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Area forecast discussion 
National Weather Service Grand Junction Colorado
1057 am MST Friday Feb 27 2015

Update...
issued at 1055 am MST Friday Feb 27 2015

A weak frontal band set up near the I-70 corridor this morning
producing snow across the central zones. The NAM picked up on this
feature yesterday and the 12z run shows it dissipating this early
afternoon as mild SW flow overruns the region. Isolated
thunderstorms are still possible late afternoon for southeast Utah and SW
Colorado where lapse rates are 7.5 c/km especially between -70 and
Highway 50.

Otherwise the developing storm looks ontrack. For the onset...the
key looks to be forcing and moisture advection along and just
north of the jet. The 120kt jet noses across northwest nm this
evening...a bit further south than modeled yesterday. The heaviest
evening snow band is now prognosticated to be south of the Colorado- nm state
line. The jet lifts into northern nm after midnight and then into
the San Juans during Saturday. So good snow accumulations are
still on track for Mancos to pagosa and vicinity tonight after
midnight through Saturday morning. The San Juan Mountains will see
some snow lighter snow tonight but stronger accumulations on
Saturday with the lifting jet.

As for snow levels guidance temperatures continue their trend
downward for afternoon highs and will undercut a bit once more for
Saturday. 700mb temperatures climb to -3c across the south Saturday which
would climb the rain/snow line up to 7500ft. But convective
showers will drop that locally lower as much 2000ft lower in the
strongest showers.

For the central and northern zones the 120kt jet streak lifts NE
through the I-70 corridor Saturday evening. This brings the best
snow to the central mountains Saturday afternoon and evening...the
northern mountains overnight into Sunday morning. We may need to
add some advisories across the northwest Colorado zones for this portion
of the storm.

Lots to look at with this important storm. Conditions and
forecast details will be evolving every day...perhaps every few
hours. So stay tuned. Its going to be big!

&&

Short term...(today through saturday)
issued at 437 am MST Friday Feb 27 2015

500 mb vorticity maximum over eastern Utah this morning has initiated some
light snow showers from low stratus along the I-70 corridor. This
line of showers is moving to the southeast and is ahead of the
bigger moisture source and energy which will move into the region
this afternoon. The showers this morning will not completely move
out of eastern Utah and western Colorado today but slowly
progress south and east through the early afternoon.

An upper level trough over the Pacific northwest will drop south
today along the California coast and open the Pacific moisture tap
from the west. Specific humidity values progressively increase
over the next 36 hours...especially over The Four Corners where
values are prognosticated to reach 4 g/kg over the southern Colorado
valleys by Saturday evening. As this trough reaches central
California by Saturday afternoon...a low closes off and leaves the
Great Basin in west southwesterly flow which will stream energy
and moisture into eastern Utah and western Colorado over the next few
days. The first bout of heavy snow will be tonight over The Four
Corners area. Showers will become more widespread from southwest
to northeast on Saturday as the trough elongates across the
region.

Kept isolated storms in for large swath of eastern Utah and western Colorado
for this afternoon and Saturday afternoon with instability and
steep lapse rates in place ahead of the low pressure center. Not
expecting widespread lightning but a few strikes are definitely
possible over the next couple of days.

Long term...(saturday night through thursday)
issued at 437 am MST Friday Feb 27 2015

Models remain in excellent agreement for the next storm with
copious amounts of snowfall for the southern mountains and to a lesser
extent the central mountains by copious...we're talking anywhere from
3 to 4 feet of snow...a remarkable amount that certainly looks
reasonable.

Starting Saturday night...the jet stream will have rounded an
upper level trough located over the Great Basin and will lie
directly across southeastern Utah and Colorado. The location of the jet will
also allow Pacific moisture to stream up from the southwest. This
jet stays over US through Tuesday until shifting east which will
keep moisture in play for much of the period. GFS shows what looks
like three fairly distinct waves with the first moving overhead
Saturday night through Sunday morning. The second wave...also the
strongest...is prognosticated to move over Sunday night through Monday
night and the final wave will move across the region Tuesday
morning through Wednesday morning. The second wave will likely
produce snowfall rates easily over an inch an hour for the San
Juans where again...3 to 4 feet of snowfall can't be ruled out.
The central mountains will likely see 1 to 2 feet of snow with south
facing slopes being favored. Decided to include zone 10 to going
warnings starting Saturday morning as the second wave moves
overhead. Northern mountains may also see some accumulation but SW flow does
not favor that area so maybe 8 to 12 inches there. Did not raise
any highlights for areas along and north of I-70 to allow a few
more model runs to get the finer details there. Important to
mention that through this entire period...Saturday night through
Wednesday morning...there will be lulls in precipitation as one
wave moves off and another approaches. The 'lucky' areas...think
Wolf Creek Pass...may see continuous snow through the entire
period.

As far as southern valleys are concerned...temperatures will be hovering near
freezing but still expect plenty of snow for those areas though
there may be a rain/snow mix at times. Model guidance still
suggesting too warm for accumulating snow for southeastern Utah and also the
Paradox Valley/lower Dolores River areas. Of course...the last
storm looked too warm for those areas and ended up dropping about
a foot of snow so the possibility does exist.

If traveling this weekend...especially over any mountain
passes...please pay particular attention to the latest weather
forecast.

&&

Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1055 am MST Friday Feb 27 2015

A band of showers continues to bring showers to the region between
I-70 and Highway 50 corridors this late morning impacting kgjt
kmtj and kase with occasional MVFR/IFR until 20z. Otherwise scattered shsn
will favor the central and southern mountains this afternoon occasionally
obscrg mountain tops. All Colorado airports could see ceilings below 030 visible below
4sm in shsn/rain this afternoon. After 00z...flight conditions will
deteriorate across the south spreading into the central airports
on Saturday and Saturday night. Increasing shsn/rasn conditions
will persist for quite some time. Mountains will be obscured into next
week.



&&

Gjt watches/warnings/advisories...
Colorado...Winter Weather Advisory from 6 PM this evening to 8 PM MST
Monday for coz014-021.

Winter Storm Warning until 8 PM MST Monday for coz009-012-
017>019.

Winter Storm Warning from 6 PM this evening to 8 PM MST Monday
for coz022-023.

Winter Storm Warning from 11 am Saturday to 8 PM MST Monday for
coz010.

Utah...Winter Storm Warning until 8 PM MST Monday for utz028.

&&

$$

Update...Joe
short term...jam
long term...tgr
aviation...Joe

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