Area Forecast Discussion
Issued by NWS Riverton, WY

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598
FXUS65 KRIW 200553
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1153 PM MDT Thu Apr 19 2018

.SHORT TERM...This evening through Sunday
Issued at 1206 PM MDT Thu Apr 19 2018

Quite the challenging forecast with our large upper level low
currently moving from CA into the Desert Southwest. Morning analysis
showed the system still digging with the jet on the backside.
However, the jet round the base this evening with most of the
guidance showing the upper low wobbling/lifting nwd into central UT
overnight. It is bumping into a larger scale ridge farther north so
I don`t expect it to lift too far north. The bigger question after
that is does it redevelop swd towards the 4-corners and move across
srn CO Friday or does it ignore the favored path and lift across the
spine of the CO Rockies reappearing in ne CO Friday evening. The NAM
takes the nrn route the GFS and GEM take the srn route. The SREF is
in between. It makes a huge difference for potential WCB wrapping
back into at least our ern zones Friday night. Normally it would
take the path of least resistance and track along srn CO before
lifting ne across the plains. A couple favoring the nrn track are
late April and the jet on the se side. Tough forecast and it makes a
potentially huge difference for say CPR. The moisture is cut-off
right now in the plains with low dew points and n-ne wind all the
way into srn TX. Models do reverse the flow but actual moisture
amounts are rather low for late April as the sly flow returns, so no
sudden low reintensifaction expected as the low moves into the
plains of CO. But where does the low end up Friday night? Leaning
towards some nrn reflection maybe close to the SREF with the main
low still in srn CO. So cold rain and CPR mtn snow still quite
possible but not significant like the NAM12 shows. Further west in
Fremont County and Sweetwater County, we deal with a shot of cold
pcpn on Friday with upslope developing, cyclonic nose of the curved
jet streak and QG forcing moving up into Sweetwater and Fremont
County with temps in that danger zone for rain changing to snow
around sunrise (-5C or so at H7). Have lowered lows and highs
tonight and Friday with some snow/rain-snow producing accumulation
above 5.5K or so. LND is tough. They could see several inches of wet
snow in the morning burst then lighten up with some rain or rain
snow mix the rest of the day. The Wind River Basin may be mostly
rain below 5.5K but it`s close. The I80 corridor will probably see
some pretty good snow/snow showers Friday with the higher spots
possibly picking up 3-6 inches of wet snow on the grassy/wood
surfaces and very little on the roads except during any heavy
spurts. Will cover this in an SPS. Much improved conditions over the
weekend with warming temps and dry conds.

.LONG TERM...Sunday night through Thursday night
Issued at 1206 PM MDT Thu Apr 19 2018

A low amplitude trough will move over the Northern Rockies Sunday
night into Monday. Breezy to windy conditions will occur across
the southern half of the forecast area ahead of the cold front.
Breezy to windy conditions will occur across the Big Horn basin
late Monday afternoon into the evening hours, behind the cold
front. The best chances for precipitation remain across the
northwestern and northern portions of the area, with chances
increasing east of the Divide during the afternoon. Mountain
snow/lower elevation rain continues in the forecast, as H7 temps
will drop dramatically behind the front.

The rest of the forecast period is trending drier with warming
temperatures. This will be due to a cut off low over the EPAC and
the northern branch of the jet staying well to the north over
western Canada. The only discrepancy in the models is the GFS that
brings a quick moving dry cold front from the north on Wednesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1152 PM MDT Thu Apr 19 2018

An upper level low pressure system will move across southern
Colorado Friday-Friday evening.  Widespread MVFR conditions, local
IFR conditions in rain and snow will develop vicinity and south and
east of a KRKS-KLND-KRIW-KBYG line by 20z Friday with snow levels
around 6000 feet.  Higher terrain above 8000 feet will also become
mostly obscured in this area; ceilings will likely lower mostly into
category after 00z Saturday.  Precipitation will decrease NW-SE 03z-
09z Saturday but areas of MVFR-IFR ceilings and patchy fog are
likely to linger through 12z.

Further north and west, VFR conditions will prevail through the
period with scattered MVFR shrasn pushing north to vicinity KBPI-
KWRL line by 20z Friday, followed by clearing 00z-06z Saturday.

&&

.FIRE WEATHER...
Issued AT 1206 PM MDT Thu Apr 19 2018

A large low pressure system will move towards the 4-corners area on
Friday and then move east across Colorado Friday night. Widespread
precipitation is expected across the Red Desert with some snow
accumulation on Friday along with gusty northeast wind. Some rain or
rain and snow mixture and gusty northeast to east wind will extend
as far north as Fremont and Natrona Counties Friday into Friday
night with a brief period up into Johnson County. The northern zones
will be dry with a period of gusty north wind as the storm moves
in.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Skrbac
LONG TERM...LaVoie
AVIATION...Meunier
FIRE WEATHER...Skrbac



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