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FXUS65 KTFX 060248
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
848 PM MDT Fri Jun 5 2020

.UPDATE...

Will continue to see shower and thunderstorm activity across the
region tonight as south to southwest flow aloft with embedded
disturbances transports moisture into the region along with
remnant showers from convective activity upstream over ID and NV.
With lower levels stabilizing and moistening over SW MT, threat
for strong wind gusts will continue to decrease through the rest
of this evening. Elevated instability remains fairly high across
northern portions of the forecast area tonight, but threat for
stronger storms will diminish overall as an initial weak
disturbance lifts out by midnight. Further west however, a
disturbance lifting across NW MT could bring some late night
stronger thunderstorms to areas near Glacier county. Made some
adjustments to pops/wx to account for this and observed trends,
but overall forecast remains on track. Hoenisch

&&

.SYNOPSIS...

Isolated to scattered showers and thunderstorms are expected this
afternoon and through the overnight hours across the Northern
Rockies, with a few thunderstorms possibly becoming severe. After
a brief break Saturday morning, widespread showers and
thunderstorms are expected to move from southwest to northeast
across the region. Some thunderstorms could become severe once
again Saturday afternoon and evening. In addition, periods of
moderate to heavy rain is possible Saturday through Sunday
morning. Accumulating mountain snow is then expected Sunday night
through Monday morning as another disturbance moves across the
Northern Rockies.

&&

.AVIATION...
Updated 555 PM MDT Fri Jun 5 2020 (06/00Z TAF Period)

Widely scattered showers and thunderstorms affecting SW MT early
this evening will decrease in coverage late this evening but
additional showers and thunderstorms will move northeast across
western MT overnight. Strong and erratic wind gusts are possible
with thunderstorm activity in SW MT this evening with potential for
these winds to reach areas far from the original cell. Brief Mtn
obscuration and VFR ceilings are possible with showers/thunderstorms
through this evening. Showers and thunderstorms will redevelop
Saturday afternoon and become fairly widespread by late Saturday
afternoon and evening. Some storms on Saturday could be severe with
large hail and damaging winds, particularly across eastern portions
of north-central MT. Rain and MVFR conditions become increasingly
likely late Saturday as precipitation becomes more widespread.
Hoenisch

NOTE: Amendments not scheduled for Ennis (KEKS) due to ongoing
runway closure.

&&

.HYDROLOGY...

Shower and thunderstorm activity is expected to begin increasing
this afternoon, but the potential for heavy down pours will
likely hold off until Saturday. Then a widespread moderate to
heavy rain event is expected to take place Saturday through Sunday
morning. Additional precipitation is anticipated for Sunday and
Monday but colder temperatures should slow higher elevation
snowmelt.

River levels remain elevated and additional rainfall/snowmelt from
Saturday through Monday will bring water levels upward. Conditions
will need to be closely monitored for possible impacts.

&&

.PREV DISCUSSION...
/ISSUED 555 PM MDT Fri Jun 5 2020/

Rest of the afternoon through Saturday night...primary concern
throughout the period will be the threat for thunderstorms, some of
which could become severe and produce large hail and damaging winds,
in addition to heavy rainfall (especially Saturday and Saturday
night).

H500 height rises will continue to lift from southwest to northeast
across the Northern Rockies through the afternoon as ridging
advances east out over the Northern High Plains. Strong warm air
advection (WAA), especially in the mid-levels, will overspread the
region from this afternoon and into the mid-evening hours as the
upper level ridge moves overhead. At the same time, a low level jet
characterized by southeast winds at H850 of 30-50kts will develop by
early this evening and advect moisture into the region, especially
across Central and North Central Montana where surface dewpoint
temperatures will climb into the 40s to low 50s. All of this
(synoptic scale ascent due to the WAA and increasing
instability/moisture) combined with diurnal heating and orographic
effects will help to promote isolated to scattered shower and
thunderstorm development and coverage across portions of Southwest
through North Central Montana from this afternoon and into the early
morning hours on Saturday. Some thunderstorms over this timeframe
could become severe and produce large hail up to quarter size and
damaging winds gusts, but coverage of these severe storms is
expected to isolated in nature at best.

Height falls will begin to overspread the Northern Rockies by
Saturday morning as a digging trough over the Pacific Northwest
interacts with a H500 closed/cutoff low over Southern California. As
the Pacific Northwest trough digs into the Great Basin Saturday
afternoon it will absorb the closed/cutoff low over Southern
California, and then lift northeast to over Eastern Montana by
Saturday night. Abundant Pacific moisture will be advected northeast
in the mid- to upper levels and across the Northern Rockies through
Saturday and Saturday night. While a brief break in precipitation is
possible during the mid-morning hours on Saturday, showers and
thunderstorms are quickly expected to expand in areal coverage by the
early afternoon as they move east northeast from Idaho/Western
Montana and across the CWA. As storms move into Central and North
Central Montana Saturday afternoon/evening and interact with
increasing instability, expect some storms to possibly become
severe. Any storm that becomes severe on Saturday will be capable of
producing large hail up to golf ball size and damaging winds up to
70 mph. In addition to the threat for severe weather on Saturday,
thunderstorms will be capable of producing heavy rainfall which
could lead to flooding, especially in urban areas. A transition to
more of a stratiform rain event is then expected during the evening
and overnight hours on Saturday, with additional chances for
moderate to heavy rain across the region, especially across Central
and North Central Montana. - Moldan

Sunday and Monday...A much colder system quickly moves into the
region by Sunday into Monday, which will bring another round of
rain along with mountain snow to Southwest through North Central
Montana. The colder air associated with this system will cause
snow levels to fall to as low as 5500 feet by Sunday
afternoon/evening across Southwest and Central Montana, along
with over the Continental Divide. While accumulating snow is not
uncommon for early June in the Northern Rockies the fact that
outdoor recreation is increasing in the mountains and that some
mountain passes will see slushy/snow covered conditions will most
likely require some form of winter highlight from Sunday night
through Monday morning. Snow accumulations of 4 to 8 inches are
generally expected above 5500ft over this timeframe, with isolated
amounts possibly approaching a foot. Strong westerly flow on the
backside of the system will keep it quite windy over the plains
for Monday afternoon and evening and High Wind Highlights may be
considered for Rocky Mountain Front and parts of the adjacent
plains. Expect well below normal temperatures by Monday, with a
freeze possible across the southwest Monday night. - Moldan/RCG

Tuesday through Friday...Drier conditions along with a day to day
warming trend is expected for this period as weak ridging settles
into the region. RCG

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF 55 75 46 64 / 30 100 100 40
CTB 52 70 46 60 / 40 90 90 40
HLN 56 75 45 61 / 40 90 90 70
BZN 54 81 43 61 / 30 80 90 80
WYS 49 69 37 47 / 30 90 90 80
DLN 54 75 40 53 / 50 90 90 90
HVR 59 82 51 69 / 30 80 100 30
LWT 55 78 46 64 / 30 90 90 50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

http://www.weather.gov/greatfalls
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