Severe Thunderstorm Warning 7, NWS Glasgow, MT

Area Forecast Discussion
National Weather Service Glasgow MT
146 PM MDT Thu May 17 2018

.DISCUSSION...

Afternoon Update (2pm MDT): Not much is changed from this morning
update besides keeping up with the current trends and utilizing
the hi-res short-term models to keep up with the developing
potential severe weather this evening. Recent model output still
indicates storms firing to the west, around Phillips County,
growing into a large MCS and then trucking to the east-southeast
through Valley County and onwards towards North Dakota. While
an isolated initial strong to severe storm or two may form out
ahead of the boundary in the eastern and southeastern reaches of
the region, the most likely scenario is for the strongest storms
to form in the west and then move through the CWA to the east.
Apart from tonight's event, the forecast was left relatively
untouched.

Bigelbach

PREVIOUS DISCUSSION: The stage is nearly set for at least some
marginally severe thunderstorms moving through NE Montana this
afternoon through tonight.

Earlier disorganized low pressure has now lined up into a low
pressure trough through the northern Rockies region this morning.
A warm and moist airmass becomes favorably-placed for
thunderstorm development over the high plains beginning this
afternoon.

Instability remains high, even during these overnight and early
morning hours with a few isolated thunderstorms persisting over
our eastern zones. Expect CAPE values to re-surge this afternoon,
well in excess of 2000 J/Kg. This model instability co-located
with model QPF output would signal the best chance for severe
storm possibility. Given the expected east and northeast winds up
to 20 kts this afternoon through tonight, chose to issue a Lake
Wind Advisory.

When this storm system first showed up in the models several days
ago, a large bulls-eye appeared along this axis of instability.
Then over the last few days, the model storm seemed to weaken; but
now with the last 2 or 3 model runs, a large and significant
thunderstorm has returned in the GFS model. Hints of it show up
in the NAM but the signal is not as strong there.

This storm seems to form off the Cypress Hills of SW Saskatchewan
this afternoon, then moves and strengthens across our Phillips
and Valley Counties tonight. The model QPF bulls-eye signature for
this storm would suggest that it could easily become severe, with
large hail, damaging winds, frequent lightning, and heavy rain
being the main threats. Next shift will want to re-investigate
this storm in the model data for one more good forecast update
today before storm initiation.

Rain showers will become more widespread across the remainder of NE
Montana through the late night hours tonight and into most of
Friday. The associated cold front will only allow Friday's high
temperatures to top out in the lower 50s for most locations.

Through the remainder of the weekend, the upper level flow regime
becomes benign between systems, with a high pressure ridge taking
shape by Saturday night. BMickelson

&&

.AVIATION...

FLIGHT CAT: Expect mainly VFR ceilings. Some reduction in category
to MVFR possible underneath strong thunderstorms this afternoon
and evening.

SYNOPSIS: An atmosphere favorable for showers and thunderstorms
will dominate the region beginning late this afternoon and lasting
through much of the overnight. Some of these storms late in the
afternoon through midnight could become severe and would impact
operations if they happen to move directly over a TAF site.

WINDS: Expect continued winds from the SE at 10-15 kts, then
increasing from the NE to around 15-20 kts later this evening.
With a frontal zone slowly drifting north, and many thunderstorm
outflows from this morning and those to come this afternoon and
evening interacting with each other, winds should be expected to
be erratic and blustery at times.

Bigelbach

&&

.GLASGOW WATCHES/WARNINGS/ADVISORIES...
Lake Wind Advisory until 10 AM MDT Friday For Fort Peck Lake for
Central and Southeast Phillips...Central and Southern Valley...
Garfield...McCone...Petroleum.

&&

weather.gov/glasgow

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GLASGOW MT
332 PM MDT THU MAY 17 2018

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

0255 PM     HEAVY RAIN       1 SSE ZORTMAN           47.91N 108.52W
05/17/2018  E1.00 INCH       PHILLIPS           MT   TRAINED SPOTTER

            AT 255 PM SPOTTER ESTIMATES 1" OF HEAVY
            RAINFALL IN HALF HOUR. RAIN ONGOING, THOUGH
            LIGHTER AT TIME OF REPORT.

&&

BCB