Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 101953
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
153 PM MDT Mon May 10 2021

.SYNOPSIS...

Scattered to numerous showers, even a few thunderstorms, will
continue south of a Helena to Kings Hill Pass line into the
evening hours tonight before dissipating. High temperatures will
remain below normal through Tuesday before climbing back above
normal by Wednesday. Another round of afternoon showers and even a
few thunderstorms is possible on Tuesday as a weak disturbance
crosses the Northern Rockies.

&&

.DISCUSSION...

Rest of Today through Tuesday...scattered to numerous showers
(mountain snow and valley rain/snow mix), even a few isolated
thunderstorms, will continue over Southwest through portions of
Central Montana through the evening hours tonight as an H500 closed
low spins over Northeastern Idaho/Southwest Montana. Loss of surface
heating this evening is then expected to lead to a reduction in the
areal coverage and intensity of showers (and most certainly
thunderstorms due to increasing CIN and weakening lapse rates), with
only a few isolated mountain showers remaining possible over
primarily the Madison and Gallatin Ranges through the overnight
hours tonight. Clearing skies tonight and light surface winds may
also allow for patchy fog development in areas that have seen recent
precipitation over the past 24-48hrs; however, surface wind
directions will become increasingly southwesterly (downsloping) over
the plains of Central and North Central Montana near sunrise on
Tuesday which could inhibit (fog) development here. Tuesday will see
the upper level pattern become increasingly northwesterly as the
H500 closed low over Northeastern Idaho/Southwest Montana opens up
and digs towards the Central Plains, all the a weak embedded wave
dives southeast from Southwestern Canada and across the Northern
Rockies. Timing of this subtle shortwave on Tuesday across the
region appears to align well with peak heating, which should allow
for another day of scattered to numerous showers, especially along
the Continental Divide and across Central and North Central Montana.
These showers and thunderstorms are then expected to decrease in
coverage through Tuesday evening with the loss of surface heating. -
Moldan

Wednesday through Thursday...Wednesday, an upper level closed
area of lower heights and the associated upper level trof begins
to evacuate mass from the aforementioned Pacific Ocean Ridge
creating a more zonal pattern. The weak ridge moves eastward
across The Treasure State. Mostly dry conditions are anticipated;
however, some ripples in the flow aloft, may allow for a few
showers with a slight chance of isolated thunderstorms, especially
through parts of Meagher, Judith Basin and Fergus Counties.
Perturbations in the upper level flow continue to pass through the
pattern favoring afternoon and evening showers and thunderstorm
on Thursday. - Fogleman

Friday through Monday...Friday 18Z is when numerical models
begin to become divergent in their solutions. As of this time, it
looks like the previously mentioned North Pacific Trof enters the
U.S. west coast on Friday. The associated moisture spills over the
weak ridge and across Montana. An upper level closed low develops
over central to northern California, and widespread precipitation
begins across Montana Friday afternoon. This precipitation is
expected to continue through Sunday. There is a chance of
thunderstorm activity Friday and Saturday afternoon and evening.
It looks like, Monday, the next trof quickly moves into the area,
and more precipitation is expected. for this extended weekend time
period, there is high confidence in the approach of The North
Pacific Trof on Friday. There is moderate to high confidence on
another wet weekend. As of this time, confidence is low in terms
of timing, temperatures, precipitation timing, precipitation type
and chance and location of thunderstorms. We will continue to
monitor, and update as the week progresses. - Fogleman

&&

.AVIATION...
1145 AM MDT Mon May 10 2021 (10/18Z TAF period)

Mainly VFR to MVFR conditions are observed across SW and North-
Central Montana early this afternoon, though there are some low
clouds in spots. These low clouds will gradually break up through
the afternoon, though a few rain showers, or even a thunderstorm,
develop, mainly across SW MT. Winds should remain on the light side
through the day. Expect any lingering clouds to gradually dissipate
later this evening. Ludwig

Refer to weather.gov/zlc for more detailed regional aviation weather
and hazard information.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  34  61  35  65 /  10  30  10  10
CTB  32  60  33  63 /   0  50  10  10
HLN  34  63  37  69 /  10  20  10  10
BZN  28  60  32  66 /  20  30  10  10
WYS  22  53  21  58 /  70  40  10  10
DLN  28  59  31  65 /  20  20   0   0
HVR  33  65  38  66 /   0  20  10  10
LWT  31  58  33  61 /  20  30  30  20

&&

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

$$

http://www.weather.gov/greatfalls


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