Area forecast discussion...updated
National Weather Service Billings Montana
613 am MDT Tuesday may 21 2013
Update...
We have updated the forecast to include patchy dense fog for parts
of south central Montana this morning. Light winds and a moist boundary
layer are contributing to fog as of 12 UTC...especially around the
Livingston area where visibility has fallen to less than a quarter
mile. The fog should burn off by middle to late morning once the low-
levels warm with sunshine. Schultz
&&
Short term...valid for today and Wednesday...
The weekend rain-maker will continue exiting stage right...leaving
easterly low-level flow in play the next several days out ahead of
a 500-hpa low which moves into the Pacific northwest. This pattern
will yield seasonable temperatures...and some chance of storms for
south central Montana...especially later on Wednesday and Wednesday night.
Today...early-morning radar trends suggest that likely probability of precipitation remain
necessary around Baker and Ekalaka into the morning to account for
residual bands of showers rotating westward into that area. The 00
UTC guidance is closely clustered in its quantitative precipitation forecast output...and it shows
a sharp western gradient to the showers dictated by a wedge of dry
air over central Montana. Surface observations as of 09 UTC suggest the
low- and middle-level drying is beginning to influence the Miles City
area...and most 00 UTC guidance keeps showers east of there today.
Even recent rap runs and the 06 UTC NAM keep showers east of Miles
City so we went with a dry forecast there this morning. Probability of precipitation over
the Baker and Ekalaka areas drop back to the chance category after
18 UTC as well since middle-level forcing and moisture advection will
be weakening by that time. Otherwise...the easterly low-level flow
will become established over much of the area by afternoon...which
could help assist with some cumulus development given the residual
near-surface moisture after weekend rainfall. There is just enough
instability forecast by the 00 UTC guidance over the mountains for
US to include slight chances of convection after 18 UTC there too.
Highs today were largely based on a consensus of 00 UTC MOS as the
easterly flow will tend to limit mixing...and using bias-corrected
flavors of guidance seems a bit dangerous given new soil moisture.
Tonight...quiet conditions should prevail...and in fact based on a
blend of 00 UTC MOS output...lows will probably be below normal in
many spots. We have a dry forecast in play after 06 UTC except for
extreme southeastern Montana. We will need to be mindfull for potential
fog development...but it seems that near-surface flow might end up
staying just strong enough to prevent its formation...so it is not
in the forecast at this point.
Wednesday into Wednesday night...a better chance of convection will exist over
south central Montana as middle-level lapse rates steepen to around 7 c/km
on strengthening southerly flow aloft. Most 00 UTC model solutions
generate convection toward 00 UTC...so our going chance-style probability of precipitation
are certainly warranted. The only question mark might be timing or
eastward extent of the weak short wave that guidance Keys in on to
yield convection...especially since the meridional flow might take
it further west than expected. There is probably a non-zero threat
of severe weather if storms develop...but it will be kept in check
by a lack of buoyancy /MUCAPE 500 j/kg or less/. Over southeastern
Montana...storm chances are minimal owing to middle-level ridging. Schultz
Long term...valid for Thursday...Fri...Sat...sun...Mon...
Medium range models were pretty consistent through the period
with some timing differences of shortwaves prognosticated to get kicked
out of a deep upper trough over the west moving inland.
Thursday could be an active day for thunderstorms...mainly
central and western zones. The deep western trough advances east
slowly and lowers heights through the day. Low level east to
southeast winds will allow for dewpoints to hang around 50 degrees
over central zones...with slightly lower values over the west.
Effective bulk shear increases to around 35kts with convective available potential energy reaching
as high as 1000 j/kg. Strong surface convergence with shortwave
energy will ignite thunderstorms over western zones in the
afternoon. Storms will move north to northeast and kick out of the
forecast fairly quickly in the evening. Shear would be strong
enough to produce some strong storms with the main threat being
hail. Will have to watch this event for a possible severe weather
day. Will mention strong storms in the severe weather potential statement.
Friday is a day a bit in question as models differ on timing and
strength of a shortwave kicking off the main low to the west. The
European model (ecmwf) was strongest and slowest with this wave as it crosses the
forecast area during the day. The GFS was weaker and takes it
across late Thursday night. The European model (ecmwf) would be wetter. Inherited
scattered probability of precipitation for Friday...but if the GFS was right...it will be
drier.
Drier middle level air will rush in behind the shortwave regardless
of the timing. This will set up a mainly dry period for the
weekend with the main energy staying north. High based showers or
thunderstorms can not be ruled out over the weekend though as
models kick disturbances across in southwest flow. Dewpoints
should be much lower and these showers will not contain a great
deal of precipitation and be more wind producers. Will keep probability of precipitation
low. Temperatures should slightly above seasonal averages through
period. Twh
&&
Aviation...
Showers will linger over the east this morning and taper off
through the afternoon. The precipitation should stay mainly east
of kmls and impact kbhk. Flying conditions are expected to remain
VFR with only isolated MVFR conditions developing in heavier
showers. The rest of the area will have a partly cloudy sky with
only isolated showers and thunderstorms nearest the mountains. Twh
&&
Preliminary point temp/pops...
Today Wednesday Thursday Friday Sat sun Monday
-------------------------------------------------------
bil 067 043/068 047/074 050/075 048/077 051/077 051/073
1/b 12/T 33/T 43/T 22/T 22/T 23/T
lvm 069 039/067 044/065 041/068 040/075 041/076 042/070
1/f 14/T 45/T 43/T 22/T 22/T 23/T
hdn 069 043/070 048/077 050/078 048/079 051/080 051/076
1/b 12/T 33/T 33/T 22/T 22/T 23/T
mls 059 044/067 047/073 053/075 051/078 055/078 054/076
1/east 11/b 11/north 23/T 22/T 23/T 33/T
4bq 056 040/065 046/074 052/076 050/079 053/080 052/077
3/west 11/b 11/north 13/T 22/T 23/T 33/T
bhk 050 040/061 043/067 050/070 050/075 053/073 052/073
7/west 21/north 11/north 13/T 22/T 33/T 33/T
shr 061 037/068 045/075 049/075 046/078 048/079 048/075
1/b 12/T 32/T 23/T 22/T 22/T 23/T
&&
Byz watches/warnings/advisories...
Montana...none.
Wyoming...none.
&&
$$
Weather.Gov/Billings
