Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
651 PM CDT Sat Sep 20 2014
Short term...(this evening through sunday)
issued at 255 PM CDT Sat Sep 20 2014
Short term concerns deal squarely with the severe weather threat
through early this evening. A broken line of storms formed over
nodak earlier today and has organized somewhat as it pushed into
northwest Wisconsin early this afternoon. Not too much severe
weather being reported at this time...mainly due to the overall lack of
destabilization. MLCAPE values of less than 500 j/kg are expected
to continue throughout the event. However...strong forcing with
the middle level trough and very strong winds in the middle and upper
levels /60+ knots at 500 mb and 130+ knots at 300 mb/...accompanied by
inverted v soundings will bring good downward momentum and thus a
threat for mainly damaging winds. Lack of instability and low
topped convection will limit any hail possibility. The line is
moving east/southeast around 45 knots which should reach stc around 5
PM and msp metropolitan between 6 PM and 7 PM. Very high temporal grid
resolution and categorical probability of precipitation were included with relatively
high confidence due to very good agreement amongst the convective
allowing models. A general weakening can be expected after sunset
as the line enters Wisconsin when instability becomes none.
Wrap around showers may drop south into eastern Minnesota and western WI late
this evening and overnight as the upper low pushes east. These
showers may linger into early Sunday with the cyclonic flow
continuing...but surface ridging should provide enough subsidence
by afternoon to lower the threat with time.
Long term...(sunday night through saturday)
issued at 255 PM CDT Sat Sep 20 2014
Not much going on in the long term in terms of sensible weather...
but a major pattern shift is still on tap that will bring much
above normal temperatures by late next week. As the digging trough
from tonight kicks east...middle level ridging will build into the
Great Lakes and upper Midwest early next week. The middle level ridge
will be centered over the southern rockies and will allow the cut
off low over California to be kicked northeast into the upper
Midwest by midweek. Not expecting much to be accompanied with this
system aside from a low chance of showers and thunderstorms and
cooler temperatures. The thunder threat may be overplayed a bit
considering poor lapse rates...weak forcing...and dry boundary
layer air. Did not make too many changes to the previous forecast.
Models diverge a bit beyond midweek with the Canadian and GFS
building the ridge north across The Rockies while a piece of
energy rounding this ridge is detached from an accelerating
trough over northern Canada. This wave then makes an odd dive
south or southwest across the plains into a weakness in the ridge.
The European model (ecmwf) seems to keep the feature associated with the stronger
westerlies across Canada which makes most sense. Otherwise...a
massive ridge is expected to encompass all of the central and
eastern U.S. To round off the end of the month. Several members of
the GFS ensemble...as well as the operational European model (ecmwf)...center an
anomalous 591+ dm ridge over the Great Lakes Friday and Saturday.
A set up like this would normally bring the upper Midwest a heat
wave...but it is rather difficult to get oppressive heat in late
September. Temperatures will still run 10-20 degrees above average and a
few days with highs in the 80s appear likely if the center of the
ridge does build close by.
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 651 PM CDT Sat Sep 20 2014
Thunderstorms and rain with wind gusts to 35 knots will move through Keau by 01z.
Otherwise...clearing following quickly behind the storms along
with increasing northwest winds. Main concern overnight and Sunday
morning is MVFR ceilings spreading in from the north around an
upper low that will move through during the morning. Confidence is
pretty high on the MVFR and indicated prevailing MVFR ceilings
(015-025) for all sites except krwf. There should also be some
light rain as well for krnh and Keau. VFR conditions for the
Kmsp...VFR conditions until late in the night with MVFR ceilings Sunday
morning. Northwest winds will gust up to 20 knots this evening in the
wake of the front and then again on Sunday.
/Outlook for kmsp/
Monday...VFR. Winds west 5-10 kts.
Tuesday...VFR. Winds S 10 kts.
Wednesday...VFR...MVFR possible with a chance of shra/tsra. Winds S