Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 180258

Area Forecast Discussion...Updated
National Weather Service Billings MT
858 PM MDT Tue Jul 17 2018

An active afternoon and early evening with thundestorms covering
large sections of the forecast area. The strongest storms ended up
in the southeast, with slow moving big time storms that produced
a lot of water and even some severe weather. Activity has now
congregated along the 700mb trough, as the midnight and day crews
were expecting. Rainfall rates have tapered off, and all model
guidance, global and high resolution both, taper precipitation
off by midnight. Flash flooding does not appear to be much of a
threat any longer, given the strength of convection has weakened
with warm cloud tops. Will cancel the Flash Flood Watch. Adjusted
PoPs and clouds to account for the latest radar and satellite
trends, not to mention high resolution model data. Also, went into
the day 2 forecast and bumped winds up a bit. Feel potential is
there for a bit more of a breezy afternoon than advertised. TWH


.SHORT TERM...valid for Wed and Thu...

Water vapor imagery shows a slowly evolving upper low over western
MT with diffluence across the much of the state. One shortwave has
pushed into southeast MT and there is an area of thunderstorms in
Carter/Fallon Counties. Closer to the low, we are seeing scattered
storms across our west and south as well. Bulk effective shear is
currently analyzed at less than 25 kts across our cwa (too weak
for a real severe threat), but is expected to increase modestly
over the coming few hours near the southeast MT/northeast WY
border (a few strong storms w/ hail possible in this area).
Southeasterly boundary layer winds are increasing moisture across
our east, with latest pwat analyzed to be 1-1.1 inches east of
Yellowstone County.

Biggest threat between now and midnight is for heavy rain and
potential flash flooding, courtesy of slow storm motions and
expectation of redevelopment along 700mb trof which will deepen
in southeast MT by evening. Storm motions will be erratic and
would expect chaotic outflow boundaries. Anyone with outdoor plans
should stay alert and be wary of the flood potential through the

Lower and upper level lows will become stacked as the system moves
into the northern high plains tonight, allowing for showers and
thunderstorms to persist in our far east through the night into
Wednesday. In fact, we could see afternoon/evening development
near the Dakotas border if low level east winds and elevated
moisture holds on, otherwise the system will depart to our east.
Drier air and mixed W-NW winds will affect our west and central
parts tomorrow. There is a late day weak shortwave which could
produce isolated mountain convection and perhaps some higher-based
cumulus and sprinkles across lower elevations. Temps should get
into the lower 90s across our western cwa Wednesday.

Thursday will be a mostly dry day with a shift to N-NE winds as
high pressure drops out of Canada. This will shave a few degrees
off of temps, with highs pretty close to normal. JKL

.LONG TERM...valid for Fri...Sat...Sun...Mon...Tue...

The GEFS and ECMWF ensembles showed some uncertainty around the
upper low that will move E through SW Canada during the period.
Deterministic models showed some differences regarding the
movement and strength of the low as well. Made just a few forecast
adjustments based on model blends.

Upper ridging will be over the area on Friday and shift E Fri.
night. Kept the forecast dry through Saturday morning. Friday
still looked very warm with highs in the lower 90s in the central
and southern zones. Upper low was over S BC on the GFS by 00Z
Sunday, but was over S. Alberta on the ECMWF. Despite the
differences, models agreed Sat. would be generally dry and
continued very warm. The upper low continues to move more quickly
to the E on the ECMWF through Sunday night. As a result, the ECMWF
brings the cold front through faster than the GFS Sat. night.
Precipitation chances on Sat. night will be over SE MT and over
and near the terrain. Some precipitation will linger in the far SE
and over the terrain on Sunday. Models showed big temperature
differences on Sun. due to the pattern differences. Model blends
gave highs in the 80s for now, but this could change as frontal
timing becomes more certain. Weakly cyclonic flow will be over the
area for Monday. GFS keeps cyclonic flow over the area Tuesday,
while the ECMWF starts building in a ridge. Some precipitation
will linger in the SE and over the terrain Sun. night and Monday.
Had precip. over the southern mountains on Tuesday. Temperatures
continued to vary between models. Blends had 70s to low 80s for
Monday and Tuesday. Arthur



Isolated thunderstorms will become more numerous through the
afternoon and evening. Expect MVFR/IFR conditions in the
precipitation, with lower conditions possible in areas of heavy
rain, especially from KBIL to KSHR E. Thunderstorms will decrease
from W to E late this evening with some storms lingering E of
KBIL through Wednesday. Mountain obscurations will decrease late
tonight. Arthur



    Wed     Thu     Fri     Sat     Sun     Mon     Tue
BIL 062/092 061/088 060/091 063/092 062/086 058/078 056/081
    20/N    10/U    00/U    00/U    10/U    01/U    11/B
LVM 053/091 051/087 050/089 053/089 053/085 050/078 050/079
    20/N    10/U    00/U    01/U    30/U    01/U    11/B
HDN 059/092 060/089 057/092 063/094 062/087 057/080 055/082
    31/U    10/U    00/U    00/U    11/U    11/U    11/B
MLS 064/088 062/088 060/088 065/093 065/087 059/079 056/081
    53/T    10/U    10/U    00/U    11/U    01/U    11/U
4BQ 061/089 062/089 059/089 063/094 063/090 060/081 056/080
    52/T    10/U    00/U    00/U    21/B    11/U    11/B
BHK 063/083 059/086 056/084 060/087 062/086 057/078 053/079
    35/T    21/U    10/U    00/U    21/B    11/U    11/U
SHR 056/091 057/088 054/091 058/093 059/088 054/079 052/082
    30/N    10/U    00/U    00/U    21/B    11/U    12/T


MT...Red Flag Warning in effect from noon to 9 PM MDT Wednesday
      FOR ZONE 117.


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