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Area forecast discussion...updated 
National Weather Service Billings Montana
914 am MDT Sat may 30 2015

Update...

Previous forecast remains in good shape but I did make a few
minor adjustments with this update. Satellite currently showing a
disturbance moving across Idaho into western Montana triggering a
few showers and isolated thunderstorms. Quite a bit of clear skies
are scattered across our forecast area so adjustments were made to
word for a partly cloudy day across our east with increasing
clouds elsewhere. Expect showers and thundestorms to move into our
far western areas but late morning/early afternoon and spread east
during the afternoon reaching the Billings area by middle afternoon.
Hrrr model showing numerous showers and thundestorms north and
west of Billings during the afternoon and as a result...I
increased probability of precipitation to likely from Roundup to Harlowton and areas south
to Livingston. Any thunderstorm today could be strong with an
isolated severe storm possible as shear is better than has been
the case the past week. Precipital waters are around an inch so
heavy rain will again be likely with any thunderstorm.
However...overall dynamics and lack of a strong triggering
mechanism...at this time...looks to prevent any widespread severe
activity. Hooley

&&

Short term...valid for today and sun...

The weekend weather will be headlined by much warmer temperatures
and a chance of thunderstorms...including a marginal severe storm
risk.

Today...warm air advection has been ongoing aloft overnight and it
will be put to use today as the air mass will support highs in the
70s f in most areas. However...mid- and high-level cloud cover and
afternoon convective development may hinder the full potential of
the air mass...so forecast highs /which were based on the 00 UTC
multi-model consensus/ are a bit cooler than thought at this time
yesterday. The 00 UTC model suite remained in good agreement with
its advertisement of a shortwave trough passage this afternoon and
evening. There was a subtle trend in most guidance for the wave to
arrive a bit sooner than earlier expected...and overnight moisture
channel images support that notion. We have scattered showers and
thunderstorms in the forecast after 18 UTC...mainly across south
central Montana. Bulk wind shear is still forecast to be between 40 and
45 knots...but MLCAPE is also still expected to largely be under 500
j/kg. The lack of more robust buoyancy will likely be detrimental
to more than an isolated and marginal severe thunderstorm risk. We
nonetheless are a bit more confident in that isolated severe storm
risk than at this time yesterday because 1/ sref-based calibrated
severe storm probabilities have crept upward...and 2/ the 00 UTC
Storm Prediction Center sseo has a 10 to 20 percent probability of updraft helicities
greater than 25 m2/s2. Potential hazards from any isolated severe
storms may be hail up to the size of quarters and wind gusts up to
60 miles per hour...which is marginal...and again most storms will be weaker
than that given the relatively low instability.

Tonight...scattered showers and thunderstorms will progress east
with the shortwave trough...ending in all but southeastern Montana by
about 06 UTC. A marginal risk of an isolated severe thunderstorm
will continue into the early evening until the boundary layer is
able to stabilize and convection becomes more elevated in nature.

Sunday...temperatures will take another step upward with highs in
some areas likely to reach 80 f. Weak middle-level ridging will be
in place across the region with broad 500-mb height rises and so
the large-scale environment will be less favorable than it will
be today for storm development. However...most 00 UTC guidance
/including convection-allowing models like the NCEP-run WRF-arw
and to a lesser extent WRF-nmm core/ simulates some convection
Sunday afternoon and evening...so we have retained some modest
probability of precipitation in the forecast. Convective initiation will likely be tied to
orographic circulations and storms might have a difficult time
propagating too far downstream across the plains. Still...there is
once again a marginal risk of severe storms Sunday afternoon and
Sunday evening. Schultz

Long term...valid for Monday...Tue...Wed...Thu...Fri...

Latest models were in good agreement with the overall pattern
until Thursday. Still looking at an unsettled week with chances of
precipitation each period. The naefs had precipitable waters on
the order of /0.75/ to 1 inch for Monday through Tuesday...then had
values around /0.75/ inches for the remainder of the period.
So...any showers and thunderstorms will continue to have the
potential to produce heavy rain.

At this point...Monday into Monday evening looked like the best time
period for possible severe weather. The sref and WRF showed
surface convective available potential energy of at least 1000 j/kg and decent shear. Zonal
flow/weak upper ridging gives way to SW flow ahead of the next
Pacific system. Shortwaves will move through the flow and the
left-front quadrant of an upper jet will provide some lift through
divergence. Models showed quantitative precipitation forecast increasing from the west Monday afternoon
and evening. A 700 mb thermal ridge with values of +8 degrees c
will allow high temperatures to reach well into the 80s ahead of
the system/S cold front. Had increasing probability of precipitation in the afternoon and
evening with lower scattered probability of precipitation overnight. Strong gusty winds
and large hail will be the threats with any severe storms.

SW flow with energy moving through the flow will continue over the
area Tuesday and Wednesday. A cooler airmass will move into the region with
highs in the 70s on Tuesday and in the upper 60s to middle 70s on Wednesday.
Went with scattered probability of precipitation each afternoon and evening. GFS showed
much lower convective available potential energy for these two days...but did have some decent
shear. Will need to watch model trends...but for now...Tuesday and Wednesday
did not show a severe threat.

On Thursday...the GFS had a broad Pacific low to the northwest of the forecast
area...while the ecwmf had a trough digging through California...with some
ridging over the forecast area ahead of the trough. Had scattered
probability of precipitation with a slight chance of thunderstorms. Temperatures will be
in the lower 70s. Similar model scenarios continued through Sat so
have continued the scattered probability of precipitation. Arthur

&&

Aviation...

VFR will prevail over the area this morning. Localized MVFR
ceilings are possible. Showers and thunderstorms will overspread
the area from west to east this afternoon and evening. MVFR/IFR
conditions will accompany the precipitation...with lower
conditions possible. VFR conditions will return to the area after
06z with lingering MVFR/IFR possible from kmls east and southeast. Areas of
mountain obscuration this afternoon and evening will decrease
overnight. Arthur
&&

Preliminary point temp/pops...

Today sun Monday Tuesday Wednesday Thursday Friday
-------------------------------------------------------
bil 075 052/080 058/083 054/078 051/072 050/072 051/070
4/T 43/T 23/T 53/T 32/T 33/T 33/T
lvm 072 050/078 055/079 050/071 046/067 043/070 046/068
6/T 54/T 34/T 55/T 43/T 33/T 44/T
hdn 078 051/081 056/087 054/080 052/075 051/075 051/073
4/T 33/T 23/T 52/T 22/T 33/T 33/T
mls 075 050/078 057/087 058/079 053/074 053/073 053/074
2/T 41/b 22/T 52/T 32/T 43/T 34/T
4bq 074 050/078 055/086 057/079 054/073 053/074 053/074
1/b 32/T 22/T 42/T 33/T 43/T 44/T
bhk 066 044/073 052/084 055/078 051/073 051/072 050/072
1/north 41/b 12/T 52/T 32/T 43/T 34/T
shr 075 049/077 053/082 052/076 049/070 048/071 048/071
3/T 33/T 22/T 33/T 33/T 33/T 34/T

&&

Byz watches/warnings/advisories...
Montana...none.
Wyoming...none.

&&

$$
Weather.Gov/Billings

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