Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
217 PM CDT Tuesday Jul 29 2014
Short term...(this evening through wednesday)
issued at 201 PM CDT Tuesday Jul 29 2014
Tonight and Tuesday...surface analysis depicts a large area of high pressure
from west-central Canada through the middle Mississippi River valley...
with a cold front lingering just north of the Great Lakes and a weak surface
trough from central Michigan westward to near kord and the quadrant cities Iowa.
Aloft...WV imagery nicely shows the longwave trough over the eastern
Continental U.S. And the large ridge over the western Continental U.S....placing the central
Continental U.S. And specifically the upper Mississippi River valley within solid
northwest flow. Through this is a drier flow...as evidenced by dewpoint
depressions of 20-25 degree f...isolated to scattered showers have
developed over northwestern WI and these are driving
southeastward. These showers are in response to the lingering surface
front but more importantly to weak shortwave axes rounding the
western periphery of the longwave trough. These axes will swing
through WI into northern Illinois through this evening...and they will be
responsible for showers...and possibly isolated thunderstorms. Not
looking for a lot of coverage nor much in the way of quantitative precipitation forecast...but the
point is that with activity just now developing...it cannot be
ignored so have included 20-40 probability of precipitation for the rest of this afternoon
before the precipitation diminishes with loss of insolation. Stable high
pressure will then guide the weather tonight on through tomorrow...with
the only possible precipitation again in far eastern portions of the weather forecast office
mpx County warning forecast area tomorrow afternoon. Again...this is due to weak shortwave
axes riding the western fringes of the large longwave trough.
Temperatures will again run very similar to the past couple days... with
highs again in the upper 70s to lower 80s tomorrow with lows
tonight mainly in the 50s to near 60.
Long term...(wednesday night through tuesday)
issued at 201 PM CDT Tuesday Jul 29 2014
Things look to remain fairly quiet overall through the forecast
period as the large scale pattern changes very little. There will
be some Delaware-amplification of the upper level pattern over time with
eastern trough lifting and western ridge flattening into our area
a bit over the weekend into early next week. However... no
significant weather appears to be on tap for our area... with
mainly just some periodic low chances of shra/tsra. The middle-upper
level cold pool should shift sufficiently east for things to be
dry across the entire area through at least Thursday morning.
We/ll then see a few shortwaves drop south from Canada and help
increase lapse rates across the eastern portion of the area once
again from Thursday afternoon through Friday... so will include
some chance probability of precipitation during that time frame... particularly over the
eastern portion of the area. The GFS and European model (ecmwf) are in decent
agreement that we/ll then see height start to build across the
area along with light mainly anti-cyclonic boundary layer flow
from late Friday though Saturday night. Chances for precipitation will then
return to the area from Sunday into Tuesday as return flow and
warm advection work in from the west. There is potential for some
heavier precipitation during the Sunday night-Tuesday period as the
baroclinic zone/warm front becomes somewhat stationary from west-northwest-
east-southeast across the region. However... at this point it is too early to
figure out exactly where that will setup... and there is a
significant difference in that level of detail between the European model (ecmwf)
and GFS as well as quite a bit of spread in the ensemble guidance.
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1231 PM CDT Tuesday Jul 29 2014
Mainly VFR conditions expected at all taf sites but there is still a
decent chance at seeing showers at the WI sites...particularly
Keau...later this afternoon. Weak disturbances rounding the western fringes
of the longwave eastern Continental U.S. Trough will swing through WI later this afternoon
into the evening hours. Enough lift from a few of these disturbances may
produce isolated to scattered showers over northwestern WI that will
drift south-southeast and may move across the WI taf sites. Not looking for
flight category reductions but it does look enough to drop
visibility to minimal VFR conditions and produce middle-level ceilings.
The threat for any showers is confined to late afternoon and early
evening...thus resulting in VFR conditions overnight through midday
tomorrow. Winds remain primarily northwest...increasing to around 10 knots
this afternoon before settling down to less than 5 knots overnight.
Kmsp...no significant weather concerns expected. High confidence
in VFR throughout this set with no precipitation expected.
/Outlook for kmsp/
Thursday...VFR. Winds northwest 5 kts.
Friday...VFR. Winds northwest 5 kts.
Sat...VFR. Winds north 5 kts.