192
FXUS65 KGGW 131934
AFDGGW

Area Forecast Discussion
National Weather Service Glasgow MT
134 PM MDT Sat Jul 13 2024

.DISCUSSION...

AFTERNOON UPDATE:

A corridor extending from the Glasgow area southeastward toward
Glendive was upgraded to an Enhanced area of severe weather by the
SPC earlier this morning. Given morning CAMS mostly showing
similar solutions to this afternoon`s convection, SPC felt
necessary to upgrade for the risk of large hail (in excess of 2
inches) and severe, damaging winds (possibly as high as 90 mph).
For those present in and around Fort Peck Lake for the Gov Cup
Walleye Tournament, sudden and erratic wind direction/speed
changes are possible generally from a time range of 3pm to 6pm.

Afterward, most of the CAMS show a linear MCS forming from the
merging of previously discrete supercells. This appears to really
look more like a bowing structure as the storms approach the I-94
corridor. Plenty of hazards in play tonight. More info to come as
needed. Remain weather aware!

-Enriquez

PREVIOUS DISCUSSION:

KEY MESSAGES:
1) Thunderstorms are expected this afternoon and evening. Some
will become severe with damaging winds and large hail. Timing will
range around 2PM to 11PM over the bulk of northeast Montana.

2) Hot conditions will continue into the weekend with the heat
advisory extended in the far south into Saturday.

WEATHER PATTERN OVERVIEW:
Upper ridge will remain in place over the western half of the
CONUS for the next several days. A cold front will arrive Sunday
evening and drop temps off about 5-10 degrees through Wednesday.
Heat will be back in sway by late next week with 100s creeping
back in as the ridge to the west reestablishes itself.
FORECAST CONFIDENCE & DEVIATIONS:
The heat advisory currently for the bottom third of counties for
Saturday has been left up due to momentum, but the cold air from
the thunderstorms outflow yesterday has backed temps off for
Saturday to a max of around 97 degrees which is probably too low
for it. In addition, there is a chance that widespread
thunderstorms in the early afternoon may produce enough cloud
coverage to take the impact edge off the highs as well. Will be
giving the day shift a chance to let it ride and the option to
cancel it early if things do not look to be going that way. While
Sunday does have temps reaching up to 100 degrees before the front
arrives that evening, there is cooler temperatures Saturday night
which will allow for some recovery in the heat risk. Therefore,
will be delaying issuing an extension for Sunday for now.

Keeping an eye to smoke models has allowed for some haze to sneak
into the area over the next couple days. Air quality only remains
moderate but will need to be monitored through the next several
days.

CAMs models for thunderstorms this afternoon have spread out the
area being affected toward the south by about a county and half
compared to 24 hours ago. They have also clumped the initiation
of the storms around Noon to an hour or two after. However, there
is an outlier in the HRRR which doesn`t bring storms through till
nearly 6PM. It looks like mesoscale details such as lingering
outflow boundaries from yesterday will be the deciding factor for
this afternoon and this is adding quite a bit of uncertainty to
timing and spatial placement of storms in addition to the strength
of the heat.

First 24 hours of PoPs were again updated to include a 50/50
blend of the SREF NPoP along with the NBM PPI01 in order to
capture the afternoon thunderstorms better with the marginal
threat for severe weather in the area. This bumped PoPs up from a
meager 20%-30% to nearly 60% around the most certain areas.

First 48 hours ended up getting a bump with the shrink stretch
tool over the CWA for afternoon highs and overnight lows, as
RTMA/URMA map passes are continuing to miss, skip, or average out
ASOS/AWOS highs by roughly 2 to 4 degrees and this is clearly
biasing baseline NBM downward compared to the reality of these 100
degree+ extremes which have occured over the last 2 days.

GAH



&&

.AVIATION...
LAST UPDATE: 1930Z

FLIGHT CAT RANGE: VFR - MVFR

DISCUSSION and WINDS: Storms are again expected this afternoon
and evening. Initially, discrete supercells can be expected across
Phillips and Valley Counties. As the storms approach KGGW and
surrounding areas (3pm to 5pm), the main threat will be large hail
in excess of 2 inches and damaging winds. As the storm approaches
KSDY and KGDV, the outflow associated with these storms will
cause erratic and gusty winds possible in excess of 60 kt (5pm to
8pm). Winds after the storms will light and variable (after
midnight local time), becoming more NW`ly Sunday afternoon at 5 to
10kt.

DENSITY ALTITUDE: Afternoon highs could break 100 degrees in some
places through Sunday. This will make for less dense air and lead
to a need for increased power and longer runway takeoffs.
-Enriquez


&&

.GGW WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 9 PM MDT this evening for Garfield-Petroleum-
Prairie.

&&

$$

weather.gov/glasgow