Severe Thunderstorm Warning 10, 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

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

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

0814 PM     HAIL             7 SE MOORE              46.90N 109.59W
05/19/2020  E0.50 INCH       FERGUS             MT   TRAINED SPOTTER

            COVERED THE GROUND WHITE.

&&

MM

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

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

0839 PM     HAIL             LEWISTOWN               47.06N 109.42W
05/19/2020  E0.70 INCH       FERGUS             MT   TRAINED SPOTTER

&&

MM

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

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

0844 PM     HAIL             1 NW LEWISTOWN          47.07N 109.44W
05/19/2020  E0.88 INCH       FERGUS             MT   TRAINED SPOTTER

&&

MM

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

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

0835 PM     HAIL             LEWISTOWN               47.06N 109.43W
05/19/2020  M1.00 INCH       FERGUS             MT   TRAINED SPOTTER

            SPOTTER REPORTS HAIL UP TO QUARTER SIZE
            COVERING THE ROADS.

&&

FMK

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