Severe Thunderstorm Warning 29, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1155 AM MDT Tue Jul 7 2020

Aviation Section Updated

.SYNOPSIS...
Strong to severe thunderstorms will be possible today, especially
this afternoon, as a strong disturbance combines with unusually
high humidity to produce scattered to numerous thunderstorm
activity. Damaging winds and hail along with frequent lightning
and an isolated tornado or two will be possibilities with these
storms, especially for eastern parts of north-central Montana.
Conditions improve this evening as the system moves east of the
area. Cooler and breezier conditions can be expected for Wednesday
before a warm- up heading into this weekend.

&&

.UPDATE...
Current forecast is on track. No updates planned for this morning.

Main concern is severe weather potential this afternoon. Overall,
the latest forecast is on track from previous forecasts. Storms
are approaching the divide at 11 AM, and should start to move
east of the divide after 12 PM. Some storms could become severe
towards 1 PM, mainly east of a line from Shelby to Great
Falls...then as the storms approach the Havre/Stanford area around
4 PM, the potential for severe storms with hail larger than 2
inches and 70 mph winds will be possible. There is the potential
for a tornado, with the most favorable conditions for that to
occur will be from 1 PM to 4 PM. Overall, most of the severe
storms should exit Blaine/Fergus counties around 7 PM.

The rest of the forecast is on track. Brusda

&&

.AVIATION...
Updated 1155 AM MDT Tue Jul 7 2020 (18Z TAF Period)

A cold front will move from west to east across the region this
afternoon. Thunderstorms, some with hail/strong winds will likely
accompany the front. An isolated tornado is possible across Central
MT. The storms exit our region generally by 01z Wednesday. Generally
VFR conditions expected overnight and into the day Wednesday. Brusda

For more detailed aviation weather and hazard information, please
see: weather.gov/zlc

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

&&

.PREV DISCUSSION...
/ISSUED 445 AM MDT Tue Jul 7 2020/

Lead weak disturbance is currently moving across the Idaho
panhandle into west-central Montana this morning, and will bring
the initial round of moisture, clouds, and some
shower/thunderstorm activity as it moves northeast across the
northwest half of the forecast area this morning. This will be
followed by a stronger and increasingly negative-tilted shortwave
disturbance which will reach western parts of the area around
midday and cross central and north-central Montana region during
the afternoon hours and early evening hours. This negative tilting
of the upper level trough will help to strengthen a low-level jet
that will advect unusually high amounts low-level moisture across
the High Plains of north-central and central Montana, with
surface dewpoints rising into the low 50s to low 60s by early to mid-
afternoon. The most significant limiting factor for strong to
severe storm development seems be a possible early arrival of
shortwave forcing and its attendant surface cold front, which
would be slightly out of phase with peak surface heating and could
produce more mid- to upper level cloudiness. However, the vast
majority of modeling, including high-resolution CAMS, continue to
trend toward this not being as much of a concern as it appeared
to be yesterday. Nevertheless, outside of this potential for
limited boundary layer heating on Tuesday, all the other severe
weather parameters will be checked; with moderate to strong
instability (1500 to 3500 J/kg), sufficient 0-6 km bulk shear
(30-60 kts), and mid-level lapse rates between 8 and 9C/km being
present. This will set up a scenario, especially if boundary layer
heating can be maximized, for all severe weather modes to be
observed over portions of southwest, central, and especially
eastern parts of north-central Montana. Very large hail in excess
of 2" in diameter and damaging winds approaching 80 mph or
stronger will be possible for much of north-central, central, and
parts of southwest Montana, along with the potential for an
isolated tornado or two, with this threat more limited mainly to
eastern parts of north-central Montana. LCL's will be on the
higher side, generally above 1-km but surface to 3-km shear could
be as high as 30-kts, which has prompted the SPC to paint our
eastern counties with a 5% chance for tornadoes. Current guidance
suggest that a round of showers and thunderstorms may move across
the northwestern portion of the CWA during the early and mid-
morning hours this morning. Then, discrete multi-cell and
supercell thunderstorms may develop somewhere along the higher
terrain of west-central Montana or along a strengthening cold
front near and along the Interstate 15 corridor in the early
afternoon before moving east and northeast and taking on an
increasingly organized and linear formation. Most of the activity
should move into northeast Montana and southern Canada by 7 PM,
giving way to partial clearing and breezy conditions.

Wednesday through next Monday...

Breezy and cool conditions are expected on Wednesday over the plains
with high temperatures generally 5-10 degrees below normal for all
areas in wake of Tuesday's cold front. Temperatures climb back to
near normal on Thursday as transient ridging traverses the area.
Primarily dry conditions are expected from Wednesday through
Thursday although an isolated shower or thunderstorm can't be ruled
out. Precipitation chances and westerly breezes increase slightly on
Friday as a shortwave trough traverses the region. Temperatures will
continue to warm through the weekend as ridging attempts to become
established over the area. Very dry and breezy conditions will be
possible at times Saturday through Monday which may pose fire
weather concerns.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  76  51  72  46 /  70  20  10   0
CTB  71  52  67  46 /  70  10  30   0
HLN  76  48  73  46 /  60  10   0   0
BZN  83  45  76  44 /  80  20  10   0
WYS  74  38  70  38 /  50  20  10   0
DLN  77  44  74  43 /  40  10   0   0
HVR  84  52  72  50 /  80  60  40  10
LWT  80  47  70  46 /  70  20  10   0

&&

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

&&

http://www.weather.gov/greatfalls

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