Hail in Montana, June 13, 2019

Area Forecast Discussion
National Weather Service Great Falls MT
1150 AM MDT Thu Jun 13 2019

Aviation Section Updated.

.UPDATE...

A band of midlevel clouds persists across North-central MT this
morning, associated with remnants of convection that had occurred
further west last evening. These clouds will continue to thin
through early afternoon. Over the southwest, regional radar shows
some new convection developing over the MT/ID border. Expect this
activity to continue to expand through the afternoon and evening
as a shortwave trough moves across the Great Basin towards
Wyoming. Convection will be favored first over higher terrain.
Once initiated, surface dewpoint temperatures in the lower 50s
will help sustain showers and thunderstorms that move off the
mountains and across the plains/valleys. Locally heavy rain and
small hail will be possible with some of these thunderstorms.
Minor updates were posted to the forecast to better match observed
trends.

&&

.SYNOPSIS...

Scattered to numerous showers and thunderstorms are expected
today, primarily during the afternoon and evening hours across
the region. A few storms could become strong and produce gusty
winds, small hail, and frequent lightning; in addition to brief
heavy downpours. A cold front will then drop south from Canada
Friday morning and into the afternoon hours, bringing another shot
of showers and storms over this timeframe.

&&

.AVIATION...
Updated 1100 AM MDT Thu Jun 13 2019 (18Z TAFs)

Showers and thunderstorms are developing over southwest Montana.
This activity will continue to spread into portions of Central
Montana through evening. VFR conditions prevail, except where
showers and storms could produce brief near-MVFR conditions.

&&

.PREV DISCUSSION...
/ISSUED 500 AM MDT Thu Jun 13 2019/

Today through Friday night....primary forecast concerns over the
timeframe will be showers and thunderstorms this afternoon/evening,
a few of which could become strong and produce some gusty
winds/small hail and brief heavy downpours.

H500 ridge will continue to be flattened out throughout the day
today, with numerous and subtle mid- to upper level waves rippling
through the nearly zonal flow. As has been the case now for the past
several nights, models discrepancies continue with the exact timing,
placement, and strength of these waves through the early evening
hours tonight. These discrepancies have led to and overall low
confidence forecast with respect to the location/lack thereof of the
highest PoPs across Southwest and North Central Montana. Have
therefore gone with a rather broadbrushed PoPs approach for this
forecast, but have incorporated latest convective allowing Hi-Res
models (i.e. ARW, NMM, and NAMNest) in an attempt to paint local
PoPs maximums. With respect to precipitation timing, while there is
not a consensus in the exact timing of the synoptic forcing arriving
overhead today, diurnal heating will play a huge role in the
shower/thunderstorm development, especially initially over the
higher terrain. With this in mind, believe the best window for
precipitation over the next 24 hours (today/tonight) will be focused
between 18z Thursday through 03z Friday (plus or minus 3 hours on
either end). With the loss of diurnal heating this evening, expect
precipitation coverage and intensity to diminish around 06z Friday.
While 0-6km bulk shear will once again be less than ideal (15-25kts)
for the maintenance/(development) of (severe) thunderstorms, a
marginally unstable atmosphere could support a few stronger
thunderstorms this afternoon and evening, especially over/in close
proximity to the mountains. High temperatures for today will also be
tricky, as increasing mid-level cloudiness could inhibit us from
reaching the highs that are currently forecasted. None-the-less,
expect highs to still exceed climatological norms for the day today.

By this evening, a more defined and agreed upon (model consensus)
shortwave will begin to dive southeast from Central British Columbia
and towards the Northern Rockies. This shortwave and accompany
synoptic ascent is then expected to move over North Central and
Southwest Montana during the morning through afternoon hours on
Friday, with the best forcing being focused across North Central
Montana over this timeframe. At the surface, an area of low pressure
over South Central Alberta Thursday evening will move southeast into
the Dakota's by Friday morning, dragging a cold front across the
region Friday morning (North Central Montana) through Friday
afternoon/evening (Southwest Montana). This will allow for another
round of showers and thunderstorms across most of Southwest and
North Central Montana, with the best chances for seeing precipitation
coming over North Central Montana with and following passage of the
cold front. The cold front will also bring a period of gusty
northwest/north winds, in addition to slightly cooler high
temperatures for the day on Friday. - Moldan

Saturday through next Thursday...

An upper level ridge temporarily rebuilds along the Pacific NW coast
and shifts inland this weekend, maintaining a northwest flow aloft
with weak embedded disturbances tracking SE across the Northern
Rockies and MT. Showers and a few thunderstorms are possible each
afternoon and evening, especially in/near the mountains but with
little support for organized storm development, main threat will be
isolated locally heavy rain with slow storm movement or storms
anchored to higher terrain. Going into next week, models and
ensembles continue to support the idea of strong ridge amplification
well off the west coast and a deepening trough downstream across the
Rockies/N-central US. While a turn to cooler and wetter conditions
appears likely by the middle of next week, confidence remains low
with respect to specific details of timing and impacts. One feature
that is well fairly well agreed upon is the depiction of an
unseasonably strong (120+ kt) upper level jet to move inland with
the the developing trough and a fairly packed height field which
combined with afternoon mixing could result in windy conditions and
lead to some organized thunderstorm activity by the middle of next
week. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  83  56  76  50 /  30  30  40  20
CTB  83  53  69  47 /  10  20  60  30
HLN  84  55  80  51 /  30  30  20  10
BZN  81  50  77  46 /  50  30  20  10
WEY  70  40  66  35 /  80  50  50  30
DLN  76  48  75  45 /  50  40  10  10
HVR  87  56  75  50 /  20  30  60  30
LWT  79  53  73  47 /  40  20  40  40

&&

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

http://www.weather.gov/greatfalls