Flash Flood Warning 6, NWS Billings, MT

Area Forecast Discussion
National Weather Service Billings MT
236 PM MDT Tue Jul 17 2018

.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
evening.

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

&&

.AVIATION...

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

&&

.PRELIMINARY POINT TEMP/POPS...

    Wed     Thu     Fri     Sat     Sun     Mon     Tue
-----------------------------------------------------------
BIL 062/092 061/088 060/091 063/092 062/086 058/078 056/081
    50/U    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
    30/U    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
    61/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
    73/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
    82/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
    55/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
    60/U    10/U    00/U    00/U    21/B    11/U    12/T

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Flash Flood Watch in effect until midnight MDT tonight FOR
      ZONES 30>32-36-37-58.
     Red Flag Warning in effect from noon to 9 PM MDT Wednesday
      FOR ZONE 117.
WY...None.

&&

weather.gov/billings

Hazardous Weather Outlook
National Weather Service Billings MT
225 AM MDT Tue Jul 17 2018

MTZ030>033-036>038-057-058-WYZ098-099-170930-
Treasure-Northern Rosebud-Custer-Fallon-Powder River-Carter-
Southern Big Horn-Northern Big Horn-Southern Rosebud-
Northeast Bighorn Mountains-Sheridan Foothills-
225 AM MDT Tue Jul 17 2018

This Hazardous Weather Outlook is for portions of south central
Montana...southeast Montana and north central Wyoming.

* WHAT... Slow moving thunderstorms producing heavy rainfall and
  localized flooding.

* WHERE...Southeastern Montana and northern Wyoming.

* WHEN...Tuesday afternoon and evening.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall could cause standing water on roadways and
underpasses. You should never attempt to drive through flood
waters. Turn Around Don't Drown!

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Special Weather Statement
National Weather Service Billings MT
239 PM MDT TUE JUL 17 2018

MTZ038-058-172130-
Southern Rosebud MT-Southern Big Horn MT-
239 PM MDT TUE JUL 17 2018

...SIGNIFICANT WEATHER ADVISORY...

At 238 PM MDT, Doppler radar was tracking a strong thunderstorm 20
miles south of Busby, or 32 miles north of Sheridan, moving east at
10 mph.

Very heavy rainfall, dime size hail and winds in excess of 40 mph
are possible with this storm.

Locations impacted include...
Birney, Tongue River Reservoir and Northern Cheyenne Indian
Reservation.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 4500 10689 4544 10695 4547 10651 4499 10644
      4499 10675
TIME...MOT...LOC 2038Z 249DEG 7KT 4526 10681

TWH