 flow over northeast Colorado
returns to a more favorable east-northeast upslope flow. Speeds
also appear to increase. This is when we should see snowfall
rates increase again lasting through mid-morning Saturday. This
coincides with the strongest deep layer QG lift over the area.
Could see 1-2 inch per hour snowfall rates along the Palmer
Divide out across Elbert and Lincoln Counties. Meanwhile closer to
the Front Range, the mean layer flow is expected to assume a more
northerly component through the morning which would lead to a
gradual reduction in snowfall in this area.

In the high country...light to moderate snowfall is being
reported at a number of observing sites. CDOT cams also show
slushy to snow covered roads. Not seeing much of this snowfall on
radar due the blocking of the Continental Divide. Be assured it`s
snowing up there this evening. Should see little chance in this
snowfall overnight.

Adjustments to the current forecast grids include lowering pops
this evening across the northeast corner of the state where precip
has yet to develop. Still expect precip/snow to develop in this
area later tonight, but the chance for measurable snowfall out
there generally less than 50/50. Also introduced areas of fog on
the plains this evening, partly in response to the 1/8 to 1 mile
visibilities mainly the result of fog reported at KDEN. Also
expanded the area of blowing snow across Elbert and Lincoln
counties late tonight and Saturday morning with forecasted
northeast sfc wind gusts in the 20-30 mph range. Did not tamper
with snow amounts at this time. Will hold off making any
adjustments until new model data arrives later this evening.
Nevertheless, have already received reports of 3-5 inch snow
amounts across the southern metro area. Here at the NWS office in
Boulder, snow depth not quite an inch. Roads in the foothills and
across portions of the I-25 urban corridor vary from wet to slushy
as a general rule. Should see more snow cover as surface
temperatures fall below 32 deg f.

&&

.SHORT TERM...(This evening through Saturday)
Issued at 337 PM MDT Fri Apr 28 2017

Showers breaking out over northeastern Colorado this afternoon
while low clouds, lighter snow, and cold northeast winds are
moving in behind a cold front from the northeast. All of this
should be coming together over Denver during rush hour.
Temperatures should be relatively warm, and the ground is warm, so
the impact on the roads should be reduced. Expect that roads
should stay wet through rush hour, even with some decent snow
rates in some of the showers.

This area of showers is actually in between the areas of higher
level forcing over Wyoming and southern Colorado. This evening we
will see the low level cold/moisture/upslope at its best with the
instability aloft over it. The deeper lift over Wyoming will
rotate southward but weaken as it does so...leading to a period of
steady but lighter snow later tonight. There will still be pretty
good upslope though, so while the intensity should be lower,
there will be significant accumulation through the night.

Bigger questions about what happens on Saturday. Today`s model
runs are similar to those we saw earlier this morning, with
perhaps a little more slowing to the eastward progress of the
system. We made a shift in that direction this morning, so we are
sticking with those adjustments at this time. This forecast has
Denver in weak QG lift with northerly low level winds, so less
snow from Denver northward, and perhaps ending early in
Greeley/Fort Collins, but a continuing steady snow south of
Denver. The real wild card is the stronger QG lift and better
moisture wrapping around the deep slow moving low that will be
moving across eastern New Mexico. Current thinking is that the
stronger lift will remain SE of Denver for two reasons: The track
of the low is far enough south, and there is not enough time for
it to wrap all the way back to Denver. QG diagnostics and 700-500
mb temperature advection suggest that the strongest lift may get
as far up as Washington and Lincoln counties late Saturday into
Saturday night. So the slower motion could prolong snow out there,
but drier air should be working into Denver in that scenario.

Of course, this is too close to comfort and there is a chance that
there could be a lull over Denver during the day Saturday and then
more later in the day. Even so, think the risk of more heavy snow
in Denver is low, but higher across our eastern plains if that
were to happen.

.LONG TERM...(Saturday night through Friday)
Issued at 337 PM MDT Fri Apr 28 2017

An upper trough axis is over the forecast area at 00Z early
Saturday evening with a closed circulation over northeast and
east central New Mexico. By 12Z Sunday morning the trough axis is
just east of the eastern Colorado boarder and the upper closed
low is over the northeastern panhandle of Texas. There is north-
northwesterly flow aloft Sunday becoming just northwesterly at 90
knots on Sunday night. Weakening northwesterly flow aloft
continues over Colorado Monday and Monday evening, becoming more
zonal early Tuesday morning. There is still pretty decent upward
vertical motion on the QG Omega fields Saturday night into Sunday
morning certainly for the eastern half of the CWA. There is weak
downward motion after that well into Monday. The low level winds
still look to be northerly upsloping Saturday evening, then the
winds become more northwesterly overnight. North-northwesterlies
are progged Sunday and Sunday evening. Downsloping west and
northwesterlies are progged the rest of Sunday night and Monday
with more normal diurnal patterns by Monday night. For moisture,
models now show it to be quite deep Saturday evening, then slowly
decreasing across the CWA from the northwest to east well into
Sunday. The far east and southeastern CWA have wrap-around
moisture from the slow moving upper low through Sunday afternoon.
Some moisture returns from the west Sunday evening and sticks
around through Monday night. The QPF fields have minor amounts of
measurable snow Saturday evening, then is decreases from west to
east through Sunday afternoon. There is not much. There is some
measurable precipitation in the mountains Sunday night and Monday.
There is very little noted after 06Z Monday night. So will keep
"chance"s going for pops Saturday evening, decreasing them only
"slight chance"s over the far east by Sunday. Will go with 30-60%s
in the mountains Sunday night into Monday evening. For
temperatures, Sunday`s highs are 4-9 C warmer than Saturday`s
highs. Monday`s highs are similar to Sunday`s. For the later days,
Tuesday through Friday, models have zonal flow early on Tuesday,
then an open upper trough moves across later Tuesday into
Wednesday morning. Northerly flow aloft is in place the rest of
Wednesday and into Thursday morning. Then an upper ridge axis gets
into Colorado and stays well into next weekend. Will need pops
for all the CWA Tuesday through Wednesday, then and Tuesday night.
it will be dry.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 850 PM MDT Fri Apr 28 2017

ILS and IFR conditions due to snow, fog and low ceilings expected
for the remainder of the night and Denver area airports. With
bursts of moderate to heavy snowfall and fog, could see visibilities
fall below 1/4 mile and ceilings below 300 feet AGL. Lowest
thresholds likely at KAPA and KBJC as a moist east-northeast
upslope flow will be the principle snow maker overnight. Could see
snowfall rate around an inch per hour at these air terminals.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT Saturday for COZ033-034-
037-045.

Winter Storm Warning until 6 PM MDT Saturday for COZ035-036-
039>041-046-047.

&&

$$

UPDATE...Baker
SHORT TERM...Gimmestad
LONG TERM...RJK
AVIATION...Baker



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