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000
FXUS65 KTFX 192035
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
235 PM MDT Sun May 19 2013

.DISCUSSION...
Tonight through Tuesday...A snow moving upper level trough over
the Pacific Northwest continues to slowly progress eastward. A
deep low center over the Dakotas continues to wrap good amounts of
moisture into central Montana. The models are in pretty good
agreement as to the placement of the weather patterns in the short
term. This pattern will continue the showery pattern of the past
couple of days. Some greater instability across north central
Montana has enhanced the probability of thunderstorms in this
area. These thunderstorms will bring locally heavier downpours,
and even some small hail late this afternoon and evening. Showers
will diminish somewhat on Monday as the upper low continues its
eastward slide. However, showers will continue along the Rocky
Mountain Front and southward towards the Little Belts. Amounts
will be lighter on Monday. Overnight Monday and into Tuesday,
drying will take place as a weak upper level ridge builds into
central Montana. Temperatures will be warmer Tuesday with the
drier air. db

Tuesday Night through Sunday...Only slight changes noted in forecast
model solutions from previous runs for this period.  Two elements, a
ridge of high pressure centered in Saskatchewan extending into the
Upper Midwest states and a closed upper level low pressure system
over the PacNW will be the main weather features for our region.
Neither feature moves much until late next weekend, leaving us in a
persistent pattern of mild and moist southerly flow that will
produce partly to mostly cloudy skies with a chance of light rain
almost every day. Timing for the precipitation remains a bit elusive
at this point, as models continue to show the PacNW system making a
big, slow loop from the Pacific coast thru WA/OR then back into
BC/Alb.  Shortwave trofs spinning off the main low center generally
stay along/west of the Continental Divide, but pronounced east-
southeast flow at the low- and mid-levels east of the Divide is
expected to pull moisture around the base of the Canadian high and
across MT, leading to upslope (cloudy) conditions and the
aforementioned periods of light rain. The one notable difference
from the earlier model runs is that both the GFS and ECMWF now push
a cool front thru our region on Friday.  The front will be
accompanied by a negatively-tilted (oriented NW-SE) trof and a
possibly some jet stream effects, which typically leads to better
chances of thunderstorms and higher rainfall amounts, which the
models are indeed suggesting at this point.  Will keep an eye on how
the models handle the evolution of the Friday conditions,
particularly for the precipitation totals and what effect that will
have on our local streams/rivers.  The overall pattern begins to
modify on Sunday with the Canadian high pushing east and the PacNW
low weakening and also tracking east along the US/Can border,
setting up a more typical westerly flow regime. Lastly, expect high
temperatures to remain close to normal (mid 60s) each day this week.
Waranauskas

&&

.AVIATION...
UPDATED 1800Z.
Slight instability and widespread moisture will
keep skies BKN-OVC across southwest and central MT with mix of
VFR/MVFR ceilings today. Scattered showers expected near all TAF
sites through the day with occasional periods of light rain.
Ceilings may briefly drop to IFR category while precipitation is
occurring. There is also a slight chance for a few short-lived
thunderstorms to develop this aftn, but probabilities currently
too low to mention in latest TAFs. Light winds expected today.
Waranauskas

&&

.HYDROLOGY...
First of two slow moving upper level low pressure systems will
bring widespread showers to much of north central and SW MT
through Monday with the second upper level low moving to a position
over the Pacific NW Wednesday and remaining nearly stationary into
next weekend bringing additional precipitation to the region. Cooler
temperatures have reduced snowmelt from higher elevations, however
there are still concerns that rain on the melting snowpack may cause
enhanced rises in stream/river levels in some areas. Most area
rivers are currently well below flood stage but some smaller
tributaries are likely near or approaching bankfull, so rainfall
totals and river stage forecasts will be closely monitored for
significant changes and impacts. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  44  64  42  72 /  70  60  20  10
CTB  43  64  42  71 /  70  50  20  10
HLN  45  65  43  73 /  60  40  20  10
BZN  41  62  38  71 /  70  60  20  10
WEY  35  56  32  61 /  50  40  20  10
DLN  40  65  39  71 /  50  30  10  10
HVR  46  70  43  72 / 100  60  20  10
LWT  43  59  40  66 / 100  70  40  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$


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