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FXUS63 KMPX 212300

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

.UPDATE...For 00z Aviation discussion below


.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 00Z TAFS through 00Z Thursday evening)
Issued at 600 PM CDT Wed Mar 21 2018

MVFR conditions will prevail through Thursday morning west of the
Interstate corridor. Areas along and to the east of I-35 will see
mostly VFR. Mostly skies over WI will gradually work westward 
across MN sites Wednesday afternoon, with all sites expected to be
VFR by the start of the next TAF period (00z Thursday). Winds 
will be light and variable to calm, and then increase to 5-8 knots
from the east on Wednesday.

MVFR cigs keep attempting to sneak into KMSP from the west, but at
worst would be a temporary ceiling around 3KFT this evening
(00z-04z). Otherwise ceilings should stay around 3500 feet, then
scatter out on Thursday afternoon. 

FRI...VFR early becoming MFVR with -RA/-SN. Wind ESE 10G15 kts. 
SAT...MVFR with Chc -RA/-SN. Wind ESE 10G15 kts.
SUN...VFR. Wind SE 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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