Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 221744
Area Forecast Discussion
National Weather Service Riverton WY
1145 AM MDT Sat Oct 22 2016
.UPDATE FOR 18Z AVIATION DISCUSSION...
After looking at the local ongoing 12Z RAOB, and upstream
observations showing dewpoints remaining in the teens as well as CPR
and Outer Drive already having 24 and 17 RH, respectively decided to
issue a red flag warning for fire weather zone 280. WFO CYS timing
(16Z-00Z) looked good as criteria thresholds could be met as early
as late morning with red flag conditions tapering off quickly late
Have updated grids and issued RFW and new FWF.
.PREV DISCUSSION... /issued 205 AM MDT Sat Oct 22 2016/
SHORT TERM...Saturday through Monday night
Dry weather looks to hold through the weekend. However, there are a
couple of things we have to concern ourselves with. For today, the
concern is with enhanced fire danger. A cold front will approach the
area from the northwest today. The front is rather weak, with little
cool air behind it. It will, however, tighten the pressure gradient
across the area and with the southwest flow, especially in the
southwestern wind corridor from Rock Springs to Casper. At the same
time, it will be a warm day with low humidity. However, with the
front expected to move through around noon, this will be before peak
heating. After the frontal passage, the area of main concern,
Natrona County, will be the wind turn to the west and decrease.
Since, peak heating and wind are expected not to coincide, we will
not issue a Red Flag Warning, but have a headline for near critical
fire conditions. Elsewhere, it looks like a nice autumn day with
sunshine mixed with high clouds and above normal temperatures.
Sunday looks to be a nice day as well. As mentioned before, there is
not a lot of cool air behind the front, so high temperatures will be
fairly similar. There will be less wind as well so fire danger
should decrease. High clouds should thicken however as a system and
a surge of moisture moves in from the southwest. However, Sunday
looks dry and any showers should hold off until later Sunday night
and at this point they look rather light.
Monday there will be some showers and possibly a thunderstorm around
West of the Divide. At this point, there does not appear to be a
tremendous amount of moisture with the system with most remaining
over Utah and Colorado. As a result, we have kept POPS in the chance
category or less. The best chance of any showers would be across the
southwest. East of the Divide, downsloping flow should keep most
areas dry and still mild, although breezy at times. One interesting
feature Monday night is both the NAM and GFS are showing an area of
precipitation with a weak jet streak moving into southern Wyoming.
There is some disagreement on the exact placement though.
LONG TERM...Tuesday through Saturday
Models are in decent agreement through much of the extended with
longwave troughs on each Coast with ridging of varying intensities
in between them. On Tuesday, a moist southwest flow and some lift
associated with left exit region of a small jet streak will be
enough for a chance of mountain snow/valley rain showers over the
west and south. Any precipitation amounts will be on the light side.
Upper ridging takes hold Tuesday night through Thursday with mainly
dry and mild to warm conditions expected. 00Z models tonight show a
significant shortwave trough ejecting out of the western longwave
trough into the Rockies by around Friday. However there remains
significant differences in the details about this shortwave trough
and associated cold front. Will respect the potential for this
system will widespread slight PoPs across the area and trend toward
cooler temps Friday/Saturday, but nothing major.
Temperatures will be well above average through at least Thursday,
cooling to more seasonal levels Friday into Saturday.
Weak high pressure continues over the area today with VFR
conditions, with dry mid level west to southwest flow. Surface
pressure gradient has established this afternoon in eastern WY to
produce surface wind gusts over 30 kts from Natrona county into SE
WY. This moderate wind will shift to the west northwest as the
afternoon continues. Persistent cirrus band over western WY at noon
will continue and expand eastward through the day. A weak
disturbance will move through Montana today, bringing a weak dry
cool front to northern and NE WY late this afternoon and into the
evening hours. KCOD will see the wind shift to the NE around 23Z and
KCPR after 01Z Sun. Not expecting as much lower MVFR ceilings as
previously thought, although the east side of the Bighorn Mountains
from KBYG to KCPR could see ceilings lowers to 5-6 K feet from 00Z
to 09Z Sun. High pressure builds over the area Sunday with SW gusts
around 25 kts from KRKS to KCPR. Cloudiness and a slight chance of
showers will move into SW WY late Sunday night into Monday morning
with a disturbance from the Great Basin.
...RED FLAG WARNING IN EFFECT FOR FIRE WEATHER ZONE 280 FROM 10 AM
TO 6 PM TODAY...
A combination of low relative humidity, warm temperatures and a
gusty southwest wind will bring critical fire conditions to
Natrona County today. Elsewhere, expect a warm and dry day. A
gusty southwest wind will blow from Rock Springs through Casper.
Mixing and smoke dispersal will be good across eastern areas; fair
to poor elsewhere.
Red Flag Warning from 10 AM this morning to 6 PM MDT this
evening for WYZ280.