Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
433 am CDT Sat Aug 1 2015
Short term...(today and tonight)
issued at 400 am CDT Sat Aug 1 2015
The main concern in the short term is the chance for severe
weather late this afternoon and tonight. A strengthening surface
low is still forecast to move southeast toward our area from North
Dakota...with a warm front pushing east into western Minnesota this
afternoon...and continuing east tonight. Low level Theta-E
advection will be quite strong as southwesterly flow develops
behind the warm front. Surface dew points should rise well into
the 60s across southwestern Minnesota today and expand eastward tonight.
Meanwhile...the longwave trough over eastern North America will
start to sink south as the main upper low over Hudson Bay starts
to push south. Deep cyclonic flow for our area will result. Middle
level lapse rates greater than 7c/km will be in place by this
evening. So...probability of precipitation expand quickly from west to east across the
area late this afternoon. The hi-res cams Don/T offer great
agreement...but they do suggest the idea of initial thunderstorms
developing late this afternoon ahead of a larger cluster of
thunderstorms expected after 00z. A strengthening low level jet with a
veering profile and wind speeds increasing with height will
provide deep layer shear values in excess of 40 knots...combined
with MUCAPE values of 2000-3000 j/kg. Supercells will certainly be
possible...especially with the initial development...and all forms
of severe weather would be possible with these. As the evening
progresses...expect storms to congeal into a cluster and possible
mesoscale convective system after dark. The main concern would then be downburst wind
potential...as the previous discussion mentioned...the forward
propagating corfidi vectors suggest that if an mesoscale convective system forms...it
could scream across the area at 60-70 miles per hour. However...the cams
leave a lot to be desired in terms of agreement right now. The
timing/placement/and initiation of storms vary quite a bit so probability of precipitation
were increased into the likely category for a portion of the County Warning Area.
By Sunday morning...most of the activity should be to our south
and east...but still carry some low end probability of precipitation in western WI as we
move the overnight storms east of our area.
As far as temperatures are concerned...winds will turn
southwesterly by this afternoon...and temperatures are generally
expected to be about 5 degrees warmer than yesterday as the warm
front pushes east...and low level warm air advection increases
this afternoon. Dew points will be on the rise as well...with most
of the area seeing dew points in the middle 60s by this evening...and
overnight lows will be held in the middle 60s as well thanks to the
boundary layer moisture.
Long term...(sunday through friday)
issued at 400 am CDT Sat Aug 1 2015
Sunday...the aforementioned surface low pressure system will center itself
just NE of the msp metropolitan by daybreak Sun morning with its cold front
snaking west-southwestward along the Minnesota/Iowa border. Much...if not all...any
overnight activity should be well east of the weather forecast office mpx County warning forecast area by Sun
morning. However...effective cold air advection and dry air intrusion will not
commence until late in the day with the departure of the cold front so
dewpoints in the low 60s...aiding modest instability...plus the
low level jet in vicinity of western WI will still allow for the potential for storm
re-development Sun afternoon in western WI. Coverage will be scattered at
best...with best storm strength well off to the southeast in the warm
sector so will not be looking for severe weather development in the
County warning forecast area. Storm potential drops off with the loss of daylight sun...
so am able to run with a dry forecast for Sun night.
Monday-Tuesday...cooler and drier air will be felt across the County warning forecast area
Monday and Tuesday as Canadian high pressure moves into the area underneath
dry northwest flow aloft. This will keep high temperatures in the 70s /after
highs prevailed in the 80s over the weekend/. Dewpoints will fall
back to the low-middle 50s...making for very comfortable conditions
for early August.
Wednesday-Friday...confidence in the forecast begins to wane for
the latter half of the upcoming week as model differences in the
synoptic setup plus timing differences of low pressure center
progressions plus upper level disturbances make trying to pinpoint
which days are better than others for the potential for precipitation.
There is enough confidence to run with at least chance probability of precipitation for much
of the period but anything higher is quite problematic to
advertise. Of greater confidence is that the cool air will remain
in place through the latter half of the week. Highs will remain in
the 70s while lows will remain in the 50s to around 60.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1200 am CDT Sat Aug 1 2015
Models insist in development of scattered showers tonight...but
there is little to support such development or drops reaching the
surface given the deep layer of dry air below 7kft. Thus a dry
forecast continues at all the taf sites. Better chance for storms
will arrive Saturday evening. Otherwise VFR.
Kmsp...winds will become southwest Saturday around 8 kts...
otherwise no concerns until Saturday evening when chance for storms
/Outlook for kmsp/
sun...VFR. Wind northwest 10-15kt.
Monday...VFR. Wind north 5-10kt.
Tuesday...VFR. Wind NE 5kt.