Winter Weather Advisory 11, NWS Twin Cities, MN

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
308 PM CDT Sat Apr 29 2017

.SHORT TERM...(This evening through Sunday)
Issued at 308 PM CDT Sat Apr 29 2017

The primary concern during this time period is timing of the onset
of precipitation. Models, particularly the NAM, have come in slower
with the north/northeastward expansion of rain lifting out of Iowa
overnight. Given the tendency for precip onsets to be delayed in
(dry) northeasterly flow regimes, have tweaked the hourly forecast
trends to reflect a slower onset closer to what the NAM suggests.
This means that much of the forecast area with exception of southern
MN could stay dry until after 12z, with rain not reaching the I-94
corridor until the mid to late morning hours on Sunday. Would not be
surprise to see a few snowflakes mix in early Sunday morning, but
overall liquid precip will be the main type until later Sunday
night. Could hear some thunder as well on Sunday afternoon,
primarily across southern MN.

Have also increased anticipated wind speeds on Sunday, given the
notable pressure falls as the surface low ceter lifts into southern
Iowa on Sunday afternoon. Expect sustained speeds to near 30 mph
from west central into south central MN, with breezy conditions
across the entire forecast area.

.LONG TERM...(Sunday night through Saturday)
Issued at 308 PM CDT Sat Apr 29 2017

If you would like to see more "Spring-like" weather, there is some
light at the end of the tunnel (forecast period). However, until
we get to that the focus continues to be the significant Spring
storm system that will be impacting the area at the beginning of
the latter portion of the forecast. The guidance has come around
into fairly good agreement in terms of the track and timing of the
upper shortwave trough and surface cyclone, with differences in
timing and precipitation amounts much less than they had been.
Ensembles have also come into better agreement with GEFS
probabilities for various threshold QPF amounts showing higher
values. Leaned heavily on a blend of the GFS and ECMWF for the
forecast details, which lines up fairly well with GEFS
probabilities and WPC guidance (for both total liquid and snowfall
amounts).

Sunday night we will see the surface low track from southwest Iowa
into western Wisconsin. A band of moderate to heavy rainfall will
lift north through the area in association with strong warm
advection and 500-300mb DPVA. Elevated instability looks to be
present above 750mb or so, and SREF calibrated thunderstorm
probabilities reflect that potential. So, continued to include the
possibility of embedded thunder as this area of precipitation
moves north through the area. Meanwhile, the deformation zone
north of the 500mb low and northwest of the surface cyclone will
be setting up during this same time period. Models continue to
show a good area of mid-level frontogenesis from eastern South
Dakota and southwest Minnesota northeast into north central and
northeast Minnesota Sunday night into Monday. This is coincident
with forcing suggested by upper level divergence associated with
a coupled upper jet, and some elevated upright and symmetric
instability. So, still expecting significant precipitation in this
southwest to northeast oriented band. As things quickly cool due
to vertical motion and saturation through the column, expect a
quick changeover to snow in the deformation zone. With the
guidance in better agreement, snowfall amounts have been nudged up
a bit more, and are now quite similar to the 50th percentile WPC
probabilistic snow guidance. This is still below 6" across most of our
area, but it seems somewhat likely that further upward
adjustments could occur as we're better able to pin down the
changeover timing, location of heaviest precipitation in that
axis, and the liquid:snow ratios. So, those from west central into
central Minnesota should be prepared for plowable snow amounts,
and some totals that could certainly be higher than the current
values (there is always just a lot of uncertainty with amounts in
this moisture laden yet mild spring systems). With these thoughts
in mind, as well as some mixed precipitation, and fairly strong
winds for part of the time period, will be going with a Winter
Storm Watch for our west central into north central counties for
Sunday night and Monday.

We should see some showers (rain/snow) linger Monday night into
Tuesday in the wake of the system. High pressure, albeit weak,
works in Tuesday night into Wednesday. However, some light rain
could occur Wednesday and Wednesday night as a surface trough
moves through together with the leading edge of warm advection. We
will then see the pattern finally change a bit with a large scale
upper ridge working into the region Friday into Saturday bringing
temperatures back to near or perhaps above normal readings.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1246 PM CDT Sat Apr 29 2017

VFR conditions are expected through the majority of the period,
with quickly deteriorating conditions Sunday afternoon as the next
system arrives. High clouds will be on the increase, with gusty
northeast winds. Will see some decline in wind speeds this
evening, but an early return of the gusts on Sunday morning as the
pressure gradient tightens with the approaching trough. The main
change this TAF issuance was to slow down the onset timing of
precip by 3-6 hours, along with slowing the degradation to MVFR
cigs on Sunday morning. For central MN/WI sites, onset now looks
to be delayed to between 15z and 18z.

KMSP...
VFR is expected through at least daybreak Sunday. Delayed precip
onset and attendant MVFR cig/vsby reductions to 18z, but could
start seeing showers near the field with MVFR conditions as early
as 15z. Northeast winds lose gustiness this eve, but return by
daybreak Sunday.

/OUTLOOK FOR KMSP/
Mon...IFR/RA mixing with SN AFTN/EVE. Winds N 15-20G30kts.
Tue...MVFR ceilings possible. NW 10-15 kts.
Wed...VFR ceilings. S 10 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
&&

SHORT TERM...LS
LONG TERM...
AVIATION...LS

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