Severe Thunderstorm Warning 116, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
604 PM MDT Sat Aug 10 2019

...Aviation Section Update...

.SYNOPSIS...
An upper level trof will approaches and moves through the region
this weekend. As a result, expect showers and thunderstorms to
move through the area this afternoon and evening and again on
Sunday. Some storms could be severe, with gusty winds, hail.
Additionally, the rainfall could be heavy at times, causing
isolated areas of flash flooding. The work week will begin with
drier condtions and breezy westerly winds for most areas.

&&

.DISCUSSION...

Remainder of this afternoon through Sunday...Showers and
thunderstorms are just beginning to develop this afternoon over
the southwest. This activity is forecast to increase as
conditions are favorable for the potential for some severe
thunderstorms, especially over southwest and central Montana,
although the far southern areas of Beaverhead, Madison and
Gallatin Counties may be a bit drier than areas further north. MU
cape values are roughly falling in the 1000 to 2000 J/kg range,
while mid level lapse rates range from 7 to 8 degrees C. Moisture
remains plentiful as well, with .75 to 1 inch PWATS across much
of southwestern Montana and areas to the north (highest values
over the north-central). These factors along with good effective
shear has lead to the issue of a severe thunderstorm watch for
portions of the area through 9 pm tonight, some of the stronger
storms may produce wind gusts in excess of 60 mph, hail up to lime
size, along with heavy rains. One limiting factor is a stubborn
deck of low stratus over the central and north-central portions of
the region. This may lower the severe weather threat for portions
of the north but heavy rain will still be possible. A flash flood
watch remains in effect for areas along the Rocky Mountain Front
and then for Lewis and Clark and Cascade Counties, including
Helena and Great Falls (please see our hydrology section). Shower
and thunder storm activity should diminish by midnight but
showers and a few storms will be possible during the overnight
hours. Expect similar conditions on Sunday, as the main upper
level trof moves through the region. However, the focus of severe
weather is expected to shift a bit to the north and east.
Afternoon temperatures are likely to be a few degrees below normal
for most areas today and Sunday due to precipitation and cloud
cover.

Monday through Friday...With a light westerly flow over the
region, expect a slow warming trend, with only a very small chance
for a passing storm from Monday through Wednesday. On Thursday,
the next upper level trof approaches the region, allowing for the
potential for another round of severe storms to move through the
region. Showers then linger into Friday and Saturday, along with
below normal temperatures. Brusda

&&

.AVIATION...
Updated 604 PM MDT Sat Aug 10 2019 (11/00Z TAFs)

During this TAF period, expect MVFR conditions in association with
thunderstorm, rain and mist. Ceilings as low as OVC015 with
visibility to 4SM are forecast, along with all hazards assumed in
and near thunderstorm activity. Some storms in our CWA are producing
severe weather. Expect mountain and valley obscurations.
- Fogleman

&&

.FIRE WEATHER...
Periods of showers and thunderstorms will move through the region
from now through Sunday night. Most of the storms will be wet, but a
few storms could contain little rainfall or lightning could strike
away from the parent storm, resulting in new fire starts. Slightly
below normal temperatures are expected from today through early next
week, before a return to near normal temperatures is expected for
Tuesday and Wednesday.

Another round of storms is expected to move through the region
from late Thursday into Friday. Brusda/RCG

&&

.HYDROLOGY...
A flash flood watch remains in effect for areas along the Rocky
Mountain Front and portions of North Central MT from this
afternoon through late this evening. Some forecast models
continue to suggest that some areas may receive nearly 1 inch of
rainfall in less than 2 hours with the stronger storms this
afternoon and evening. This amount of rainfall could lead to
flooding problems in urban areas that have poor drainage, as well
as steep terrain and burn scar areas from this year or previous
years in areas that regrowth has not occurred. Brusda/RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  53  75  52  74 /  90  80  70  10
CTB  50  71  50  71 /  70  80  70  30
HLN  52  76  49  76 /  90  90  70   0
BZN  48  78  45  77 /  40  70  50   0
WEY  43  71  35  71 /  10  30  20   0
DLN  47  72  42  74 /  40  70  50   0
HVR  51  76  52  73 /  50  80  90  50
LWT  50  74  49  71 /  80  80  80  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until midnight MDT tonight Cascade...Central
and Southern Lewis and Clark...Northern Rocky Mountain Front...
Southern Rocky Mountain Front.

Severe thunderstorm watch is in effect until 9 pm MDT for the
following counties in central Montana - Fergus, Judith Basin,
Meagher, and Lewis and Clark...In southwest Montana - Beaverhead,
Broadwater, Gallatin, Jefferson, and Madison

&&

http://www.weather.gov/greatfalls

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