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Area forecast discussion 
National Weather Service Riverton Wyoming
214 PM MST Wednesday Jan 28 2015

Short term...tonight through Thursday

Improving conditions expected through the evening with lingering
showers tapering off. Skies will clear across much of the forecast
area overnight with high pressure building over western and central

These clearing skies and remnant moisture from the earlier shower
activity will promote the development of patchy fog around midnight.
Fog may persist through the middle to late morning hours for some
areas...though should generally have lifted by the afternoon.

Benign conditions will then be found through the afternoon across
the area.

Long term...Thursday night through Wednesday

This period starts with our current upper low off the Southern California which
is forecast to move slowly towards Southern California/northern Baja California on Friday...then
it looks like it will move slowly into Arizona on Saturday before sagging
south again into Mexico as the northern branch begins to take over. A
couple shortwaves then look like they'll have varying amounts of
impact over US over the weekend as we'll have to contend with two
potential cold fronts along and east of The Divide mainly. Although
the 12z runs today have kept the main impact with these front over
the far northestern zones. Early next still looks like the flow
will flatten early next week leading to an increased chance of snow
in the west. But then by Wednesday...the flow is expected to sharpen
again with building heights along the West Coast and another
potential system dropping down from the north-northwest.

The first system well to our south looks like it will remain well to
the south with the leading edge of the moisture edging close to the
area with clouds but most if not all the precipitation will stay south of the
area. There is a small chance that as the jet to the south pulls off
to the east late Friday/Friday evening...we get in the right entrance region over
southern Sweetwater County and that could be just enough to help what
cloud shield may be down there to get a few light showers going.
Have some slight chances in for that reason. Will probably have some
continued at least patchy fog/low clouds Friday into Saturday
morning ahead of this first weekend system. The first shortwave to
drop south originates from the eastern Pacific upper low with a
shortwave shooting out of that and moving through the developing
West Coast ridge and then down the front side. 12z runs have backed
off on how much of an impact this Saturday front will have with flow
aloft a little more northwest and showing the front having the greatest
impact over the northestern zones with a chance of snow late Sat or Sat
evening. The front still looks like it may make it to most areas east of
The Divide but most of the energy with this storm is forecast to
stay just to the NE of most of the area. The 12z NAM is even further
to the NE with the front. Next shortwave originates in the
developing cross polar flow. This system has its origins west of
Alaska and north of the Arctic Circle. This front will likely
contain very cold air but at the same time...the cross polar flow is
being shunted slightly to the east as the ridge tries to nose into
the west. Tough call as just a slightly further westward movement would
give at least parts of our northern or eastern zones a very cold Arctic
frontal passage. 12z model runs are generally keeping the front NE
or just into about Johnson count Sunday but we'll need to watch it
closely as there is some very cold -40 to -55 below zero surface air up
in the far north of Alaska and the northwest territories where this
system will move through. The flow then flattens quickly with the
potential for a major snowstorm in the west as early as later Sunday
night and then continuing Monday into Tuesday. It'll start in a
favorable warm advection pattern then dynamics increase ahead of a
eastward moving disturbance and active jet Monday night into Tuesday
morning. Amplifying flow behind this system will likely lead to a
cold...cyclonic... unstable northwest flow Tuesday night through Wednesday
night with some light mountain snowfall and a chance
elsewhere...especially east of The Divide. Although the European model (ecmwf) shows
just a brief break and then reloads our warm...moist northwest flow for
more significant snowfall potential in the especially the Western Mountain
Wednesday and Wednesday night. Active medium range period shaping up with several
things to watch closely over the next couple days.


Aviation.../00z issuance/

Conditions will steadily improve through the evening...with
showers ending over the lower elevations by 06z this evening. The
central and northern mountain passes may see continued light
showers through 12z Thursday...which could slightly increase
turbulence for impacted aviation passes. Otherwise...patchy
morning fog will be possible across the majority of terminals due
to clearing skies overnight. Most fog should lift by 17z Thursday.
VFR conditions will then be found through the remainder of the
forecast period.


Fire weather...

Fire concerns will be low for the next few days. Generally dry
conditions expected tonight through Thursday...though morning
inversions will be present. In addition to the normal impacts of
morning inversions...some patchy fog will be possible. Dry
conditions with the patchy fog will persist for the next few days
with fairly consistent daytime temperatures. A bit of cooling will
be possible into the early portion of the weekend.


Riw watches/warnings/advisories...


Short term...branham
long term...skrbac
fire weather...branham

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