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Area forecast discussion...updated 
National Weather Service Billings Montana
927 PM MDT sun may 24 2015

Update...
turned into a busy evening here with some flood issues and cold
air funnels sighted. The cold air funnel/S/ occurred a little west
of Billings around 7 PM. These popped out from some isolated
cells. Why cold air funnels...because nothing on radar that
appeared threatening...no lightning...no super cell structures.
We have a cool moist atmosphere with some vertical directional
wind shear. This is the type of conditions cold air funnels are
normally seen.

As for flooding issues...we have had reports of over 2 inches of
rainfall in the Sheridan foothills in the last 12 hours. The
little Goose Creek south of Sheridan is out of its banks and over
flood stage. However...impacts at this time are minor. Other
smaller creeks and drainages have been reported to be flooding
from story to the town of Big Horn as well. Our concern is a quick
rising Goose Creek in Sheridan itself. We currently have room for
some rising with the latest stage at 5.64 feet and flood stage at
7.5 feet. We just received a new forecast for a crest at around
8.3ft at 2 am. Confidence is low as river models do not work well
for this area. So this will need to be monitored closely as rising
water along the little goose and big goose creeks both flow into
the Goose Creek in Sheridan.

As for the big picture...upper low continues to spread moisture
into the forecast area with impacts mainly in the southeast
sections of our forecast area. Probability of precipitation trends with inherited
forecasts look good. I expect more scattered showers
tomorrow...though maybe not quite as locked in as today in the
southern portions. Bt

&&

Short term...valid for Monday and Tuesday...

A chance of showers is in the forecast most of the time during the
next few days as an active late Spring weather pattern continues.

Tonight...a moist atmosphere characterized by precipitable water
values of 0.70 to 1.00 inches will remain in place and so showers
will have no trouble continuing where large-scale forcing exists.
The 12 UTC global...convectively-parameterized model guidance and
higher-resolution...convection-allowing models like the NCEP WRF-
arw and nmm all suggest the greatest concentration of forcing and
thus showers will be from Sheridan east across southeastern Montana. We
thus have 60+ percent probability of precipitation in play there and mainly chance-style
probability of precipitation elsewhere tonight. The forcing in southeastern Montana will be
driven on the larger-scale by a 500-mb low lifting out of The
Rockies and into the northern plains by Monday.

Monday...the 12 UTC guidance continued the theme presented in most
recent solutions for a greater chance of showers than expected in
earlier forecasts for Memorial Day. We generally followed with the
multi-model consensus...which supported likely probability of precipitation lingering into
Monday morning over southeastern Montana and chance-type probability of precipitation of 30 to
50 percent in most areas through the day. Drying will nonetheless
begin aloft by afternoon and that should help highs reach into the
60s f in many places.

Monday night...weak middle-level ridging will cross the area and that
will offer a relative minimum in the chance of precipitation after
any lingering...diurnally-driven showers diminish in the evening.

Tuesday...the air mass will warm ahead of a weak 500-mb low moving
east out of the Pacific northwest and that will combine with a bit
less cloud cover to allow for highs ranging from the upper 60s f
to the lower 70s f in most areas. That in turn will likely combine
with steepening middle-level lapse rates to yield MLCAPE between 250
and 500 j/kg Tuesday afternoon and evening per the 12 UTC GFS and
NAM solutions. This environment will likely favor shower and storm
development over south central Montana and and north central Wyoming Tuesday
afternoon into Tuesday night. The cape-shear setting looks like it
will be marginal for any strong storms...but the scenario appears
more supportive of lightning than most recent days. That is backed
up by 09 UTC sref-based calibrated thunderstorm probabilities near
40 percent. Schultz

Long term...valid for Wednesday...Thu...Fri...Sat...sun...

Unsettled pattern with generally weak flow through the extended
though there a few periods which are Worth highlighting. Wednesday
an upper low pushing into the area from the Pacific northwest
stalls and fills over Idaho leaving the forecast area under a belt
of slightly stronger southwesterly flow aloft. Could be a better
thunderstorm set up depending on the positioning of the surface
low. GFS is favoring most of the area being in the cool sector
with the surface low over Wyoming while the European model (ecmwf) is less distinct
with this surface low and allows for warmer temperatures. WRF and
sref indicate 500 to 1000 j/kg convective available potential energy but shear is still light so
expect better thunderstorm coverage but not much intensity.
Thursday has even less shear and cape but airmass will be unstable
so any heating will generate shower activity.

Friday will remain cool and unsettled but does not look quite as
wet as a back door cold front moves through eastern Montana. Ridging
shows up next weekend for warmer temperatures though the European model (ecmwf) is
less certain of the amplitude of the ridging and has more of a
zonal flow. Could see a better chance of temperatures in the 70s
than the past few weekends. Looks like slight chance of showers
and isolated thunderstorms is a good compromise forecast but GFS
would support a mostly dry forecast. Borsum

&&

Aviation...

Expect scattered showers through the night with numerous showers
east and S of kbil. Ceilings will be MVFR/IFR with the showers...with
occasional MVFR visibilities. Kshr will have lower conditions with
occasional LIFR ceilings. Isolated to scattered showers will
continue on Monday with more numerous showers from Rosebud County east.
There is a small chance of thunder Monday afternoon as well. MVFR/IFR
ceilings will generally improve through the day. Expect widespread
obscurations over the NE bighorns through Monday and areas of
mountain obscuration elsewhere. Arthur
&&

Preliminary point temp/pops...

Monday Tuesday Wednesday Thursday Friday Sat sun
-----------------------------------------------------------
bil 046/066 047/072 050/071 049/071 049/066 048/073 049/077
34/T 24/T 54/T 44/T 44/T 33/T 31/b
lvm 040/065 044/067 045/066 043/066 043/065 043/072 045/074
23/T 36/T 64/T 44/T 44/T 43/T 32/west
hdn 048/068 045/075 049/073 049/074 049/069 046/075 048/080
65/T 23/T 34/T 44/T 44/T 32/T 21/b
mls 049/066 047/074 051/073 050/074 049/068 046/071 048/075
56/T 22/T 34/T 43/T 33/T 22/west 11/b
4bq 047/064 045/072 049/072 049/072 049/066 045/070 047/075
87/T 22/T 34/T 44/T 44/T 32/T 21/b
bhk 048/062 045/071 047/071 047/071 045/064 040/066 043/072
77/T 32/T 34/T 44/T 33/T 32/west 11/b
shr 042/061 041/069 045/068 045/068 045/064 044/069 045/075
94/T 23/T 43/T 44/T 44/T 42/T 21/b

&&

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

&&

$$
Weather.Gov/Billings

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