Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
334 PM CDT Tuesday Sep 30 2014
Short term...(this evening through wednesday)
issued at 311 PM CDT Tuesday Sep 30 2014
High pressure was centered over southern Ontario early this morning
with an extensive area of stratus covering the much of the Great
Lakes and the upper Mississippi Valley region. The high will drift
slowly east today as a vigorous low pressure system lifts through
the northern plains. The surface pressure gradient will be
increasing this morning with winds becoming more southeast with
time. This will aid in shredding of the stratus during the morning.
However...as the low clouds go...mid/high level cloudiness will be
on the increase from the west ending up in a mostly cloudy to cloudy
A band of showers and thunderstorms associated with the low pressure
system to our west will drive into west central and southwest Minnesota by
early this afternoon. The various cams are in really good agreement
with the timing and location...with the activity spreading into
eastern Minnesota by this evening and across western WI tonight. At this
point...the Twin Cities should remain dry until sunset. The one
aspect noted with this system is that due to the short wave lifting
almost due north through the northern plains...there is a continued
slowing to the eastward progression of the showers and thunderstorms
tonight. This may result in the rain not reaching Ladysmith and Eau
Claire until late in the night/daybreak Wednesday. Rain amounts
through tonight will be greatest over south central Minnesota with amounts
up to a half inch. No severe weather is expected in our forecast area
with the storms.
High temperatures will remain a little below normal today in most
areas. The exception is in the Minnesota River Valley where middle to upper
60s are forecast near the low level thermal ridge. There will be
quite the contrast in low temperatures tonight from west to east due
to the wet/dry scenario. Lows from near 60 in southwest Minnesota to the
lower 40s near Ladysmith.
Long term...(wednesday night through tuesday)
issued at 311 PM CDT Tuesday Sep 30 2014
The phasing of the northern and southern jet streams will be
critical in terms of how widespread the precipitation becomes
Wednesday night through Friday morning...especially across western
and southwest Minnesota.
Several strong shortwave/S moving into the Pacific northwest along with a
powerful 100-120 knots jet will crave out a significant storm system
by the end of the week across the Great Lakes region. Depending
upon how the phasing of the southern and northern jet and
convection across Iowa Wednesday night...will determine whether
western Minnesota remains dry or becomes much wetter. Upper level jet
structure remains favorable for a widespread rainfall event across
Iowa Wednesday night...and taking this area northeast across the
eastern 1/4 of mpx County Warning Area. Although time details remain unclear...the
bulk of the wet weather will occur Thursday afternoon/evening across
the eastern 1/4 of mpx County Warning Area. As far west this area of precipitation
expands is dependent on the location of the surface low and how much
moisture advection is held to the southeast across eastern Iowa/northern
Illinois. There has been some slowing of the overall pattern once this
system takes shape Thursday night. But...inconsistencies and the
fact the surface low remains mostly southeast of mpx County Warning Area...will lean toward
only chance probability of precipitation in the west...with likely/categorical in the far
east. Precipitable water values due increase enough for heavy
rainfall...especially if thunderstorms and rain can develop above the boundary
layer. But the best chance/S of widespread one inch rainfall totals
will be S/southeast of our region.
Once this system begins to organize and deepen across the northern
Great Lakes...strong west/northwest winds will develop...leading to wind
chill values in the 20s/30s for Friday evening.
Past Friday...the overall pattern remains favorable for
isolated/scattered rain showers as the mean flow becomes northwest and embedded
shortwave/S ride southeast along this northwest flow pattern. Timing and how
widespread this activity becomes remains unclear. The clear part
of the extended period will be cooler than normal for early
October. One of the main factors of this cooler than normal
pattern is the development of the negative /-ao/ Arctic oscillation
where the medium range models have been advertising. If this
becomes dominate...the first/second week of October will be
unseasonably cool. In addition to the -ao...the mean upper pattern
is forecasting a strong upper ridge developing across Greenland
which leads to a blocking pattern and the continuation of the northwest
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1245 PM CDT Tuesday Sep 30 2014
VFR for most locations this afternoon...but clouds should return
later tonight as a system bring periods of rain...and possibly a
thunderstorm...but confidence in thunder is too low to include
mention in the taf. Model guidance shows abundant low level
moisture...so anticipate a return of IFR and perhaps LIFR
ceilings...along with lower visbys. Conditions should gradually
improve by late morning...especially in the western sites.
at this time it appears that the first round of precipitation will split
msp this evening...so only have vcsh. Later tonight precipitation should
lift up from the south southwest. Low ceilings will be the main
restriction...but could see reduced visibilities with the rain/drizzle.
Conditions will slowly improve on Wednesday.
/Outlook for kmsp/
Thursday...IFR early becoming MVFR with -shra. Winds south-southeast at 10g15kts.
Threshold night...increasing winds after midnight. Northwest at 20g25kts.
Friday...MVFR with -shra possible. Winds west-northwest at 20g30kts.
Sat...VFR. Winds northwest at 20g30kts.