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Area forecast discussion 
National Weather Service Billings Montana
930 am MDT Thursday Sep 3 2015

Update...
weak energy produced some showers and thunderstorms over our western areas
early this morning. At 9am...only some light showers left over in
northwest Rosebud County as this energy has passed...so have adjusted
probability of precipitation down for the remainder of the morning. Potential for diurnal
showers this afternoon is hindered by middle level capping today per
cooler NE surface winds. However...area of pv over southern Nevada will
lift northward today and offer some ascent by this evening...which
along with increasing monsoonal moisture will increase the chance
of showers and some thunderstorms across our County Warning Area tonight. It should be
noted that upstream precipitable waters are as high as 0.86 inches at Grand
Junction at 12z this morning...a fairly high value for early
September.

Jkl

&&

Short term...valid for today and Friday...

Stalled cold front along the Montana Wyoming border this morning
while southwest flow aloft is in place in response to an upper
level low over the Pacific northwest. Trough is swinging eastward
slowly and this is inducing minor height falls this morning with a
weak speed maximum passing over Yellowstone Park. Convection is
developing under this zone but this is likely favoring the west
side of the Continental Divide in that area because area on the
east side is much drier so do not expect convection to move much
further northeast. Northerly flow on the northern side of the
surface front will slowly deepen moisture during the day so some
isolated showers and thunderstorms will be possible later this
afternoon over the western half of the area but storms should not
produce much moisture. Will be a much cooler day for the central
and western zones with highs around 80 but still expect some 90s
over southeast Montana and northern Wyoming where the front has
not arrived yet.

Tonight the front sags a bit further south and the southwest flow
aloft begins to tap into moisture streaming off of Tropical Storm
Kevin off the Mexican coast. This deepens the moisture and
increases the shower and thunderstorm potential further east but
still only expected isolated storms. For Friday into Friday night
the pattern has the flow aloft slowly backing and the
northeasterly low level flow continuing which supports additional
moistening of the airmass. The upstream trough becomes more north
to south oriented with an upper low becoming embedded in the
trough over northern California 12z Saturday. The flow pattern is
relatively undynamic Friday and Friday night but could still see
scattered thunderstorms as models indicate quite a bit of
instability above a capped boundary layer. Certainly should see
elevated fast moving storms so precipitation should again be
light. Borsum

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

Cooler and potentially wetter weather advertised by the medium
range models. This change in the weather pattern will be caused by
a deep upper trough...currently just off the Pacific northwest
coast...moving through the northern rockies. This process will
make the weekend weather very active across Montana.

Saturday...trough timing between the European model (ecmwf) and GFS becoming
closer with the axis moving into eastern Idaho 00z Sunday. This
sets up diffluent flow over the area. GFS projecting convective available potential energy around
1000j/kg with precipitable waters around an inch from Billings
east. The precipitable waters were 4 Standard deviations above normal. The European model (ecmwf)
was even showing a little negative tilt to the trough...which
potentially could draw in deeper low level moisture. Strong
forcing with a 100kt jet kicking into Wyoming should ignite
showers and a thundestorms over central zones. Some of these
storms could turn strong to severe as they drift northeast.
Limiting factor could be amount of clouds around early in the
day...thus limiting heating and potential surface based cape.
Elevated cape will still be there however...so not sure initiation
will be a concern. Raised probability of precipitation and will mention the possibility of
strong to severe storms in the severe weather potential statement. Raised probability of precipitation to likely for a
large part of the area with the trough moving through.

Sunday...the upper trough rips through the forecast area and
leaves behind zonal flow. A surface low wraps up and moves into
the western Dakotas with a strong surface pressure gradient. 850mb
winds increase to around 40kts over central zones and 45kts over
the east. Should provide a windy day. Some ascent present and may
keep these winds from surfacing...but the gradient alone should
provide some gusts to 40 miles per hour. Precipitation will be tapering off
from west to east. Highs will struggle to get out of the middle 60s.

Monday through Wednesday...models differ on timing of events in
this time period. The European model (ecmwf) was faster and weaker with more energy
wrapping around into southern Montana. The GFS brings a stronger
wave through Tuesday as opposed to the weaker European model (ecmwf) wave of Monday
night. Either way...ascent arrives and thus raised probability of precipitation. Wednesday
looks to be the drying out day. Twh
&&

Aviation...

Isolated showers and thundestorms that crossed western
routes...including kbil...this morning are diminishing and slowly
moving into northeast Montana.

Additional showers and thunderstorms continue to drift northeast
from Idaho into Cooke City and toward klvm through the morning.
More showers and thunderstorms are expected this afternoon from
kbil west. Otherwise...mainly VFR conditions are expected. Aag

&&

Preliminary point temp/pops...

Today Friday Sat sun Monday Tuesday Wednesday
-------------------------------------------------------
bil 080 052/073 053/074 049/066 043/070 045/070 046/073
1/b 22/T 35/T 77/T 21/u 22/west 21/b
lvm 079 046/072 046/073 040/064 036/070 038/068 038/074
2/T 34/T 45/T 86/T 21/b 22/west 21/b
hdn 085 053/079 053/078 049/070 043/072 044/072 044/075
1/b 22/T 34/T 66/T 21/u 12/west 21/b
mls 088 059/079 059/080 053/070 044/070 045/071 045/074
1/b 22/T 44/T 66/T 32/west 11/b 11/b
4bq 091 058/082 059/084 054/073 046/072 048/072 048/074
0/b 12/T 33/T 43/T 11/b 11/b 11/b
bhk 092 059/083 059/080 053/072 045/069 043/070 045/072
1/b 22/T 44/T 53/T 22/west 11/b 12/T
shr 090 051/081 051/082 047/072 039/073 040/072 042/072
1/b 22/T 34/T 63/T 11/u 12/west 21/b

&&

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

&&

$$
Weather.Gov/Billings

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