FXUS65 KBYZ 171515

Area Forecast Discussion
National Weather Service Billings MT
915 AM MDT Tue Jul 17 2018

Slow-moving upper low is approaching from our west and will bring
widespread showers/t-storms this afternoon and evening. Have
reduced pops a bit this morning as activity will get going mainly
after 18z. There is also starting to be development in southern
Powder River County, a bit earlier than thought, but in region of
high moisture w/ pwats near an inch. Moisture advection will
continue across our east, and this will lead to a threat of flash
flooding w/ strong slow-moving or back-building thunderstorms
later today. Shear is quite low so not much of a severe threat,
though 20-30 kt of bulk shear may support a stronger storm w/ hail
in far southeast MT near the WY border. In any event, heavy rain
is the big risk today.

Based on latest HRRR and NAM12 trends, and expected position of
mid level trof, have added Treasure and Carter Counties to the
Flash Flood Watch, which is now in effect from 4 pm to midnight.



.SHORT TERM...valid for Tdy and Wed...

Low pressure system over western Idaho will slowly meander
through southern Montana today. Radar already showing some showers
and thunderstorms into eastern Idaho and western Montana. It is
still somewhat uncertain how the upper-level PV will evolve as it
crosses the Divide. Some models suggesting a split flow with a
piece of forcing over central Montana and another piece over
northern Wyoming. Mid-level trough seems to develop just east of
Billings by late afternoon. As it does so, numerous showers and
thunderstorms will develop, especially from Rosebud County
southeast through northern Wyoming. CAPE around 1500-2000 j/kg
with light shear is confirmed by the hi-res models which also show
generally multi-cellular clusters and line segments forming.
Initial storm motion should be ENE but begin to turn southeast
toward the low-level jet. This change in movement as storms cross
the mid-level trough axis, high atmospheric moisture, and slow
storm motion should allow for localized heavy rainfall over
eastern Montana and Wyoming. We feel the flooding threat will be
maximized along the mid-level trough position during the evening
hours, so we put out a Flash Flood Watch for Rosebud, Powder
River, and Custer Counties from 6 pm to Midnight tonight. Any
storms that form across the area today will be capable of
producing heavy rain, and frequent lightning so be aware if you
have any outdoor plans. The severe threat looks limited given the
shear profile. However some QLCS structures could develop as low-
level jet develops over eastern MT. This increases the threat of
strong wind gusts for extreme southeastern MT and northeast WY.

Overall the timing has slowed down with this system and it seems
precip begins late morning to near mid-day in the western zones,
transitions to central zones (including Billings) mainly between 3
and 6 pm, then over eastern MT during the evening before pushing
into the Dakotas late night into early morning. Temperatures only
make it to the mid 80s in western zones due to increased cloud
cover earlier in the day with low 90s possible in the east.

Progressive westerly flow brings another shortwave through the
area tomorrow afternoon. In response westerly winds will increase
across the area making for a warm, breezy, and dry afternoon with
temperatures in the low 90s and humidity values near 20 percent
over our western zones. Walsh

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

Upper ridging will dominate the weather over the Northern Rockies
Thursday and Friday with dry and warm conditions. The upper ridge
axis will then move eastward out of Montana on Saturday in
advance of an upper trough moving eastward over the Pacific
Northwest/southwestern Canada.

Upper trough axis and surface cold front look to cross the region
late Saturday night-Sunday morning, which is a bit of a delay from
yesterday's model solutions. There is a slight chance of a shower
or thunderstorm Saturday evening ahead of the surface front/upper
trough axis. Sunday looks breezy due to an appreciable surface
pressure gradient left in the wake of the cold front.

For Monday, models differ with the GFS bringing a shortwave and
scattered showers and thunderstorms over eastern MT while the
ECMWF solution is dry. The GFS solution is quite possible so have
a chance of showers/thunderstorms over SE MT on Monday.

High temperatures will generally be in the upper 80s Thursday,
rise to the lower 90s Friday and Saturday, then decrease a little
to the middle 80s on Sunday and upper 70s-lower 80s for Monday.



Scattered showers and isolated thunderstorms will become more
numerous through the afternoon as they spread W to E. A few
showers and storms will occur over SE MT this morning as well. Expect
MVFR/IFR conditions in the precipitation, with lower conditions
possible in areas of heavy rain, especially E and S of KBIL.
Thunderstorms will decrease from W to E this evening, with some
storms lingering E of KBIL overnight through Wednesday morning.
Mountain obscurations will increase through the day, then decrease
from W to E this evening. Arthur



Tdy Wed Thu Fri Sat Sun Mon
BIL 087 061/092 061/088 060/092 063/092 062/087 058/079
7/T 30/U 10/U 00/U 00/U 20/U 01/U
LVM 081 055/089 051/085 051/089 054/089 053/085 050/079
7/T 20/U 00/U 00/U 02/T 20/U 01/U
HDN 090 059/092 059/088 059/093 063/094 062/087 057/081
8/T 41/U 10/U 00/U 00/U 21/U 01/U
MLS 090 066/088 062/088 060/088 065/092 065/087 060/081
5/T 72/T 10/U 10/U 00/U 21/U 01/U
4BQ 091 061/089 061/089 059/089 063/093 063/089 060/083
7/T 82/T 00/U 00/U 00/U 21/U 12/T
BHK 089 063/082 059/086 056/084 060/087 061/085 057/079
1/B 55/T 11/U 10/U 00/U 21/B 13/W
SHR 088 058/090 057/088 055/092 057/093 059/087 054/081
6/T 60/U 10/U 00/U 00/U 21/U 11/U


MT...Flash Flood Watch in effect from 4 PM MDT this afternoon
through this evening FOR ZONES 30>32-36-37-58.
Fire Weather Watch in effect from Wednesday afternoon through
Wednesday evening FOR ZONE 117.


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