Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 270554

Area Forecast Discussion
National Weather Service Great Falls MT
1150 PM MDT THU MAY 26 2016

Aviation Section Updated.

.UPDATE...Latest radar imagery is showing a band of showers with
embedded isolated thunderstorms that will move through the Great
Falls area in the next hour or so. Latest model runs suggest that
this band will weaken significantly over the next few hours with
only isolated light rain showers expected after midnight. Have
updated forecast PoPs/weather/QPF and the hourly diurnal
temperature curve to incorporate observed and expected evolution
of the airmass through the overnight hours. Latest model runs
continue to indicate that another round of showers and
thunderstorms can be anticipated Friday afternoon/evening but a
drier and warmer holiday weekend is expected through Sunday. mpj


Several upper level disturbances in an unstable west-northwest flow
aloft will move over Montana this evening through late Friday
afternoon. Scattered showers and thunderstorms may cause short-
duration MVFR conditions due to heavy rain and small hail reducing
visibility. Convection over the region has settled down for the
night, but is expected to re-develop Friday afternoon. VFR conditions
will prevail outside of any storms moving across the forecast area.


This Afternoon through Saturday Evening...Latest analysis shows
the upper- level low pressure system that wandered around north
central and eastern MT last two days has moved downstream into N.
Dakota. Almost all of the moisture/rain showers directly
associated with the low are now east of our area, with just a few
showers skirting eastern Blaine/far eastern Fergus counties. Weak
high pressure ridging currently over Idaho and western MT will
work its way east of the Divide this evening bringing us mostly
dry conditions overnight. However, before that happens, residual
moisture from the exiting low and a second area of moisture
crossing through the ridge will combine with marginal instability
and a bit of aftn heating/mixing to generate another round of
scattered showers and thunderstorms along and west/south of a line
from Cut Bank to Lewistown/Judith Gap. With CAPE (potential
energy) values of only 300-600 J/kg and very weak wind shear in
the lower atmosphere, thunderstorms that form today will be of the
short-lived, pulse variety, producing some locally moderate
rainfall and pea- to dime- sized hail. All the convective activity
should steadily dissipate this evening.

Friday begins dry but our next weather system is already moving into
the PacNW with its leading edge of moisture along the ID/WA border.
The system`s low pressure center is forecast to track along the
US/Can border tomorrow and Sat and will push a cold front through
our region tomorrow aftn, sparking more widespread showers and
thunderstorms.  Forecast models are indicating better low-level wind
shear (30-40 kts) at that time but CAPE values remain marginal for
strong to severe storms.  As the front heads into eastern MT Fri
eve, drier conditions move in for Sat under westerly flow aloft.
Some spotty areas of moisture may produce a few showers along the
Rocky Mtn Front and over the southwest mtn ranges Sat aftn.  With
increasing clouds from late morning onward, high temps on Fri will
only reach the upper 50s to low 60s.  More sunshine on Sat will
result in slightly warmer temps, mainly in the low-mid 60s.

Saturday night through Thursday...From Saturday night through early
Monday morning, an upper level trof will swing through the northern
portions of the region and then gradually move east of the region.
Thus there will be a chance for showers early in the period, but
some dry conditions are possible late Saturday night and into Sunday
morning, before isolated showers/thunderstorms redevelop on Sunday
afternoon. For Monday through Wednesday, significant differences
have developed in the models from the GFS/EC. The GFS progs
moderate/heavy precipitation to move through Central MT Monday
afternoon thru Tue, while the EC is significantly less. For now, I
have blended the solutions, with more confidence given to the EC
solution. The precipitation gradually exits the region on Wednesday,
with weak upper level ridging trying to build over the region by
next Thursday. Thus precipitation chances on Thur are confined
mainly to the western mountains. Temperatures will generally be
below normal through the period, with Tuesday being the coolest day,
but temperatures will start to warm to near normal by next Thursday
as the upper level ridge starts to rebuild.


GTF  45  62  41  62 /  40  60  50  10
CTB  42  62  38  59 /  30  20  40  10
HLN  47  62  41  63 /  30  50  60  20
BZN  42  63  39  62 /  20  60  60  20
WEY  34  55  33  53 /  30  40  30  20
DLN  41  59  37  60 /  30  60  60  30
HVR  45  68  45  64 /  10  50  60  20
LWT  42  63  42  61 /  20  60  70  30



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