Special Weather Statement, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
343 PM MDT Tue May 19 2020

.SYNOPSIS...

Showers and thunderstorms are developing over southwest Montana
and will move from south to north across the region through this
evening. Some thunderstorms could become severe and produce large
hail, damaging winds, and even an isolated tornado. In addition
to the threat for severe weather, showers and thunderstorms will
be efficient rain producers and could cause some flooding,
especially in urban areas and on streams and creeks emanating from
the mountains.

&&

.DISCUSSION...

A broad closed low has brought a swift southerly flow aloft for
much of Montana. This combined with east and northeasterly flow at
lower elevations creates effective shear values generally in the
40 to 55 kt range. The addition of modest CAPE of 500 to 1200 j/kg
has lead to the development of isolated to scattered showers and
thunderstorms over portions of Central and Southwest Montana. This
development will continue through the afternoon and the storms
will quickly move toward the north and northeast upon development.
Some storms can be severe and contain wind gusts up to 70 mph,
hail up to ping pong ball size, and possibly an isolated tornado.
With coordination with Storm Prediction Center, a Severe
Thunderstorm Watch has been issued for the entire CWA with he
exception of Glacier county. This watch currently runs through 10
PM MDT. In addition to the severe threats, these storms will be
efficient rain producers. The saving grace will be the fast motion
of the storms but still the heavier cells can produce a quick .25
to .5 inches of rain in a short period of time, which can cause
some flooding concerns in urban areas and or melting snowpack. The
thunderstorm activity is expected to diminish during the
overnight hours but then general stratiform rain will set in,
especially for areas northwest of a Great Falls to Havre line,
where addition .25 to .5 inches of rain will be possible through
Wednesday morning. RCG

Wednesday through Saturday...the H500 trough/closed low will lift
north from the Central Great Basin and over the Northern Rockies
from Wednesday through Thursday, all the while another trough/closed
low over the Gulf of Alaska digs southeast over the Pacific
Northwest. These two systems will begin to interact from Thursday
afternoon through Friday, with multiple waves dumbbelling around the
broad closed low/trough. Needless to say, chances for precipitation
will continue across the Northern Rockies from Wednesday and into the
day Saturday. Falling snow levels over this timeframe will allow for
precipitation to changeover to snow for elevations above 6000ft,
with significant snow accumulations possible in some of the higher
elevations of Southwest Montana from Thursday night through Friday
night. Thursday will also be a windy day for portions of Central
and all of North Central Montana, with the potential for some areas
to approach High Wind criteria. - Moldan

Sunday through next Tuesday...temperatures will begin to moderate and
precipitation chances will be limited as upper level ridging moves
overhead. - Moldan

&&

.AVIATION...
Updated 1151 AM MDT Tue May 19 2020 (19/18Z TAF Period)

Showers and thunderstorms are still expected to develop over Central
and Southwest Montana around 19 to 21Z and then lift north/northeast
across the region this afternoon and evening. Some storms could
become severe and produce wind gusts approaching 50kts and large
hail to the size of quarters to ping pong. Central/North-central
terminals (i.e. KHLN, KGTF, KLWT, and KHVR)are most at risk of
seeing severe thunderstorms over this timeframe. VFR conditions are
generally expected outside of areas of precipitation through this
evening, with MVFR conditions expected beneath showers and
thunderstorms. MVFR/IFR conditions are then expected to develop
after 3Z along the upslope favored regions of North Central and
Central Montana (i.e. KCTB, KGTF, and KLWT) and persist into
Wednesday morning. Mountain obscuration will develop this afternoon
and persist into Wednesday morning.

NOTE: Amendments not scheduled for Ennis (KEKS) due to ongoing
runway closure.

&&

.HYDROLOGY...

Periods of rain, heavy at times, will continue to affect portions
of Central and North Central MT for the next several days. The
rainfall will be convective at times, resulting in some areas
receiving heavy rainfall and locations just a few miles away
receiving much less precipitation. Overall, the Rocky Mountain
Front continues to favor the highest precipitation amounts. - Brusda

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  50  66  45  61 /  90  50  60  30
CTB  46  59  44  56 /  90  80  60  40
HLN  49  69  45  63 /  90  60  80  50
BZN  47  69  42  63 /  60  30  60  60
WYS  35  56  31  51 /  30  60  80  80
DLN  43  61  40  59 /  70  60  80  40
HVR  53  74  48  65 /  60  40  50  40
LWT  48  69  43  62 /  50  60  50  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Severe Thunderstorm Watch in efffect until 1000 PM for
Beaverhead, Blaine, Broadwater, Cascade, Chouteau, Fergus,
Gallatin, Hill, Jefferson, Judith Basin, Lewis and Clark,
Liberty, Madison, Meagher, Pondera, Teton, toole.

