Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 230922
Area Forecast Discussion
National Weather Service Riverton WY
322 AM MDT THU JUN 23 2016
.SHORT TERM...Today through Saturday night
...Primary threat during this period result of increasing fire
danger beginning today...
Imagery shows large broad strong high pressure aloft in control
of most of the CONUS with a shortwave trough just moving
into/across the PAC NW. Surface has weak low pressure intruding
into western WY with general high pressure across the plains to
the east. A couple of high based showers moving ewd near the WY/CO
Today, stronger upper level ridging and west to southwest flow
returns to Wyoming with hot weather also returning to the forecast
area. Although, wind speeds look to remain marginal with respect to
critical levels for Red Flag conditions across zones with critical
fuels, fire danger will be elevated for all of the lower elevations
and extreme across fire weather zone 280 - the lower elevations of
Natrona county because of hot/dry conditions, HAINES values hovering
between 5 and 6 in the afternoon and chances for dry thunderstorms.
These thunderstorms will arrive across the southern/central zones as
a surface low and associated warm front builds into/across
southern/central Wyoming this afternoon. A Red Flag Warning has been
issued for zone 280 beginning at noon today...replacing the previous
Watch. This warning will be extended through early evening Saturday.
Later this evening, the gusty mostly dry storms will quickly move
off to the east.
Friday, fire weather concerns continue and have added a Fire Weather
Watch to zones 281, 285 and 300. Similar conditions to those of
today, but even more extreme as a stronger lee side trof/low forms
across eastern Wyoming...greatly increasing wind speeds across the
forecast area. This combined with an already a very dry boundary
layer, HAINES instability indices of 6 and some potential for more
dry thunder across Natrona county will create extreme fire weather
conditions that will last through early Saturday evening.
Saturday, Red Flag conditions continue. Under troffing aloft and
post frontal at the surface, it will be cooler...but still quite dry
with west winds increasing to 15 to 25 mph with gusts 25 to 35 in
the afternoon. No precipitation expected anywhere in the CWA. Red
Flag conditions should abate Saturday evening under mild upper level
ridging and weak high pressure building across the FA that will
lessen wind speeds...keeping them within check through Sunday.
.LONG TERM...Sunday through Thursday
Synopsis...Dry and warmer weather prevails on Sunday. A few Pacific
disturbances will move across the northern Rockies Monday through
Wednesday, bringing isolated to scattered thunderstorms across the
north. The most active day for thunderstorm coverage across north
and central Wyoming will be on Tuesday with a few strong to severe
thunderstorms possible across Johnson and Natrona counties. Above
normal temperatures are expected through the period with basin and
valley highs mostly in the 80s to lower 90s.
Discussion...GFS and ECMWF in good agreement with synoptic features
through Thursday. Upper level high pressure will rebuild over the
southern and central Rockies on Monday, flanked by troughs across
the NE U.S. and Gulf of Alaska/NE Pacific. The latter trough will
eject several shortwaves into the northwest U.S. and across the
northern Rockies. The first in this series of shortwaves will shear
into Montana late Sunday with dry airmass remaining over northern
Wyoming. Surface high pressure will build into the northern plains
behind this system with return, upslope flow bringing higher CAPES
across eastern and far northern Wyoming where forecast maintains
slight chances over and near the Absaroka and Bighorn ranges.
Return flow is expected to strengthen around backside surface high
over the plains on Tuesday bringing moderate instability along
mainly along and east of the Laramie and Bighorn ranges. A more
potent shortwave shown by both GFS and ECMWF turning SE across NE
Wyoming late Tuesday along this axis moderate instability, so a few
strong to severe thunderstorms could develop mainly across Johnson
and Natrona counties. As upper ridge amplifies into central Canada
Wednesday and Thursday, incoming Pacific shortwaves will be shunted
off well to the north of the area by Thursday. However, the door
will be open for a few sub-tropical `ridge riders` to rotate north
across the Great Basin into the area Thursday with focus for
thunderstorms shifting to the SW.
Conditions will remain VFR conditions through the forecast period.
Southwest wind will increase late this morning into the early
afternoon. Wide wind speeds of 15-25kts are expected, with 18G28kts
anticipated at KBPI, KPNA, and KRKS Thursday afternoon. Speeds will
decrease toward sunset with more common speeds around 10-12kts by
02Z/Fri. Conditions will remain dry with the best chance of
convection coming at KRKS and KCPR from 20Z/Thu to 02Z/Fri.
Temperature-dew point spreads will be on the order of 55-60F
Thursday afternoon. Therefore, the the main hazard will be gusty
outflow wind to 40kts due to the very dry sub-cloud layer.
...Red Flag Warning for the lower elevations of Natrona county
beginning at noon today...
...Fire Weather Watch for Johnson county...the southern Bighorn
Mountains...and Casper Mountain beginning noon Friday...
Smoke dispersal during the daytime hours will be fair to excellent
east of the Divide and good west of the Divide during the daytime
periods through the weekend. Today, hot and dry conditions return
once again. Winds look to remain at or below critical levels for
most of the areas with critical fuels. However, high instability and
increasing dry thunderstorm potential across portions of Natrona
county and will enhance fire danger across the lower elevations of
Natrona county this afternoon and evening. Therefore the Fire
Weather Watch has been upgraded to a Red FLag Warning for the lower
elevations of Natrona county beginning at noon today...continuing
through early Saturday evening. Hot and very dry conditions continue
Friday and winds increase even further...so the lower elevations of
Johnson county, the southern Bighorn Mountains and Casper Mountain
have now now been added to a Fire Weather Watch which begins at noon
on Friday...also continuing through early Saturday evening.
Fire Weather Watch from Friday afternoon through Saturday
evening for WYZ281-285-300.
Red Flag Warning from noon today to 9 PM MDT Saturday for WYZ280.