• Fri. Sep 22nd, 2023

Not much has changed in tonight’s forecast


1. An outbreak of severe thunderstorms is likely early this evening.
A line of thunderstorms will race northeast across southern and
eastern Minnesota into Wisconsin between 5 PM and 9 PM. Widespread
wind damage from gusts possibly exceeding 80 mph will accompany this
serial derecho. Tornadoes are also possible, a couple of which could
be strong/intense.

2. A potentially historic high wind event tonight with gusts of 60
to 65 mph across the entire region.

3. Flash freeze across western Minnesota as temperatures plummet
from the 40s to the teens overnight. Some snow accumulation also

The storm system impacting the region today and tonight is one a
meteorologist will remember throughout their career. The highly
dynamic and very unusual storm system will bring almost every
weather type we experience around here within the next 24 hours.
Most weather elements are record breaking for this time of year,
including temperatures, moisture, wind, and various severe weather
parameters. Because there are multiple hazards to be concerned about,
each will be broken down individually below.


Southerly flow has enveloped the entire region. Moisture is lifting
north quickly over a melting snowpack and dew points have already
risen into the lower 40s across southern and eastern MN. Conditions
will deteriorate from south to north early this morning as the
moisture advection continues. A Dense Fog Advisory is in effect.
Tough to say when conditions will improve, but guidance suggests the
fog will lift from south to north through early afternoon. That may
be optimistic with the amount of snow still on the ground. Areas of
drizzle will also be likely.


Southerly flow will continue into the evening. Temperatures will rise
steadily up and until the line of thunderstorms moves through. Highs
are expected to reach the lower 60s (record breaking) across
southern/eastern MN and parts of WI, despite the snow pack. The
degree of moisture working north and the volatile wind profile aloft
will negate just about any impact the melting snow will have on the
boundary layer. Moderately steep mid level lapse rates of 7 to 8 C/km
will overspread a moist and very warm inversion just above the
surface to generate 600-1200 J/kg MUCAPE. The system will advance
northeast and a cold front will initiate thunderstorm development
across eastern NE this afternoon. The line will race northeast
between 65-80 mph and reach Redwood Falls and Fairmont around 6 PM,
Twin Cities to Rochester around 7 PM, then Eau Claire between 8-9 PM.
Extreme wind shear in the low levels and aloft will bring a
significant risk of widespread wind damage and gusts could exceed 80
mph with this serial derecho. Some QLCS tornadoes are also possible,
a couple could be strong/intense, especially if dew points can
indeed reach the upper 50s as currently predicted. There is a
Moderate Risk highlighted by the SPC. Moderate risks are rare even
during peak severe weather season. They are unheard of in December.

To emphasize…the motion of this line is expected to be incredibly
fast. Conditions will deteriorate very rapidly. Unless preparations
are made ahead of time, it may be hard to take adequate shelter when
one notices storms beginning to approach! These are expected to be
high end damaging wind producers, with embedded strong tornadoes
also possible, so pay attention to warnings issued later today.


As the low center pushes northeast into northeast Minnesota, very
strong cold air advection will follow. Steepening low level lapse
rates overnight will tap into 50-60 kts aloft. There will be a 2-4
hour period where wind gusts could reach 65 mph before the strongest
winds ease from southwest to northeast. The High Wind Warning has
been expanded to the all of the counties within the MPX forecast


Showers and thunderstorms along the track of the low will turn to
snow and possibly freezing drizzle immediately behind the low as cold
air spills in. Temperatures will fall from the 40s to the teens and
20s, setting up a flash freeze. Snow accumulation potential is
rather uncertain. The convective nature of the precipitation could
allow for a rapid switch over to heavy snow that could accumulate for
several hours. Some hi-res model guidance indicates 3 to 6 inches
possible near Morris, Alexandria, eastward toward Brainerd. Posted a
Winter Weather Advisory for the plethora of winter hazards as temps
drop, but if the heavy snow coincides with 60 mph gusts for longer
than forecast, then Blizzard Warnings could be necessary.

Historical data:


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