Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 112118

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
318 PM MDT Fri Aug 11 2017

.SHORT TERM...(Tonight through Saturday night)
Issued at 318 PM MDT Fri Aug 11 2017

Storms continued this morning across the north and increased in
coverage down south, due to an embedded shortwave disturbance
moving through the flow. The 3km NAMNest picked up well on this
activity and has resulted in higher confidence in this model in
the near to short term. Showers with embedded thunderstorms have
continued down south with storms developing across the central and
northern higher terrain where more surface heating is taking place
due to less cloud cover. Expect storms to have more organization
across the central and northern areas as this area remains in the
left exit region of a 60-70 kt upper level jet. Better shear also
exists here with steep lapse rates upwards of 9 degrees C/km.
Small hail and heavy rain remains a threat with these storms,
while areas of moderate to heavy rain are possible down south with
the embedded convection. Storm activity is expected to pick up
this evening as a shortwave moves through the southwest flow with
moisture increasing around the western periphery of high pressure
sitting to our southeast. Thinking some convection will continue
overnight as well due to this shortwave, as has been the case the
last two evenings, although more isolated to scattered in nature.

On Saturday, the upper level flow will lay over from west to east
again as the aforementioned shortwave helps to shift and elongate
the high pressure area further westward across New Mexico.
Precipitable water (PW) values will be in the 0.75 to 1.0 inch
plus range, which is more than sufficient for decent thunderstorm
activity across most areas. Convection may take a little longer to
get going until late Saturday afternoon, but expect it to pick up
towards the evening as another shortwave moves across the north.

.LONG TERM...(Sunday through Friday)
Issued at 318 PM MDT Fri Aug 11 2017

The 3km NAMNest model is picking up on convection developing early
Sunday morning across eastern Utah with storms moving in from the
west into west-central Colorado by late Sunday morning. This makes
sense as PW values will be at their highest with values
approaching 1.25 inches across this area and models are showing
another shortwave moving through with a weak upper level jet
streak. Expecting Sunday to be a more active day with these
embedded shortwave and upper level speed maxes moving through the
region through Sunday evening. CAPE values are forecast to be in
the 1000 to 2000 J/kg range, with some good directional and speed
shear, allowing for more longer lived storms and some stronger
storms at that. Expect heavy rain, gusty outflow winds and some
hail with the stronger storms. Coordinated with the neighboring
offices to increase PoPs during this period from Sunday morning
through Sunday evening due to these factors.

Some shortwave ridging occurs on Monday morning with an upper
level trough deepening over the west coast. Southwest flow will
increase Monday afternoon as the high shifts back towards the
southeast over Texas. Expect a slight downturn Monday morning
before convection picks up by Monday afternoon across the higher
terrain. The best potential for thunderstorm activity looks to be
Monday evening as a leading shortwave moves through and the trough
approaches with good upper level support as our region will be in
the left exit region. Convection looks to continue overnight into
the day on Tuesday with some stronger storms possible Tuesday
afternoon and evening as the upper level trough and associated
frontal boundary move through the area. Increased PoPs for this
period as well in coordination with surrounding offices.

Drier air looks to move in to the area from the west behind this
upper level trough and frontal boundary by Wednesday morning with
PW values decreasing below 0.6 inches across most areas, although
a few pockets of 0.75 and above remain up north. Another shortwave
moves through during the day Wednesday but expecting mostly a
drying trend from Wednesday through the end of the week for a
downturn in convective activity. Enough moisture still lingers to
allow for afternoon convection favoring the higher terrain.

There is not much change in H7 temperatures with the passage of
this trough, so expecting near normal temps to continue with
cooler temps in clouds and showers with the most active periods
from the weekend through Tuesday.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1148 AM MDT Fri Aug 11 2017

Active day expected as several features are over the area - jet
aloft is strongest across NE UT and NW CO and monsoonal moisture
is increasing from the southeast into the Four Corners.
Anticipating rain and embedded heavy rainers across SE UT and SW
CO, whereas storms across NE UT and NW CO will be more organized
and longer lasting but less coverage across the region. ILS
breakpoints could be reached in heavier storms, with even the
chance for MVFR at times if stronger storms reach the airports.
Instability continues after 00z over the San Juans and in the Four
Corners region allowing showers and thunderstorms to continue into
the nighttime hours.




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