Scientific Forecaster Discussion
fxus63 kmpx 231748
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1248 PM CDT Sat Sep 23 2017
Update...for 18z aviation discussion below
Short term...(today and tonight)
issued at 355 am CDT Sat Sep 23 2017
As of 3am this morning, the Twin Cities was still 81 degrees and
much of southern Minnesota and western WI were in the mid to upper 70s.
However, the front has made its way into western Minnesota, where
continuous showers and thunderstorms have been ongoing through the
overnight hours. The front was positioned from just east of
Alexandria through the southwest corner of the state.
This front will continue to be the key factor for our weather the
next few days. The first surface low has passed to our north with
the second developing across eastern Colorado and hence a frontal
boundary connecting the two. The front will remain mostly
stationary from its current position today, meaning western Minnesota will
continue to see chances for showers and thunderstorms, and highs
limited to the 60s west of the boundary. East of the boundary will
likely be dry today, with southerly winds and highs in the mid 80s
to lower 90s. Dew points in the warm sector will once again be
around 70 degrees. The thermal ridge won't be as strong as
yesterday, so expect temperatures a few degrees cooler, but will
still yield apparent temperatures in the 90-95 degree range across
eastern and south central Minnesota, as western WI as well.
The upper jet will strengthen north of the Dakotas later today and
the flow will become more amplified with time. This will begin
pulling the frontal boundary back northwest, so expect the Dakotas
to light up with widespread precipitation by this evening. The main
change for US was to push pops back farther west tonight as the
front looks to move back into the Dakotas, which will bring
southerly flow back across the entire forecast area overnight, while
the vast majority of the precip will be amidst the large scale
forcing behind the front. So, don't be surprised if eastern Minnesota
including the Twin Cities sees no precipitation through at least
Long term...(sunday through friday)
issued at 355 am CDT Sat Sep 23 2017
The main forecast concern in the extended period is timing of the
precipitation late Sunday/Monday, and how long will it last early
First, models are fairly consistent on weakening the upper ridge
over the Great Lakes/Ohio Valley late Sunday. This will allow for
the mean trough out west to slowly lift and move northeastward. This
will also allow for the surface cold front to finally move eastward
across the upper Midwest late Sunday. Sunday remains very warm and
humid for the eastern 1/2 of mpx forecast area. Highs in the 80s
will be common with heat indices once again near 90 to 95 degrees.
Contrast this to western Minnesota where periods of showers and
thunderstorms will keep temperatures in the 50s/60s. It still looks
reasonable for excessive rainfall in portions of west
central/central Minnesota as several waves of precipitation are
expected. See the latest wpc outlook for flooding potential.
Once the mean trough weakens and lifts to the northeast across the
northern plains, the surface front will begin to move east. This
will lead to showers and thunderstorms to become more numerous in
eastern Minnesota and western Wisconsin late Sunday night/Monday.
Depending upon how fast the mean long wave trough moves northeast
across the northern plains, and the mean flow becomes
west/northwest, will determine how fast the precipitation exits
Minnesota, and western Wisconsin. The best time frame will be
western Minnesota by Monday morning, and eastern Minnesota by
Tuesday morning. West central Wisconsin may have showers/storms
linger until Tuesday night.
By midweek, the mean upper flow becomes north/northwest. This will
usher in the coldest air mass since last Spring. Highs in the
40s/50s in the north, with 50s/60s in the south look reasonable for
Wednesday/Thursday and Friday. Depending upon the amount of cloud
cover and surface wind speeds late in the week, will determine if
frost is possible in portions of central/southern Minnesota, as
well as western Wisconsin.
Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1248 am CDT Sat Sep 23 2017
IFR/MVFR cigs will primarily affect the far northwest part of the
taf area (kaxn), although fleeting broken ceilings around 3kft or
just under will be possible slightly farther east and south
(kstc/krwf/kmkt) during the afternoon. Rain showers/ts chances are
expected to stay confined to kaxn as well. As the front wobbles
back into the eastern Dakotas overnight, precipitation and low
cigs retreat back west as well. This will mean kaxn will mostly
likely be precip-free after 05z, with VFR cigs arriving by early
Sunday. Other sites should therefore stay dry tonight with VFR
conditions. With the front back over the Dakotas, southerly winds
are expected to return for Sunday, with gusts into the upper teens
to lower 20s (knots). Showers and thunderstorm chances will inch
into the western portion of the area again on Sunday p.M.
expecting precip and MVFR cigs to stay to the west and northwest
of msp through the period.
/Outlook for kmsp/
Mon...MVFR/tsra. Winds S 5-10 kts.
Tue...chc MVFR/thunderstorms and rain early. Winds northwest 5-10 kts.
Wed...VFR. Winds west 5-10 kts.