Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
325 am CDT Tuesday Sep 1 2015
Short term...(today and tonight)
issued at 323 am CDT Tuesday Sep 1 2015
Conditions today will be somewhat similar to Monday...starting off
with a bit less stratus but slightly lower visibilities due to fog.
Early morning surface analysis shows a weak cold front stretching
from west central Minnesota into The Arrowhead region. Lack of moisture and
forcing have translated to merely an increase in middle/high clouds
overnight. Expect this to be the case through the day...with
temperatures moderating into the low/middle 80s again and somewhat
sticky conditions ahead of the front given dewpoints in the upper
60s to lower 70s. Winds will lack the gustiness that Monday
brought...with the decaying front nearby.
Tonight brings an increase in low level moisture convergence over
southern Minnesota on the nose of the low level jet...and a pocket of pv advection
from southern Minnesota into west central WI. Have included 20-30 probability of precipitation
after dark when these triggers are forecast to kick in. Lows tonight
should remain mild...ranging from the middle 60s to lower 70s.
Long term...(wednesday through monday)
issued at 323 am CDT Tuesday Sep 1 2015
A few shortwaves embedded in southwest flow are the cause for slight
chances of precipitation. The timing and placement of these waves
are still uncertain because they are driven primarily from upstream
convection. A better chance for widespread precipitation is expected
alter this weekend as height falls from an upper level wave
currently located off the British Columbia coast overspread the
Deterministic numerical models are in fairly good agreement that a
shortwave will move across the region late Tuesday night into
Wednesday morning. However...forecast soundings show a warm layer
near 800mb that will act to inhibit convection...so only have a
slight chance of probability of precipitation during this timeframe. Middle level saturation
just above this capping inversion should hold off high
temperatures...so lowered them a couple degrees on Wednesday.
Once this wave moves through Thursday and most of Friday will be
dry...especially east of I-35. Another pv anomaly...that seems to
originate from convection on the High Plains of Texas...will lift
northward toward the Dakotas. This feature is the primary driver
behind Friday probability of precipitation. On Saturday the remnants of tropical depression
14_3 will bring chances for showers and thunderstorms once again
across primarily the western half of the forecast area. Looking
ahead...the northern stream upper level wave has slowed somewhat and
taken a more northerly track...so have tried to delay probability of precipitation across
the east. As of now the best chance for precipitation is late in the
weekend and early next week...but if the ridging holds across the
eastern Continental U.S. This too may be pushed back in time. The moisture is
still in place...so locations that see storms will also have the
potential for heavy rain.
Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 1019 PM CDT Monday Aug 31 2015
Forecast thinking hasn/T change much... with VFR conditions
predominating through the period as the weak trough/front to our
west continues to weaken as it pushes east across the area tonight
and Tuesday. Middle/high clouds will move across in association with
that feature... but Don/T foresee anything more than virga. Some
fog/stratus is possible again tonight... but thinking it should
mainly be confined to the eastern sites where middle/high clouds
will not thicken as much... and perhaps at kaxn where the higher
clouds may move out in time. Some precipitation getting going over Colorado
and Nebraska overnight will try to move northeast into the area
tomorrow... but at this point most guidance suggests it will
weaken as it works toward the area... so definitely Don/T have
enough confidence to include anything at this point.
Kmsp...only item of concern would be whether we see some patchy
fog late tonight... but at this point it appears we should be able
to maintain a sufficient dewpoint depression to avoid that.
/Outlook for kmsp/
Wednesday...VFR. South wind 10 to 20 knots.
Wednesday night...MVFR possible with a chance of shra/tsra.
Southeast wind 10 to 15 knots becoming southwest.
Thursday...VFR. South wind 15 to 20 knots.
Thursday night and Friday...MVFR possible with a chance of
shra/tsra. South wind 10 to 20 knots.