Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1235 PM CDT Friday Jul 3 2015
Update for aviation discussion below...
Short term...(today and tonight)
issued at 406 am CDT Friday Jul 3 2015
Two main issues during the short term deal with thunderstorm
chances/coverage along a weak boundary that will be slipping into
the area this afternoon and the potential impact the continued dense
smoke from Canadian fires will have on highs and in turn potential
Visible satellite imagery toward sunset last night clearly showed
that often dense smoke from Canadian wildfires was spread out across
the northern Continental U.S. From Montana to Michigan...so expect another hazy day
with dense smoke in the middle-upper levels of the troposphere. This
will again keep US from achieving our full potential when it comes
to highs this afternoon...but a good 2-4 degree c warming in the 925-
850 layer will still yield highs a few degrees warmer than Thursday...with
most places climbing to around 80...which is still a few degrees
On water vapor imagery...a strong shortwave can be seen heading for
Northern Lake Superior...with a cluster of thunderstorms out ahead of it
that stretches from The Arrowhead into adjoining sections of
Ontario. This shortwave will continue on toward Lake Huron through
the day...but the reason it will play a role in our weather today is
the weak surface boundary it has trailing back to its SW. This boundary
was working across northwest Minnesota this morning and will be to central Minnesota by
the afternoon and will provide what meager forcing we will have for
thunderstorms this afternoon. We will see lower 60 dewpoints work in
out ahead of the boundary...which will aid in developing some
instability this afternoon. However...think dense smoke will make it
tough for US to realize the upwards of 2000 j/kg of MLCAPE the
NAM/GFS have...and instead will keep US capped around 1000 j/kg.
This does not Bode well for seeing widespread thunderstorm activity.
Cam reflectivity forecasts reflect this idea as well. For
probability of precipitation...took 30 probability of precipitation from central Minnesota and worked them over toward western
WI into the evening as this is where the best moisture transport
shows up around 850 mb. Also switched our weather grids from
probability to coverage wording to get the forecast to read isolated
to scattered thunderstorms for this afternoon/evening.
As we loose daytime heating...we will lose the thunderstorms as
well. Main question then for Friday night becomes how quickly can we
clear out to allow for fog formation. Would not be anything
widespread...but the typical low spots and river valleys from
central Minnesota into western WI could see some fog develop again Saturday
Long term...(saturday through thursday)
issued at 406 am CDT Friday Jul 3 2015
Concerns continue to increase in the long term for the Saturday
through Monday period with respect to highs...severe weather and
The period will begin with a nearly stationary front near the
I-90 corridor. The front will lift slowly northeast during the
day as low level south-southwest flow increases. While this is occurring...a
weak short wave will arrive by midday from South Dakota...which
will push into southern Minnesota/northern Iowa during the afternoon. As
instability and forcing increase...there is a small threat for
some showers and isolated thunderstorms to develop along and north
of the front. Various cams suggest southeast Minnesota and adjoining areas of WI
will see widely scattered activity in the afternoon and this is
where small probability of precipitation were placed. A few solutions have some isolated
showers reaching the west and south metropolitan...but confidence was not
high enough to include them at this point. Highs on Saturday are a
concern as the fire smoke the past few days has surely cost US
several degrees on the high temperature. The flow aloft on
Saturday is still northwest...so had to take this into consideration on
the highs as mix-down and MOS guidance are way warmer than the
going forecast. However...this has been the case for the past two
days with highs being cooler than guidance. Current forecast highs
are between 82 and 85 degrees which is very close to climatology.
Sunday into Monday is the next concern as a cold front pushes
across the region. Timing on the front remains consistent...
stretching from northwest Minnesota through eastern South Dakota Sunday afternoon...from
Duluth through the Twin Cities to Omaha Monday morning and then
east of Eau Claire by Monday afternoon. Temperatures on Sunday are
tricky. Low level south-southeast flow increases across the forecast area which actually
brings in cooler air at 850 mb. The exception is across western Minnesota
ahead of the front where the low level thermal ribbon exists.
Highs on Sunday ended up being very similar to those on Saturday
with west central Minnesota being several degrees warmer. One reason for
keeping the highs up across eastern Minnesota and western WI is the flow
aloft becomes more westerly which should clear out the fire smoke
and allow highs to be closer to mix-down values.
Convection is likely to develop along the front Sunday afternoon
and evening. Storm Prediction Center now has much of the forecast area in a marginal risk with
west central and southwest Minnesota in a slight risk for severe weather.
Surface cape values in the slight risk area increase to between
1500-2000 j/kg by late in the day along with decent middle level
lapse rates. Cape in the surface-3km range is on the order of 150-200
j/kg. Although a veering profile is forecast...deep layer shear
values are not that great for our western area. 0-1km shear is a
little better. A conglomeration of severe weather parameters point
to eastern South Dakota...southwest Minnesota and northeast Nebraska as the Hot
Spot Sunday afternoon and evening. This is also pretty much where
the highest sref calibrated severe probabilities are. Hence...a
few strong to severe storms can not be ruled out heading through
The cold front will move through Sunday night and early Monday
with likely to categorical probability of precipitation remaining in place. Precipitable water values
continue to be forecast above 2 inches ahead of the front... with
bouts of heavy rainfall likely. Cips analogs continue to show high
chance probabilities over a large area of our forecast area for greater than
an inch of rain. The current quantitative precipitation forecast grids through 12z Monday indicate
over 1.25 inches of rain west of a St cloud to Redwood Falls line.
Before the rain ends early Monday afternoon...eastern/southern Minnesota
and western WI will likely have an inch or more of rain.
In the wake of the front...dry and less humid conditions will
occur for Monday night through Tuesday night. Another short wave
and associated cold front will pass across the northern plains
and upper Mississippi Valley region Wednesday and Thursday. At
this point...small probability of precipitation begin across western Minnesota on Wednesday with
low chance probability of precipitation across the forecast area on Thursday as the front moves
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1215 PM CDT Friday Jul 3 2015
Several hires models/cams continue to generate some isolated/widely
scattered -tsra over the eastern County Warning Area into the evening. Best chance into
west central WI. Will mention vcsh at WI taf sites for now through
03z. Then some potential of fog generating later
tonight...especially if we do get some rain showers at the taf sites and
the sky clears. Will continue to mention that potential at Keau.
Other issue into central Minnesota this afternoon/evening will be the
smoke potential. Did mention MVFR visibility possible latitude kstc for now.
Some question how far south it will go and potential mixing with
the lower level smoke layer. Will have to monitor this into the
evening as well.
Kmsp...window for isolated thunder still looks to be between 23z
and 03z as boundary drops south. Because of isolated nature will
leave out of taf for now. Some threat of visibility reduction because of
smoke/fu moving in late this afternoon/overnight. Will mention
some 6sm br developing after 08z...but there could be some fu
affecting the area before that time. West winds this
afternoon...become a bit more SW and light overnight...and more
south into Saturday.
/Outlook for kmsp/
Sat night...VFR. Winds SW 5-10 kts.
Sun...VFR. Thunderstorms and rain with MVFR likely overnight. Winds S 10-15 kts.
Monday...MVFR/IFR with thunderstorms and rain early. Then VFR. Winds northwest 10 kts.