Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 162117
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
317 PM MDT Sat Sep 16 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 307 PM MDT Sat Sep 16 2017

Some showers will linger over the mountains and east early, then
clearing with cold freezing temperatures is expected.  Some patchy
fog is expected in the valleys, especially where heavier precipitation
amounts have occurred.  Sunday will become warmer with areas of
breezy west winds.  Clouds will begin to increase on Monday ahead of
a fast moving Pacific low pressure trough.  Some showers will
move in over the northern and central mountains in the afternoon
increasing significantly early in the evening with embedded
thunderstorms.  Snow levels will lower to around 8500 feet in the
north and 9500 feet in the south after midnight.  Colder showery
weather will continue through Tuesday with isolated thunderstorms.
Local heavy precipitation  is expected, especially in and near
the mountains.

.LONG TERM...(Tuesday night through Saturday)
Issued at 307 PM MDT Sat Sep 16 2017

Once the shortwave exits east into the Plains, flat ridging will
take over from the west with the gradient remaining rather tight.
This strong westerly steering flow will keep upslope precip going in
the far west. West southwest winds will diminish late Tuesday night
in the southern wind corridor and the higher elevations west of the
divide. A surface warm front will push in from the southwest
Wednesday, making Wednesday the warmest day in the long term with
highs ranging from the 60s at the lower elevations of the east to
the 40s in the west where westerly upslope flow, clouds, and precip
will keep that area cooler. By Thursday, the long wave trough will
be over the Great Basin, This trough will eject a shortwave toward
the northeast, almost identical to the one on Tuesday across the
area with h7 temps cooling to around the levels we will witness
early next week resulting in valley rain and mountain snow showers
spreading east of the divide during the night Wednesday night and
into Thursday. West southwest winds will increase once again in the
southern wind corridor and in the mountains Wednesday night. By
Thursday, the models begin to diverge a bit with the Canadian
closing off the long wave trough over Idaho. Meanwhile, on Thursday
night, the band of showers will exit southeast across Natrona and
Sweetwater Counties. Then on Friday, the GFS indicates southwest
confluent flow with precip diminishing central and east and some
upslope valley rain and mtn snow showers continuing in the west. By
Saturday, things could get interesting. The Canadian continues to
close off the aforementioned low within the long wave trough (albeit
positively tilted) and widens the low over the Great Basin resulting
in a southerly difluent flow over our CWA which could set up a
pattern for a potentially significant precip event next Saturday.
This is entirely possible with the long wave trough in the 5 wave
pattern slowly migrating east across the Great Basin and toward the
Rockies. However, GFS and the Euro keep it more of an open trough.
So for now, will play it down, somewhat, as far as precip
distribution east and west of the dvd goes with the uncertainty.
Temps will not vary much Thursday through Saturday with highs
remaining on the cool side with highs in the 40s west, 50s
Sweetwater County and 50s and 60s east.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 307 PM MDT Sat Sep 16 2017

Isolated rain and snow showers will remain possible through 03Z
Sunday over northern half of the state. VFR conditions are expected
through the forecast period, with areas of mountain obscuration.
These clouds are expected to quickly exit the area between 03-06Z
Sunday. Mid to high clouds will then quickly move over the area by
06Z Sunday, which could impede fog development. Have left it in KJAC
forecast for now, with KBPI/KPNA/KRIW being other possible
locations. SCT clouds will remain in the area through 00Z Monday,
with generally light SW winds.

&&

.FIRE WEATHER...
Issued AT 307 PM MDT Sat Sep 16 2017

Once the shortwave exits east into the Plains, flat ridging will
take over from the west with the gradient remaining rather tight.
This strong westerly steering flow will keep upslope precip going in
the far west. West southwest winds will diminish late Tuesday night
in the southern wind corridor and the higher elevations west of the
divide. A surface warm front will push in from the southwest
Wednesday, making Wednesday the warmest day in the long term with
highs ranging from the 60s at the lower elevations of the east to
the 40s in the west where westerly upslope flow, clouds, and precip
will keep that area cooler. By Thursday, the long wave trough will
be over the Great Basin, This trough will eject a shortwave toward
the northeast, almost identical to the one on Tuesday across the
area with h7 temps cooling to around the levels we will witness
early next week resulting in valley rain and mountain snow showers
spreading east of the divide during the night Wednesday night and
into Thursday. West southwest winds will increase once again in the
southern wind corridor and in the mountains Wednesday night. By
Thursday, the models begin to diverge a bit with the Canadian
closing off the long wave trough over Idaho. Meanwhile, on Thursday
night, the band of showers will exit southeast across Natrona and
Sweetwater Counties. Then on Friday, the GFS indicates southwest
confluent flow with precip diminishing central and east and some
upslope valley rain and mtn snow showers continuing in the west. By
Saturday, things could get interesting. The Canadian continues to
close off the aforementioned low within the long wave trough (albeit
positively tilted) and widens the low over the Great Basin resulting
in a southerly difluent flow over our CWA which could set up a
pattern for a potentially significant precip event next Saturday.
This is entirely possible with the long wave trough in the 5 wave
pattern slowly migrating east across the Great Basin and toward the
Rockies. However, GFS and the Euro keep it more of an open trough.
So for now, will play it down, somewhat, as far as precip
distribution east and west of the dvd goes with the uncertainty.
Temps will not vary much Thursday through Saturday with highs
remaining on the cool side with highs in the 40s west, 50s
Sweetwater County and 50s and 60s east.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Freeze Warning from midnight tonight to 9 AM MDT Sunday for
WYZ003>006-010-011-013-016>020-023-025>030.

&&

$$

SHORT TERM...Baker
LONG TERM...Lipson
AVIATION...LaVoie
FIRE WEATHER...Baker


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