Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 212120

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
418 PM CDT Wed Mar 21 2018

.SHORT TERM...(This evening through Thursday)
Issued at 418 PM CDT Wed Mar 21 2018

Due to plenty of low-level moisture in place and a weak shortwave
trough traversing over the forecast area this afternoon, low-level
cloud cover remains prevalent over Western and Central MN. Mid-level
cloud cover is widespread over Eastern MN and low-level clouds are
expected to spread east into Eastern MN and far Western WI this
evening. Patchy fog is possible over parts of West Central MN and
Southern MN during the early to mid-morning hours as sufficient
moisture remains in place near the surface. Winds will remain
light overnight with temperatures cooling into the upper 20s in
Western MN and mid to upper teens in West Central WI.

Upper level ridging will begin building-in on Thursday as surface
high pressure remains over the region. So, we are expected to remain
dry for Thursday with temperatures expected to warm into the mid
forties. Cloud cover is expected to gradually decrease with mostly
clear skies expected by Friday evening.

.LONG TERM...(Thursday night through Wednesday)
Issued at 418 PM CDT Wed Mar 21 2018

It continues to look increasingly likely most of western and
southern Minnesota, roughly south of I-94, will see heavy snow
late Friday and Friday night. A Winter Storm Watch has been issued
for these areas.

The forecast remains complex, but model consistency has improved
and is pretty good today - enough so that confidence has increased
for a watch. NAM made a jump westward on the 12Z run, whereas the
ECMWF and the majority of its ensemble members continue to spread
light QPF too far east. The GFS splits the difference (which is
now what the 18Z NAM is showing as well) and is generally
preferred for this forecast package. Confidence is high there will
be a very tight gradient with this system, and that gradient once
again will set up near the metro. The NAM and GFS advertise this
quite well in their QPF fields.

Snow will develop over the Dakotas late Thursday night and spread
eastward to west central Minnesota late Friday morning as a well
developed shield. Temperatures will warm well into the 30s or
lower 40s across the CWA, but a dry easterly flow will allow for
wetbulbing these surface temperatures down quickly once the heavy
precip moves in. There may be a period of rain in the initial
stages of precipitation before transitioning to heavy snow. This
transition should take less than a couple hours to complete.

There are considerable amounts of moisture for this system to
work with. PWATs increase to as high as 0.75 inches thanks to a
15kft deep, nearly isothermal supercooled water layer. That PW is
quite high for a snow storm. In addition, strong lift through a
20-25 kft deep layer, including the DGZ, should make for efficient
snowfall production and the deep isothermal layer below the DGZ
will make for large aggregate flakes. Snowfall rates could
approach 2 inches per hour in this setup.

The biggest question is how far east this snow will build before
stalling and dropping south. As mentioned above, models are in
pretty decent agreement overall but there still will be minor shifts
and a further tightening of the gradient going forward. The watch
includes the I-94 counties due to the possibility of a shift, and
because the far western/southern portions of those counties could
see over 6 inches.

The snow will wind down by Saturday morning, but could linger in
an inverted trough into the mid to late morning hours. The loss of
ice in the mid levels could also lead to patchy freezing drizzle.

The next system will arrive Sunday night and linger into
Wednesday. It will take the form of a panhandle hook, but will
move much slower than is typical. A mix of rain and snow
continues in the forecast and will probably be dictated mostly by
the diurnal curve.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1247 PM CDT Wed Mar 21 2018

Widespread IFR/MVFR conditions continue over Western and Central
due to low ceilings. Due to plenty of low-level moisture and weak
wave progressing through the area, we can expect the extensive
cloud cover to continue through the remainder of the day west of
EAU. Winds in Western MN will remain predominately southeasterly
at 5-10 kts through the evening while winds in Central MN and West
Central MN continue to be light and variable. Winds by late
tomorrow morning will be predominately easterly. Otherwise, due
to the low-level moisture, patchy fog will likely cause MVFR
visibilities and possibly IFR ceilings at AXN during the mid-
morning hours on Thursday.

KMSP... Midlevel cloud cover will remain in place with a chance
for MVFR clouds this afternoon and through the early overnight
hours. Otherwise, cloud cover will gradually clear on Thursday
with VFR conditions likely during Thursday afternoon. Otherwise,
winds will remain light and variable before becoming predominately
easterly Thursday afternoon.

THU Night...VFR. Wind ENE 5 kts.
FRI...VFR early becoming MFVR with -RA/-SN. Wind ESE 10G15 kts.
SAT...MVFR with Chc -RA/-SN. Wind ESE 10G15 kts.


MN...Winter Storm Watch from Friday afternoon through Saturday
     morning for MNZ041-048-057-065-067-074-075-082>084-091>093.

     Winter Storm Watch from Friday evening through Saturday morning
     for MNZ049-058>060-066-068>070-076>078-085.

     Winter Storm Watch from Friday morning through Saturday morning
     for MNZ047-054>056-064-073.



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