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Area forecast discussion...updated 
National Weather Service Billings Montana
908 PM MDT Thursday Sep 3 2015

Update...
scattered showers and a few thunderstorms continued to work up
from the south/southwest in a monsoonal flow. Monsoonal impulse
energy really only forcing besides basic height falls from
approaching upper trough. We do have some weak shear in the south
and east zones. A good cluster of lightning occurred over by
Miles City on the edge of the cap /cap situated to the east/.
Latest model proggs keeps weak impulse energy flowing into our
western forecast area overnight and this supported by whats
upstream in Wyoming. Adjusted probability of precipitation upward in mountains overnight.
I do not expect severe weather as lapse rates are not impressive
as we are getting some cool air advection in the the lower levels.
Rest of forecast in good shape. Bt

&&

Short term...valid for Friday and Sat...

SW flow aloft exists over the area today as trough continues to
deepen along the Pacific coast. This flow is opening the door to
monsoonal flow...embedded within which there are a couple of
shortwaves. One in Colorado will lift into the Dakotas tonight but
could help to generate some showers or thunderstorms tonight in our east
where precipitable waters are near 0.75 inches. Another shortwave along the
Nevada/Utah border is lifting north and will provide some ascent for our
west and central parts by this evening. As of 3pm we are already
seeing some development over the Beartooth Mountains broadbrushed
isolated to low scattered probability of precipitation should cover what should be weak
activity tonight.

SW flow will continue to moisten up our airmass over the next
couple days with precipitable waters increasing to near an inch in our east. NE
surface winds will keep capping in place into tomorrow so expect a
lull in convection late tonight through much of Friday. More
shortwave energy expected to arrive Friday night will allow for
our next period of showers and thunderstorms. It should be noted
that moisture from Tropical Storm Kevin in the eastern Pacific
will surge northward up The Rockies over the next 48 hours.

Most interesting time period is late Saturday and Saturday night
as dynamic Pacific low finally kicks through the region...taking
a track from southern Idaho into central or eastern Montana by Sunday.
Models still differ in exact track of the middle level low and this
will affect precipitation amounts and duration. That being said our west
will be most favored with a half inch of rain or more
anticipated...with embedded thunderstorms. 700mb temperatures falling to
near -4c suggest some back side snow showers for our western mountains
by late Saturday night. Be aware of this if you will be camping in
the beartooth absarokas.

Potential exists for strong to severe storms especially for our
east Saturday afternoon and night...with best potential along and
east of surface trough...ie east of mls/4bq...though again model
differences exist here. Latest model trends show a stronger push
of low level cooling for our west half on Saturday...and this is
consistent with passage of Friday night shortwave and pressure
rise bulge to follow...so more uncertainty exists in potential for
strong storms in our west although there will certainly be
embedded thunderstorms once stronger forcing arrives Saturday evening.

Will advertise overall increase in thunderstorm potential for Saturday and
Saturday night...including chance of strong/severe storms...in severe weather potential statement
and graphic. Labor Day recreationists take note.

Jkl

Long term...valid for sun...Mon...Tue...Wed...Thu...

Potentially wetter weather should exit the region and remain cool
thereafter with a slight warming trend following...as advertised
rather similarly by the medium range models. This change in the
weather pattern will be caused by a deep upper trough moving
through the northern rockies with moderate ridging occurring in
its wake...with perhaps a weak shortwave riding northwest flow on Tuesday
or early Wednesday...in the Euro and GFS respectively.

Sunday...the upper trough passes through the forecast area and
leaves behind zonal flow. A surface low wraps up and moves into
the western Dakotas leaving a strong surface pressure gradient
across the state which should provide for a windy day with some
gusts to near 40 miles per hour in the morning through the afternoon.
Precipitation will be tapering off from west to east as the
surface low ejects with the upper level wave to the northeast.
Highs will struggle to get out of the middle 60s.

Monday through Thursday...models differ on nature of flow regime
on Tuesday in this period. The European model (ecmwf) and GFS both produce some
moderate ridging of slightly different strength behind passing
weekend shortwave but similarly timed..resulting in a slight
warming trend until at least Wednesday. The GFS brings a shortwave
through Tuesday as opposed to the weaker European model (ecmwf) wave later that
night. Either way...ascent arrives and thus raised some mountain
probability of precipitation. Flow regime is more consistent in Euro and GFS by
Thursday...which is fairly zonal. Mrowell

&&

Aviation...

Isolated to scattered showers and thunderstorms...are expected to
linger across the forecast area tonight. Brief MVFR conditions may
be found within shower activity...but otherwise VFR conditions
will rule. Look for northeasterly surface winds to prevail at most
locations. Bt

&&

Preliminary point temp/pops...

Friday Sat sun Monday Tuesday Wednesday Thursday
-----------------------------------------------------------
bil 053/073 053/071 050/065 043/068 042/072 047/075 047/075
22/T 33/T 77/T 21/b 11/b 11/b 11/u
lvm 046/072 046/072 043/062 036/067 035/073 041/074 042/076
33/T 46/T 86/T 21/b 11/b 11/b 22/west
hdn 053/078 053/077 049/068 042/071 041/074 045/077 045/075
32/T 33/T 66/T 21/u 11/u 11/b 11/u
mls 059/081 059/080 053/066 044/070 043/071 046/077 047/073
52/T 44/T 66/T 31/b 11/u 11/b 11/b
4bq 058/082 060/084 054/069 046/071 046/071 049/077 049/073
21/b 33/T 43/T 11/u 11/u 11/b 11/u
bhk 059/084 060/082 054/068 043/069 043/069 044/076 047/070
22/T 44/T 53/T 11/u 11/b 11/b 11/b
shr 052/082 052/081 047/069 039/071 038/073 042/076 043/074
22/T 33/T 53/T 11/u 11/u 11/b 11/u

&&

Byz watches/warnings/advisories...
Montana...none.
Wyoming...none.

&&

$$
Weather.Gov/Billings

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