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Area forecast discussion 
National Weather Service Riverton Wyoming
325 am MDT Sat Jul 4 2015

Short term...Saturday through Monday night

In Honor of the 239th birthday of the country...actually the
declaration was approved on the second...but anyway...we present
the discussion of the Fourth of July weather this morning in the
form of the preamble of the declaration of Independence.

We hold this weather to be self-evident...that western and central
Wyoming will be endowed...by a ridge of high pressure...with certain
unalienable conditions...warm to hot temperatures...light to
moderate winds...and isolated afternoon and evening thunderstorms.

That to secure these conditions...a westerly flow instituted by low
pressure to the southwest and high pressure to the
northeast...deriving their Powers from the jet stream flow...that
whenever the strong cap is punctured by low level instability...it
is the right of the atmosphere to alter the hot and dry
conditions...and Institute isolated afternoon and evening
thunderstorms...laying its foundation on high level heat source and
drifting east in such form...as to them shall seem most likely to
effect Cook outs...parties and fireworks displays.

Prudence...indeed...will dictate that get togethers long planned
should not be changed for most areas will be dry most of the
time...and accordingly all experience hath shown...that areas near
mountains are most disposed to see thunderstorms...while storms are
sufferable...then to right themselves by noting that they will pass
quickly as we are accustomed. But when a long train of hot days and
pop up storms...occurring invariably at the same time...shows a
design typical of the Wyoming summertime...it is common...and well
known...to throw off such forecast...and to provide the need for
updates.

Now back to your regularly scheduled discussion. The heat will
peak today as the ridge looks to be at peak strength. Any leftover
showers should end by daybreak with the morning and most of the
afternoon being mainly dry. As for the pesky afternoon
thunderstorms...two areas look to have the best chance. One will
be across southern areas where a push of moisture from Colorado
may fire a few storms...especially across eastern Sweetwater
County. The other will be across the north...from the absarokas to
the big horns...immediately downwind of these locations and the
Big Horn basin. With the cap stronger today...we expect fewer
storms than yesterday...but any could turn strong like the
previous few days. But...we can not rule one out for anyone...just
not enough confidence to it in everywhere. Will be another hot one
as well...with highs in the 90s widespread across the lower
elevations.

However...changes are coming as we head into Sunday. This will be
courtesy of a cold front dropping in from the north and some
moisture moving in from the south. There are some differences in how
fast the front will pass through however. In some locations...there
is a difference of up to 12 degrees for high temperatures. For
now...we have played the middle. However...temperatures will be 10
to 20 degrees cooler than on Saturday. This will also play into how
strong the storms will be. At this time...lifted indices are fairly
impressive...falling down to minus 4 in some locations.
However...cape is relatively limited. In addition...increased cloud
cover my limit instability and the best jet dynamics remain over
Montana. The big threat looks to be heavy rain...as precipitable
water values rise to well over an inch across many areas. Continuity
had heavy rain in the forecast and we kept that wording.
Thunderstorms may continue into the night as well as the front moves
south. The models have some disagreement on this however with both
amount and placement...with the NAM quite wet and the GFS drier.
Again...for now we split the middle.

Unsettled weather also looks to continue into Monday as well.
Previous runs that showed heavier showers on Monday morning have
backed off so we backed off probability of precipitation and quantitative precipitation forecast. More moisture surging in
from the south will fire more showers and thunderstorms Monday
afternoon. This time...the best coverage looks to be across the
south and west. Again...there are some differences in coverage with
the NAM drier than the GFS. Continuity looked reasonable so we made
few changes. With the moisture pushing eastward...so will the chance
of thunderstorms into the evening hours.

Long term...Tuesday through Saturday

For Tuesday and Wednesday the weather looks unsettled over the area.
It looks like better coverage of showers and storms in the afternoon
right through the late night hours. The timing of the long range
European model (ecmwf) and GFS models are close with a weather disturbance moving
west to east across Wyoming Tuesday night. Then a new system drops southeast
over Montana Wednesday and into northern Wyoming Wednesday night. Model timing
right now with this system is close. Some cooling as a front gets
kicked southward on Wednesday. Upper ridge builds Thursday and Friday
sending temperatures back to very warm to hot with less coverage of
thunderstorms east of The Divide. There may be more storm coverage
over the west Friday as an increase in moisture may occur. This
depends on how the models handle the movement of a weather system
approaching the West Coast Thursday night through Friday. The track
yet to be determined and will watch it over the coming days. Next
Saturday will be hot and mostly dry east of The Divide with a chance
of showers and storms in the west and northwest as the weather
system passes through Idaho into Montana.

&&

Aviation.../12z issuance/

A couple of upper level disturbances will rotate across southern
Wyoming where isolated to widely scattered thunderstorms will
Blossom in the afternoon mainly vicinity and south of a krks-kcpr line.
A drier and more stable airmass further north and west will keep
isolated -tsra confined mainly over and near the mountains

A cold front will push south into northern Wyoming around 06z
Sunday...spreading isolated showers south into vicinity kcod-kwrl-kbyg
early Sunday morning...conditions remaining prevailing VFR behind
the front.

&&

Fire weather...

A ridge of high pressure will bring one more hot and mainly dry day
for most areas. Expect good to excellent smoke dispersal across the
area. Relative humidity will fall into the middle teens in some of
the lower elevations. A gusty breeze may develop in the south and
west. A few thunderstorms will develop later in the afternoon and
evening east of The Divide but coverage should be less than in
previous days. A cold front will drop southward across the area for
Sunday and bring in cooler temperatures...higher humidity and a
better chance for showers and thunderstorms.&&

Riw watches/warnings/advisories...none.

&&

$$

Short term...hattings
long term...Arkansas
aviation...aem
fire weather...hattings

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