Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 121605

Area Forecast Discussion
National Weather Service Denver/Boulder CO
905 AM MST Thu Jan 12 2017

Issued at 850 AM MST Thu Jan 12 2017

Main change to forecast was to increase snowfall potential for
the Front range foothills and urban corridor by later this
afternoon and evening. Upper level disturbance can be seen on
water vapor across Southwest Utah with even a few lightning
strikes overnight. This feature and associated jet streak will
move across Colorado through this evening as snow spreads over the
mountains and nearby plains this afternoon. Model solutions on
potential for snowfall are still quite varied but looks like at
least 1-3 inches over the urban corridor and possibly a bit more
in the foothills. Hi res models like the RAP/HRRR are showing even
more snow but not sure if this might be overdone if there is
convective elements getting in the way. Will at least trend for at
least some light accumulations with the jet dynamics...CSI...and
shallow upslope.


.SHORT TERM...(Today and tonight)
Issued at 309 AM MST Thu Jan 12 2017

for this morning, the models show a short wave ridge over CO with
a weak period of subsidence this morning. The HRRR indicates snow
decreasing in the mountains through 18z, the weak mid level qg
ascent redevelops in the afternoon. Will stick with the current
trend of light snow today with accumulations generally less than 3
inches. Across the urban corridor and northeast plains, a surface
high pushes a cold front into the Denver around 18z with an
overrunning profile in the forecast soundings as a shallow and
weak post frontal upslope develops. Most of the models continue
to show areas of light snow developing primarily west of
Interstate 25. The HRRR however has been indicating the
development of a Denver Cyclone and notably higher qpf over
southern and western parts of metro Denver this aftn and early
evening. Since this is the only model trending this way will not
deviate too far from the current trends but have opted to go with
higher chc pops in these areas. Could see one half to one inch in
the vicinity of the convergence line that potentially develops.
Day shift can increase pops if some of the other model runs trend
in this direction. Main impact could be the evening rush hour,
primarily 3 to 7 pm window. Otherwise, could see snowfall increase
a bit more in the mountains tonight as a weak upper level
disturbance moves across CO with the best lift in the evening.

.LONG TERM...(Friday through Wednesday)
Issued at 309 AM MST Thu Jan 12 2017

An upper level low will be off the southern California coast Friday.
To the east of it, flow aloft will go from west to southwest over
Colorado. This will start to bring drier into the area. Mountain
snow will decrease Friday and is expected to end Friday night.
Additional snowfall Friday and Friday night will be light compared
to recent snows, with accumulations expected to be less than 6
inches. East of the mountains, mid and high cloud cover will limit
heating, helping to keep temperatures cool.

For the weekend, the upper level low will move across northern
Mexico Saturday and then lift northeast across eastern New Mexico
and western Texas Sunday. The southerly flow ahead of it will likely
tap into Gulf moisture and bring a good amount of precipitation to
the Southern Rockies and Southern Plains. Just a slight chance for
precipitation south of I-70 to the Palmer Divide on Saturday. There
will be a better chance for precipitation Sunday. Still large
differences among the models. The ECMWF shows over a half inch of
QPF, while the GFS has around a hundredth. Not much consensus yet
among the GFS ensemble members. One thing that sticks out, is that
ECMWF shows the low will have a very negative tilt to it. This would
help get precipitation farther west. Normally would not expect much
precipitation along the Front Range with an upper level low lifting
out of Mexico and then across central Kansas. For now, will keep the
high pops for Sunday/Sunday night with low QPF amounts.
Precipitation type could be challenge as well, especially early on
during the event over the eastern plains. Will add a mention of
freezing rain for the eastern plains Saturday night and early Sunday.

On Monday and Tuesday, flow aloft will be weak as a long wave trough
resides over the center of the country. The best dynamics will be
well south of the area in Mexico. Moisture is expected to be limited
as well with the Pacific and Gulf being cut off. Will keep the
forecast dry with temperatures near normal.

Strong westerly flow aloft starts to develop over California
Wednesday. This should bring and round of heavy precipitation to the
west coast. Will have low pops for Wednesday and Wednesday night
over the mountains. Some snow may make it into the area then. Better
chances for snow look to come Thursday and Friday.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 850 AM MST Thu Jan 12 2017

will trend for more snow potential and for the local terminals
this afternoon and evening. Latest model data indicates some
light snowfall and will go 1-3 inches, lower amounts at KDEN and
with highest potential at BJC. Best potential will be between
20z-04z and will adjust timing of snowfall a few hours to account
for this. Improving conditions after 04z.




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