Tornado and Hail in Montana, June 27 to 28, 2019

Area Forecast Discussion
National Weather Service Glasgow MT
401 AM MDT Thu Jun 27 2019

.DISCUSSION...

...Severe thunderstorm threat exists for much of NE Montana with
very large hail and damaging winds through Friday from the late
afternoon through overnight hours...

...Locally heavy rainfall with thunderstorms will raise flooding
concerns for locations north of the Missouri River through Friday
night...

A deep Pacific upper level trough with closed low centered off the
coast of Oregon will lead diffluent southwest flow aloft over
northeast Montana today through Friday night. Meanwhile, a slow
moving surface low pressure system will track through the region
over this same period. Southeast surface winds will help moisten
the low levels with dew points expected to reach near 60 degrees
for portions of the CWA today, and approaching 70 for eastern
portions of the area on Friday. The very moist low level
environment will help lower LCLs, especially late Friday, and
combined with the presence of a strong theta-e ridge/instability
axis over the region and well developed veering wind profile,
steep midlevel lapse rates, the stage is set for several rounds
of severe weather over the next couple of days.

Initial discrete supercell and multicell modes will likely
initiate over the complex terrain of central Montana this
afternoon and approach western portions of the CWA around 6PM this
evening per latest mesoscale guidance. During the evening hours
the storms will spill eastward, most concentrated north of the
Missouri River over Valley County between 8pm and midnight, and
reaching Roosevelt, Daniels, and Sheridan Counties between
midnight and 4 am. That is not to say that storms cannot occur
over these areas sooner than indicated here. This is just when the
greatest concentration of convection is likely for given areas
during the evolution of the event tonight. In any case, in the
evening and overnight hours, the strengthening nocturnal low level
jet will lead to about 40 kts shear in the 0-6 km layer. The
total CAPE/Shear profiles suggest that individual storm structures
may thus evolve into linear structures and perhaps bowing
segments as they track eastward. Therefore it makes sense that an
initial very large hail threat may dominate in the beginning of
the event, transitioning to a greater damaging wind threat with
time.

Lastly, as forecast Bufkit soundings show over an inch of
precipitable water over the region, locally heavy rainfall is
possible with the storms. With the most concentrated storms
expected north of the Missouri River, a Flash flood Watch has been
expanded and now includes the following counties: Phillips,
Valley, Daniels, Sheridan, and Roosevelt. The watch has also been
extended into Friday night, That is because additional storm
development will likely track over similar areas late Friday into
Friday night, with an additional severe weather and heavy rainfall
threat.

Over the weekend, the main cold front will have pushed downstream
of the CWA, but unstable southwest upper flow will remain present,
and so showers and general thunderstorms will still be possible at
times, though coverage will likely be dialed down. Latest 00z
models continue to point to a continuation of an active weather
pattern with unsettled conditions continuing at times through the
upcoming week. That said, the medium and longer terms were trended
toward consensus blends given inherent uncertainties in specific
mesoscale details for that period, and due to the much needed
concentration over the expected short term severe weather and
flooding threats. Maliawco

&&

.AVIATION...
VFR conditions will prevail for today. Low pressure tracking
through the region will lead to an increase in showers and
thunderstorms over the area, especially late this afternoon into
tonight. MVFR to IFR conditions are possible tonight with
lowering ceilings and reduced visibilities from showers and
thunderstorms. Some of the storms could be severe and may produce
locally heavy rainfall. The greatest chances for showers and
storms for KGGW and KOLF will be between 02Z and 06Z, with KGDV
and KSDY more likely to experience storms between 06 and 09Z
tonight. Maliawco

&&

.GLASGOW WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from this evening through late Friday night
for Central and Southeast Phillips...Central and Southern
Valley...Daniels...Eastern Roosevelt...Northern Phillips...
Northern Valley...Sheridan...Southwest Phillips...Western
Roosevelt.

&&

weather.gov/glasgow