Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
412 am CDT Tuesday Aug 4 2015
Short term...(today and tonight)
issued at 400 am CDT Tuesday Aug 4 2015
Today and tonight...surface analysis shows the center of a large northwest-southeast
oriented high pressure airmass sitting over the upper miss River Valley
this morning underneath pronounced northwest flow aloft. This is keeping
cool/dry air over the mpx County warning forecast area this morning with barely any clouds in
sight or present on infrared imagery. Temperatures have dropped into the 50s County warning forecast area-
wide /outside of the Twin Cities metropolitan/ with even a few sporadic
readings in the 40s. Plenty of sunshine is expected today as high pressure
exits to the southeast. However...500 mb heights will rise slightly from the west
which means temperatures will continue to run warm. Highs will range from
the middle 70s in western WI to the lower 80s in western Minnesota. Mostly clear skies will
continue through the evening hours then clouds increases slightly overnight in
advance on an approaching warm front and a flattening ridge aloft. Isentropic
lift along with deeper moisture...associated with the warm front and a
capable combination in order to produce rain showers...will remain
off to the SW through tonight so have kept the County warning forecast area dry through tonight.
However...the increases in clouds plus a more westerly rather than northwesterly flow
aloft means temperatures will be warmer early Wednesday morning than this morning. Lows
early Tuesday morning will run 50-60 degrees.
Long term...(wednesday through monday)
issued at 400 am CDT Tuesday Aug 4 2015
The guidance generally agrees that the longwave trough and upper
low over eastern North America will continue to shift east through
the weekend. With this transition...we'll break out of the
steady northwest flow beginning tomorrow. The pattern becomes a
bit complex starting tomorrow night.
The first shortwave in a series of potential precipitation makers will
push through the upper Midwest from tomorrow night into Thursday
morning. The main change with this wave was to shift the
forcing/saturation further north...with drier air reaching
southern Minnesota. So...increased probability of precipitation in north central Minnesota as the
GFS/ec/NAM/Canadian all showed better agreement on this northerly
track. Decreased probability of precipitation and removed all mention of precipitation across far
southern Minnesota for this period.
Another...more energetic wave is forecast to quickly fill in
Thursday afternoon and night...and move across the forecast area through
Friday. The deterministic guidance showed improved agreement on
the timing and strength of this wave...even more so than the first
wave. By Thursday night...strong pv advection will be pushing into
western Minnesota along the leading edge of this shortwave...with a weak
west to east oriented jet streak pushing through Iowa...implying
upper level divergence across southern Minnesota. At the surface...the
NAM and GFS solutions suggest a surface low will progress east
through South Dakota through our area Thursday night through
Friday...with a frontal boundary draped east to west through Minnesota.
The ec suggests this possibility as well...but lifts the front as
a warm front into northern Minnesota. This solution is an outlier...as
the Canadian agrees more with the NAM/GFS at this time. If this
low and front track does come to fruition...we'll have a decent
heavy rainfall setup with the front draped across the area...and
precipitable water values increasing to 1.75" or more...broad low
level lift with positive Theta-E advection...upper level
support...and modest instability. Will need to see run to run
consistency and even better agreement before coming too concerned
about heavy rainfall issues...but at this point we did increase
probability of precipitation to the low end likely category across southern Minnesota and western
WI Thursday night.
As that second wave departs the area Friday...our attention turns
to the next upper low which is expected to slide east along the
Minnesota/Canadian border Saturday. There is actually decent agreement
between the ec/GFS for being 5 days away on this scenario. Given
these solutions...trimmed the probability of precipitation in extreme southern Minnesota...but
stuck with slight chance mention for most of the area including
the Twin Cities...as confidence in the placement/timing with a
feature like this...embedded in a complex flow pattern...is
From Sunday and beyond...the longwave ridge still looks to build
in...increasing heat and humidity for next week.
Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 1104 PM CDT Monday Aug 3 2015
VFR through the period. SC in wisc will be exiting early in the
period...then just a few clouds are expected the rest of the
night. Very weak trough appears to be across west central and
central Minnesota...where winds will be nearly calm. Remainder of area
should mostly be light west or northwest. Ridge axis begins moving across
Minnesota during the day on Tuesday. Minnesota will have mostly light northwest winds
but west central wisc may have some 10-15 knots gusts from the northwest
Kmsp...VFR through the period. Some chance of variable wind
direction late tonight but should generally be west at around 5 kts.
Northwest winds will pick up again Tuesday late morning but mostly remain
below 10 kts...with maybe a few gusts a little above 10 kts during
/Outlook for kmsp/
Wednesday...VFR. Wind southeast 5-10 kts.
Thursday...mainly VFR. Chance MVFR shra/tsra. Wind southeast 5-10 kts.
Friday...mainly VFR. Chance MVFR shra/tsra. Wind east 5-10 kts.