Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1100 PM CDT Friday Sep 19 2014
Short term...(this evening through saturday)
issued at 353 PM CDT Friday Sep 19 2014
During the short term...confidence on seeing much activity in the
mpx County Warning Area this evening and overnight continues to plummet...but this
has been offset by a significant increase in confidence for seeing
thunderstorms Saturday afternoon.
This afternoon...a warm front had pushed up to about the Minnesota
river...with a cold front extending from a surface low near the
Minnesota/ND/can border to the south across the eastern Dakotas. Band of light
returns on radar from west central Minnesota toward Duluth correlates well
with fgen the rap is indicating in the 700 mb-h6 layer on the northwest side of
a rather strong capping inversion. This capping inversion looks to
keep things in check through 00z...but cams begin to show the cap
breaking down between 00z and 03z this evening. Storms look to
initiate along a prefrontal trough that is currently taking shape
from Duluth...down through Glencoe and off toward Sioux Falls. This
boundary can be identified in the wind field...with strong southerly
winds that have been gusting to nearly 40 miles per hour across Iowa into southern Minnesota
to the east...and much lighter west-southwest winds that are back across western
Minnesota. By the time 3z comes around...which is when the hrrr and hopwrf
show storms initiating...said wind shift looks to be sitting near a
Fairmont/Hastings/Rice Lake line. Any sort of short wave to help
kick of thunderstorms does not look to arrive until after 6z...so
looks like isolated-scattered storm coverage is about the best the eastern fringe
of the mpx County Warning Area can hope for...with most cams really starting to
highlight the greatest storm cover being down in Iowa...where better
instability will reside. Given trend in short term models...
continued to reduce probability of precipitation/quantitative precipitation forecast this evening...and also sped up the
eastward progression of the dry weather as all cams show any activity
that does develop this evening being east of the mpx County Warning Area by 9z. The
only thing to watch out for with probability of precipitation for late tonight into Saturday
morning is the NAM does show a region of middle level fgen working
across south central Minnesota as a shortwave finally works across southern Minnesota.
However...this would not be anything too serious and would be Post
frontal and no worse than what we are seeing across central Minnesota this
For Saturday we will see a break in the action through the
morning...but the strong shortwave and associated jet streak
currently moving into western Alberta will be diving southeast across northern Minnesota by
Saturday afternoon. There is good agreement between the
deterministic models and cams with a line of storms diving southeast out of
northern Minnesota during the afternoon out ahead of the waves pv feature and at
the nose of a 120+ knots nwrly jet streak. Given strong model
agreement...increased probability of precipitation significantly Saturday afternoon from the
low chance category to knocking on the door of going categorical.
Main question is how far SW will we see activity. Given the track of
the pv anomaly and jet streak...forcing from those features looks to
be centered along and north of the I-94 corridor...but sref probs
and cam reflectivity forecasts do show the line tailing back into SW
Minnesota...so did drag probability of precipitation as far SW as the Minnesota river. For the severe
threat...the big limiting factor will be the meager instability
/MUCAPE around 500 j/kg/ and weak middle-level lapse rates.
However...forecast soundings show winds of 50 kts or more getting
down to near 10k feet. With the cams showing a nice line of storms
developing...could see these storms be able to Transfer some of this
momentum to the ground...which will pose a risk for damaging
winds...but the lack of instability and weak middle-level lapse rates
should keep any hail sizes in check.
Not many changes were made to the rest of the forecast...other than
to cool highs tomorrow some where we have likely probability of precipitation in the
afternoon...as the arrival of cloud cover and rain looks to put an
earlier end to diurnal rises in those locations.
Long term...(saturday night through friday)
issued at 353 PM CDT Friday Sep 19 2014
Any thunderstorm activity ongoing at the beginning of the period
will be quick to swing east thanks to the very fast 90-130 knots
upper level flow contributing to the southeastward digging trough.
Cyclonic flow will remain over portions of central and eastern
Wisconsin into Sunday and may allow for a few showers to develop
in the afternoon. However...the surface ridge building east into
the plains will keep the remaining areas dry and mainly clear. The
trough will lift out by Monday with seasonal temperatures and clear skies
across the entire upper Midwest through early next week.
A major pattern shift will take place next week and likely persist
through the rest of September. The surface high will make steady
progress east and eventually park itself over New England for
midweek and beyond. Meanwhile...middle level heights will build over
the Midwest with the center located somewhere nearby for late week
and next weekend. This will allow for a warmer and drier than
normal pattern to set up for a good portion of the long term.
There will be a brief hiatus in this evolution midweek as low
level moisture increases and a disturbance lifts north up and
around the building ridge from the Desert Southwest /currently
located over California/. Clouds and scattered showers/thunderstorms
will keep temperatures down a bit Tuesday and Wednesday. With the
building middle level heights...middle level lapse rates will be rather
meager which will limit any severe threat greatly. Kept probability of precipitation low
due to the lack of instability and a slow weakening/absorption of
the system into the zonal flow across Canada.
As mentioned above...the middle level ridge will likely be centered
over the Midwest by next weekend. Summerlike temperatures should
make a return as 850 mb temperatures reach between +15 to +20c. Since the
majority of southern Minnesota did not have a hard time reaching
the Lower/Middle 80s today...we stand a pretty good chance at
reaching the 80s again late next week with a stronger ridge and
warmer temperatures aloft. Dry weather will prevail until the western
trough begins to advance eastward - not until well after the
current forecast period.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1100 PM CDT Friday Sep 19 2014
Convection is now east and south of the taf sites late this
evening. A growing concern overnight is how expansive stratus and
fog will become that has formed across portions of central Minnesota
where skies have cleared and winds have become light. Thinking is
that conditions will actually improve by 09z as high level clouds
increase across the area along with a cold frontal passage that
will begin to lower dew points. There could still be some MVFR
ceilings develop north and east of the Twin Cities on Saturday
with a very negative cumulus rule. A second and much stronger cold
front is set to blast southeast across the taf sites Saturday afternoon.
A line of convection is still expected to be near kaxn by
21z...kmsp at 00z and Keau by 03z. Very strong wind gusts may
occur as the rain showers and thunderstorms and rain move through. Continued with tempo thunderstorms and rain
and narrowed the window down to 2 hours. Winds becoming west-northwest near
10 knots overnight and northwest 12-15 knots on Saturday. Gusts will
reach into the 20 to 25 knot range.
Kmsp...temporary bouts of MVFR visibilities are possible early this
morning as sky cover diminishes and winds weaken. Indicated 6sm br
by 08z through daybreak. Cold frontal passage by daybreak as well
with northwest winds 12-15 knots through the day. A second and strong
cold front will likely bring a line of thunderstorms and rain across the airfield in
the 23z-01z time frame. The storms may contain strong wind gusts.
/Outlook for kmsp/
sun...VFR. Winds northwest 10-15 kts.
Monday...VFR. Winds west 5-10 kts.
Tuesday...VFR. Winds southeast 5-10 kts.