Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
642 PM CDT Friday Sep 4 2015
Short term...(this evening and saturday)
issued at 117 PM CDT Friday Sep 4 2015
Although we have plenty of instability in place today with
dewpoints in the low/middle 70s...shear/forcing are currently
lacking. However...a shortwave pushing in from the Dakotas should
help drive at least light showers into west/north central Minnesota this
afternoon and evening. Seemingly better chances arrive tonight as
a 30-40kt low level jet develops and a weak pv advection signature moves
overhead. These mechanisms are somewhat disjointed however...and
cams models are more dry than wet. Therefore confidence is rather
low...so have maintained 20-30 probability of precipitation. Low temperatures will remain quite
mild given the elevated dewpoints and cloud cover. Anticipate lows
similar to Thursday night...ranging from the upper 60s to middle 70s.
An even warmer and more humid day is expected for Saturday...with
little to no change in dewpoints or winds...and even slightly
warmer temperatures nosing into east/south central Minnesota. Will have some
areas nearing heat advisory criteria...particularly the Twin
Cities metropolitan area where heat indices are forecast in the middle 90s.
Instability will remain high...with even greater precipitable water values
approaching 2 inches. Have maintained 20-30 probability of precipitation for the
afternoon hours...but higher chances will arrive on Saturday
evening as a stronger middle level shortwave approaches.
Long term...(saturday night through friday)
issued at 317 PM CDT Friday Sep 4 2015
As a surface low deepens over the Dakotas and moves into Manitoba
from Saturday night into Sunday...very moist air will continue to be
pumped northward from the Gulf of Mexico into the upper Midwest.
This moist southerly flow will produce unseasonably high precipitable water values
in our area during this time...with the NAM and GFS both showing a
band of greater than 2 inch precipitable water values from southwest Minnesota through
the Twin Cities area into western Wisconsin from Saturday night
through Sunday night. With the Storm Prediction Center sounding climatology showing precipitable water
records of 1.71 inches on 12z Sunday morning and 1.81 inches on 00z
Sunday evening...it is certainly possible that both of these records
could be broken. Surface dewpoints in the lower 70s will be
widespread...and a slow-moving cold front to the south and east of
the aforementioned low will provide forcing as it moves eastward
through our area. Instability is also expected to be fairly
impressive...with cape values likely exceeding 2000 j/kg at times.
As a result of this setup...periods of scattered thunderstorms with
the potential to produce heavy rain are expected from Saturday night
through Sunday night.
With the impressive influx of moisture and relatively slow storm
motion expected...cannot rule out some localized flash flooding
through Sunday night. In addition to producing heavy rain...a few
of the storms may become strong to severe due to the strong
instability mentioned earlier. Shear will initially be fairly weak
and could limit severe potential Saturday evening...but 0-6 km bulk
shear could increase to at least 20-30 kts on Sunday...which should
be sufficient for multicellular development when combined with the
After highs in the middle-upper 80s and active weather over the
weekend...cooler and drier air is expected next week as the cold
front moves to our east on Monday morning. One exception to the
generally dry week may be Tuesday across southern Minnesota...when there is
a chance of rain to the north of a weak low pressure system moving
through Iowa. Highs are expected to mainly be in the 70s from
Monday through Friday...with lows generally in the 50s.
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 642 PM CDT Friday Sep 4 2015
Scattered rain showers and isolated thunderstorms and rain from kaxn through krwf will continue
slowly eastward this evening and migrate through the Twin Cities
during 02z-05z time frame...then weaken/dissipate across west
central WI. The activity is along the axis of the low level
moisture transport and gradient of instability. The low level jet
will increase from southwest into central Minnesota overnight. The hrrr
has been trying to show additional rain showers/thunderstorms and rain developing after 06z
between kaxn and krwf and spreading east-northeast through daybreak. With
pretty good continuity with this second area...added tempo groups
during the overnight hours to account for the increase rain showers/thunderstorms and rain
threat. Of course...if this doesn't occur we will be left with a
very moist atmosphere with fog and stratus developing once again.
Hence...confidence is not high on this scenario with the
exception of Keau. Southeast winds tonight and Saturday morning near 10
knots then south-southeast 12-18 knots gusting 20-25 knots for the afternoon.
Kmsp...two bouts of rain showers/isolated thunderstorms and rain expected overnight. The
first is from 02z-05z with the other in the 09z-12z period. Kept
the ceiling VFR with the visibility dropping into MVFR territory from
time to time. The most significant wind is Saturday afternoon with
south-southeast winds 15g22 knots.
/Outlook for kmsp/
Sun...VFR with MVFR/thunderstorms and rain likely at times. Wind SW at 15g20kt.
Monday...MVFR/tsra possible early. Then VFR. Wind NE at 5-10kt.
Tuesday...VFR. Wind north-northwest at 10kt.