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000
FXUS65 KTFX 191048
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
448 AM MDT Wed Jun 19 2013

...Strong to possibly severe thunderstorms today into this
evening...

.DISCUSSION...

Today through Friday...Thunderstorms from overnight have exited
the area, but another round of potentially severe weather is on
tap for today. The Pacific low pressure system that brought
thunderstorms to the western portion of the forecast area
yesterday will slowly move into the Pacific Northwest today. The
moist, unstable, and diffluent southerly flow ahead of the system
will combine with a strong embedded disturbance and the associated
cold front over the area for more widespread thunderstorm
development than yesterday. Therefore, the Storm Prediction Center
has all of north central/central Montana under a Slight Risk for
severe thunderstorms today. While some storms over the western
plains of north central/central Montana may see marginally severe
thunderstorms (wind gusts around 60 mph, hail around 1 inch in
diameter), the greater focus will be on the eastern plains.
Surface winds there will be breezy and out of the east ahead of
the cold front, which will advect moist air (dewpoints in the
upper 50s to lower 60s) into the area and enhance the instability.
The surface winds will also combine with the stronger winds aloft
to provide strong wind shear (with regard to both speed and
direction) to that area as well. The enhanced instability and
shear will focus the potential for significantly strong
thunderstorms (hail possibly 2+ inches in diameter, wind gusts in
excess of 70 mph) on the eastern TFX plains. Wind shear may even
be strong enough to possibly generate an isolated tornado.
However, all of these ingredients seem to come together later this
afternoon, which means that thunderstorm development there may not
occur until after 4 or 5 pm MDT, while storms farther west will
develop after 1 pm MDT. Forecast models are also indicating that
these storms may produce heavy rainfall, so have issued a Flash
Flood Watch for the area as well (please see Hydrology section for
details). While storms will generate over the higher terrain of
southwest Montana, the potential for severe storms there is low.
Overall, storms will exit the area to the east with the passage of
the cold front this evening. Westerly downslope winds will help
limit overnight precipitation to the western mountains. A much
cooler airmass will also move into the area, possibly bringing
light measurable snow to the mountain peaks. Winds will likely
approach high wind criteria along the Rocky Mountain Front on
Thursday, but am not anticipating a need for high wind highlights
at this time. The Pacific low pressure system will linger through
the day on Friday, keeping a chance of showers and (likely weak)
afternoon/evening thunderstorms over the area, as temperatures
remain about 10 degrees cooler than normal.  Coulston

Friday night through Wednesday...An upper low over the Pacific
Northwest early Friday evening is forecast to weaken as it moves
over the forecast area on Saturday. Models are fairly wet over
southwest and north central Montana on Saturday so have increased
the chances of precipitation especially over north central
Montana. The remnants of the upper low move out by late Saturday
night and by early Sunday morning it appears the only threat for
precipitation will be over north central Montana and possibly the
mountains of the Rocky Mountain Front. On Sunday an upper ridge
will move through the forecast area and expect a mostly dry
period but then southwest flow aloft returns next week. Models
differ on shortwave details and associated precipitation
patterns. Due to these inconsistencies will go with generally a
chance of showers/thunderstorms over the mountains of the Rocky
Mountain Front with a slight chance of showers/thunderstorms
elsewhere. Cool air associated with the upper low will result in
highs on Saturday of only 55 to 65 except over north central
Montana where highs will be closer to 70. Thereafter expect
gradually warming temperatures. Blank

&&

.AVIATION...
Updated 0450Z.
Widespread showers and thunderstorms will continue to move
through western portions of north central Montana through 10Z.
During this time, areas of MVFR conditions in heavy rain remain
possible along with mountain obscurations. A surface cold front
will push east of the continental divide Wednesday afternoon then
moves into eastern Montana after 03Z. Another round of strong to
severe thunderstorms will develop ahead of the frontal boundary
but clearing skies and calmer weather will move in behind the
front. mpj

&&

.HYDROLOGY...
Showers and thunderstorms are expected to develop once again this
afternoon over the Rocky Mountain Front and the adjacent plains
along a surface cold front. These showers and thunderstorms are
then expected to spread east across the plains of north central
and central Montana later this afternoon and continue into this
evening. Forecast models are predicting significant rainfall with
these showers, which is expected to occur in rapid bursts with the
thunderstorms. Some localized flash flooding occurred with storms
along the Rocky Mountain Front Tuesday evening. However, the
greater area of concern is across eastern portions of north
central and central Montana that still have moist soil that would
not be able to absorb much rapid rainfall. Forecast models also
indicate that this area will receive the greatest amount of
rainfall. Taking these factors into consideration, have decided to
issue a Flash Flood Watch from noon MDT today until midnight MDT
tonight for all of north central and central Montana. The greatest
threat for flash flooding for the Rocky Mountain Front and
adjacent plains will be this afternoon, while the greatest threat
in the east will likely be after 5 pm MDT.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  76  47  67  43 /  90  40  20  20
CTB  74  45  63  41 / 100  70  20  20
HLN  75  46  65  43 /  70  40  30  20
BZN  81  42  68  39 /  40  30  30  20
WEY  69  32  60  31 /  40  30  20  10
DLN  72  39  62  37 /  60  50  30  20
HVR  88  52  72  47 /  60 100  20  20
LWT  83  45  68  42 /  60  80  20  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH from Noon MDT today through this evening
Blaine...Cascade...Chouteau...Eastern Glacier...Eastern
Pondera...Eastern Teton...Fergus...Hill...Judith Basin...
Liberty...Northern Rocky Mountain Front...Southern Rocky
Mountain Front...Toole.

&&

$$

SHORT TERM...COULSTON
LONG TERM...BLANK
AVIATION...mpj

weather.gov/greatfalls