Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 kmpx 210349 

Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1049 PM CDT Fri Sep 20 2019

issued at 1037 PM CDT Fri Sep 20 2019

Updated to include 06z aviation discussion below.


Short term...(this evening through Saturday night)
issued at 300 PM CDT Fri Sep 20 2019

Morning fog and low clouds was slow to dissipate this morning due
to deep low level moisture and weak mixing in the boundary layer.
However, the boundary layer winds are expected to increase
significantly over the next 12-24 hours, leading to gusty winds
overnight and Saturday.

The main forecast concerns are timing and areal extent of the
thunderstorm activity later tonight, and into Saturday.

First, I can't rule out an isolated storm this afternoon, and
early evening across the upper Midwest. The combination of
diurnal heating and a weakening cap, should lead to very isolated
storms, but there remains no trigger for initiation. Forecasters
need to watch for any building cumulus field and weak convergence
near the surface trough for development.

Regional satellite imagery noted a concentrated area of clouds and
weak convection across eastern Kansas this afternoon. This area of
convection will move rapidly northward during the evening, and
will trigger showers and thunderstorms along it. The cold front,
and main upper level trough, will still be far enough away in the
Dakotas to not have any impact until later in the day. The
overnight and first part of Saturday will be associated with the
low level jet and deep moisture advection. I can't rule out a few
strong storms as deep layer shear increases overnight. Heavy
rainfall will be the main threat as pwats continue to be very
anomalous for late September. The best time frame of precipitation
will occur after midnight, and through noon Saturday. By Saturday
afternoon, the weakening low level jet and dry mid-levels should
decrease chances of precipitation. However, the cold front should
initiate some activity by late Saturday afternoon in eastern
Minnesota/west central Wisconsin. Due to deep layer shear along
the cold front, any storms will be capable of becoming severe. See
the latest Storm Prediction Center outlooks for more information.

Behind the cold front Saturday night, humidity levels will rapidly
decrease as a Canadian air mass moves overhead.

Long term...(sunday through friday)
issued at 300 PM CDT Fri Sep 20 2019

Overall pattern shifts to more zonal aloft into early next week,
with cooler air in the wake of the Saturday trough. The pattern
shifts abruptly/more amplification again by late in the week as
more trough energy digs into the southwest Continental U.S. With ridging over
the central and east.

Clouds will gradually depart Sunday with cold air advection pattern in the wake
of the cold front and upper low. More west to east flow develops
into early next week with the deterministic models timing of the
midweek frontal boundary still in question. The GFS remains
fastest with the front, moving it through Tuesday/Tuesday night
with perhaps some residual showers to the northeast Wednesday.
Otherwise, the European model (ecmwf) bring the boundary through about Wednesday.
Following this feature, the models lift a closed low northeast
across the Central Plains toward the western Great Lakes Thursday
night into Saturday as more significant trough energy drops in
over the Pacific northwest and eventually southwest into the
weekend. This feature promises to be moisture loaded and has a
good shot of producing heavy rain across portions of the County Warning Area
Friday/Saturday period. Will continue to monitor overall model
trends into the coming week.

Temperatures remain above normal for the most part for the first
part of the extended period. May see near normal highs Sunday.
Otherwise, warmer than normal trending back to cooler late in the
week depending where the frontal boundary sets up across the
northern plains.


Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1037 PM CDT Fri Sep 20 2019

All sites to start out as VFR this set but the complications begin
after midnight with the timing of potentially multiple rounds of
precip from around daybreak then thru the day Saturday. As a cold
front slowly pushes east over the Dakotas tonight, rain showers/thunderstorms and rain will
develop over western Minnesota overnight then steadily push east,
reaching eastern Minnesota around daybreak then continuing into western
WI during the late morning hours. Conditions will likely drop to
MVFR levels with the morning round of precip, and this round will
have the highest potential for cb/ts. Once this round moves thru,
then there may well be a break in the action and not just lesser
coverage. In addition, IFR ceilings look likely to develop during
the day tomorrow in close vicinity of the cold front. Additional
showers may reach the terminals tomorrow afternoon but intensity
is expected to be less and cumulonimbus/ts chances are much lower.
Conditions look to improve around tomorrow evening and beyond.

Kmsp...two main rounds look to push thru msp, with best timing
around and shortly after 12z then a second round late tomorrow
afternoon. Chances for ts/cumulonimbus higher for the morning precip, so
have omitted mention for the afternoon tempo at this point. Breezy
conditions also likely with slowly veering wind direction.

/Outlook for kmsp/
sun...VFR. Wind west 5-10 kts.
Mon...VFR. Wind light and variable.
Tue...VFR. Wind SW 5-10 kts.


Mpx watches/warnings/advisories...


National Weather Service Glossary of Abbreviations