Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 261713
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1113 AM MDT SUN JUN 26 2016

.UPDATE...

Forecast on track with no changes. Included 18Z avaition
discussion below.

&&


.PREV DISCUSSION... /issued 232 AM MDT SUN JUN 26 2016/

SHORT TERM...Today through Tuesday night

Imagery shows flat ridging across the western CONUS with a few
embedded shortwave disturbances moving through the flow. The surface
has modest high pressure across the west and over WY...with a weak
boundary...later to become a front...into northeastern MT this
morning. No precipitation fall in the state ATTM.

Today, fire weather concerns decrease below extreme but will still
remain enhanced for most of the lower elevations east of the Divide
and across south and southwest WY. The area will be post-frontal
with weak high pressure building across the surface and modest
ridging taking place aloft. No precipitation is expected across the
FA and SFC winds should largely remain below 15 kts for all
locations. Late this afternoon and evening...another dry clipper
type front will move through northeastern/eastern WY...but with
little impact of any kind save a wind shift.

Monday, ridging returns along with warming at the sfc. Low pressure
building over the Great Basin and high pressure out over the plains
will begin to feed a little moisture back into eastern WY...and to a
lesser extent as far west as the Divide...as low level flow goes
easterly. Still, it will be mostly too stable and capped to fire too
many thunderstorms. However, there will be an outside chance for a
storm or two over the Bighorn/Casper Mountains and adjacent lower
elevations to the east and north of the respective ranges. If these
storms happen they should be fairly robust and quite gusty...perhaps
approaching severe in winds or possible downburst as DCAPE will
probably range between 1000 and 1500 j/kg. Small to penny size hail
will also be possible for a short period. There will also be a small
chance for a few showers across the northwestern FA with the
approach and passage of a ridge topping shortwave overnight.

Tuesday, warm to hot across the FA. Low pressure builds into
southwest WY with fronts/boundaries around and a shortwave
disturbance moving through the ridge by mid-day/afternoon, setting
the stage for TS initiation. Isolated to widely scattered chances
for thunderstorms will cover most of the state of WY except for the
southwest. Stronger storms of the day...including a few severe storms
will be most possible east of the Divide. Dry thunder most probable
across the south...especially Sweetwater county. Most likely
strong/severe threat will be in the form of strong wind gusts.
However, a rather early show...by noon or soon after...across the
Bighorn Basin and Johnson County could also produce some possible
large hail and even a tornado or two in Johnson county with strong
shear and moderate CAPE around. Did not include severe in the grids
quite yet...but may add later.

LONG TERM...Wednesday through Sunday

Synopsis...Upper ridge will remain over the Rockies through the
period, bringing warm temperatures with basin and valley highs
mostly in the 80s to lower 90s.  Isolated to widely scattered
thunderstorms will develop mainly across the north and east
Wednesday.  Some monsoonal moisture will bring chances of
thunderstorms in from the south on Thursday and Friday.  A drier
airmass is expected over the area Saturday and Sunday.

Discussion...ECMWF and GFS in overall agreement with synoptic
features through Friday.  An upper ridge will be centered roughly
along the 110W meridian flanked by upper troughs across the Great
Lakes region and the Gulf of Alaska-NE Pacific.  This latter trough
will continue to eject several shortwaves into/over the ridge before
both models begin shoving the trough into British Columbia over the
weekend.  GFS is more progressive with trough/ridge/trough pattern
over Canada than ECMWF and preference is for the slower EC solution.

No big trends in temperatures are expected as far as
heights/thickness go.  Increasing high clouds as monsoonal plume
makes a northward surge Thursday and Friday will likely moderate
temperatures, especially across the south.

The main forecast challenge will be sources of moisture/fuel for
thunderstorms.  Quasi-stationary surface front/dry line is expected
to reside roughly near a Cody-Buffalo-Casper line Wednesday with the
higher dewpoints/CAPES north and east of this boundary.  Shortwave
topping the ridge is expected to interact with this boundary
Wednesday afternoon/evening with thunderstorms developing along and
north-east of the boundary.  This boundary will set up further east
on Thursday as monsoonal moisture pushes north across the Great
Basin.  A few thunderstorms may push into the south and west on the
edge of the monsoonal plume Thursday.  At least higher level
monsoonal moisture is expected across much of the area on Friday to
bring at least a slight chance of thunderstorms, and then this
moisture is expected to get suppressed further south across Colorado
and Utah over the weekend.

AVIATION.../18Z ISSUANCE/

SKC with light winds will continue through this evening. Some cirrus
could overspread the area from the west late tonight...while some
low level clouds could begin spread/develop westward into portions
of Johnson/Natrona Counties. Boundary layer moisture is marginal at
best for low clouds, and will only hint at the lower clouds at the
CPR terminal with a FEW group. On Monday there will be a bit more
moisture for some high based cumulus...with even isolated late
afternoon convection possible across the extreme east. KCPR could be
impacted by this activity, but will not mention TS in the TAF due
the expected limited coverage, and the possibility that the
convection will remain east/southeast of the area.

FIRE WEATHER...

Moderate to elevated fire danger across the forecast area as warmer
and very dry conditions continue today. however, winds will be quite
tame by Wyoming standards at generally less than 15 mph for most
locations. Monday and Monday night, warmer with low level winds
beginning to turn out of the east, but not quite as dry with slowly
increasing moisture through the period. There will be small chances
for isolated thunderstorms with gusty erratic winds mainly over
Casper Mountain and perhaps the Bighorn Mountains...with less
chances over the lower elevations of Johnson and Natrona counties.
Tuesday will see better chances for thunderstorms east of the
Divide...some on the strong side especially with regard to strong
wind gusts. Smoke dispersal during the daytime hours remains good to
excellent east and west of the Divide.

&&

.RIW Watches/Warnings/Advisories...None.

&&

$$

Update...WM
SHORT TERM...Braun
LONG TERM...AEM
AVIATION...WM
FIRE WEATHER...Braun



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