Hail in Montana, May 4, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1145 AM MDT Sun May 4 2014

...UPDATE TO AVIATION...

.UPDATE...

Made further updates as of 1020 am.
The band of precipitation mentioned earlier is moving to the east
across north central and central Montana. Have therefore decreased
shower coverage for the remainder of the morning along the Rocky
Mountain Front and adjacent plains, mainly west of a Helena-Great
Falls-Sweetgrass Hills line. Have kept widespread precipitation in
for the area east of this line, while scattered showers persist
across southwest Montana. Have kept mention of numerous showers and
isolated thunderstorms in for this afternoon across most of the
area. A secondary wave of showers is currently west of the
Continental Divide and am anticipating it to move east of the Divide
during the afternoon. With clouds already forming some breaks along
the Rocky Mountain Front, am anticipating that enough instability
should develop to cause isolated thunderstorms. However, have
decreased high temperatures 2 to 4 degrees across much of north
central and central Montana due to significant cloud cover.
Coulston

Updated discussion at 819 am.
Quick update to the forecast this
morning to increase the coverage of precipitation across most of
north central and central Montana. Widespread precipitation will
persist across this area at least through this morning, likely due
to the continuing low level easterly upslope flow. The airmass is
forecast to become weakly unstable this afternoon, causing the
precipitation to become more showery, but will monitor this and
adjust the forecast further as necessary.
Coulston

&&

.AVIATION...
UPDATED 1745Z.
A moist and unsettled southwesterly flow aloft will continue over
Montana through mid-day Monday as an upper level trough moves inland
along the west coast. Band of rain/showers that moved through
central Montana this morning brought rapidly changing conditions
ranging from LIFR to VFR in short periods of time. This system is
now moving east allowing breaks in the cloud shield to develop.
Afternoon heating will cause the airmass over the entire region to
become unstable with several rounds of showers and isolated
thunderstorms likely moving through the forecast area between 18Z
and 03Z. Once again, very fast categorical changes from VFR to
MVFR/IFR or even LIFR are likely through this entire period with
timing remaining an extremely low confidence proposition. To address
this, I have added a tempo group during the climatologically favored
times for spring convection to the terminals that are most
likely to be affected. Conditions will stabilize overnight but mid
and high level cloudiness will persist through noon Monday. Areas of
fog are also possible overnight but confidence is too low to add to
terminal forecast at this time. mpj

&&

.PREV DISCUSSION...
/ISSUED 530 AM MDT Sun May 4 2014/

Today through Sunday Night...Sfc boundary over the SW near the
Bozeman area will continue to weaken early this morning as the
pressure gradient decreases over the area. An upper level trough
will deepen along the NW Pacific Coast region into tomorrow. Ahead
of the mid-level trough axis, a 300mb jet streak and vorticity max
will cross N. Montana and S. Alberta, inducing weak lee-side
low development. This will end the easterly upslope flow over
central/north-central sections...so expect westerly sfc winds to
develop midday today which will mix-out some of the low-level
moisture. Highs should be a solid 10 to 15 degrees warmer for
north-central areas compared to Saturday.

Warmer sfc temps will lead to higher daytime instability and the
threat for sct showers and isolated thunderstorms. MUCAPE
values peak around 300 J/kg per NAM/GFS. Do not anticipate strong
convection but gusty winds and small hail are possible. SPC Day 1
outlook highlights general thunderstorm potential over most of the
CWA. 00z NAM tracks vort. max out of Washington early this morning
and into the western portion of my CWA by the late morning. Mid to
upper Q-vector convergence, upper divergence, and mid level DCVA are
maximized over the area between 16z and 22z. Shower and
thunderstorms should be fairly numerous during this period...QPF
amounts of 0.10 to 0.30 inches will be common.

Monday and Tuesday...Conditions should be relatively quiet on Monday
as area is sandwiched between upper disturbance over Saskatchewan
and larger trough axis over Washington and Oregon. Isolated SHRA and
thunderstorms will be possible in mainly mountainous areas
during the afternoon. Otherwise, temps will be seasonal in the low
60s.

Approaching trough axis will have more influence on Tuesday.
Lee-side cyclogenesis occurs over west-central Plains, sfc pressure
gradient organizes over Montana with a steady northerly flow
developing. NAM/GFS/ECMWF in agreement with steady precip somewhere
over the central and SW...exact placement still not certain. SREF
has snow as likely p-type above 5500 feet Tuesday morning before
snow levels rise by the afternoon. Little impact expected on roads
due to mild ground temps. Uttech

Tuesday Night through Sunday...Forecast models maintaining good
continuity and agreement over past several runs on progression of
the broad upper-level trof that will bring cool, moist and unsettled
conditions across MT through the first half of the week.  Confidence
is increasing that this weather system will take on a split-flow
configuration, with the northern half of the trof extending from a
closed low pressure area in central Alberta/Saskatchewan down to the
MT/ID border, and the southern half extending from the Great Basin
all the way to the US/Mexico border.  As noted yesterday, a split
flow regime can lead to reduced precipitation amounts over
central/southwest MT as some of the moisture is diverted to the
south and east, which is depicted in the latest model runs.  The
high precip areas that were initially forecast to move through the
Bozeman-to-Lewistown area on Tues/Wed are now predicted to be
further east toward Billings.  This will reduce, but not eliminate,
the "rain on melting snow" threat in the aforementioned area, as
some locations may still see 0.25 to 0.40 inch of precip, leading to
some localized stream rises during the latter half of the week.
Remaining parts of the forecast area will see about half as much
precipitation through early Thurs morning when the system exits into
ern MT and the Dakotas. Thursday still looking mostly dry as weak
ridging follows the departing system. Conditions transition to a
more progressive, west-northwest flow pattern for Fri through Sun.
A series of shortwave trofs embedded in the flow will move across
the northern Rockies, bringing periods of showers Fri evening and
again on Sun.  Temperatures remaining at or slightly below normal
through the period.
Waranauskas

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  59  38  62  37 / 100  30  30  30
CTB  54  32  58  31 /  70  30  30  30
HLN  62  40  63  39 / 100  40  30  40
BZN  64  36  64  37 /  60  40  40  50
WEY  58  34  57  34 /  60  30  30  50
DLN  64  39  62  39 /  50  30  30  40
HVR  58  35  64  36 / 100  40  20  30
LWT  56  35  60  36 /  80  40  40  50

&&

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

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
227 PM MDT SUN MAY 04 2014

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0222 PM     HAIL             8 WSW FAIRFIELD         47.58N 112.15W
05/04/2014  M0.25 INCH       TETON              MT   TRAINED SPOTTER

            PEA SIZED HAIL COVERING THE GROUND TO ONE INCH IN
            DEPTH. THUNDER AND LIGHTNING. LITTLE RAIN.

&&

NJL

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
242 PM MDT SUN MAY 04 2014

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0241 PM     HAIL             3 ESE FAIRFIELD         47.59N 111.93W
05/04/2014  M0.25 INCH       TETON              MT   TRAINED SPOTTER

            PEA SIZED HAIL ABOUT ONE HALF INCH DEEP ON PAVEMENT.

&&

NJL