Hail in Montana, June 28, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
1159 AM MDT Wed Jun 28 2017

Aviation Section Updated

.SYNOPSIS...

Upper level low pressure system will bring scattered showers and
thunderstorms to Montana Wednesday through Thursday but drier
conditions should return Friday. Seasonable temperatures on
Wednesday will cool 5 to 10 degrees Thursday as a surface cool front
pushes through the region. Building high pressure aloft brings
warming temperatures Friday through the weekend.

&&

.UPDATE...

A weak SFC low continues to move eastward through central Montana
at this time. Isolated thunderstorms associated with that low have
pushed into NE Montana. Meanwhile, radar shows isolated
showers/storms beginning to develop across SW MT where it is more
unstable. High clouds are moving through SW MT at this time on the
north side of convection moving through eastern ID. This may delay
the onset of better heating just a bit, but it still looks like
this area will be the focus for showers/storms today as a small
mid level jet passes just south of the area, providing modest
lifting. Like yesterday, weak/modest instability should develop in
this area, but a lack of better shear may tend to limit the severe
threat. That said, still looking at gusty winds and hail with the
strongest cores there. Further north across central Montana,
downsloping may tend to limit the coverage of showers/storms, but
I kept an isolated mention in ahead of a cold front that will dive
south through the area this evening. In that area, the threat of
stronger storms should be limited by a lack of better instability.
MARTIN

&&

.AVIATION...
Updated 1759Z

TSRA are beginning to develop across SW MT at this time, associated
with an upper level shortwave approaching the area. The greatest
coverage of SHRA/TSRA with this wave is expected in that area. Any
TSRA that develops could cause brief reductions in VIS/CIGs as well
as gusty winds and hail. MTN obscurations will be possible with the
TSRA activity as well. This evening, a strong cold front will drop
south out of canada bringing a better chance of SHRA to central MT.
In addition, confidence continues to increase regarding the
potential of MVFR CIGS behind this front. At this time, am not
thinking those lowest CIGS will make it into SW MT, but this will be
monitored in future forecasts. MARTIN

&&

.PREV DISCUSSION...
/ISSUED 544 AM MDT Wed Jun 28 2017/

Today through Friday...An upper level low pressure system remains
over central Alberta/Saskatchewan today. Unsettled westerly flow
aloft over Montana will bring variable clouds along with scattered
showers and thunderstorms to southwest Montana while only isolated
afternoon and evening showers will affect north central portions
of the state. The upper low drops southward and opens into a
positive-tilt trough across Montana Thursday before sliding
eastward into the northern Great Plains on Friday. This system
will help push a Canadian cold front into Montana for cooler
temperatures and a continued chance for precipitation Thursday.
Building high pressure along the West Coast edges eastward into
western Montana on Friday allowing temperatures to warm back up to
seasonable temperatures with mostly dry conditions over the plains
and isolated PoPs over the mountains. mpj

Friday night through Tuesday night...a progressive upper level
pattern best describes the long term once again, with very quick
transient ridging and shortwaves moving across the Northern Rockies
and Northern High Plains. A quick moving shortwave will translate
from the Intermountain West Saturday morning and across the Northern
Rockies through the remainder of the day. This shortwave should
spark off scattered showers and thunderstorms off of the higher
terrain of North Central and Southwest Montana by the afternoon
hours on Saturday, with any shower/storm then moving from west to
east. Thunderstorms on Saturday are expected to be of the garden
variety type, with a few storms possibly producing some breezy winds
and small hail. Both GFS and EC guidance have continued to trend up
with high temperatures through Monday, so felt comfortable in
raising temperatures further. For the day on Monday, upper level
ridging will begin to break down, with H500 flow becoming southwest
to westerly. H850 temperatures will soar into the mid 20C to low
30C. This should translate to high temperatures in the mid 80s to
mid 90s across the CWA. These warm/hot temperatures, combined with
dry/breezy downsloping winds and low afternoon relative humidity
values, will lead to heightened fire weather concerns for the day on
Monday. Given the ongoing dry conditions across our far eastern
zones, am growing concerned about the potential for grassland fire
spread given the aforementioned weather conditions. Finally, an
early look at Independence Day reveals highs potentially being just
as warm as Monday, with continued dry and breezy winds. Am not as
confident in temperatures being as warm on Tuesday (with respect to
Monday) given a broad trough moving into the region. However, given
the expected breezy winds and antecedent dry conditions, fire
weather concerns will once again linger for the day on Tuesday. -
Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  82  49  70  48 /  20  30  40  10
CTB  79  48  69  47 /  30  40  20  10
HLN  82  52  74  51 /  10  20  30  10
BZN  75  46  71  45 /  30  30  20  10
WEY  63  37  65  36 /  60  60  10  10
DLN  70  44  71  45 /  40  30  20  10
HVR  83  52  72  46 /  20  20  30  10
LWT  77  47  68  44 /  10  10  30  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

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