Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1106 PM CDT Friday may 29 2015
Short term...(this evening through saturday)
issued at 249 PM CDT Friday may 29 2015
The short term concerns are timing of precipitation and the clearing
Large area of rain with embedded thunder over most of the area.
More scattered nature of thunder with best chance located mainly
over the southeast third of the County Warning Area into the evening. This matches
well with the 1 hour thunderstorm forecast from the sref. Should see
thunder end through 03z across the County Warning Area. Timing concerns as well as
upper trough lifting over the area at this time. Will slow clearing
some...but still anticipate mainly high clouds remaining to the
southeast by 12z Sat. Rainfall rates still ranging around an 1 inch
an hour in more organized convection. This will linger through
about 00z with precipitable water values around 1.5 inches...per
model trends and Storm Prediction Center mesoanalysis.
With some clearing expected and strong cold air advection...temperatures will drop
off into the upper 30s over the northwest County Warning Area. Cooler weather with a
few clouds expected to the southeast at least early Saturday with
high pressure building to the southeast. High temperatures remaining
in the lower and middle 60s most areas...some 10 degrees below
Long term...(saturday night through friday)
issued at 249 PM CDT Friday may 29 2015
In the extended period the weather pattern will transition from
seemingly early fall to early Summer as northwest flow and cool high
pressure is replaced with zonal flow and increasing moisture. As a
result...after a frosty morning across the northwest...temperatures
will increase with periodic chances for showers and thunderstorms
for most of next week. At this point...the instability appears to be
in place...but the limiting factor for severe weather hinges on wind
shear and forcing.
At the start of the period...a textbook example of a h500 rise/fall
couplet on the upstream/downstream side of a potent upper level wave
will result in surface high/low pressure over Ontario/Newfoundland.
Light winds and clear skies within this surface high will promote
radiational cooling...and a few areas north and east of I-94 will be
subject to frost development in the typical cold spots.
Meanwhile...zonal flow will develop causing Lee cyclogenesis over
the High Plains...and southeasterly surface winds will develop over
the Midwest as the aforementioned surface high drifts eastward.
Shortwaves embedded in the zonal flow will lead to showers and
thunderstorms across the Dakotas...and forecast models indicate this
area of convection will propagate eastward as an mesoscale convective system.
This has two implications on the forecast. First of all...overcast
skies from convective debris could limit the afternoon heating...so
did not stray from the blended guidance despite the seasonably warm
temperatures a h850. Also...convective debris would limit cape
development/severe weather potential. One thing that is supportive
of instability is the middle level lapse rates of 7 to 8 c/km above a
h800 warm nose. Forecast soundings show roughly 1500 j/kg of MUCAPE
by middle afternoon. However...the environment may be capped off by the
aforementioned warm nose...so as of now expect the best chance for
showers and thunderstorms across the northern half of Minnesota/WI unless a
shortwave comes farther south across the region. Wind profiles show
90 degree of directional shear...but the magnitude is on the lower end
of the severe weather spectrum...especially in middle and upper levels
where h500 and h300 winds are forecast to range from 20 to 35kts.
In summary...expected periodic showers and thunderstorms next week.
A few storms could be strong to severe shortly after
initiation...but the more likely scenario is an unorganized complex
of storms with localized heavy rain as the primary threat.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1106 PM CDT Friday may 29 2015
MVFR continues to clear from west to east in far eastern Minnesota and
WI. The clearing has already made it through kaxn...kstc...krwf
and will soon follow at the remaining sites over the next few
Clearing looks like it is in the west metropolitan...so it should not be
more than a couple hours before reaching the Airport.
Therefore...we will start the 06z taf off VFR. No problems arter the
low clouds clear.
/Outlook for kmsp/
sun...VFR.Winds southeast 5-10 knots.
Monday...VFR. Winds southeast 10 knots.
Tuesday...VFR. Chance -tsra. Winds 10-20 knots.