Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
710 am CDT Tuesday Sep 30 2014
Short term...(today and tonight)
issued at 414 am 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 through monday)
issued at 414 am CDT Tuesday Sep 30 2014
Several changes were made during the long term to reflect changes
seen with the 30.00 models...will separate them out into the
significant and minor categories. The significant changes were made
to Thursday and Friday. Thursday...probability of precipitation were increased
significantly...especially east of I-35...where the 30.00 guidance
is in good agreement on another rainy day unfolding. Friday changes
included cutting back on highs...significantly increasing cloud
cover and wind speeds...and increasing/expanding probability of precipitation. More minor
tweaks were needed for Wednesday and Sunday. For Wednesday...simply
increased existing likely probability of precipitation east of I-35 into the categorical
range. For Sunday...expanded low probability of precipitation to account for the threat of
showers with a clipper that will be dropping across the area then.
What changed with the models? The biggest thing that changed is they
are all phasing southern stream energy coming out of the Central Plains
and a powerful northern stream wave coming out of central Canada a day
earlier. This has taken the low riding up the cold front on Thursday
from heading for Lake Michigan to going up through central WI
instead...which pulls precipitation chances quite a bit west on
Thursday...with a strong deformation band of precipitation currently
prognosticated to come across southeast Minnesota and western WI. Only changes to guidance on
Wednesday is that everything has come in line with what the European model (ecmwf)
has been showing for days with a wave and associated precipitation working
up the front during the day on Wednesday...impacting mainly areas
along/east of I-35 Wednesday afternoon. Given the forecast ewrd
progression of the front...or rather the lack there of...it makes
sense that more precipitation would make it into the eastern half of the County Warning Area
both days. This front looks to be just into western Minnesota 12z Wednesday...about
100 miles west of I-35 12z Thursday...with a surface low tracking north along
the boundary during the day Thursday across central WI before the
front gets kicked east of the area with gusto Thursday night.
Both Wednesday and Thursday...severe chances look pretty much
non-existent...as the instability high enough to consider a severe
threat looks to remain south of Minnesota. Given the dynamics...we will
however see some healthy rainfall amounts for early October from
these two waves...which look to come about 12 hours apart...with
Wednesday night being a bit of a lull. In each case...a good half to
one inch of quantitative precipitation forecast looks likely...with total precipitation easily getting up
into the 1-2 inch range east of the I-35 corridor. In fact...run
total qpfs from the GFS/ECMWF/Gem all show a swath of 2-3 inches
going across western WI. Guess the moral of the story here is eau looks
to build on its year-to-date precipitation surplus...which already sits at
a robust 12.43 inches!
Thursday night we again look to briefly dry out as we get into
subsidence and strong cold air advection in the wake of the front...but precipitation
looks to come back on Friday. For Friday...did have to modify the
forecast from the initialized blend. With the core of the upper low
coming overhead and 850 mb temperatures nose diving to below zero...first
order of business was to up sky cover and decrease highs. Bumped
highs down into the lower 50s...though given the degree of
cold air advection...would not be surprised if we do not rise much above our lows
Friday morning...which will be in the 40s. Given the strong pv
anomaly coming in also increased/expanded probability of precipitation across the entire
area...though if the European model (ecmwf)/Gem are right...these will need to be
upped significantly in future forecasts. Finally...Friday also looks
to be our first potential Wind Advisory day of the season...with GFS
bufr soundings showing mix down potential winds up to 45 kts. At the
very least...expect northwest winds to be gusting to be 30 and 40 miles per hour in
our normal wind favored areas...with some gusts getting up toward 45
miles per hour not out of the question.
For Saturday...we will dry out as a surface ridge works across the area.
Winds will slowly diminish during the day...though it will be a
chilly day...with lows starting out in the 30s and highs struggling
to get into the low 50s...or 10 to 15 degrees below normal.
Sunday...the European model (ecmwf) continues to show a 160 knots nwrly jet streak
coming down from Canada. The left exit region of this jet will send
a clipper down across northern Minnesota. We still look to be in the warm sector
of the system...though isentropic lift out ahead of the wave looks
to be enough to generate some light showers mainly along and north
of the I-94 corridor. Despite the inclusion of some low probability of precipitation...
Sunday is still looking like a perfect day for those running the
Twin Cities Marathon.
Next week we will see temperatures moderate as the western ridge
flattens out and works east...with no significant storm systems
looking to be in the offing.
Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 710 am CDT Tuesday Sep 30 2014
Extensive IFR/low MVFR ceilings cover the region with IFR visibilities
around kaxn. East-southeast winds will continue to increase this morning
which will aid in breaking up the low clouds with sites becoming VFR.
The strongest sustained winds will be from kaxn on southward
where near 20 knot winds are expected by late in the morning. A
band of showers will advance into west central and southwest Minnesota
this afternoon ahead of a low pressure system moving northward
across the Dakotas. Confidence is highest for the showers at
kaxn...krwf and kstc. Thunder chances are best from krwf on
southward with thunderstorms in the vicinity used. Confidence wanes on the showers chances
early in the night for the eastern taf as the system begins to
shear apart with vcsh indicated. Another area of showers from a
second low pressure system will spread northeast across eastern Minnesota
and western WI late tonight and Wednesday morning. With very moist
low level conditions across the area tonight...low MVFR/IFR
ceilings will redevelop once again and likely continue on
Kmsp...ceilings expected to climb above 017 by noon. Confidence is
low on shower chances this evening but high on showers along with
low MVFR/IFR ceilings late tonight and Wednesday.
/Outlook for kmsp/
Wednesday...IFR morning. MVFR afternoon. Rain showers likely. Winds S-southeast 10 kts.
Thursday...MVFR. Chance IFR. Rain showers likely. Winds northwest 5-10 kts.
Friday...MVFR. Chance -shra. Winds northwest 15-20 kts with gusts 30-35 kts.