Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 171003
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
403 AM MDT Wed May 17 2017

.SHORT TERM...(Today and tonight)
Issued at 400 AM MDT Wed May 17 2017

Upper low over southwest Kansas still responsible for continued
showers over the northeast plains, although the areal extent of
showers continues to diminish. Still a fair amount of mid level
clouds further back towards the Front Range but this will also
shift east towards sunrise.

Attention then turns to even stronger system which is located over
Northern Idaho and Oregon this morning, and will be dropping
southeastward into Utah tonight. Model tracks are fairly
consistent at least through tonight, although the NAM is a bit
further North with this feature. Increasing QG ascent expected
from late this afternoon over the mountains and spreading over
much of northern Colorado tonight as the low approaches Colorado.
The associated cold front already moving through Northern Utah
with good pressure rise behind it. This front will sweep across
Northern colorado mountains this afternoon and then over the urban
corridor late afternoon with increasing northwest winds. Winds
will then switch northeast this evening over the front range as
surface low pressure intensifies across southeast Colorado.

Rain showers and isolated thunderstorms will become more
widespread by later this evening and overnight with deepening
upslope flow and modest QG ascent. Focus area for tonight will be
locations from Denver Northward and especially in the foothills of
Boulder and Larimer counties. As cold air filters in, snow levels
will be dropping through the night to around 7000 ft over the
foothills. Will be upgrading the watches to warnings for the
mountains and northern foothills (also see long term discussion
below). Confidence not as high for Southern Front Range foothills
especially if the 700mb low circulation ends up further North,
like the latest NAM is showing. So will keep zone 36 in a watch
for now.

.LONG TERM...(Thursday through Tuesday)
Issued at 400 AM MDT Wed May 17 2017

The upper low over Utah on Thursday morning will swing into
Colorado through the day. Overall, models have slowed their
progression with the system, as well as gotten slightly stronger.
This trend will result in warmer temperatures across the eastern
plains, and possibly into the urban corridor, with higher snow
levels than the previous forecast. Trying to key into where the
700 mb low will end up, as this will have a big impact on areas
around and south of I-70. The GFS is keeping it further south
which will bring good easterly upslope flow up through 300 mb
across most of the forecast area. The NAM is further north with
the 700mb low which is causing a drying effect along and south of
I70. This is the reason for keeping a Winter Storm Watch over the
southern foothills where confidence is lower.

Have upgraded the remaining Watches to Winter Storm Warnings as
moisture surges up from the Gulf of Mexico to combine with the
Pacific moisture, good upward QG motion, unseasonably cold
temperatures and deep upslope flow combine together through
Friday. With the low slowing its progression, dynamics will be
able to continue to produce appreciable amounts of precipitation
across most of the area. Models have anywhere from 2 to 6 inches
of liquid falling over the Front Range mountains and foothills,
with a half inch to 6 inches of liquid for the plains - the NAM
being the driest along and south of I70 due to its 700mb low
position. All models have placed their highest amounts over
Larimer County, with 3-6 inches common. This will result in
impressive snowfall amounts, even if possible convection is
doubling the amounts. Half that amount of liquid will still
produce impressive snowfall amounts, especially for the second
half of May. One interesting thing is however, the models are only
showing PWs of near half an inch along the foothills and urban
corridor, quite a lot less than the storm a little over a week
ago. Snowfall rates over the mountains will likely be 1-2 inches
per hour through the day, making travel difficult. Some concern on
the deep convection expected over the Great Plains Thursday, may
rob some moisture and disrupt the flow into the low. Snow levels
are expected to start around 8000-9000 Thursday with lowering from
northwest to southeast through the day and night. Dynamic cooling
with the falling precipitation may be able to lower these levels
farther, however for now, see snow levels getting down to about
5500 feet by Friday morning. Will keep most of the plains rain,
with some rain/snow mix and some light accumulations up to a
couple inches in the western urban corridor and Palmer Divide
seeing an inch or two more. Good soaking rains will be expected
over elevations below this. High temperatures this day will be 20
to 30 degrees colder than normal for this time of year. Winds will
be northeasterly upslope through the day, with speeds of 10 to 25
mph, aiding in snowfall over the Front Range high terrain.

The system will continue to slowly move northeast across the area
Friday with high temperatures a few degrees colder than Thursday.
Confidence is low on where the heavier precipitation will be due
to the differences in the track and speeds of the system in the
models. However precip will continue across the area likely
through the afternoon and into the early evening, before a sharp
decrease is seen. With the models slowing the departure of the
system, this may effect low temperatures Friday night and into
Saturday morning, but still expecting at or below freezing along
the urban corridor, with slightly warmer temperatures further
east.

Saturday, a few showers may still remain in northwesterly flow
over the mountains, but drying otherwise. Temperatures will
increase about 10 degrees under clearing skies.

A weak shortwave in westerly flow Sunday will bring warmer
temperatures along with scattered showers and thunderstorms. Flow
will then turn northerly as a trough drops out of Canada, for
slightly cooler temperatures and an increase in showers Monday and
Tuesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 400 AM MDT Wed May 17 2017

Mid level deck of clouds will be moving off to the East early this
morning as the upper low in Kansas moves further east. Cold front
will move into terminals in late afternoon from the northwest and
then shift northeast this evening. Rain showers will become more
widespread late evening and overnight with IFR conditions
developing.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late tonight through Friday afternoon
for COZ036.

Winter Storm Warning from midnight tonight to 6 PM MDT Friday
for COZ035.

Winter Storm Warning from 6 PM this evening to 6 PM MDT Friday
for COZ033-034.

&&

$$

SHORT TERM...Entrekin
LONG TERM...Kriederman
AVIATION...Entrekin



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