Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Riverton Wyoming
334 am MDT Thursday Apr 17 2014

Short term...today through Saturday night

Sat imagery west of the Mississippi shows upper level trough from the upper
miss valley...south then SW into the southern rockies. Ridge to the west
over the coastal states has Wyoming stuck between the two and under northwest
flow. North/S oriented jet quickly swinging across Wyoming...pushing the last
upper vorticity maximum out to the east with it...and taking the last of the
instability and showers with it. Further upstream at 140 west is the
next trough and jet that will affect our region later Friday. At the
surface...the low and front that pushed through the state Tuesday is now well
off to the east in Iowa. Weak high p is generally draped across the state
with a weak north/S inverted trough splitting the state in two.

Today...patchy fog possible eastern zones this morning under moist l
levels weak flow and building high press...with Natrona County looking to
have the best chance. Otherwise...weak surface pattern with upper levels
transitioning to ridging and drying. With 700 mb temperatures warming some 3 to
4 degree...warming at the surface also begins today...reaching near or
into the lower 60s across the lower elevations. Winds will also
remain tame for the most part today.

Friday...warming continues with 70s within reach as the ridge moves
through overhead. Lee trofing builds across the eastern counties under ll
SW flow and ahead of the next upper trough. At the same time...the upper
trough and jet will spark rapid cyclogenesis over the Pacific northwest...moving
eastward quickly during the day within the very progressive upper level
pattern. Lee cyclogenesis will also begin by Friday afternoon over
the eastern portion of the state along the Lee trough...while weak Great
Basin low p will merge with the developing northwest low. With this split
system taking shape...it appears that we will not be able to tap
into much in the way of deep moisture...as most will either head off
to the north with the one cyclone...or get shunted S and east with the
other. Middle level moisture will work its way into the western zones...but
the western mountains will wring most of what is available out...leaving
little for the lower elevations to work with. In addition to upper
level forcing...frontogenesis will also commence to our north
northwest...kicking through the forecast area from the afternoon through early
evening period...giving the area east of The Divide their best but
modest shot at any precipitation...much in the form of virga.
Otherwise...the surface to middle level p and T pattern will begin to favor
increased winds through the day on Friday. At this time...it appears that only
isolated high wind conditions will manifest...but it is Worth
monitoring across the usual wind prone zones for expansion as this
complex but mostly dry system evolves. Saturday will be somewhat
cooler overall...but nothing too drastic with high temperatures still reach
into the 60s for many locations...and all with less wind than
Friday.

Long term...Sunday through Thursday
the main weather maker during this period will be upper low
currently slowly wobbling its way eastward along the Aleutian
chain.

Beforehand...a shortwave will race east along the Montana/Canadian
border on Sunday and drive a weak/dry cold front into northern
Wyoming. Some lingering moisture and slight instability across the
south will result in slight chances of showers and isolated
thunderstorms mainly along and south of Interstate 80.

GFS has trended toward European model (ecmwf) earlier solutions of a more
disorganized system coming onshore. The Aleutian low wobbles its
way to near the northwest coast by 12z Tuesday with lead shortwave swinging
onto the California coast...across northwest Wyoming late Tuesday...then closing
off and becoming quasi-stationary near Havre on Wednesday.
Meanwhile...Aleutian low shears southeast int The Four Corners region
before lifting into the plains. The European model (ecmwf) is much quicker in doing
so than the GFS...but the GFS is trending faster over the last few
runs.

Overall...forecast remains on track with surface low expected to
deepen in the vicinity of Billings on Tuesday with cold front
pushing into the west Tuesday...central and eastern Wyoming in the
warm sector. This translates into increasing showers and some
thunderstorms in the west with snow levels initially quite high on
Tuesday...at or above 10kft...while windy and very warm conditions
prevail east of The Divide. This pattern will be favorable for
accelerated snowmelt in the central mountains along with possible
significant rises in creeks and streams...especially in the Bighorn
Mountains and downstream river basins.

Pacific cold front will push across central Wyoming Tuesday
night...but downslope westerly winds are expected to keep shower and
thunderstorm activity mostly isolated with the main hazard of strong
wind gusts with the frontal passage. In the west...snow levels are
expected to lower to valley floors Tuesday night with cyclonic
westerly flow bringing more widespread snow...especially to ynp and
the Teton Range...into Wednesday. North and central areas will
have better chances of rain and snow showers late Wednesday into
Thursday in northwest flow with 500 mb cold pool aloft.

&&

Aviation.../12z issuance/

Some IFR/MVFR conditions in stratus and/or br across Johnson and
Natrona counties early this morning are expected erode between 09z
and 12z as the boundary layer dries a bit as winds shift more
westerly. Otherwise...VFR and dry conditions with relatively light
winds will occur today as a quick moving upper ridge builds across
the cowboy state. Considerable high level cloudiness is expected
to develop especially along and east of The Divide this afternoon
with some middle level clouds over the north. The next storm system
will approach the area late tonight resulting in increasing
cloudiness over the west...as well as increasing mountain winds
with possibility of some low level wind shear developing at kcod
and klnd. Contemplated adding at 10z or 11z group having vcsh to
the kjac taf...but looks like the timing of shower activity with
next system will hold off until after 12z Friday.

&&

Fire weather...

A weak surface pattern today will keep winds in check for another
day as high temperatures warm into the 60s across many lower
elevations. Friday...the basins will climb near or into the lower
70s...ahead of the next Spring weather system due to push through
the forecast area through the day. Winds will also pick up to the
breezy category across most of the forecast zones...with the usual
wind prone areas of the southern dispatch zones...Mountain Ridge
tops and northern Cody foothills possibly getting some isolated high
winds. In addition...by Friday afternoon there will be an increasing
chance for shower and isolated thunder lasting into the evening
period. Saturday...high temperatures will be somewhat cooler...but
winds will also be more tame.

&&

Riw watches/warnings/advisories...
none.
&&

$$

Short term...Braun
long term...aem
aviation...wm
fire weather...Braun