Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Riverton Wyoming
218 PM MDT Monday may 4 2015

Short term...tonight through Tuesday

Showers and thunderstorms will remain mostly near the higher terrain
this evening...especially across southern Wyoming where storm
movement is nearly non-existent. Across northern Wyoming...storms
are moving off to the northeast at about 10 to 15 miles per hour so there could
be some activity in the foothills but it will remain fairly stable
over the basins.

Overnight...a small disturbance will move into southwest Wyoming at
around midnight...keeping some showers and storms going in the Upper
Green river basin and into the Wyoming and salt ranges.

On Tuesday...a low pressure system currently near Southern
California/Baja California will continue to trek northeast and will eventually
provide some additional lift for Tuesday afternoon. In
addition...moisture will feed in from the southeast with precipitable water
values expected to be well above normal for this time of
year...between 0.5in and 0.75in with the higher values moving into
eastern portions of the forecast area late in the day. Went ahead
and included some heavy rain wording with the thunderstorms late
in the afternoon in the addition to Johnson and
Natrona counties. Elsewhere...coverage will be about the same as
today...if not slightly increased given the additional support
aloft...with storm movement remaining on the slow side at 5 to
10kt...thus with yet another day of slow moving storms...the main
threat will be from flooding in any areas that have been saturated
by a slow moving storm over the past couple of days.

Long term...Tuesday night through Monday

Beginning of forecast...current upper SW low over Southern California/Baja California will
swing northward with axis just S of the forecast area Tuesday night. Cyclogenesis and
associated surface low will develop and deepen over the County Warning Area from the
S...with surface convergent boundary lifting northward into central Wyoming. Heavier
precipitation and stronger storms expected near/south of this
boundary...with best activity roughly between eastern Fremont the southeast and into cys jurisdiction. Characteristic
atmosphere in this region with have increasing precipitable waters ...but Low Cape
and relatively low shear. Showers/storms will also be slow
movers...with possible movement becoming Erly through the evening. As
stacked system further develops with surface to Maryland level frontogenesis
proceeding overnight...surface low will move to the Lee side of the
mountains in southeastern Wyoming with trowel type situation setting up west to northwest
of the low's center Wednesday morning. Expecting deep/copious amounts of
moisture from the S and southeast to wrap up into this developing system
with precipitable waters increasing to nearly 1 inch over the eastern counties. Expecting
0.5 to 1 inch totals for a wide area including eastern Fremont
County...through Natrona and Johnson counties...including the
mountains surrounding these locations. Severe parameters overall will be
low... however...ll shear near the surface low and its effects be quite
large for short periods before the front finally pushes through in the
afternoon. As cloud bases will be very low...will not be out of the
question to see a few funnel clouds to spin up on/near this boundary
between eastern Fremont and points east from late morning through middle

Thursday...Post frontal and much more stable east of The Divide...with
morning fog around. Increasing instability west of The Divide and
across the south as upper divergence moves over the area along with a
modest increase in moisture in the afternoon and through the evening.
Possible overnight mountain showers producing at inch or so of snow
across the western/northwestern mountains by Friday morning. Otherwise...similar
situation Friday with large upper level closed low approaching the forecast area
from the SW...again with the main impetus of precipitation occurring
across the west and S.

Sat and sun...agreement within the lr models is remarkably good...both
aloft and at the surface. The basic idea is that the area is headed back
into a Spring/winter-like scenario as large upper closed low moves
across the central and southern rockies...while strong cyclogenesis begins
over southeastern Colorado by middle-day Sat. Moisture return flow from the southeast will be
prevalent across the forecast area...with greatest amounts once again in eastern
Wyoming. While both models get cold enough to snow over the mountains...and
possibly mix with rain over the northern lower elevations...the GFS is a
couple of degree lower at 700 mb which will make the evolution of this
mother day's weekend storm interesting.

Monday...upper ridging and Post frontal surface...with drying beginning...
lasting for at least a few days...possibly.


Aviation.../00z issuance/

East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes

There is not much wind aloft to move the ongoing showers and storms
along so most of the activity will occur over/near the mountains
through about 03z in most areas...and storms will quickly weaken as
they move off of the higher terrain. Winds will generally be
easterly in many locations...but could be erratic and gusty due to
nearby showers and storms. Expect more showers and thunderstorms on
Tuesday...generally after 18z...with a bit more coverage than what
is expected today...especially vicinity kcpr...with heavy rain possible
after 21z Tuesday.

West of The Divide...kbpi/kjac/kpna/krks routes

All terminals could have a shower or storm in the vicinity through
03z with the focus then shifting to southwestern terminals...with the
focus on kbpi and kpna by midnight with vcsh or thunderstorms in the vicinity possible
through sunrise. Wind fields will generally be driven by showers and
thunderstorms. Expect more showers and thunderstorms on
Tuesday...generally after 18z...with similar coverage to today.


Fire weather...

A generally wet and stormy week ahead with light winds driven by
terrain and nearby shower or thunderstorm activity. A push of
moisture will arrive tomorrow afternoon...which will benefit the
Casper dispatch area the most with wetting rains. Storms are slow
moving so any that do form will likely produce a wetting rain. There
could be a bit of a break from the thunderstorms on Wednesday...and
Thursday for Casper and Cody dispatch areas. A much larger system
moves in with mountain snow and a very good chance for a wetting
rain Saturday and possibly again on Sunday in the low elevations.


Riw watches/warnings/advisories...


Short term...Allen
long term...Braun
fire weather...Allen

National Weather Service Glossary of Abbreviations