994
FXUS65 KGGW 050901
AFDGGW

Area Forecast Discussion
National Weather Service Glasgow MT
301 AM MDT Sun May 5 2024

.DISCUSSION...
KEY MESSAGES:

- Prolonged rainfall event in store for northeast Montana from
  Monday through Wednesday night. The WPC has issued a Slight
  Risk for Excessive Rainfall for the 24 hr period ending on 6am
  Tuesday.

- The general probability of exceeding 1 inch, 2 inches, and 3
  inches of total rainfall during this aforementioned period
  ranges from 70-90%, 40-60%, and 10-30%, respectively. Locations
  in the Little Rockies have a 40-60% chance of exceeding 3
  inches.

- Strong wind gusts in excess of 65 mph are expected for locations
  south of the Missouri River Breaks and west of Jordan (70-90%
  chance) on Tuesday, leading to extreme crosswind concerns for
  roadways in this area.

WEATHER PATTERN OVERVIEW:

Synoptic Setup: Forecast begins with a closed low moving into the
Intermountain West with a ridge formed up ahead of it. The ridge
runs from New Mexico into Wyoming and eastern Montana.

Today: The ridge looks like it is going to tap into Desert
Southwest air which will warm the region fast today. Meanwhile,
the closed low from the Intermountain West will stretch into a
very amplified negatively tilted trough in response to the ridge
bending flow south to southeasterly aloft.

Sunday night through Monday: A mainly southern flow interface
will setup between the extremely warm temps of the exiting ridge
and the incoming trough over the Intermountain West. This will
start pulling gulf moisture into the Great Plains potentially all
the way up toward eastern Montana. The intense cool down will
generate a closed low over south eastern Montana with enough lift
energy to not only generate rain showers but also thunderstorms.
Around Monday morning the low will begin to deform and stretch
back northwest pulling more moisture in ahead of it.

Monday night through Thursday night: The deformed Low will
migrate northward and start to cut off/occlude from the warm air.
However, this just means that most of the convective elements
will become mainly stratiform. Main problem here onward is that
positioning of the surface low is still in question with a range
for its center stretching from western North Dakota , to northeast
Montana, to southwestern Saskatchewan.

Pressure gradient forces southwest of the low look to become
strong enough to reach 60 mph pretty easily(50-70% chance). This
has led to a High Wind event Monday night/Tuesday morning.
NBM signaling on this feature has increased in strength from last
night and now convective HREF CAMs have joined in for the party.
However, in the end this will mainly come down to the positioning
of the low and strength of the PGF behind it. Extreme crosswinds
may affect high-profile vehicles south of the Missouri River
Breaks.

The low still looks to generate a trowal in most models starting
Monday night through Wednesday which will bring a heavy amount of
rain to areas directly under or behind it. The majority of the
ensembles place this somewhere between north central to
northeastern Montana. Probabilities of 1, 2, and 3 inches have
risen up a bit more to (70-90%), (40-60%), and (10-30%),
respectively with this passage. WPC currently has a slight risk
(15% chance) for rainfall exceeding flash flood guidance from
Monday 6AM to Tuesday 6AM. Given this, impacts for motorists
would be in areas where small streams and creeks approach roadways
and areas where construction zones are located.

Friday onward: A new ridge looks to move into the area which will
raise temps and dry conditions out through the weekend. GAH

&&

.AVIATION...

LAST UPDATED: 0830Z

FLIGHT CAT RANGE: VFR

DISCUSSION: Low level wind shear will be impactful early this
morning until surface wind ramps up around mid-morning or 12-14Z.
Otherwise expect SCT to BKN mid level clouds and dry conditions
into the afternoon. In the evening showers and thunderstorms will
spread across the area lasting into the night. Ceilings will begin
to deteriorate late Monday morning.

WIND: East to southeasterly and increasing through the morning to
15-25 kt gusting to 35 kts by this afternoon lasting into the
evening. After midnight speeds will begin to decrease until wind
switches to the west mid-morning Monday.

GAH



&&

.GGW WATCHES/WARNINGS/ADVISORIES...
Lake Wind Advisory until 6 PM MDT Monday for Central and
Southeast Phillips-Central and Southern Valley-Garfield-McCone-
Petroleum.

High Wind Watch from Monday evening through Tuesday evening for
Central and Southeast Phillips-Central and Southern Valley-
Dawson-Garfield-McCone-Northern Phillips-Petroleum-Prairie-
Southwest Phillips-Wibaux.

&&

$$

weather.gov/glasgow