Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
640 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
relatively low.

From Sunday and beyond...the longwave ridge still looks to build
in...increasing heat and humidity for next week.

&&

Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 632 am CDT Tuesday Aug 4 2015

VFR conds for all terminals as high pressure moves atop the area.
Light/variable winds this morning become west-northwest 5-10 knots before going light/variable
again overnight tonight. Some midlvl fair weather cumulus to develop this afternoon
before skies go sky clear overnight.

Kmsp...no significant weather impacts expected.

/Outlook for kmsp/
Thursday...mainly VFR. Chance MVFR shra/tsra. Wind southeast 5-10 kts.
Friday...mainly VFR. Chance MVFR shra/tsra. Wind north 5-10 kts.
Sat...mainly VFR. Slight chance MVFR shra/tsra. Wind west 5-10 kts.

&&

Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&

$$

Short term...jpc
long term...speed
aviation...jpc

National Weather Service Glossary of Abbreviations