Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
351 am CDT Wednesday Sep 3 2014
Short term...(today and tonight)
issued at 351 am CDT Wednesday Sep 3 2014
Severe weather is expected to develop later this afternoon along a
warm front that is forecast to lift northward through the County Warning Area.
MUCAPE and deep shear profiles support storms capable of producing
large hail...with low level hodographs and LCLs favorable for
tornadoes if convection manages to become surface based along the
warm front. This convection will eventually grow upscale into an mesoscale convective system
after sunset and slide east southeast into central Wisconsin by
Thursday morning along the nose of the low level jet...which will
mark the transition from a severe weather to a heavy rain/flooding
The main driver behind the severe threat today is two-fold.
First...a potent shortwave which as of this morning was located over
Washington state on the poleward side of a 100kt upper level jet
will track eastward and cross The Rockies into Montana by midday.
Low level theta_e advection will overspread the upper Midwest
ahead of this wave...and boundary layer dewpoints near 70 will
surge into southern Minnesota. However...the warm air advection will act
to cap off the convective potential until a weak upper level wave
arrives later this afternoon. This wave is the second ingredient
needed for severe storms...and although subtle...should be enough
to develop at least elevated convection on the periphery of the
cap. Based on the bufr soundings off both the NAM and GFS...this
will be mainly along and north of I-94. Here...the 800mb warm layer
is more subdued with ample deep layer shear. At kstc for example...
0-3km hodographs show 300-400 m2/s2 of storm relative helicity by
22/23z. The 03.00 runs of the hires wrfarw...wfrnmm...and emcnmm
support a more northern solution yet...with the highest threat for
severe weather north of a line from Fergus Falls...to Lake Mille
lacs...and Hayward WI. At this time...am slightly leaning towards
this northern solution because feel that despite the increasing
boundary layer moisture to the south...the weak shortwave will not
be strong enough to overcome the sbcin stemming from the combination
of filtered sunshine with the strengthening cap. Therefore storms
will remain elevated...with perhaps a hail threat...drift northeast
of the forecast area with the mean wind...and not realize the low
level wind fields. If storms start to move right...be on the
lookout for tornadogenesis as this is evidence that the storm has
indeed tapped into the helical flow of the boundary layer.
In summary...the strengthening cap should keep convection at Bay
across the south today...while a weak shortwave moving up from the
southwest will trigger convection farther north. Confidence is
wavering regarding the exact location of the severe weather...and
this mornings elevated convection may limit the northward
propagation of the warm front today...but for now feel that the
northern half of the Storm Prediction Center day1 severe outlook has a higher risk of
severe weather than the southern half.
Long term...(thursday through tuesday)
issued at 351 am CDT Wednesday Sep 3 2014
Thunderstorm activity at the beginning of the period will be
confined to portions of WI with the remnants of the mesoscale convective system pushing
through tonight...and across west central Minnesota where another weak
cluster may be pushing in from overnight development over the
Dakotas. That will probably be about it for convection Thursday until
late afternoon. A very warm thermal ridge will build north into
eastern Minnesota and western WI ahead of the cold front and will lead to
boundary layer capping on the order of 200-500 j/kg for much of
the day. Given the positively tilted nature of the system...
forcing with the front and meager height falls will very likely
not be enough to generate much along the front. Reduced probability of precipitation
rather markedly into the schc category. Did not remove them
completely due to hi res models still developing weak elevated
echos or altocumulus castellanus. This may just be the result of the very unstable
airmass models generate.
The high temperature grid was the biggest challenge Thursday. The
front will be coming through during the late morning/early
afternoon with good mixing and the aforementioned impressive
thermal ridge just ahead of it. Models are in pretty good
agreement with timing and the challenge stems more from how deep
the mixing will get before the front pushes through. It is usually
easy to mix at least to 925 mb with some sun...in which case upper
80s would be achieved by early afternoon. Could see some 90s in WI
where the longest potential for heating will reside. Obviously any
change in timing the front or considerable cloudiness would impact
these highs a bit. It is somewhat interesting to note that all
global models...including the GFS cold bias...bring 2m temperatures into
the upper 80s at 18z.
Much cooler air will follow the front for the rest of the week.
Dry conditions are forecast through Sunday night.
Next chance for thunderstorms will accompany a strong cyclone
traversing southern Canada early next week. Kept probability of precipitation in the chance
category...although models are in pretty good agreement for this
range. If this trend continues...the next couple shifts may
consider allowing likely probability of precipitation.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1045 PM CDT Tuesday Sep 2 2014
Little change in the overnight scenario of elevated instability
spreading into west central and southwest Minnesota. Pretty good chances
for scattered showers and isolated thunderstorms to reach krwf
before daybreak and then spread northeast past kaxn and kstc
during the morning. There is a small chance that this activity may
brush the Twin Cities and adjoining areas of west central WI
during the middle morning hours. Latest model data continues to show
concentrated convection developing by Wednesday evening just to
the north of a line from kaxn through kstc to krnh. This pushes
east-southeast during the late evening hours. Therefore...included prob30
groups for thunder at all but krwf Wednesday evening. Due to the
strong inflow of low level moisture on Wednesday...concern is
increasing that low MVFR ceilings may develop Wednesday
night/early Thursday morning. This is at or beyond the end of the
taf with the exception of kmsp which GOES through 12z. This will
need to be tackled with the next set of tafs. In addition...MVFR
visibilities could occur as well. Southerly winds overnight 5 to
10 knots increasing to 10 to 15 with gusts to around 20 knots on
Kmsp...confidence high on VFR conditions overnight and Wednesday
morning with chances increasing Wednesday afternoon and night for
MVFR or lower conditions. A few showers are possible Wednesday
morning with thunderstorms possible Wednesday evening...although
confidence is higher for the storms to the north of the airfield.
/Outlook for kmsp/
Thursday...MVFR/IFR ceilings/visibilities possible early then scattered thunderstorms and rain in the
afternoon. Winds S 10-15 kts becoming west.
Friday...VFR. Winds northwest 10 kts.
Sat...VFR. Winds west 4-8 kts.