Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221724 AAA
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1125 AM MDT Thu Jun 22 2017

.SHORT TERM...Thursday through Saturday night

A cold front is now dropping southward through the state as I write
this. And normally, this would mean some sort of precipitation.
However, the airmass with it is very dry so this will be a dry
passage. There will be some decent northerly wind after the front
passes, but that will be pretty much the only effects. Of course, a
couple of the models are doing the Bighorn Mountain rain forest
effect, but with the lack of moisture everyone should stay dry. The
main effect will be cooler temperatures, generally 10 to 15 degrees
cooler. We kept temperatures above MOS in most areas however, given
a good amount of sunshine and being right around the solstice. There
will still be a gusty breeze at times, but nothing unusual.

Temperatures will continue to decrease on Friday. And again, with
the very dry atmosphere, all areas should stay dry despite a
shortwave moving through. Again the models are trying to bring
showers to the mountains, but at this point we think that is bogus.
The aforementioned temperatures will average around 10 to 15 degrees
below normal for many areas once again.

Yet another shortwave will move through on Saturday. There could
be a bit more moisture recovery ahead of this system. However, the
models again are not very impressed with the coverage of any
precipitation. Even the European, which has been the wettest
model, has continued to back off on QPF. As a result, we trimmed
POPS back mainly to slight chance and halved QPF. And yes, there
could be a bit of snow, but given that it is late June and most
would fall during the day, it would be restricted to the higher
elevations and would likely not stick. The chance of any showers
or thunderstorms would run roughly north of a Riverton to Casper
line.

.LONG TERM...Sunday through Thursday

Overview...Dry and milder conditions will occur Sunday, but
temperatures will still be below seasonal normals.  Much warmer
conditions are expected Monday and Tuesday with isolated to widely
scattered afternoon and evening thunderstorms across the north.
Temperatures will likely trend slightly cooler mid-week with slight
chances of thunderstorms across the far north.

Discussion...Synoptic pattern on Sunday features upper ridge over
the Intermountain West with NW flow aloft over Wyoming.  Surface
high pressure over the central plains will bring light east winds
across eastern and central Wyoming, but better moisture and
instability will be to our southeast across SE Wyoming and eastern
Colorado.  All in all, a Chamber of Commerce day Sunday across
western and central Wyoming with most lower elevation highs in the
70s to lower 80s, light wind, and plentiful sunshine.  Upper ridge
will translate the Rockies Monday-Monday night with building heights
and the return of summer heat through Tuesday.  Pacific shortwaves
riding through the ridge will bring some showers and thunderstorms
across the north.  A lee side surface trough is expected to
intensify Tuesday bringing breezy to windy and very warm and dry
conditions across the southern zones, elevating the fire danger.
Spread between the GFS and ECMWF increases mid-week with GFS slower
and more amplified with a trough across the northern Rockies while
ECMWF keeps stronger upper high over Arizona/New Mexico, which
blocks northern stream trough from digging south of the U.S. /
Canadian border.  Still much uncertainty at this stage, with
climatology favoring ECMWF`s stronger upper high over the southern
Rockies.  Forecast keeps temperatures above normal Wednesday with a
hedge toward near normal values on Thursday.

&&

.AVIATION...18Z Issuance

VFR will prevail at all terminals through 12Z/Fri. A cool front will
bring northerly oriented winds of 15-25kts (with higher gusts)
has pushed south to the divide earlier this morning. Generally few clouds
expected around the forecast area through the period. A second cool front
with gusty northerly winds will again burst through the region this
(Thursday) evening...affecting most terminal sites east of the Divide.
LLWS possible with the frontal passages at some locations.
&&

.FIRE WEATHER...

Expect cooler temperatures behind a cold front today. Some areas,
especially across the west and in southern Wyoming will continue to
a gusty breeze. Relative humidity should remain above critical
levels for most locations, but will likely drop below 15 percent at
times this afternoon across portions of Sweetwater and Lincoln
Counties. All areas should have a rain free day today. Mixing and
smoke dispersal will be generally good to excellent.
&&

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

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Braun/Lipson
FIRE WEATHER...Hattings



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