High Wind in Montana, May 11, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
542 PM MDT Thu May 11 2017

...Update to Aviation...

.SYNOPSIS...

Most of the region will remain dry and very warm into Friday, as
high pressure starts moving out of the area. However, a few
showers and thunderstorms will continue to develop in the
mountains west of Interstate 15 later this afternoon into this
evening. This system and its associated cold front will then bring
more widespread showers and thunderstorms for Friday afternoon
into Friday evening. Some storms may produce strong wind gusts,
hail, and heavy rainfall. Cooler and showery conditions this
weekend will likely continue through much of next week as well.

&&

.DISCUSSION...

Rest of this afternoon through Friday night...Scattered showers
with a few thunderstorms have already started developing early
this afternoon over the mountains of southwest Montana. With
increasing southwesterly flow aloft, expect these showers/storms
to move off of the mountains. The main impact from these storms
will be strong gusty winds from evaporative cooling, as plenty of
dry air exists below the shower/storm cloud bases. These showers
and storms will develop northward with time as a weak warm front,
the leading edge of increasing 700-mb southerly flow) moves north
through the central and northern parts of state through the
evening hours. Showers and perhaps a storm will remain primarily
along and west of Interstate 15 over the plains this evening, but
even during the late evening into the overnight a stray shower
can't be ruled out over the remainder of the plains.

The primary concern for Friday will be the potential for severe
thunderstorms. Strong dynamics will be in play as a strong upper
level trough moves toward the area from the west along with an
active jet stream. This will push a cold front across the
Continental Divide and along the Interstate 15 corridor. Showers
and thunderstorms will develop by mid-afternoon, with these storms
moving quickly north-northeast. Given strong unidirectional flow
aloft and lots of dry air in the lower levels, the main threat
will be bowing line segments that will be capable of producing
damaging winds. Given the very cold temperatures aloft, marginally
severe hail will also be a possibility. The severe threat will
begin near the Continental Divide around midday and expand east
through the day as the cold front progresses east. Again, the main
impacts will be the potential for damaging winds, severe hail up
to quarter-sized, and locally heavy rains. The threat will subside
through Friday evening and into the overnight over our eastern
counties. Cassell

Saturday through Thursday...Cool and unsettled weather looks to
continue through Mother's Day Weekend...however...shower coverage
may not be as widespread as initially forecasted. Frontal boundary
that moves through on Friday stalls out across southern MT for
Saturday. This will bring chances for widespread showers and a few
thunderstorms on Saturday along and south of a line from Helena to
Great Falls to Lewistown. North of this line will be cool with some
breezy winds at times...but overall see mainly dry conditions.
Mother's Day sees cool conditions continuing...with only isolated to
widely scattered activity now expected. The other impact to note is
the lowering of snow levels this weekend down to 5-6000 feet...which
may see some snow in southern portions. Accumulations this time of
year should be light and mainly confined to the mountains. This
persistently cool and showery pattern continues next week as another
deep upper level low digs south across Oregon and into the Great
Basin. Monday looks to see scattered activity...with widespread
showers expected for Tuesday and Wednesday...then becoming more
scattered on Thursday. Each model run does show potential for a
bullseye of heavier precip...and the exact locations are still being
determined. This system will need to be watched, for if it tracks
further north than presently forecast a significant rainfall event
becomes possible with a deep moist southeasterly upslope flow. The
potential for more flooding will also have to be watched with any
areas that see rain over mountain snow or rain to already elevated
river and stream levels. As mentioned temperatures will be near to
slightly below normal. High temperatures will likely range in the
50s and 60s with 20s and 30s generally expected for lows. Anglin/PN

&&

.AVIATION...
Updated 2342Z.

Moist southwest flow aloft is bringing scattered/numerous showers
and scattered thunderstorms to the forecast area this evening. VFR
conditions prevail through the forecast period but precipitation
could bring local MVFR conditions at times. Thunderstorm activity
producing locally heavy rain, small hail and strong but variable
wind gusts should diminish by 04Z. Stronger system moves into North
Central Montana after 18Z Friday with numerous showers and
thunderstorms. Some storms may become severe with large hail and
strong outflow winds. mpj

&&

.HYDROLOGY...
Updated 240 PM MDT Thu May 11 2017.

Indications are that rivers and creeks across much of North
Central and Southwest Montana near mountains have continued
rising levels due to the recent warm up. Some river gauge
forecasts show that they will only reach Action Stage at worst.
However, the Big Hole River near Melrose is forecast to exceed
Flood Stage late Friday night; therefore, a Flood Watch for that
point is in effect. Significant rainfall may accompany some of the
storms expected Friday afternoon/evening, and at least light
rainfall is expected to continue through the weekend, which may
cause further rises across the area. Coulston/Cassell

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  50  78  41  58 /  20  70  70  20
CTB  47  70  37  57 /  40  90  80  10
HLN  53  77  42  57 /  30  80  70  40
BZN  47  77  41  54 /  10  20  60  50
WEY  34  67  32  49 /   0  10  40  40
DLN  47  71  38  51 /  30  60  70  70
HVR  50  85  44  64 /  20  40  50  10
LWT  49  81  41  56 /  10  10  40  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch in effect through Monday evening for the Big Hole
River near Melrose.

&&

weather.gov/greatfalls