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Area forecast discussion 
National Weather Service Billings Montana
829 PM MST Tuesday Jan 27 2015

area of pv lifting through southeast Idaho with a 60 knots 300 mb southwesterly jet
is producing showers to our SW which are approaching the West
Yellowstone area. Given this and latest high res models looks like
some snow showers will spread into the beartooth absarokas before
midnight. Precipitation chances will spread across our southern tier late
tonight into Wednesday morning before tapering off as pv moves
into SD/NE. Have made some minor pop/quantitative precipitation forecast adjustments but overall
forecast seems to be in good shape. Area of clearing in southeast
Montana will give way to increasing clouds later tonight...but in the
meantime some valleys have already fallen to the 30s despite high
temperatures near 70 degrees today. Have tweaked forecast lows to account
for this. We will see a frontal passage tonight...modest pressure
rises already spreading across western and central Montana this
evening. This will yield a much cooler day tomorrow but with temperatures
still well above late January normals. We will see a gusty northwest wind
behind the frontal passage and have tweaked winds/gusts up some for
Wednesday...especially across our east. Jkl


Short term...valid for Wednesday and Thursday...

Another mild day across the forecast area with an unusually warm
air mass in place across our region. Model proggs are in good
agreement with regards to a subtropical weather system
undercutting the ridge aloft. Look for increasing clouds later
today as this system will track through the Great Basin and
approach our SW zones this evening. The moisture is mainly middle
level and lift is moderate at best. Thus I expect to see areas of
virga or sprinkles over the plains...but some decent rain and snow
showers in the high country. Snow levels may drop to around 7
kft late tonight...but being of subtropical origin...I do not
expect to see accumulations pile up much lower than this. The
system will then track east/southeast thanks to the cyclonic flow
over the area...and this will take it by the big horns as well on
Wednesday with some scattered showers into the Sheridan area
briefly at some point as well. The cloud cover and height falls
will result in cooler temperatures tomorrow...but still generally
on the mild side for late January.

Things dry out Wednesday night and Thursday as the system exits to
the southeast. However...the air mass will have been modified a
bit and temperatures will be cooler for Thursday...though still
somewhat above seasonal norms. Bt

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

The normalized Standard deviation for the European model (ecmwf) ensembles showed
a low confidence forecast through the extended period.
Similarly...the GFS spaghetti plots showed a fair amount of spread
through much of the period. Thus forecast confidence will not be
very high with this package.

Period starts off on Friday with a flat northwest flow over the forecast
area...while the large trough along 131w on water vapor imagery
this afternoon...moves into the SW U.S.. models keep the moisture
from this system S of the area. A shortwave in the northwest flow will
move through the area Friday night through Sat driving a backdoor
cold front into the region. Friday looked dry on both the European model (ecmwf) and
GFS. The models then differed on frontal and precipitation timing
into the area for Friday night and Sat. Went with a model blend for
probability of precipitation which resulted in the best precipitation chances on Sat. Flow
will remain basically northwest over the region through Tuesday with ridging
upstream of the area through Monday. The frontal boundary will get
reinforced through sun and Pacific flow will overrun the boundary
bringing additional precipitation to the area. Will again need to
go with a model blend for probability of precipitation. Waves of low pressure will move
along the frontal boundary through Monday...then the boundary was
forecast to move out on the GFS on Tuesday. The European model (ecmwf) brought the
boundary back into the area on Tuesday. So chances for precipitation
will continue through Tuesday. Previous forecast carried this idea

There were large temperature differences between the models for
portions of the extended due to the differences in the ways
the models handled the frontal boundaries. Will go with super
blend for the temperatures. Arthur


A few rain and snow showers will move into the Southwest
Mountains and adjacent foothills tonight. This will create some
mountain obscurations and local MVFR conditions. The precipitation
will spread into the bighorns and adjacent foothills late tonight.
Southwest winds will gust to 30kts at klvm this evening before
decreasing some overnight. A cold front will push across the area
late tonight and early Wednesday. This front will turn winds to
the northwest with some gusts of 20-25kts. Twh

Preliminary point temp/pops...

Wednesday Thursday Friday Sat sun Monday Tuesday
bil 043/054 029/046 029/049 028/038 019/032 019/035 021/035
11/north 00/b 00/u 13/west 23/j 33/j 22/west
lvm 041/051 027/047 029/048 024/040 018/037 023/041 024/040
33/west 00/b 01/u 13/west 23/j 44/west 32/west
hdn 036/051 024/044 025/048 023/035 014/031 014/033 015/034
01/east 00/b 00/u 13/j 22/j 33/j 22/j
mls 036/048 026/043 026/044 024/032 014/027 010/027 012/028
00/north 00/b 00/b 13/j 22/j 22/j 22/j
4bq 034/051 025/046 027/047 025/037 016/032 014/034 017/035
00/north 00/b 00/b 13/west 22/j 22/j 22/west
bhk 032/045 025/039 025/043 023/029 010/024 007/023 011/028
00/north 10/b 00/b 13/j 22/j 22/j 22/j
shr 031/049 021/043 022/045 020/036 013/033 014/036 017/034
23/west 00/b 00/b 13/j 32/j 33/west 22/j


Byz watches/warnings/advisories...



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