High Wind in Montana, May 21, 2020

Area Forecast Discussion
National Weather Service Great Falls MT
1130 AM MDT Thu May 21 2020

Updated Aviation Section.

.UPDATE...
Very minor changes made to the forecast this morning, mainly with
regards to slightly lowering temperatures today and tonight over
parts of southwest and northcentral Montana. Overall the forecast
remains on track with showery conditions persisting through the
end of the week, windy conditions today especially along the
Northern Rocky Mountian Front, and potential flooding due to rain
and snowmelt.

&&

.SYNOPSIS...

Widespread precipitation this morning will become more scattered
in nature with the approach of the afternoon hours. Westerly
winds will begin to increase across Central and North Central
Montana by the late morning hours, becoming strong during the
afternoon and then decreasing during the early evening hours
tonight. Snow levels will begin to fall to around 6500 feet by
this evening as another storm moves across the Northern Rockies,
bringing accumulating snow to portions of Southwest Montana and
along the Continental Divide.

&&

.AVIATION...
Updated 1130 AM MDT Thu May 21 2020 (21/18Z TAF Period)

Mostly VFR and windy conditons will prevail through much of the TAF
period, with some terminals experience MVFR to IFR conditons under
low clouds and showers. Wind gusts in excess of 35kts are expected
at the KCTB, KHVR, and KGTF terminals today, with even some 50+ kt
gusts possible at KCTB. With colder air moving into Southwest
Montana tonight, rain is expected to mix with or change over to snow
at the KEKS and KWYS terminals. Mountains will continue to be
obscured for much of the TAF period. Finally, mountain wave
turbulence is likely along and immediately downwind of the Rocky
Mountain Front. -TP

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

&&

.HYDROLOGY...

Creeks, streams, and rivers emanating from the mountains are now
showing rises in water levels due to 72hrs worth of rainfall and
snowmelt now making its way into the water system. Of these
creeks, streams, and rivers, the Dearborn River is the closest to
reaching flood stage. With the forecast for the Dearborn
currently expected to level off near/at minor flood stage we have
decided to issue a Flood Advisory to cover the minor expected
impacts; however, we will continue to monitor water levels here
and act accordingly should water levels rise further. Other areas
that are beginning to approach or are in action stage are Tenmile
Creek near Helena, Big Hole River near Melrose, Belt Creek near
Monarch, Badger Creek below Four Horns Canal near Browning, and
Saint Mary River at the International Border. - Moldan

&&

.PREV DISCUSSION...
/ISSUED 440 AM MDT Thu May 21 2020/

Today through Friday night...primary concerns throughout the period
continue to be ongoing precipitation, strong winds (late this
morning through early this evening), and accumulating snow (tonight
and Friday night).

Broad and deep upper level troughing pattern remains anchored over
the western half of the CONUS this morning, with two main H500
shortwaves/closed lows within the overall pattern. The first
shortwave/closed has taken on a negative tilt as it lifts
north/northeast across the Northern Rockies this morning, while the
second was digging southeast along the Pacific Northwest Coast. The
shortwave/closed low over the Northern Rockies this morning will
continue to slowly lift north and over Alberta through Friday
night, while the Pacific Northwest shortwave/closed low digs
southeast towards the Central Great Basin and over the Central
Rockies by Saturday morning. At the surface, the area of low
pressure responsible for yesterday's severe weather over Eastern
Montana has moved north in Southern Saskatchewan (as of 09z
Thursday), but will retrograde west and become vertically stacked
beneath the H500 closed low by this afternoon in Southern/Central
Alberta. This pattern will continue to maintain areas of
precipitation across the region throughout the day; however,
precipitation will become more scattered in nature by the late
morning/early afternoon hours, especially across Central and North
Central Montana. As the H500 and surface low become vertically
stacked this afternoon, the pressure gradient along the Rocky
Mountain Front will begin to rapidly strengthen. This rapidly
strengthening pressure gradient combined with increasing H700 flow
of 30-60kts will lead to very windy conditions along the Rocky
Mountain Front and points east towards the Interstate 15 corridor
across North Central Montana. With sustained winds of 30-40 mph and
gusts of 65 mph expected this afternoon, have decided to issue a
High Wind Warning for the Northern/Southern Rocky Mountain Front,
Eastern Glacier, Toole, and Eastern Pondera counties until 8pm.
Areas outside of the High Wind Warning will also experience strong
westerly winds today, but these speeds should remain below High Wind
criteria. These strong winds will quickly decrease by the early
evening hours with the loss of vertical mixing and solar insolation.

As the H500 shortwave/closed low over the Pacific Northwest digs
southeast into the Central Great Basin tonight, expect a band(s) of
precipitation to develop along/ahead of an eastward surging mid- to
upper front across Southwest Montana. Given the strength of the
frontogenetic forcing expected with this front and very cold air
aloft, expect precipitation to change over to snow for elevations
above 6500ft this evening and continue into the morning on Friday.
With generally 2-5" of snow expected over this timeframe for
elevations above 6500ft (which includes West Yellowstone, Raynolds
Pass, and Targhee Pass), felt it was prudent to issue a Winter
Weather Advisory, especially given the convective nature of the snow
tonight which could lead to locally higher amounts of 6+". While
snow will likely continue through much of the day on Friday, a high
May sun angle should limit accumulations to grassy surfaces during
the day. For this reason, the Winter Weather Advisory was issued for
03z-18z Friday; however, another advisory may be needed for Friday
night and into Saturday morning as the impacts from solar heating
will be negated and lead to accumulations on road surfaces once
again. - Moldan

Saturday through next Thursday...One more wet day is expected
across Southwest, Central, and North Central Montana on Saturday,
before transient ridging moves over the Northern Rockies through
Monday and brings some drying conditions to most lower elevations
and moderating temperatures. A closer look at Memorial Day
specifically reveals high temperatures near to slightly above
normal across the CWA, with light southwest to westerly winds and
isolated/scattered mountain showers and thunderstorms. A fast H500
shortwave moving along the International Border Monday night
through Tuesday will flatten the ridge and leave in its wake zonal
flow through the middle of the work week. This fast moving
shortwave and the following zonal flow will bring additional
chances for showers and storms further east over the plains and
valleys through Thursday, along with over the mountains. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  59  38  58  36 /  60  20  30  10
CTB  53  34  54  35 / 100  10  20  10
HLN  62  37  57  36 /  40  20  50  20
BZN  62  36  57  36 /  50  40  70  60
WYS  50  31  46  28 /  90 100  80  80
DLN  58  34  54  32 /  30  30  60  50
HVR  62  39  63  39 /  70  20  30  10
LWT  60  39  58  37 /  40  40  80  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 8 PM MDT this evening Eastern Glacier...
Eastern Pondera...Northern Rocky Mountain Front...Southern Rocky
Mountain Front...Toole.

Winter Weather Advisory from 9 PM this evening to noon MDT
Friday above 6500 feet for Gallatin...Madison.

&&

http://www.weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
1038 AM MDT THU MAY 21 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1035 AM     HEAVY RAIN       7 SSE ULM               47.33N 111.48W
05/21/2020  E0.93 INCH       CASCADE            MT   TRAINED SPOTTER

            WIND GUST WSW 39 MPH.

&&

AJP

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
1144 AM MDT THU MAY 21 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1141 AM     HEAVY RAIN       AUGUSTA                 47.49N 112.40W
05/20/2020  M1.50 INCH       LEWIS AND CLARK    MT   TRAINED SPOTTER

            0.6 MONDAY 0.6 DURING THE DAY TUESDAY 1.5
            MORE BY WED MORNING.

&&

AJP