Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Junction Colorado
525 PM MST Friday Feb 27 2015
Short term...(tonight through Saturday night)
issued at 358 PM MST Friday Feb 27 2015
The developing storm looks generally ontrack. The upper low was
dropping through western Oregon Friday afternoon with the
associated jet carving into northern Arizona. The key to this
storm 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 on Saturday. This saturates the dendritic Crystal
growth zone and produces some frontogenetical forcing in SW Colorado
Saturday morning. 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 have undercut for Saturday.
700mb temperatures climb to -3c across the south Saturday which would
raise the rain/snow line up to 7500ft. But convective showers
will drop that locally lower as much 2000ft lower in the strongest
showers. Isolated thunderstorms are forecast across the southern
forecast area Saturday afternoon.
For the central and northern zones the 120kt jet streak lifts NE
through the I-70 corridor Saturday evening and is reinforced
Sunday. It will be oriented from Lake Powell to The Flat Tops.
This brings the best snow to the central mountains Saturday afternoon
and evening...the northern mountains Saturday afternoon Onward. We
have issued a long-duration watch for the Roan-Tavaputs plateaus and
Flat Tops where jet support is strongest starting Saturday
afternoon and going all the way out until Wednesday morning. Other
highlights will likely be needed including at least the eastern
I-70 corridor into early next week.
As expected details are evolving with this long-duration storm.
Conditions and forecast details will be changing every
day...perhaps every few hours. So stay tuned for important updates.
Long term...(sunday through friday)
issued at 358 PM MST Friday Feb 27 2015
Some interesting...but not unexpected changes...to the models has
occurred over the past 24 hours. Overall however we are sticking
with spirit of the forecast but have had to make some adjustments
based on the model consensus to when and where periods of heavier
precipitation are more probable in the upcoming days. The
southern San Juan Mountains are still in the bullseye for the
heaviest snow today through Monday and beyond. However a subtle
shift in the pattern to the west looks to be favorable for some of
our northern County Warning Area to pick up some heavier snow Saturday evening and
again Monday through Wednesday.
By sunrise Sunday an upper low will be stretching from southern
Nevada to the waters off central California. This low will become
detached from the northern stream and retrograde back off the coast
north of Los Angeles in the afternoon and remain there through
Monday morning. This separation leaves US under the influence of
south to southwest flow. This is good for moisture transport...which
appears now to be tapping into some subtropical moisture as early as
Sunday and merging it with the Pacific tap by Monday morning. Precipitable water
reach at least to 0.50 which is well above normal. This moisture
will be good for making snow at high elevations where it is
cold...but could also limit lower level snowfall pushing dew points
above freezing. The forecast for some of southern valleys have been
reduced slightly for this near surface environment. Problems of mud
and back country travel have been received for areas in southeast
Utah...and this may be exacerbated by the snowfall.
One difference the models have grabbed onto comes in the Sunday
night to Monday night time frame. This is when the northern stream
comes back into play. A strong upstream wave splits over over the
western Canadian coast with one piece separating and dropping
down the left coast of the Continental U.S. Through Monday afternoon...while
the main trough swings into the central Canadian prairies. The
western wave rotating around the closed low allows it to Orient
more north to south than 24 hours ago and thus directs the
moisture and jet dynamics back to the northwest. This allows
central Utah to northern Colorado to begin to pick up better
precipitation rates Sunday night into Monday...while a noticeable
decrease in precipitation takes shape to the south. Now this is a
big change and did trend this way...more runs will add confidence.
The NAM was the outlier in this time period. Snow should pick up
again in the south Monday evening as the two troughs appear to
phase across the northern rockies and intermountain west. Cold
advection and frontal forcing will then favor the northern half of
the County Warning Area Tuesday into Wednesday for the heavier snow rates.
Moderate to light snow continues to impact the south as well but
may focus more on the northern San Juans like Telluride and Ouray.
A much colder airmass moves in for the middle week period with
temperatures remaining below normal through the long term forecast
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 525 PM MST Friday Feb 27 2015
South...deteriorating conditions with conditions becoming IFR/MVFR
with widespread areas visible below 3sm -sn ceilings at or below ovc010. Kdro and
ktex will have steady snow through the forecast period.
Central...scattered snow showers will decrease this evening with a
lull in the activity overnight. Snow will develop after 15z and
spread northward with deteriorating conditions as precipitation
becomes widespread with areas of VFR shrinking. Ceilings will fall
below airfield minimums at the mountain airports.
North...much of the weather stays south of the Highway 40
corridor. VFR will prevail through the forecast period.
Colorado...Winter Weather Advisory until 8 PM MST Monday for coz014-021.
Winter Storm Warning until 8 PM MST Monday for coz009-012-
Winter Storm Warning until 8 PM MST Monday for coz022-023.
Winter Storm Warning from 11 am Saturday to 8 PM MST Monday for
Winter Storm Watch from Saturday afternoon through Wednesday
morning for coz003-013.
Utah...Winter Storm Warning until 8 PM MST Monday for utz028.