Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 291603
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1003 AM MDT THU SEP 29 2016

.UPDATE...
Issued at 1002 AM MDT Thu Sep 29 2016

We have updated the forecast today to include the threat of heavy
rainfall and a flash flood watch for the slickrock and canyonlands
areas of southeast Utah. A coupled jet moving into the 4 Corners
region...well defined wave lifting across central AZ and a high
probability of dominant warm rain processes help boost confidence
for this headline. Extensive cloud cover and lingering showers in
the much of the western 2/3 of the CWA should also limit heating
today and as a result lowered temperatures for many areas. Snow
levels remain at or above timberline under this warm/moist
environment as well.

&&

.SHORT TERM...(Today through Friday)
Issued at 445 AM MDT Thu Sep 29 2016

The forecast area was sandwiched between a high pressure ridge
over the High Plains and a trough of low pressure off the West
Coast. This has opened the door to the latest monsoonal surge
which began Wednesday and will impact the region through the end
of the week. Moisture levels have already risen significantly in
the past 24 hours as measured by dew points at observation points
across the forecast area. As of 10Z/Thu, dew point values were up
by 8 to 13 deg F over eastern Utah and all but the eastern most
zones of western Colorado. Expect these values to continue to
climb as southerly flow continues during the day and remnant
moisture from Roslyn peaks.

The latest radar imagery showed light showers moving northward
across southwest Colorado and southeast and east-central Utah.
This activity appeared to be sustained by 50 kt jet flowing
northeastward across central Utah. Daytime heating combined with
increasing moisture and a weak coupled jet over southeast Utah and
southwest Colorado will lead to increased coverage and intensity
of showers with thunderstorms developing around midday. As
precipitable water levels approach the 1 inch mark over the Four
Corners region storms will become more efficient rain producers
raising the threat of excessive rainfall from the stronger cells.
Storms should move along at a decent clip (15 to 20 MPH) so expect
any flooding issues will be localized.

Waning dynamic forcing and diurnal cooling will result in
decreased showers and thunderstorms tonight...however its
unlikely that moist convection will completely diminish. Showers
and thunderstorms will increase again from late Friday morning
through the afternoon in response to diurnal warming. However, a
shift in flow to a more southwesterly fetch will result in a
decrease in precipitable water values.

Increased clouds and showers will result in more seasonable high
temperatures today and Friday. In contrast, clouds and increased
atmospheric moisture will buoy overnight lows.

.LONG TERM...(Friday night through Wednesday)
Issued at 445 AM MDT Thu Sep 29 2016

Deep tropical moisture transitions to windy and cold early next
week.

Behind the dynamical remnants of Rosyln, late Friday night into
Sunday will still see some, mainly diurnal, showers and
thunderstorms. A colder and stronger West Coast low develops along
the OR-CA coast. Sunday, a strong vort max digs down the west side
of the trough progressing it into the Great Basin on Monday.
Strong SW winds develop early Monday as the cold front passes.
Storms along the front will favor the northern half of the
forecast area. These storms may be strong with increasing
instability and a veering and speed shear environment.

Monday afternoon-Tuesday, the upper low lifts through western
Wyoming and onto the northern plains. A cold but drier NW flow
will lower snow levels to around 6kft that will bring potentially
significant snowfall to the NW Colorado mtns from Vail Pass
northward. The GFS has more energy dropping into the backside of
the trough that could continue snow over the northern mtns into
Wednesday. The EC is faster and dries out by Wednesday. Elsewhere
expect below normal temperatures and freezing Tuesday night
temperatures into most areas above roughly 5500 ft.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1002 AM MDT Thu Sep 29 2016

VFR with intermittent MVFR conditions in showers/cigs will remain
possible through the early portion of the 29/18Z forecast. Heating
will be limited by the cloud cover but do expect instability to be
enough to produce some thunderstorms...with moderate to heavy
rainfall...through the afternoon and early evening hours. This may
help produce areas of mvfr/ifr flight conditions as well. The
highest probability will be at the KTEX...DRO and KCNY terminals
through 30/03Z. Gusty outflow winds becoming less of a threat in
this moist environment but can be ruled out by the stronger
thunderstorms. Moisture and therefore lower cigs and mountain
obscuration remains a threat over the next 24 hours.

&&

.GJT Watches/Warnings/Advisories...
CO...None.
UT...Flash Flood Watch through this evening for UTZ022-027-029.

&&

$$

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


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