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Area forecast discussion 
National Weather Service Billings Montana
312 PM MDT Sat Aug 29 2015

Short term...valid for sun and Monday...

The main impact in the short term forecast period is critical fire
weather conditions over south central Montana...which will be an issue
all the way through Sunday evening...when a relatively strong cold
front of Pacific origins will cross the region.

A weak shortwave trough will push a weakening cool front eastward
into the area tonight. This scenario could yield isolated showers
and thunderstorms over south central Montana...but instability is weak
and so updrafts will be high-based and could have a hard time with
lightning production. Gusty winds are nonetheless possible with a
dry boundary layer. Additional...isolated showers and storms are
possible Sunday...with some uptick in coverage likely late Sunday
afternoon into Sunday evening as a strong shortwave trough crosses
the region. Middle-level winds will have increased by that time and
the 09 UTC sref supports 500-800 j/kg of MLCAPE in places with 40
knots of effective shear...so a strong storm or two is possible...as
suggested by the 12 UTC NCEP-run WRF-arw simulation.

Where fire weather is concerned...mixing will yield gusty winds as
we go into this evening...but they will relax with sunset and the
passage of tonight/S weak frontal passage. Based on the 12 UTC set
of model guidance...a much stronger cold frontal passage is set to
occur Sunday evening. Sunday will be cooler with highs only in the
middle 80s to lower 90s f...but humidities over south central Montana will
still be between 15 and 25 percent by late afternoon and evening
in advance of the front. Moreover...MOS-based guidance suggests we
will tap into strengthening west-southwest flow ahead of the front
in many places...with sustained speeds over 30 miles per hour possible in the
Livingston and Big Timber vicinity by 00 UTC. The thermal gradient
with the frontal passage itself and model-forecast winds near 3000
feet above ground level support west-northwest wind gusts to 40 miles per hour in the wake of
the front Sunday evening as well. Given all of this...we chose to
extend the current red flag warning in time through midnight MDT
Sunday night to cover both critical burn periods. The Pacific cold
front will make its way all the way across south central Montana and
north central Wyoming too with gusty...shifting winds in its wake...but
it will do so well after dark in those areas /when humidities will
be recovering/ and so we did not expand the red flag warning in
area at this point.

As for the smoke from western wildfires...conditions may continue
to improve heading into Sunday with southwest flow aloft...but the
smoke will likely return in higher concentration once again Sunday
night into Monday behind the expected cold frontal passage.

By Monday...dry and cooler Post-frontal conditions will occur with
highs only near 80 f. Smoke will likely be more prevalent again in
the wake of the Sunday night cold frontal passage too based on the
trajectory of the low- and middle-level wind fields in its wake...as
supported by the Navy-run smoke model. Schultz

Long term...valid for Tuesday...Wed...Thu...Fri...Sat...

Extended has not changed much the past few days...and that story
remains the same. Another deep trough will drop into the Pacific
northwest and set up southwest flow over southern Montana and
northern Wyoming. Several disturbances will cross the forecast
area and provide at least a slight chance of convection. These
waves will be embedded in fairly dry flow however...as the deeper
monsoon will be pushed into the Central Plains. Will keep probability of precipitation low
due to limited moisture. Frontal timing looks to be late in the
week as heights lower and allow a cold front to push across the
area in the Thursday to Friday time frame. Will keep temperatures
above seasonal levels through much of the week. Twh

&&

Aviation...

A few weak showers and isolated thunderstorms are expected mainly
west of Billings late this afternoon and this evening. Strong wind
gusts to 40kts will be possible with this activity. Southwest
winds will gust to 30kts at Livingston into the evening hours.
Winds will decrease by late evening as a weak surface trough
slides across the area. Mountains will be mostly obscured due to
smoke...otherwise VFR flying weather is anticipated. Twh

&&

Preliminary point temp/pops...

Sun Monday Tuesday Wednesday Thursday Friday Sat
-----------------------------------------------------------
bil 068/088 052/081 054/088 057/087 053/078 049/074 051/072
22/T 20/k 01/u 11/b 11/b 12/T 22/T
lvm 059/087 044/079 046/085 049/083 045/074 041/073 044/073
22/T 20/k 00/u 11/b 22/T 22/T 22/T
hdn 065/091 052/082 053/091 056/090 052/082 049/078 051/076
22/T 20/k 01/u 21/b 11/b 11/b 22/T
mls 067/092 057/082 055/091 059/092 055/084 051/079 054/078
02/T 30/k 00/u 11/u 11/b 11/b 23/T
4bq 063/094 056/080 055/091 059/091 056/087 053/078 055/077
02/T 30/k 00/u 11/u 11/u 11/b 23/T
bhk 063/094 055/079 054/090 057/091 054/087 052/078 052/078
02/T 40/k 01/u 11/u 11/u 11/b 23/T
shr 059/092 049/081 049/089 053/089 049/084 047/077 049/073
12/T 20/k 01/u 21/b 11/u 11/b 22/T

&&

Byz watches/warnings/advisories...
Montana...red flag warning in effect until midnight MDT Sunday night
for zones 28-29-34-35-39>42-56-63>68.
Wyoming...none.

&&

$$
Weather.Gov/Billings

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