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Area forecast discussion...updated 
National Weather Service Billings Montana
927 am MDT Tuesday Apr 21 2015

weak shortwave was cruising down in northwest flow aloft and
producing some clouds and virga over central zones. Returns have
weakened over time and reports were of not much reaching the
ground. Have removed sprinkles from the morning grids. The rest of
the forecast looks on track with mainly higher terrain showers
this afternoon and evening. Easterly low level flow should keep
our highs in check today and near forecast levels. Twh


Short term...valid for today and Wednesday...

Our forecast area remains under a weakly cyclonic northwest flow
as a ridge to our west attempts to push east...blocked by the
powerful cut off low near the Great Lakes. The models hint at some
weak energy in the northwest flow aloft which is typical and
supported by satellite imagery upstream in Alberta. The other weak
feature we have is a back door cold front trying to push into our
eastern zones this morning. There is some isolated showers
associated with this feature near the Montana/Dakota borders...but
I question if any of this is even reaching the ground given the
wide dewpoint depressions at the surface.

So basically we have very little change necessary for this
package. Look for that back door cold air to probably come as far
west as Billings...and maybe result in some virga or sprinkles at
some point this morning/midday due to the baroclinicity and low
level convergence. So temperatures may not warm up quite as much
as we have been anticipating today...but we should still see
seasonal numbers. We should also see a a little bit more activity
over the mountains this afternoon with weak energy aloft and some
better low level convergence.

The ridge axis moves over Montana Wednesday with some moisture
sneaking up under it and decent instability to include some low
mountains probability of precipitation. Thermal ridge will maximum out over US Wednesday for
highs around 70 in many locations. Bt

Long term...valid for Thursday...Fri...Sat...sun...Mon...

Flow aloft through the extended period show a ridge axis shift
east of the area Thursday with a pocket of cooler air moving
through northwest Montana. Flow aloft becomes southwesterly on
Friday and then turns a bit more cyclonic on Saturday as remnants
of a dissipating Gulf of Alaska low moves through British Columbia and Alberta.
Behind this feature models favor weak ridging developing.

Moisture is not terribly deep Thursday evening but models do show
some convection developing as the first wave moves across
northwest Montana. Southeasterly low level flow helps deepen the
moisture on Friday but next round of forcing waits until Friday
night as heights begin to fall associated with the low remnants
moving across Canada. A slow moving surface low develops over
Wyoming on Saturday to maintain the moisture over the area until
it shifts east on Sunday.

Models not in agreement on how strong the surface low develops
over Wyoming during the early part of the weekend so there are
differences on the amount of moisture the area will receive. It
will be unsettled Thursday night through Saturday but forcing and
instability are unimpressive so will continue expectation of
scattered shower activity. Temperatures will be near normal
through the weekend after a warmer Thursday with warming expected
for the beginning of the next week. Borsum.



Northwest flow aloft will keep some middle level cloud cover over the
area with a few mountain showers possible this afternoon. VFR
conditions will prevail with isolated mountain obscuration due to
shower activity. Chambers

Preliminary point temp/pops...

Today Wednesday Thursday Friday Sat sun Monday
bil 063 040/073 045/070 043/066 042/064 043/068 044/073
1/b 00/u 02/west 33/west 33/west 32/west 21/b
lvm 065 037/074 039/068 038/067 039/063 037/066 039/069
1/b 01/b 23/west 33/west 44/west 32/west 22/west
hdn 064 036/075 039/074 038/067 037/067 039/069 039/075
1/b 00/b 02/west 33/west 33/west 32/west 21/b
mls 057 034/070 038/069 039/067 039/067 039/070 041/073
0/u 00/u 01/b 22/west 32/west 32/west 21/b
4bq 059 033/070 037/070 038/066 039/068 040/068 040/073
0/b 00/b 01/b 24/west 33/west 42/west 21/b
bhk 052 028/064 032/064 034/062 034/066 037/066 038/072
0/u 00/u 01/north 23/west 33/west 32/west 21/b
shr 060 035/070 039/069 039/064 038/064 038/065 038/071
1/b 00/b 12/west 33/west 33/west 32/west 21/b


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