Flood and Heavy Rain in Montana, June 29 to 30, 2020

Area Forecast Discussion
National Weather Service Great Falls MT
523 AM MDT Mon Jun 29 2020

Updated Aviation Discussion

.SYNOPSIS...

A spring-like storm system will continue to impact the region with
wet and unseasonably cool conditions through Tuesday. Locally heavy
rain across western portions of central and north-central Montana
may lead to flooding in some areas with rises likely on many rivers
and creeks likely through the middle of this week. Unseasonably cool
conditions will persist through Tuesday with snow possible at times
across the higher mountains of southwest Montana.

&&

.DISCUSSION...

Today through Tuesday...

An unseasonably deep upper trough is in place across the northern
Rockies with mid level closed circulation center over southern Idaho
this morning. Deep moisture continuing to circulate across the
region will maintain widespread coverage of rain/showers across most
of the forecast area today. Central MT remains under the most
favorable upper level forcing including diffluent flow and upper
level jet position and this along with a deep/moist easterly flow
will continue to focus heaviest rainfall over central MT through
early this afternoon with a gradual shift northward later today into
tonight as the mid level circulation center slowly moves north from
Idaho. Focus for continuous precipitation shifts north to the
northern Rocky Mtn Front tonight as the mid level circulation
elongates and lifts north with some drier air working north into SW
MT this evening and at least eastern portions of central MT
overnight. Coldest air aloft near the upper low has forced snow
levels down to near 7000 feet at times overnight across SW MT and
this may occur through tonight with some wet snow possible at Chief
Joseph and Big Hole Passes at times, but any accumulation would most
likely be limited to the peaks above 8000 ft. Overall forecast
precipitation amounts have changed little with discussion of
potential hydrologic impacts included below. Hoenisch

Wednesday through the Independence Day Weekend...

The ongoing upper level trough over the region persist through
Wednesday, with some drier air wrapping into the region.
Widespread rainfall will transition to more showery-like
conditions Wednesday morning, with increased chances for
thunderstorms to develop Wednesday afternoon and evening. By
Thursday and into next weekend, the large scale pattern continues
to support continued upper level troughing west of the divide,
with ridging expanding back over the area from the east. Gradual
warming is likely as troughing shifts back to the west of the area
with temperatures likely returning to seasonal norms by the
weekend. Still a lot of details to be worked out however with
regard to the impact of shortwave features moving through the
trough centered to our west, which would maintain at least some
chance for convective precipitation throughout the period. -TP

&&

.AVIATION...
Updated 523 AM MDT Mon Jun 29 2020 (29/12Z TAF Period)

MVFR to IFR conditions remain persist through much of today across
the region with widespread rain and Mtn obscuration. Steady rain
over central MT this morning into early this afternoon with periods
of moderate to heavy rainfall and lower visibility at times.
Precipitation coverage and intensity decreases from south to north
this evening with some improvement to ceilings, though lower clouds
and occasional showers linger into Tuesday morning. Hoenisch

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

&&

.HYDROLOGY...

Much of central MT has already seen .5" to 1" of precipitation
with locally higher amounts in central/northern Lewis and clark
county. An additional 1" to 3" of rainfall are likely today
through tonight for areas roughly along and west of I-15 from
central Lewis and clark county northward. Expect some flooding
impacts to develop today in the area highlighted by the Flood
Watch along smaller creeks and streams and adjacent roadways as
well as in lower lying areas where water collects. The Dearborn
River is likely to see the sharpest increase today and will be
closely monitored with latest forecasts projecting a rise to flood
stage near Craig by this evening. The Sun, Teton and Marias
Rivers will likely also rise later tonight through Tuesday, though
not as sharply as the Dearborn.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  56  48  62  48 / 100  70  70  50
CTB  57  49  60  47 / 100  90  90  60
HLN  56  46  62  46 / 100  80  90  60
BZN  60  41  63  43 /  80  50  80  70
WYS  53  32  54  35 /  80  50  90  90
DLN  55  40  59  42 /  70  60  90  60
HVR  64  52  69  51 /  90  80  60  60
LWT  56  45  63  45 / 100  60  70  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Wednesday afternoon Cascade...Central and
Southern Lewis and Clark...Eastern Glacier...Eastern Pondera...
Eastern Teton...Northern Rocky Mountain Front...Southern Rocky
Mountain Front...Toole.

&&

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