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Area forecast discussion 
National Weather Service Grand Junction Colorado
1038 am MST Wednesday Jan 28 2015

Update...
issued at 1033 am MST Wednesday Jan 28 2015

An upper disturbance over Wyoming and evident on satellite water vapor
images is forecast by short-range models to clip northwest Colorado today. This
and trailing northwest flow will keep some light snow across the northern Colorado
mountains a spotter in Cortez reported some cumulus building to the
northeast and indicative of some instability aloft but think the
airmass too dry for any significant threat of showers for the San
Juans. Trimmed back probability of precipitation across the Grand Mesa and west elks today
per current webcam observation and latest available hrrr guidance.
Otherwise...the current forecast looks on track.

&&

Short term...(today through thursday)
issued at 447 am MST Wednesday Jan 28 2015

The low pressure disturbance that moved northward through Nevada
yesterday has been picked up by the westerly flow over the
northern tier states...and has moved across southern Idaho and into
far southwest Wyoming early this morning. This disturbance will
continue to brush our northern zones through the next few
hours...with a northwest flow developing by late morning. Radar
echoes early this morning indicate some light shower activity
over the Colorado mountains...with a few showers over the
northern Colorado valleys. As the trough axis passes later this morning
a northwest flow will develop which will combine with the still
moist air mass to keep areas of precipitation over the favored
northern mountains...especially along the northern Divide. Early
morning temperatures have remained mild so rain or mixed
precipitation will be possible in the middle Yampa River basin
from around Milner west. We do get some weak cooling in the
northwest flow...but daytime highs will remain above normal...
but slightly cooler than tuesday's.

Middle-high level moisture will remain over the region through
Thursday as the ridge temporarily rebounds along the central
rockies. Deeper moisture will begin to move into the Desert
Southwest ahead of another low pressure system coming onshore
along the southwest coast. But more about that in the long term
discussion.

The cloudy skies will keep temperatures similar to a couple
degrees cooler than today's.

Long term...(thursday night through tuesday)
issued at 447 am MST Wednesday Jan 28 2015

Moisture surge from the south combined with isentropic upglide
results in expanding cloud/precipitation shield Thursday night. A
developing closed upper low over the southern Colorado River Basin will
keep the air mass moist through Saturday. Most areas south of Highway
40 (vernal to Steamboat springs) stand a very good chance of observing
some precipitation...but drop off significantly north of this Highway.

The forecast challenge is how much quantitative precipitation forecast falls and the precipitation type
(especially the higher southwestern Colorado valleys). While some model
spread still exists regarding the evolution on the developing
closed low circulation...the NAM...GFS and European model (ecmwf) show similar
trends. The best moisture transport stays south and east of the
forecast area...targeting the Southern Plains. Some cyclogenetical
forcing does exist...but the bullseye stays south of the forecast
area. The GFS hints of a surface low near Durango by Saturday
morning...but no other model shows this and therefore not confident
in this solution. Given these assessments...pattern favors precipitation
and high pop values for the southern half is warranted late Thursday
night into Saturday morning. With regard to snow amounts...quantitative precipitation forecast
with snow ratios on the order of 8:1 to 10:1 may only result in 4
to 8 inches with localized areas reaching 12 inches. Will allow
the next shift to make another assessment but elected not to
issue a Winter Storm Watch at this time for the southern mountains.

For the southern valleys between 6000 to 8000 feet...precipitation type may
depend on the time of day. Rain in the afternoon...but
transitioning to snow or rain/snow mixture during the overnight
hours.

Saturday afternoon into Sunday...northern stream dives toward Colorado
and shunts moisture southward. The closed low circulation begins
to weaken and move farther south. The progressive northern stream short
wave briefly enhances orographic snowfall and lowers the snow
level...but drier air spreads across the entire forecast area
overnight on Saturday into Sunday.

With the amplified ridge axis gone...Pacific energy continues to
move into the Pacific northwest on Monday and Tuesday. Cyclonic
northwest flow allow short wave energy to move across the central
rockies. The long wave pattern may be in state of flux here and
not clear how these Pacific waves will impact the northern mountains.

&&

Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1033 am MST Wednesday Jan 28 2015

Weak disturbance will clip northwest Colorado today followed by northwest flow for
some mountain obscurations in - snow from kccu to k3mw this afternoon. Isolated
to scattered -shsn/-shra may briefly produce MVFR ceilings at kase until
00z...so will continue to carry vcsh there as well as at kege.
Otherwise...VFR should be the prevailing conditions at all taf
sites through Thursday morning as high clouds invade from the southwest.

&&

Gjt watches/warnings/advisories...
Colorado...none.
Utah...none.
&&

$$

Update...jad
short term...eh
long term...pf
aviation...jad

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