Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 130923

Area Forecast Discussion
National Weather Service Billings MT
323 AM MDT Fri Oct 13 2017

.SHORT TERM...valid for Tdy and Sat...

Water vapor imagery was showing an upper trough over the Pacific
Northwest this morning. This trough will slide east and be the
weather maker over the next 24 to 36 hours. System will be
fighting dry low levels, but should still serve to produce a
decent chance of rain and snow showers across the plains, with
another shot of accumulating snow over the high country.

GFS and ECMWF were in pretty good agreement in shifting the
trough axis east into western Idaho this afternoon. Ascent on the
front side of the trough will get into western zones and generate
scattered showers. Snow showers will develop in the mountains.
The trough axis slides into western Wyoming tonight and allows a
cold front to push into the forecast area. This process will slide
ascent east across the plains. Have sped up the timing a bit for
higher PoPs for central zones this evening and tapered things off
over the west quicker overnight. 850mb temperatures fall to
around 0c after midnight on the GFS and right around midnight
according to the ECMWF. QPF is not progged that high with this
system and agree with this, as true upslope flow never really
materializes as low level flow stays mainly northwest. Low levels
will start out pretty dry too, as dewpoints will be in the 20s
during the early evening hours.

GFS was broken with the line of showers shifting across central
zones tonight. ECMWF generally agrees with this and have kept
accumulations below a tenth of an inch. One thing to keep an eye
on is the instability. 850-500mb lapse of around 8c/km will be
present this evening and only fall to around 7c/km overnight.
This instability could lead to more aggressive type of rain/snow
showers and produce locally higher bursts of QPF and some local
travel impacts with reduced instability. Warm ground should keep
accumulations down, but will need to watch snow rates with the
instability present.

System exists the area Saturday and tapered things off from west
to east Saturday morning. Lingered low PoPs in the far east
Saturday afternoon as a secondary, weaker wave, follows the main
trough in developing northwest flow. Low level downslope flow will
limit precipitation potential. Downslope flow will warm low
levels and limit the cool down on Saturday. It will be a bit
breezy Saturday but not overly windy. Mixing winds will get
cranking over western zones with some gusts to 40 mph. Will have
to watch the corridor from Livingston to Big Timber to Harlowton,
as 700mb winds increase to 40kts with a favorable gradient for Big
Timber to Harlowton early. The gradient then turns more favorable
for Livingston late Saturday night. This covered well in existing
grids and will let ride. TWH

.LONG TERM...valid for Sun...Mon...Tue...Wed...Thu...

Only minor adjustments for the long term, generally concentrated
late in the period, as next weather system begins moving into
the region next Friday.

Sunday will be a transition day, as trof exits the region and
relatively shallow upper level ridging builds into the region.
Shallow ridging points to dry conditions and will keep downslope
flow over the region through much of the period, which will keep
temperatures above normal. Temps around 70 degrees are possible
Tuesday ahead of a very weak and dry cold front.

Conditions through the work week appear to be breezy, as strong
westerly flow keep a fairly persistent lee- side trof over the
plains through much of the week. This will keep the winds
elevated in our gap- flow favored regions like Livingston, Nye,
and other foothill locations.

Models diverge on timing of next Pacific system into the region.
GFS is the earlier, with showers into the west as early as
Thursday evening, and spreading east through Friday. This timing
would point to possibility of warmer highs Thursday. ECMWF is
slower by 6 to 12 hours. AAG



Rain/snow showers will spread into western mountains this
morning, with local MVFR near KLVM, and the Beartooth/ Absaroka
and Crazy Mountains by early afternoon. Showers will spread
eastward through the afternoon/evening. Showers will overspread
all routes overnight, with localized IFR conditions overnight
tonight. Otherwise, mainly vfr conditions will prevail.

SW winds are expected to increase along the western foothills
early this morning, with gusts of 25-35 kts at KLVM for much of
the day. AAG



    Tdy Sat     Sun     Mon     Tue     Wed     Thu
BIL 055 033/049 034/061 040/066 041/069 044/065 044/066
    2/W 53/W    00/U    00/N    01/U    00/U    01/B
LVM 048 026/046 029/058 035/064 038/067 039/064 040/062
    5/W 63/W    00/N    00/N    01/N    10/N    11/N
HDN 058 033/051 030/062 035/068 038/070 042/066 040/069
    1/B 44/W    00/U    00/U    01/U    10/U    00/B
MLS 058 033/048 031/059 037/066 039/071 041/064 040/068
    1/B 34/W    01/U    00/U    00/U    10/U    00/B
4BQ 060 033/046 029/059 034/068 037/071 039/064 039/069
    0/U 24/W    00/U    00/U    00/U    10/U    00/U
BHK 058 032/046 030/056 036/066 038/070 040/063 039/068
    0/B 13/W    01/N    00/U    00/U    10/U    00/U
SHR 058 030/046 025/059 032/068 035/071 038/064 037/068
    0/U 34/W    10/U    00/U    00/U    10/U    00/B




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