Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 261101
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
501 AM MDT Wed Jun 26 2019

UPDATED AVIATION

.SYNOPSIS...

Unstable southwesterly flow aloft and embedded disturbances affect
our region through this weekend. Scattered to numerous showers and
thunderstorms are expected today and Thursday. Some of these storms
will likely become severe. Isolated to scattered showers and
thunderstorms are then forecast for Friday through this weekend.
Temperatures should be near or slightly below-normal.

&&

.DISCUSSION...

Today through Thursday...Southwesterly flow aloft and embedded
shortwave disturbances affect our region as our CWA remains east of
a longwave trough axis. A mid- to upper-level low pressure center
embedded within this longwave trough will drift eastward from just
offshore the Pac NW coast to near the mouth of the Columbia River by
the end of the period. Accordingly, southwesterly flow aloft will
strengthen over our CWA as the longwave trough and embedded low
interact with a nearly-stationary longwave high pressure ridge
over/near the central U.S. In addition, this southwesterly flow
aloft will continue to tap into abundant moisture from the Pacific.
At the surface, a low-level easterly flow is expected, especially in
north-central MT, and this easterly flow will tap into abundant
moisture from the Gulf of Mexico. This pattern will contribute to
the development of scattered to numerous showers and thunderstorms
daily, especially during the afternoon and evening hours, given
daytime heating of higher terrain will be the primary mechanism for
initiation of showers and storms. Unusually-high humidity will allow
showers and thunderstorms to produce downpours, especially in north-
central MT. Localized flash flooding may occur today and Thursday,
particularly where showers and thunderstorms manage to train (i.e.
follow the same path).

Moderate to strong instability and vertical wind shear will likely
allow some thunderstorms to become severe today and tomorrow. Some
of these severe storms will likely be supercells (i.e. rotating
thunderstorms). The greatest potential for severe storms exists in
north-central MT, where easterly low-level winds veering to
southwesterly and strengthening with height will enhance vertical
shear. Hail of quarter to ping pong ball-size and damaging wind
gusts of 60 to 70 mph are the primary severe threats today. On
Thursday, a more unstable atmosphere should allow severe storms to
produce hail as large as quarters to perhaps baseballs and damaging
wind gusts of 70 to 80 mph or greater. A few tornadoes cannot be
ruled-out, especially during the late afternoon and early evening
hours of Thursday. In addition, the portion of north-central MT east
of I-15 is the preferred area for potential tornadoes. Here, cloud
bases may be favorably-low and low-level vertical shear may be
sufficiently-large for a tornadic supercell or two. Temperatures
will be near or slightly below-normal through Thursday.

Friday through Sunday...Southwesterly flow aloft and embedded
shortwave disturbances continue to affect our CWA as we remain east
of the longwave trough axis. The embedded mid- to upper-level low
pressure center may eject northeastward from the Pac NW to northern
MB by the end of the period. Simultaneously, the aforementioned
longwave high pressure ridge persists over the central U.S. and
vicinity. The shortwave disturbances will assist in the development
of isolated to scattered showers and thunderstorms daily. In
addition, moderate instability and strong vertical wind shear may
allow a few severe thunderstorms to affect far-eastern portions of
north-central MT on Friday. Temperatures remain near or slightly
below-normal.

Next Week Monday through Wednesday...Compared to 24-hours ago,
ensemble means are in much better agreement that our CWA will remain
east of the longwave trough axis over/near the Pacific NW and
western Canada, allowing southwesterly flow aloft and embedded
shortwave disturbances to persist over north-central and southwest
MT. Isolated to scattered showers and thunderstorms are possible on
a daily basis, especially during the afternoon and evening.
Temperatures should remain near or slightly-below normal.
- Jaszka

&&

.AVIATION...
Updated 501 AM MDT Wed Jun 26 2019 (12Z/26 TAF issuance)

Mainly VFR for the next 24-hours. Simultaneously, southwesterly flow
aloft and embedded disturbances affect our area. Scattered to
numerous showers and thunderstorms are probable between 18Z/Wed and
06Z/Thurs, while isolated showers and thunderstorms are possible
before and after that period. Downpours and brief MVFR may accompany
any shower or storm. Some thunderstorms will likely become severe
this afternoon and evening, especially in north-central MT. Quarter
to ping pong ball-size hail and erratic surface wind gusts of 50 to
60 knots are the main severe threats. Regional surface winds remain
light outside outflow (i.e. precip-cooled air emanating from any
shower or storm).
- Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  75  51  76  51 /  40  60  80  60
CTB  73  47  70  45 /  40  80  80  70
HLN  76  50  80  52 /  40  50  70  40
BZN  76  47  81  49 /  60  50  30  20
WEY  70  38  73  40 /  20  20  10  10
DLN  72  46  75  47 /  70  30  30  10
HVR  79  51  79  52 /  40  70  70  80
LWT  74  48  77  49 /  70  90  60  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

None.

&&

$$

http://www.weather.gov/greatfalls



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