Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 190828
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
228 AM MDT Fri May 19 2017

.SHORT TERM...Friday through Sunday night

The late season snow continues in some areas but the trend has been
downward and this will continue for the most part through the night
although we cannot rule out a burst of snow or two if things set up
right. We can thank the intense thunderstorms across the Midwest for
robbing a lot of the moisture that would have produced even more
snow in many locations. We will leave the advisory up however,
mainly for possible icy areas on roads with temperatures falling
below freezing. Low pressure moving away should end most of the
steady precipitation this morning. However, with unstable
northwest flow and cyclonic curvature remaining, there will still
be some showers around. Temperatures will be warmer than
yesterday, but with a lot of cloud cover lingering as well as snow
cover in some areas, it will still 10 to 15 degrees below normal.
In addition, with the moisture and rather small temperature-dew
point spreads, the fog monster will be around in some areas for
the morning hours. All showers should end late tonight.

After a dry start on Saturday, there will be another chance of
showers in the afternoon and evening as another trough approaches
from the north and west. The models differ a bit on coverage of
convection though. The best chance would be west of the Divide but
all areas will have a chance of something. With cool temperatures
and low wet bulb zero levels, small hail would be the main threat.
However, most areas should be rain free most of the time.
Temperatures will also moderate although remain below normal. At
this point, Sunday also looks rather unsettled. The models again
differ on the coverage of any showers and storms though with the GFS
the wetter model and the NAM drier. For now, we made few changes to
continuity as most areas will have at least a slight chance but be
mainly dry most of the time.


.LONG TERM...(Monday through Friday)
Issued at 1256 AM MDT Fri May 19 2017

Models show a shortwave trough over northeast Montana at 12Z
Monday sliding rapidly south southeast across the northern High
Plains. The associated cold front will sweep south, east of the
Divide Monday. This system will bring a good chance of mountain
snow/valley rain showers east of the Divide especially over
Johnson County, becoming slight over the far west/southwest. Looks
like snow levels over the Bighorn Mountains/northern Absarokas
could dip down to 7000-7500 feet. This is a quick system so total
rain/snow amounts will be on the light side.

The 00Z GFS and Canadian models have trended towards the ECMWF
which has shown rapidly drying Monday night behind this shortwave
trough with dry and slightly below average temps for Tuesday.
Models are also showing a stable atmosphere Tuesday so even with
peak heating not expecting any convection.

Also Monday night, models show the potential for a frost and/or
light freeze in the lower elevations as skies clear and winds
become light during the predawn hours Tuesday.

Global models have also trended with more ridging over the Rockies
Wednesday and Thursday ahead of a couple upper lows. One sliding
southeast across British Columbia and the other one meandering
well off the West Coast. Thus Wednesday and Thursday look drier
and warmer, and have trended forecast that direction. Models are
generally showing 700mb temps warming to between +7C and +11C
Wednesday into Thursday. This would result in highs in the mid
70s to lower 80s east, and low to mid 70s west. Some model
solutions then try to flatten out the ridge late Thursday into
Friday with slightly cooler temps and possibly some convection.

&&

.AVIATION...12Z Issuance

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS Terminals

An upper low pressure system over the central Rockies will slowly
lift into the High Plains today.  Areas of MVFR, local IFR ceilings
with light snow will continue to wrap west across the Interstate 80
corridor, vicinity KRKS, through the morning while areas further
north remain under prevailing VFR conditions with some higher
mountain obscurations.  Lower clouds should dissipate vicinity KRKS
and eastward late Friday morning-early afternoon. Slight instability
in the afternoon will result in scattered rain and snow showers
developing over the high terrain and moving southeast, vicinity KJAC-
KPNA-KBPI in the afternoon possibly pushing south into vicinity KRKS
Friday evening.  Brief MVFR conditions are possible with these
showers.  This activity should dissipate from NW-SE 03z-09z
Saturday.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL Terminals

An upper low pressure system over the central Rockies will slowly
lift into the High Plains today.  Areas of MVFR light snow with
local IFR conditions will persist primarily along and south of a
KDUB-KRIW-40SM N KCPR line through 15z with upslope snow persisting
lastly vicinity KLND and the eastern Wind River Range possibly into
early afternoon.  Otherwise areas of MVFR ceilings should begin to
lift and dissipate 15z-19z Friday leaving prevailing VFR conditions
in the afternoon with some higher mountain obscurations.  Clearing
from northwest to southeast is expected Friday evening and overnight
with some patchy valley fog possible early Saturday morning.

&&

.FIRE WEATHER...

Unsettled weather will continue into the weekend. Steady
precipitation should end by morning but there will a lot of cloud
cover and lingering rain and snow showers through the day, with the
largest coverage in the east and more sun in the northwest. Wind
should remain light. Mixing and smoke dispersal will generally range
from poor to good. It will turn somewhat milder this weekend
although there will be a chance of mainly afternoon showers and
thunderstorms each day with a mixture of clouds and sunshine.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 AM MDT early this morning for
WYZ015-019-022.

Winter Weather Advisory until 6 AM MDT early this morning for
WYZ007-011-017-018-020-030.

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Meunier
FIRE WEATHER...Hattings



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