Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 212338
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
638 PM CDT Thu Mar 21 2024

.KEY MESSAGES...

- Snow is expected tonight. The heaviest area has shifted south
  a bit and amounts have increased with some 5 or 6 inch totals
  likely along and just south of I-94. The Winter Weather
  Advisory was expanded to a few more counties south/west.

- Confidence is increasing quickly for a significant winter
  storm late Saturday night into Monday night or Tuesday.

- Biggest change today with the weekend system has been a shift
  of the track a bit to the east, with the rain/snow line also
  shifting east. Snowfall totals over a foot appear likely
  across a large portion of Minnesota and northern Wisconsin.

- The snow will be wet and heavy near the rain/snow line, making
  it difficult to move.

&&

.DISCUSSION...
Issued at 300 PM CDT Thu Mar 21 2024

TONIGHT THROUGH SATURDAY...

A band of light snow south of I-94 has been battling dry air for
much of the day, but snow has been reaching the ground across west
central MN. Visibilities there have been as low as 3/4 mile at
times, signaling some light accumulation likely occurred. The main
event is gathering itself over Montana and southern and western
North Dakota this afternoon. By early evening (~7 PM) it will expand
eastward into west central MN, and by mid to late evening (9-10 PM)
will begin reaching eastern MN and western WI. The large scale
picture of this system`s evolution hasn`t changed a whole lot, but
the guidance today has focused the core of the heaviest totals a
little farther south of where it was last night. Areas just south of
I-94 are now expected to be the winners of some of the higher
totals of 5 or 6 inches. This is largely a function of how long
the snow will persist more so than its intensity. Elsewhere, 2-
4 inches remains a solid bet. The back edge of the snow will
clear the area late tonight or early Friday morning, making for
a rather slow morning commute.

High pressure will settle over the Upper Midwest Friday afternoon
into Saturday. Winds will ease Friday evening and clear skies with
fresh snowpack could send temperatures into the single digits in the
favored colder locations.

SATURDAY NIGHT THROUGH TUESDAY...

A deep trough over the West Coast with a very strong 150-175 kt
upper jet rounding the base of that trough will come ashore the
northern Baja and Arizona Saturday night. Surface low pressure will
spawn on the Colorado Front Range by early Sunday. A deep, strong
low level southerly flow will transport 0.75-1.0 inch pwats
northward to Iowa late Saturday night into Monday. Broad upper lift
will result in widespread precip development well north of the
surface system beginning late Saturday night across the Plains.
There may be a lighter band displaced from the main development of
precipitation from North Dakota to southern Minnesota Saturday, but
this likely wouldn`t amount to much. The slug of moisture will
continue east to the Upper Midwest by Sunday and remain in place
until the surface low tracks northeast to Wisconsin early Tuesday. A
continuous stream of deep moisture transported on a 50-60 kt LLJ,
and strong forcing on the nose of this jet across the southern half
of MN, will lead to nearly continuous moderate to heavy snow across
the region Sunday and Monday.

As the surface low nears Monday afternoon and passes through Monday
night, milder air could lift in and lead to rain mixing with the
snow. The trend over the past few model runs has been to shift the
track southeast, which would result in more snow than in previous
forecasts across southern/eastern MN and WI. The consensus is now a
track over southeastern MN, which is ideal for heavy snow locally.
Yesterday the track was more over the Twin Cities metro with much
milder air in the 40s reaching areas as far west as eastern and
central MN. Low clustering has also tightened in ensembles, and the
faster solutions (GFS) have slowed while the slower solutions
(ECMWF) have become more progressive. Hence, confidence is
increasing but there may be some shifting yet to go at this range.
Given the track forecast as it stands now, believe NBM temperatures
Monday may be too warm with the presence of widespread moderate to
heavy precip continuing and NNE low level flow. Some bias correction
may be contaminating these numbers too. Lowered temps a bit closer
to the wetbulb temperatures in the mid to upper 30s. Even with these
temperatures, p-type could be an issue and any small changes in the
eventual track could bring significant changes. Whether or not it
turns to rain or a mix of rain/snow Monday, the main takeaway here
is what ever accumulates will be very wet and heavy and quite
difficult to move. Blowing snow remains a possibility, especially
earlier in the event when higher snow ratios are expected, and where
precip type remains all snow.

As for accumulation numbers, it`s fairly safe to say a lot of the
region will surpass 12 inches. Details beyond that at this point
have yet to be worked out. There are a lot of driving factors that
could influence total snow, the largest being the storm track and
thus any potential turn over to rain. Compaction of wet snow as the
depth increases is another. Adding up 6 hour snow totals for 2 days
(as our storm total maps do) will yield a higher storm total number
than will ever be seen on the ground as a depth due to the
compaction. It will be a challenge to message this going forward.

Boiling it all down, this will be a highly impactful storm.
Winter Storm Watches are likely to be issued soon.

MID TO LATE NEXT WEEK...

The trough will lift out Wednesday and ridging will build across the
Plains. Another system passing through the Rockies and may develop
into another system to watch late week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 638 PM CDT Thu Mar 21 2024

Snow with MVFR to IFR has already started across western
Minnesota and will spread to the rest of the terminals over the
next few hours. During the heaviest snow a few hours of LIFR are
possible. The good news is that there is little wind so the
impacts are tied to falling snow exclusively. So the LIFR will
be tied to 1/2SM visibility with the heaviest snow this evening
and tonight due to the intensity of the snowfall. Snow will
come to an end Friday morning with a return to VFR for most of
the day. Winds will also pick up some with gusts up to around 18
kts at some terminals.

KMSP... Light snow is imminent at the terminal. The heaviest
snow with IFR to LIFR visibility will be from 03Z to 07Z. Snow
should lighten up with MVFR by 10Z and VFR again by 14Z. Winds
will be light before picking up tomorrow after the snow ends.

/OUTLOOK FOR KMSP/
SAT...VFR. Wind N 5-10kts.
SUN...IFR/LIFR +SN likely. Wind E 25G30-40kts
MON...IFR Likely. Chance LIFR/+SN early. RA/SN mix likely
afternoon-evening. Wind ENE 15G25-30kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...Winter Weather Advisory until 10 AM CDT Friday for Anoka-
     Benton-Carver-Chippewa-Chisago-Dakota-Douglas-Hennepin-
     Isanti-Kanabec-Kandiyohi-Lac Qui Parle-McLeod-Meeker-
     Mille Lacs-Morrison-Pope-Ramsey-Renville-Scott-Sherburne-
     Sibley-Stearns-Stevens-Swift-Todd-Washington-Wright.
     Winter Weather Advisory from 10 PM this evening to 10 AM CDT
     Friday for Freeborn-Goodhue-Le Sueur-Rice-Steele-Waseca.
WI...Winter Weather Advisory from 10 PM this evening to 10 AM CDT
     Friday for Barron-Chippewa-Dunn-Eau Claire-Pepin-Pierce-
     Polk-Rusk-St. Croix.

&&

$$

DISCUSSION...Borghoff
AVIATION...NDC


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