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Area forecast discussion 
National Weather Service Twin Cities/Chanhassen Minnesota
332 PM CDT Friday Jul 31 2015

Short term...(this evening through saturday)
issued at 332 PM CDT Friday Jul 31 2015

Todays setup is nearly identical to yesterdays...with northwest
winds gusting near 25 miles per hour at most locations. Once again a few
afternoon showers developed across far northeast Minnesota and
northern Wisconsin where there was better instability owing to the
cooler temperatures aloft.

For the rest of the day expect winds to taper off around sunset
again...with another seasonably cool and quiet night. On Saturday
southerly winds will develop ahead of an approaching upper level
wave that as discussed in further detail below is prognosticated to bring
thunderstorms across the region Saturday night. The question in the
short term is how much low level moisture will return across the
upper Midwest. Current dewpoints are only in the 50s...and the
deeper...tropical moisture per precipitable water plots is confined
south of I-70. By Saturday afternoon h925 moisture transport
develops across the Dakotas...and eventually veers over the upper
Mississippi River valley by 00z Sunday at the end of the short term
period. There is low confidence in how the evapotranspiration...
together with the moisture advection will modify the boundary
layer to the point that surface based convection is possible
Saturday afternoon. One thing that is certain is the middle level
lapse rates of 7-7.5 c/km will support high cape values if this
low level moisture comes to fruition. The hires models show
convective potential after 21/22z...so introduced some slight
chances during this time frame.

Long term...(saturday night through friday)
issued at 332 PM CDT Friday Jul 31 2015

The main concern in the long term is the threat for severe weather
Saturday evening. Deterministic solutions are unified on driving a
surface wave southeast into the forecast area Saturday night. This results
in strong low level Theta-E advection in the evening across much
of the County Warning Area as a warm front advances east. The surface low is
deepening as it heads into our area with the best pressure falls
passing just north of the Twin Cities. The low level jet
increases in the evening to around 45 knots...positioned across
the Twin Cities by late evening. The 850-500mb crossover ends up
being SW 45kts/northwest 50kts respectively. Needless to say the deep
layer shear is impressive at 45+ knots. A majority of the cams are
showing some convection sprouting across central Minnesota in the late
afternoon/early evening and then heading southeast into the Twin
Cities and west central WI during the late evening hours. NAM
profile data indicates dcape values in excess of a 1000 j/kg. In
addition...the corfidi forward propagation is from 320 degree at
60-65 knots. So the concern is for damaging straightline winds if
a complex could get rolling along. Collaboration earlier today
with Storm Prediction Center resulted in the day 2 marginal and slight risk areas
being pushed a little farther to the north and east.

The surface low should be over western WI Sunday morning with a
cold front trailing west through South Dakota. Another round of thunderstorm
development looks imminent Sunday afternoon...with the worst of
the storms to the southeast of the forecast area. At this point...just small
probability of precipitation cover eastern Minnesota and WI for diurnally driven activity.

Dry weather is still forecast for Monday and Tuesday as surface
high pressure ridges southeast from the western High Plains
through the middle Mississippi Valley in the wake of the cold
front. Confidence in the forecast beyond Tuesday lowers as the GFS
and Canadian have a different solution versus the European model (ecmwf). The European model (ecmwf)
remains steadfast on developing an upper ridge across the plains
while the Canadian and GFS are allowing short wave energy to push
eastward across the northern rockies and into the middle and upper
Mississippi Valley. By Friday morning...the GFS has a closed 500mb
low (563 dam) near Duluth versus a 582 dam contour on the European model (ecmwf).
Some of the difference can be traced to how the two solutions
handle the massive upper low over Ontario and Quebec next week.
The European model (ecmwf) is a little more progressive while the GFS allows the
incoming short wave energy to merge with the trough/low aloft. In
the end...chance probability of precipitation slowly increase across the forecast area from Wednesday
through Friday. Highs during the latter half of next week are
forecast to be in the lower to middle 70s.

&&

Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1229 PM CDT Friday Jul 31 2015

VFR conditions throughout. Northwesterly winds will taper off
around sunset...and become southerly Saturday morning. Storms are
possible beyond this taf period Saturday night.

Kmsp...

VFR conditions throughout. Northwesterly winds will taper off
around sunset...and become southerly Saturday morning. Storms are
possible beyond this taf period Saturday night.

/Outlook for kmsp/
Sat night...VFR. With MVFR/thunderstorms and rain possible. Wind west at 10kt.
Sun...VFR. Wind north-northwest at 10g15kt.
Monday...VFR. Wind northwest at 10g15kt.
Tuesday...VFR. Wind northwest at 5kt.

&&

Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&

$$

Short term...jrb
long term...rah
aviation...jrb

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