Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 130913

Area Forecast Discussion
National Weather Service Riverton WY
213 AM MST Mon Feb 13 2017

.SHORT TERM...Today through Wednesday night

Imagery shows split flow across the west with a deep closed, nearly
cut-off, low and associated trof, helping in separating air-masses,
centered over northern Mexico. Upstream ridge now along the CONUS
west coast. SFC has high pressure across most of the CONUS,
including WY, with low pressure generally over the southernmost
portions of the USA. No precip anywhere in WY.

Most of the forecast will remain seasonal or to the cool side of
seasonal, with light SFC winds through Tuesday in addition to light
flow aloft. Patchy morning fog a possibility mainly west of the
Divide. Otherwise, with little available moisture or forcing at any
level...few clouds and no precipitation will be expected through the
end of the forecast period. Tuesday night through Wednesday night,
strong upper level ridging and building deep high pressure at the
surface will be in place across the Great Basin and Intermountain
West with lower pressure well off to the north and east of the
FA. Also at the SFC, a slowly increasing P GRAD will start to
grow between the two pressure zones along the Continental Divide.
This may induce some low end gusty southwest winds across the
usual areas beginning Tuesday night and lasting through the end of
the forecast. Nothing special here except that some lee side
foothills may be able to take advantage of their location and
associated breezes to fully reach seasonal average temperatures.

.LONG TERM...Thursday Through Monday

Mean ridge over the inter-mountain west will gradually flatten and
be replaced by a mean trough just off the west coast early next
week. System just west of the dateline will move into this ridge
around Thursday night with a quick shot of snow out west and a
chance in the I80 corridor. System will be weakening as it moves
through the mean ridge. The next system moving off Asia attm will
move into the inter-mountain west over the weekend. This system is
forecast by both the GFS and Euro to split as it encounters the mean
ridge...albeit weakening mean ridge. A decent shot of snow at times
will persist in the west and south over the weekend. A series of
systems will start moving through as the ridge is replaced by the
trough off the west coast with potential shortwaves in the southwest
flow. The models have one system coming through around Sunday
night/Monday with more snow out west and south and possibly some
across the north/east with trough passage and frontal boundary that
sags swd with it. Inversions that will likely plague the short term
may break at the beginning of this period as flow increases and lee
side trough increases. Will have to watch Thursday into Thursday
night period for the central basins as very warm h7 temps could
start mixing down and create another warm up like we saw last
Thursday evening in the Wind River Basin.



Patchy valley fog may partially obscure terrain in the lower river
valleys with all TAF sites except KCOD and KCPR showing VCFG through
16-18z Monday.  Fog may become more extensive along the KRKS-KBPI
route, on the west side of South Pass, Monday morning.  Otherwise
VFR conditions will prevail through 06z Tuesday with a weak, dry
cold front pushing south across the area Monday morning, accompanied
by some mid and high clouds and a subtle wind shift from the north.
High pressure aloft will fold across the area Monday night bringing
in warmer air aloft, steeper inversions over the valleys, and
possibly more extensive areas of fog development Tuesday morning
across the lower central basins (vicinity KRIW-WRL-KGEY) and
southwest Wyoming (KRKS-KBPI route).



Fire Danger low for all of WY through the next week as a good
existing snow pack will combine with seasonal to below seasonal
temperatures and less wind to keep fire danger low for most of the
next week. Smoke dispersion poor to briefly fair through Wednesday





LONG TERM...Skrbac
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