Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 141742
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1142 AM MDT Thu Sep 14 2017

.UPDATE...
Issued at 1024 AM MDT Thu Sep 14 2017

Should be a busy afternoon as the atmosphere continues to
destabilize ahead of a well defined vorticity maxima moving
across southern Nevada late this morning. Previous discussion
summed this up well with large scale ascent ahead of the
approaching system combining with the instability to help produce
strong thunderstorms this afternoon and evening over much of the 4
Corners region. Looking at forecast soundings the right moving
cell of splitting storms will be the one to watch for rotation and
the threat of producing larger hail and stronger wind gusts. Due
to moderately fast flow aloft storms will be moving at a good pace
and enhance the outflow wind threat in a dry boundary layer
environment. The flow also supports training cells which would
enhance localized flash flooding especially in the southwest. Will
continue to monitor the coverage of this threat but at this time
confidence too low for any kind of hydro watch attm.

&&

.SHORT TERM...(Today through Friday)
Issued at 125 AM MDT Thu Sep 14 2017

A closed low centered over southern California early this morning
is expected to dictate weather conditions across the region over
the next 24 hours. In fact, scattered light showers were already
moving over southeast Utah and southwest Colorado early this
morning beneath the broad difluent area ahead of the SoCal low.

Models were consistent in lifting the low to a position over Las
Vegas at midday, before it becomes an open wave over southwest
Utah late this afternoon. Dynamic lift associated with this
feature corresponds with peak heating which will result in
numerous showers and thunderstorms over higher terrain with
scattered coverage across the valleys. A 65 KT jet moving through
the base of the low is expected to result in 0-6 km shear of 35 to
45 kts over the Four Corners region. Shear combined with
deepening moisture and dynamic forcing may result in a few
supercell storms this afternoon and evening. The primary threat
from rotating storms will be large hail and damaging outflow
winds. Outside of the Four Corners region, expect sub-severe
multicellular storms, though some may approach severe limits for
hail or high winds. Meanwhile, deepening moisture may also result
in a few storms capable of producing heavy rainfall possibly
resulting in excessive runoff leading to localized flooding and/or
mudslides. Temperatures fall to near normal levels today in
response to the influx of clouds and showers.

The trough lifts northeastward across the forecast area tonight
which will bring the best chances for rainfall this evening.
Showers will decrease from southwest to northeast in the trough`s
wake after midnight. Colder air aloft and lows in the 30s above
the timberline in the northern mountains are expected to cause
rain to turn over to snow with light accumulations possible by
sunrise Friday! A deeper secondary trough drops across the
northern portion of the forecast area Friday. Showers and
thunderstorms become likely as the cold front associated with
this system sweeps across northeast Utah and northwest Colorado.
Areas to the south will be less active as drier southwesterly
flow persists ahead of the cold front. The cooling trend continues
Friday, especially north of the cold front with more modest
cooling across the south.

.LONG TERM...(Friday night through Wednesday)
Issued at 125 AM MDT Thu Sep 14 2017

Significant changes mark the long term period as a cold front
moves through the area. The parent low will be over central Idaho
and the cold front will be just about entering the CWA by Friday
evening. While there will be a 60kt jet supporting this cold
front, available moisture will be minimal at best. Also, the best
forcing will be over Wyoming and Montana where the models do put
the most precipitation. The last few model runs have favored our
northern valleys for precipitation and the latest run is no
different; areas north of I-70 still have the best chance for any
precipitation. That`s not to say some rain won`t fall south, some
likely will, but highest amounts will remain north. What about
those freezing temperatures? Both the GFS and EC have backed off
with H7 temperatures now bottoming out around 2C or so. It now
looks like areas above 12K feet will see a quick dusting as the
front moves through.

By Saturday morning, a few lingering showers will persist over the
northern mountains but that will be about it. Morning lows will be
noticeably cooler than we`ve seen recently and highs will likely
top out 5 to 10 degrees below seasonal values. Most of eastern
Utah and western Colorado will see plenty of sunshine and if the
day didn`t start out sunny, it will likely end that way.

Sunday through Wednesday look to be dry as a deep area of low
pressure slowly drops down from the northwest and high pressure
sets up over the Gulf of Mexico. Some major differences show up
past Wednesday with respect to model solutions which will
hopefully get resolved over the next few model runs.

The long and short of it is unsettled weather starting today and
continuing through Saturday morning with a welcome relief to the
hot temperatures we`ve seen lately. By Sunday, high temperatures
will rebound 5 to 7 degrees bringing us to right where we should
be for this time of year.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1142 AM MDT Thu Sep 14 2017

A Pacific storm will impact the region through early Friday.
Ongoing scattered showers and thunderstorms will become numerous
late this afternoon and evening as the storm passes overhead. All
TAF sites are likely to experience light to moderate rainfall
with localized heavy rain possible. Heavier showers may result in
MVFR visibility, though CIGS are likely to remain in the VFR
range. However, CIGS below ILS breakpoints from KRIL, KEGE and
KASE are probable. In addition, outflow winds to 45 MPH are
possible with stronger cells. Conditions improve from north to
south after midnight with nocturnal showers lingering north of
I-70 through early Friday.

Scattered showers and thunderstorms will redevelop after 15Z
Friday as a cold front approaches the region with areas north of
I-70 once again favored for activity.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...None.
UT...None.
&&

$$

UPDATE...15
SHORT TERM...NL
LONG TERM...TGR
AVIATION...TGJT



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