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Area forecast discussion 
National Weather Service Billings Montana
937 PM MDT Thursday Aug 21 2014

Update...
a couple severe thunderstorms impacted Carter County tonight with strong
wind gusts along with very heavy rainfall. Convection across our
east is becoming weaker and less organized as 75kt 300 mb jet in
northeast Wyoming lifts out of the area...and as the airmass has become
worked over. Looking upstream...a shortwave is lifting out of
southern Idaho and this will bring increased potential for showers
and isolated embedded thunderstorms to our western parts through the night.
More impressive feature is the energy driving south along the Washington
coast courtesy of a 100kt northerly jet. This will eventually
carve out a deep trough to our west which will bring showers and
thunderstorms and overall greater precipitation to our County Warning Area Friday afternoon into
Saturday. A late August anomaly for sure.

For the update this evening have tweaked probability of precipitation and winds based on
current activity.

Jkl

&&

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

Active weather pattern with a major storm system bringing
widespread precipitation to the region. Unseasonably cold
temperatures...moist conditions and winds will certainly impact
outdoor activities. There are also periods where some strong
thunderstorms are possible...mainly for southeast Montana.

Moist airmass in place this evening with a strong shortwave
approaching from southwest Montana. Plentiful shower and
thunderstorm activity already over the western central zones and
expect intensity to pick during the early evening hours. Models
try to trigger a round of stronger thunderstorm activity for
southeast Montana but believe atmosphere too uncapped for discrete
strong storms but will have to watch because that area is
experiencing better heating. Storms are slow moving so across the
region folks may see .50 to .75 rainfall in a short period of
time.

Friday starts off in between waves with showers and thunderstorms
exiting eastern Montana. Backing flow aloft becoming diffluent
during the day will support development of more showers and
thunderstorms with strengthening low level easterly flow. One
shortwave rotates out of the low Friday evening which really
increases the shower and thunderstorm activity and widespread
heavy rain becomes likely Friday night through early Saturday
evening. Maximum temperatures will be below 70 for most areas though
southeast Montana still has a window for warming.

As Saturday progresses main mechanism for precipitation switches from
upper level dynamics to increasing upslope flow as frontogenesis
occurs over central Montana. BUFKIT soundings show saturated
airmass to 300mb with long skinny cape but good wind shear so slow
moving tall growth cells redeveloping over the same area looks
good. This scenario allows some areas to develop locally heavy
rain...with rainfall totals through the event of 2 to 3 inches in
localized areas possible. Small streams and burn areas may see a
response to this so will have to watch rainfall amounts through
the weekend. Also as temperatures cool Saturday night...some
snowfall would be possible over the high country for elevations of
10000 feet and above.

Storm system will begin to pull out late Saturday night but low
level airmass will be cool and moist so while heaviest rain may
end it could be replaced by more of a drizzle and light rain
scenario. Borsum

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

Although the models are generally in agreement with regard to the
overall pattern to start the extended...they continue to have some
significant disagreement. The GFS is more progressive with upper
trough and is probably the outlier in the near extended periods
as the NAM and European model (ecmwf) hold a quasi closed off upper low over our
forecast area Sunday and produce considerable quantitative precipitation forecast in a trowal like
fashion before it finally exits the area Sunday night. I will
trend our probability of precipitation and temperatures toward the wetter solutions a bit...as
confidence in any particular solution is rather low at this time.
The European model (ecmwf) has trended more toward the GFS Monday and Tuesday
dragging another piece of energy under the trough through the
central rockies rather than dropping it south as in its previous
run. Due to the increased consistency across models...with a
zonal/West Coast ridge type pattern over the region by
Wednesday...we will lean harder on a drier and more seasonally
temperate forecast for the latter part of the extended. Bt

&&

Aviation...

Scattered showers and some thunderstorm activity will continue
through the night...with local MVFR to IFR...and patchy fog is
also possible due to the wet ground. Mountains will be frequently
obscured. Shower coverage will increase on Friday as a strong
weather system moves into the region. Expect areas of MVFR to IFR
flight conditions on Friday. Jkl

&&

Preliminary point temp/pops...

Friday Sat sun Monday Tuesday Wednesday Thursday
-----------------------------------------------------------
bil 053/068 053/064 048/067 049/072 050/076 053/082 057/087
58/T 99/T 54/T 23/T 32/T 21/u 01/u
lvm 050/066 049/062 044/068 044/071 044/076 048/083 051/086
79/T 97/T 63/T 25/T 52/T 21/u 12/T
hdn 054/074 055/066 051/069 047/075 050/078 051/083 055/088
48/T 99/T 54/T 23/T 32/T 21/u 01/u
mls 057/074 057/070 052/065 049/072 051/075 053/082 057/087
64/T 98/T 55/T 21/b 32/T 21/u 01/u
4bq 056/078 059/074 053/066 048/072 052/074 051/081 056/087
54/T 98/T 63/T 22/T 43/T 31/u 01/u
bhk 057/072 055/070 052/065 046/070 047/072 048/078 053/083
74/T 99/T 84/T 22/b 33/T 21/u 01/u
shr 051/073 053/068 048/067 045/074 048/075 048/081 051/087
46/T 87/T 53/T 23/T 33/T 21/u 01/u

&&

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

&&

$$
Weather.Gov/Billings

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