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Area forecast discussion 
National Weather Service Riverton Wyoming
341 am MDT Wednesday may 6 2015

Short term...tonight through Friday night

The weak southern stream 700mb trough axis and asct line of
convergence appears to be established across central Wyoming with
the asct north south broken band of precipitation extending all the way
from central Montana south through Cody...thermop...Lander and into
eastern Sweetwater County. A closed low will develop along the slow
moving east bound trough over southeast Wyoming by 12z this morning which
will lay underneath the 500mb low. The surface low will track southeast
to southeast Wyoming as well. As the weak vertically stacked low
propagates to our southeast the ongoing convection currently taking
place will become more stratiform this morning as the majority of
the rainfall slowly slides off to the east. A surface backdoor cold
front behind the trough will sweep south across areas east of The
Divide today reinforcing the cooler more stable air. For this reason
have mentioned a good chance of showers and cool weather everywhere
with isolated thunderstorms west of The Divide where some negative
lifted indices and cape will reside. The most rainfall today will
likely occur over Natrona and Johnson counties closer to the trough
as well as in the mountains east of the dvd. By the end of the day
today...the 700mb energy will retrograde with a new circulation
forming around eastern Sweetwater County. We could even witness
another funnel cloud or two with the low local/cloud bases today
along with the weak directional shear asct with the 700 mb
circulation. A funnel cloud was spotted near Kemmerer Tuesday
afternoon. Showers will continue into tonight across the County Warning Area. Cool
air will continue to stream south behind the trough axis tonight
with snow levels likely flirting with 6k in far northern Wyoming with
Cody potentially witnessing some wet snow mixed in with any
showers.

In the meantime the stronger northern stream low currently spinning
along the Washington/British Columbia coast will shoot down to
Oregon by 00z Thursday and then to northern California by 12z Thursday.
This will put US under a difluent southwest flow conducive for more
numerous showers/stratiform rain as the southwest flow pushes the
front northeast as a warm front followed by stronger convection/T
storms in Sweetwater County on the heels of the north bound band of
rain. North of the front...low level upslope flow will enhance
precipitation east of the dvd as surface high builds from Montana into the
Central Plains increasing the surface flow. Showers will continue
Thursday night with the front pretty much stalling east of the dvd.

By Friday the northern stream low will actually transition into the
southern stream energy and be situated near Vegas. With the cool
stable airmass remaining over the area...continued difluent flow
ahead of the strong low along with isentropic lift moving north
across Natrona/Johnson County...showers will continue with the most
rainfall trending toward the western County Warning Area. However...with the low way
down in Vegas do not think there will be as much coverage of
rainfall as on Thursday. Isolated thunder is expected in the far
west Friday in the more unstable air. On Friday night precipitation will
increase as the low moves into Utah and difluent flow increases.

Freezing levels will hover between -4 and -6 across northern Wyoming
Thursday through Saturday morning with snow levels between 6 and 7k
much of the time. Farther south and west expect snow level due hover
around 8500 feet. An inch or so of snow is expected each night
including tonight...Thursday nt and Friday night at these elevations.

Models are still rather consistent with each other and from run to
run.

Long term...Saturday through Wednesday

Major Spring storm will impact the region through the weekend with
models trending colder as early as Saturday now. GFS and Euro are
colder than the NAM to start the period. Major closed low is
expected to be around southern or southwestern Utah at 12z Saturday and then
slowly lift northeastward over the next 36 hours ending up over western NE
Sunday afternoon. Gulf is wide open so surface cyclogenesis should
be significant and low level moisture will easily stream northward then
westward around the developing low in eastern Colorado Saturday night and Sunday.
Ahead of it...strong isentropic upglide is already showing up
Saturday across most of our area along and east of The Divide. 700 mb
temperatures are forecast to be -3.5c to -4.5c on the GFS in Fremont
County and warmer further east. The European model (ecmwf) is -5.5c Saturday
morning and even colder Saturday afternoon. The trend is colder
yet Saturday night into Sunday. Some concern if precipitation is heavy
enough Saturday morning that precipitation could change over to snow in
Fremont County above 5k feet or so. We will need to watch this
very closely over the next couple days as a significant rain
changing to snow event Saturday into Sunday could be a longer
duration snow event in the lower elevations of central Wyo. The
foothills and mountains will be all snow with heavy accumulations
likely and future watches expected. Casper will be warmer during
the first half of the storm Saturday and should be rain initially in
strong warm air advection pattern. Casper Mountain could be snow. Heavy precipitation
amounts are likely from this storm for many areas along and east
of The Divide over the weekend not counting the precipitation that starts
before the medium range period. Gradually improvement Sunday night
into Monday as low moves further east away from the area. Small
piece of a northern stream low may get lost in this complex flow and be
close enough to cause showers in the north Monday. A new upper
low...currently out near the western Aleutians...is expected to move
into the west and bring an increasing chance of showers and thunderstorms
to the west as early as Tuesday with the low hanging off to our
west through the end of this period with a chance of showers and
thunderstorms to especially the west.

&&

Aviation.../12z issuance/

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

After 12z...areas of rain will occur east of the Continental Divide
with widespread MVFR conditions and local IFR conditions. Rain will
be on the decrease after 00z Thursday east of The Divide but low
clouds will linger in many areas with MVFR conditions prevailing.

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

West of The Divide...isolated showers this morning. Then isolated
showers and storms will develop after 19z this afternoon and linger
until 02z this evening. Local MVFR conditions will occur. After 02z
this evening showers will occur in southwest Wyoming with MVFR conditions
prevailing.

&&

Fire weather...

Cool showery wet weather is expected this week and through the
weekend. Any thunderstorms will be of the wet variety and will
occur mainly west of The Divide. The snow level will range from between
6000 and 7000 feet in northern Wyoming to above 8500 along and west of
The Divide.

&&

Riw watches/warnings/advisories...none.

&&

$$

Short term...lipson
long term...plus
aviation...Arkansas
fire weather...lipson

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