Flash Flood Warning 5, NWS Glasgow, MT

Area Forecast Discussion
National Weather Service Glasgow MT
345 PM MDT Fri Jun 2 2023

.DISCUSSION...

AFTERNOON UPDATE:

The focus continues to be on excessive rainfall this afternoon
and tonight. Also, scattered thunderstorms have developed and the
strongest are likely producing small hail. Marginal severe hail is
the primary severe threat, but a funnel cloud or landspout cannot
be ruled out with a similar environment to yesterday.
Precipitable water is around or above climatological maxima across
the area, approaching 1.5" which is about the forecast rainfall
tonight in a corridor from Glasgow to Winnett. Most totals will
likely be a half inch to inch, with localized areas near 2" or
more.

Short-term models like the RAP and HRRR indicate MU CAPE
approaching 2000 J/kg by early this evening in the Glasgow
vicinity, with surface to 3km shear around 40 mph. The limiting
factor is less surface heating reducing lapse rates compared to
yesterday. Kept the current Flood Watch as-is until 6am Saturday
and will handle localized advisories or warnings as needed by
monitoring rainfall reports and radar estimates.

Each of the next several days continues to bring moderate
confidence for some scattered thunderstorms, but there is little
concern for significant rainfall or severe storms at this time.
The environment remains anomalously muggy driving the storms, with
high temperatures heating up to near 90 by midweek and lows
staying well into the 60s for a prolonged period. This also gives
moderate confidence for some minor heat concerns.

-Zanker

MORNING DISCUSSION:

Light showers will continue across our region through the morning,
gradually increasing as we head into the afternoon and evening
hours. Morning accumulations are expected to be less than a tenth
of an inch for most locations with higher amounts expected later
this afternoon as thunderstorms begin to develop.

When evaluating the QPF again this morning, QPF totals decreased
slightly but still remain high enough to justify the Flood Watch
especially given some of the rain rates we received across the
region last night. That being said, there was no reason to expand
the watch past its current boundaries as the main threat will
remain towards the east, particularly in Phillips, Valley,
Petroleum, and Garfield counties. Storm total QPF is expected to
range from half an inch in our eastern zones to as much as an inch
and a half in our western zones with locally higher amounts
possible.

Regarding the severe threat for this afternoon there are a few
things to note. 0-6 km bulk shear looks to max out across the
central portion of our CWA in the range of 30 to 55 kts. MLCAPE
values look to be generally less than 1000 J/kg with sporadic
MLCIN values that range from nothing to -150 J/kg. Some of the
CAMs are picking up on a potential for some severe weather in the
area and overall seem to be more robust with predictions compared
to what was seen this time yesterday morning with regards to
yesterday afternoon. But the disclaimer worth adding is that the
CAMs have struggled as of late to capture the full picture of how
precip has developed and moved across our region.

When painting an overall picture for the chance of severe weather
this afternoon the best consensus is there is certainly a chance
for severe thunderstorms but there is still a decent bit of
uncertainty when trying to pin down the location and extent of the
greatest threat. And with mostly cloudy skies expected to
continue through most of the day, the lack of good diurnal heating
might become a hindrance for storm development in some locations.
That being said, this afternoon will need to be monitored closely
for any changes.

-thor

&&

.AVIATION...

UPDATED: 2100Z

FLIGHT CAT RANGE: VFR - MVFR

DISCUSSION: Numerous showers and thunderstorms have initiated
across the area early this afternoon and will continue into early
Saturday morning, especially around KGGW. They are slow-moving
with the main threat being heavy rainfall, but small hail and
gusty winds are expected with the strongest storms through this
evening. Expect generally brief MVFR visibility reductions from
the strong thunderstorms and heavy rain, then some MVFR ceilings
from low clouds are possible for KGGW Saturday morning.

Some TAF amendments are likely.

WINDS: Prevailing northeast to east at 5-10 knots tonight, then
southeast at 10-15 knots on Saturday. Wind speed and direction may
shift erratically under thunderstorms.

&&

.HYDROLOGY...

The stagnant weather pattern and moisture rich atmosphere will
lend itself to efficient heavy rain and scattered thunderstorms
through early Saturday morning. The heaviest accumulations are
favored west of a Scobey to Terry line, particularly from Glasgow
to Winnett where localized amounts of 1-2" are most likely.
Localized heavy rainfall since yesterday along the Musselshell
River near Mosby has prompted a quick rise to near flood stage
this morning, with a Flood Warning issued for temporary minor
flooding forecast through tomorrow.

Anyone planning outdoor activities should closely monitor the
latest forecasts and any possible flood advisory or warning that
may be issued. Alternate plans will need to be considered in order
to address public safety concerns.

Current headlines as of 3pm Friday:

-Flood Warning for the Musselshell River at Mosby until 6pm
 Saturday

-Flood Advisory for portions of southwest Phillips county until
 4pm Saturday

-Flood Watch until 6am Saturday for Phillips, Valley, Daniels,
 Petroleum, Garfield, McCone, Prairie and western Roosevelt
 counties.

&&

.GLASGOW WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for Central and Southeast
Phillips...Central and Southern Valley...Daniels...Garfield...
McCone...Northern Phillips...Northern Valley...Petroleum...
Prairie...Southwest Phillips...Western Roosevelt.

&&

weather.gov/glasgow

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