Area forecast discussion
National Weather Service Riverton Wyoming
1134 PM MDT Friday may 24 2013
Short term...tonight through Saturday (issued at 319 PM mdt)
Today...large plains upper level ridge continues to broaden more
than move eastward. However...this will still allow for things to change
here at the surface and aloft. The upper Pacific northwest low can now begin to
advance and move up and over the ridge. This morning found SW
rotating around upper low exiting northern portions of Wyoming. SW activity
subsides today as leading edge of northwest low flattens as it rides up an
over the ridge. Stronger upper flow and vorticity...currently over
Arizona will arrive over eastern Colorado later today inducing a Front Range
trough/backdoor front to spin up a Colorado plains low. Here however...and
behind our exiting surface low...dry southwesterly flow is in place across most
of the County Warning Area. Result is a warm/dry precipitation free day.
Temperatures will range to around 5 degree cooler most
places...somewhat less in the Wind River valley. Relative humidity levels will be low
and breezy conditions will abound this afternoon in mostly southwesterly
flow after adiabatic mixing occurs. Commonly...surface winds will run to
10 to 15 miles per hour...somewhat higher southeastern County Warning Area and mountain ridges...with
gusts to 20-25 miles per hour common...30 to 35 mountain ridges. As we are
currently in green up...no critical fire weather anticipated
today.
Saturday...as upper northwest low pulls northeastward...indications are that another
upper level jet streak with move across our region...inducing a
relatively weak broad low to form over Wyoming with some low to
moderate moisture trying to return into NE Wyoming and perhaps into the
Big Horn Mountains and portions of Johnson County. Northwest County Warning Area...middle level
moisture push in. Overall Thermo instability looks to be relatively
low compared to the last few days around here...middle to deep shear
will range on the modest end of 20 to 30 kts and with little to no
capping through much of the day. So while isolated thunderstorms
will be a possiblity through the afternoon and early evening across
the northern County Warning Area...no severe weather is expected with any of the storms.
Southwest winds perhaps a bit stronger than Friday across the southern
2/3rds of the County Warning Area...with high temperatures ranging about 5 degree
higher than today but relative humidity levels remaining about the
same.
Long term...Saturday night through Friday
Sunday looks to be a fairly active convective day in northern
Wyoming...particularly in Johnson County where the best convective available potential energy will
be along the Theta-E ridge. Expect possible large hail and strong
gusty winds with some of these storms. The strong closed low now
over the Pacific northwest will weaken over the course of the
weekend and a secondary low will spin around it. This secondary
feature will eject off the main circulation and travel east as an
open wave on Monday along the Montana Wyoming border with
scattered morning showers and afternoon thunderstorms mainly in
northern Wyoming spreading from west to east. The next upstream
main trough will cut off around Las Vegas by 06z Wednesday. This
trough will then open up and travel northeast to Wyoming by 12z
Thursday with the low closing off once again along the Montana
Wyoming border. In addition...an associated 700 mb circulation will be
spinning around northern Wyoming Wednesday along with a surface low
over Wyoming as well. Expect scattered showers and late day
thunderstorms to spread southeast into central Wyoming Tuesday. By
Wednesday...with the weather feature overhead...just about the
entire County Warning Area will have scattered showers and late day thunderstorms.
Precipitation and storms will continue into Thursday with troughiness
over the area. Then on Friday another closed low will track east
along the Montana Wyoming border but we will be on the dry
southern side of the track with drier air punching in around the
back side of the circulation so low probability of precipitation are expected Friday.
Friday should be the coolest day of the week with hy temperatures falling
to -4c in northwest Wyoming behind this weather feature and down to -3.5 in
southeast Natrona County. The second coolest days should be
Wednesday and Thursday given the clouds and precipitation associated with
the low overhead.
&&
Aviation.../06z issuance/
VFR conditions will prevail through 06z Sunday. Isolated showers and
thunderstorms will develop over northwest Wyoming after 20z Saturday until 03z
Sunday. Gusty wind to 40 knots will be possible near this activity.
Breezy conditions will occur at most terminal sites in the Saturday
afternoon and into the early evening hours.
&&
Fire weather...
South to southwest flow will dominate much of the state through the
weekend ... Bighorn Basin being somewhat of an exception with
more variable surface flow. Minimum relative humidities are
expected range between 10 and 15 percent across the lower elevations
of southern and central Wyoming. Higher elevations will range up to
25 percent. Winds will remain breezy today at 10 to 15 miles per hour sustained
with gusts 30 to 35 miles per hour...especially southeast zones and mountain
ridges. It will be unstable on Saturday and Sunday afternoon...with
a Haines index of 6 expected across much of the area late on
Saturday...but it will be too dry for thunderstorms most places
except perhaps in the far northwest and northeast zones where
isolated thunder could occur...but instability is less.
Later...there could be strong storms on Sunday afternoon near the
i25 corridor...and also in the northwest zones...but little chance
across southern half of Wyoming.
&&
Riw watches/warnings/advisories...none.
&&
$$
Short term...Braun
long term...lipson
aviation...Arkansas
fire weather...Braun
