Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 190445
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1045 PM CST Sun Feb 18 2018

.UPDATE...For 06Z Aviation discussion below

&&

.SHORT TERM...(This evening through Monday)
Issued at 317 PM CST Sun Feb 18 2018

A very messy short term period is in store as the cold front
working into southwest MN slides east across the area. Aloft, a
broad positively tilted long wave trough will remain across the
area. We`ll see several weak waves ripple across this strong
thermal gradient, resulting several pulses of precipitation
through Tuesday.

Biggest change for the short term is the wave coming up for Monday
is coming in slower and with a little less QPF with the 18.12
model runs as the better moisture is getting shifted off to the
east. In fact, there are some CAMs that are almost completely dry
for the MPX area through Monday, so this is still somewhat of a
low confidence forecast through the short term. For P-type, the
trick is figuring out whether ice crystals will be present, which
means it`s a snow or freezing rain situation. Though we look to
have issues the whole period with RH in the -12C to -18C dendritic
growth zone, as the boundary layer cools with continued CAA
through the period, we see the top of the boundary layer cool into
the -10c to -13c range, which should be cold enough to get
crystals going, just fine needles as opposed to nice fluffy
dendrites. Because of the cooling boundary layer, we did go with a
snowier solution than the previous forecast, mainly by going with
higher cloud ice probabilities farther to the southeast. The
snowier solution along with the overall lower QPF resulted in
lower ice accumulations in general. We now keep ice accumulations
out of the Twin Cities (mostly snow), with the highest amounts
down from Albert Lea up toward Eau Claire, though those have come
down into the 0.1"-0.15" range.

With headlines already out, we made no changes to the existing
headlines, with the new winter weather advisory a problem for the
beginning of the long term portion of the forecast.

.LONG TERM...(Monday night through Sunday)
Issued at 317 PM CST Sun Feb 18 2018

Picking up where the short term forecast left off, the next wave in
our positively tilted trough will be moving through Monday night
into Tuesday morning. We have seen in upward trend in the snow
potential with this wave, in particular for western into central
MN. We still have 3" to 5" of snow forecast in this region, but
most of this will fall between 6z and 18z Tuesday. From the
messaging standpoint, it`s been difficult trying to explain the
highest snow totals falling where there was no advisory and with a
concentrated window for heavier snow Monday night into Tuesday
morning, we went ahead and filled in the rest of the MPX coverage
area in a winter weather advisory, though we don`t start this new
segment until Monday evening.

High pressure coming in Wednesday still looks to finally drive the
thermal gradient southeast of our area Tuesday afternoon, which
will finally cut the precip off. We`ll see below normal
temperatures thanks to the high pressure with lows
Wednesday/Thursday mornings and highs on Wednesday. This cold snap
will end quickly though, with return flow setting up Thursday
that will send highs back to near normal by Thursday afternoon.

Also with that return flow comes our next chance for precip. The
ECMWF/GFS/Canadian look similar with bringing another shield of
isentropic lift and snow across much of the area, but we see
significant timing difference still. The ECMWF continues to be the
fastest, brining it through Thursday night/Friday morning, with
the GFS/Canadian trailing 12 hours behind. Given the spread,
stayed with the blended forecast, which has precip chances spread
out pretty good between Thursday night and Friday, but this will
get narrowed down as models start narrowing in on a time frame
this will occur.

For next weekend, we will maintain the general idea of a through
west/ridge east, but with less amplification. This means a more
zonal flow and temperatures getting back a little above normal as
the colder air remains , with highs expected back in the 30s. more
over the northern Rockies. Beside milder, it`s looking dry as
well.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1045 PM CST Sun Feb 18 2018

Light snow is expected to develop across central MN overnight
(primarily affecting KAXN and possibly KSTC prior to daybreak),
and become more widespread across the area on Monday morning. The
freezing drizzle/rain threat still looks to be south of the TAF
sites, with KMKT and KEAU on the edge. Expect ceilings to continue
degrading to MVFR overnight, with prevalent MVFR on Monday. There
looks to be a brief break in the precip at most sites Monday
evening, another round developing early in the next period (06z
Tuesday). Will need to re-evaluate the freezing precip again with
this round, as some models are indicating a slight northward shift
of the threat, possibly as far north as KMSP. Northerly winds
tonight become more northeasterly for Monday, with gusts to
between 20 and 25 knots.

KMSP...
15Z still looks to be the most likely time for snow onset at KMSP.
Still expect the freezing rain to stay south and east of the site
this period. Northerly winds shift to the northeast and become
gusty tomorrow during the daytime hours.

/OUTLOOK FOR KMSP/
Tue...MVFR/-SN. Winds N15 kts.
Wed...VFR. Winds NW becoming SW 5 kts.
Thu...VFR. Winds S 5 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory until 6 PM CST Tuesday for WIZ014>016-
     023>028.

MN...Winter Weather Advisory until 6 PM CST Tuesday for MNZ053-
     060>063-065>070-073>078-082>085-091>093.

     Winter Weather Advisory from 6 PM Monday to 6 PM CST Tuesday for
     MNZ041>045-047>052-054>059-064.

&&

$$

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



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