&&

http://www.weather.gov/greatfalls

Special Weather Statement
National Weather Service Great Falls MT
428 PM MDT Tue May 19 2020

MTZ012-013-050-192315-
Chouteau-Cascade-Judith Basin-
428 PM MDT Tue May 19 2020

...A STRONG THUNDERSTORM WILL AFFECT NORTHWESTERN JUDITH BASIN...
SOUTH CENTRAL CHOUTEAU AND NORTHEASTERN CASCADE COUNTIES...

At 427 PM MDT, a strong thunderstorm was located 9 miles southeast of
Belt, or 27 miles southeast of Great Falls, moving northeast at 25
mph.

Winds in excess of 30 mph and pea size hail are possible with this
storm.

Locations impacted include...
Belt, Highwood, Sluice Boxes State Park, Raynesford and Armington.

This includes the following highways...
 Highway 200 between mile markers 0 and 15.
 Highway 89 between mile markers 61 and 71, and between mile markers
72 and 74.

If outdoors, consider seeking shelter inside a building.

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

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

To report severe weather, contact your nearest law enforcement
agency. They can relay your report to the National Weather Service
office in Great Falls.

LAT...LON 4716 11077 4723 11101 4763 11087 4744 11031
TIME...MOT...LOC 2227Z 202DEG 22KT 4727 11083

TP

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
430 PM MDT TUE MAY 19 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0425 PM     HAIL             7 ESE MARYSVILLE        46.73N 112.16W
05/19/2020  E1.00 INCH       LEWIS AND CLARK    MT   TRAINED SPOTTER

            PENNY TO QUARTER SIZED HAIL REPORTED.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
434 PM MDT TUE MAY 19 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0417 PM     HAIL             7 E MARYSVILLE          46.73N 112.16W
05/19/2020  E0.75 INCH       LEWIS AND CLARK    MT   TRAINED SPOTTER

            PEA AND PENNY SIZED HAIL.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
437 PM MDT TUE MAY 19 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0405 PM     HAIL             6 SE CANYON CREEK       46.74N 112.16W
05/19/2020  E0.75 INCH       LEWIS AND CLARK    MT   TRAINED SPOTTER

            DIME TO PENNY SIZED HAIL REPORTED AROUND
            4PM.

&&

TP

Special Weather Statement
National Weather Service Great Falls MT
441 PM MDT Tue May 19 2020

MTZ014-192315-
Central and Southern Lewis and Clark-
441 PM MDT Tue May 19 2020

...A STRONG THUNDERSTORM WILL AFFECT CENTRAL LEWIS AND CLARK AND
SOUTHWESTERN CASCADE COUNTIES...

At 441 PM MDT, a strong thunderstorm was located 8 miles southwest of
Wolf Creek, or 24 miles north of Helena, moving north at 30 mph.

Winds in excess of 40 mph and half inch hail are possible with this
storm.

Locations impacted include...
Wolf Creek and Craig.

This includes the following highways...
 Interstate 15 between mile markers 215 and 239.
 Highway 200 between mile markers 88 and 105.
 Highway 287 between mile markers 1  and 16.

If outdoors, consider seeking shelter inside a building.

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

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

To report severe weather, contact your nearest law enforcement
agency. They can relay your report to the National Weather Service
office in Great Falls.

LAT...LON 4730 11243 4723 11205 4720 11205 4720 11203
      4718 11201 4719 11198 4714 11194 4714 11190
      4712 11189 4713 11180 4678 11216 4690 11237
TIME...MOT...LOC 2241Z 189DEG 26KT 4693 11221

TP

Special Weather Statement
National Weather Service Great Falls MT
521 PM MDT Tue May 19 2020

MTZ014-192345-
Central and Southern Lewis and Clark-
521 PM MDT Tue May 19 2020

...A STRONG THUNDERSTORM WILL AFFECT SOUTH CENTRAL LEWIS AND CLARK
COUNTY...

At 520 PM MDT, a strong thunderstorm was located 19 miles northeast
of Garrison, or 21 miles west of Helena, moving northeast at 30 mph.

Winds in excess of 30 mph and pea size hail are possible with this
storm.

Locations impacted include...
Marysville, Canyon Creek and Austin.

If outdoors, consider seeking shelter inside a building.

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

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

To report severe weather, contact your nearest law enforcement
agency. They can relay your report to the National Weather Service
office in Great Falls.

LAT...LON 4698 11249 4670 11204 4659 11230 4666 11233
      4666 11241 4670 11242 4670 11252 4683 11253
      4684 11255
TIME...MOT...LOC 2320Z 220DEG 26KT 4665 11246

TP