FXUS63 KMPX 272356 AAA

Area Forecast Discussion...UPDATED
National Weather Service Twin Cities/Chanhassen MN
656 PM CDT Thu Apr 27 2017

.SHORT TERM...(This evening through Friday)
Issued at 255 PM CDT Thu Apr 27 2017

Cooler than forecasted temperatures this afternoon will aid in
slightly cooler overnight lows as skies clear overnight. The
reason for no frost headlines across most of southern and central
Minnesota, with the exception along the Minnesota River Valley,
and along the Iowa border, is because of the stronger surface
winds. These winds coupled with skies not clearing until later
this evening, will keep from widespread frost from forming. In
addition, last night low temperatures were in the 20s, and lower
30s and adding a headline of frost when temperatures have already
been close to freezing, doesn't support this scenario.

The growing season has started in some areas of southern
Minnesota, especially in the Mississippi River Valley southeast of
the Twin Cities. But, other areas of Minnesota, and west central
Wisconsin have remained cold enough for any outdoor planting to
be minimal. Only those who have tender plants outside should be
aware that temperatures will lower to freezing or below for
several hours tonight.

More sunshine is expected tomorrow with temperatures rebounding
into the 40s and 50s. Late tomorrow, mainly after 3 pm, thickening
clouds from the south will advect northeast across south
central/southeast Minnesota. There could even be a few showers
near Albert Lea before 5 pm.

.LONG TERM...(Friday night through Thursday)
Issued at 300 PM CDT Thu Apr 27 2017

The main focus in the later periods remains the significant storm
system that will impact the area Sunday into Monday. There is little
doubt that the system will produce widespread moderate to heavy
precipitation, but there remains plenty of doubt on exactly where
the heaviest precipitation will occur, and how much will take the
form of snow. Prior to then, we may have a little rain near the I-90
corridor tomorrow evening as a shortwave trough feature ripples
along the baroclinic zone to our south. Otherwise, things look to be
dry with below normal temperatures across the area through the day
on Saturday. After that we will await the system that will be
ejecting out of the southwest U.S., and ultimately leading to the
development of a surface low that will intensity as it moves from
the southern Plains on Saturday to the upper Mississippi
Valley/western Great Lakes by Monday. All of the deterministic
guidance is in agreement in terms of this big picture scenario.
However, there remain some significant differences in timing,
strength, and track of the surface low and associated features.

The ECMWF remains the deepest, slowest, and farthest west of the
deterministic runs, and at this point am certainly not willing to
pick a particular solution on which to base the forecast. So, a
blended solution, which reasonably mirrors the GEFS guidance,
remains the wisest course of action. With that in mind, we should
see precipitation spread into the area from the south Saturday
night, then overspread the entire area on Sunday. Currently expect
the majority of the initial surge of precipitation to be in the form
of rain with boundary layer temperatures well above freezing,
although a change to snow would be possible where the heaviest
precipitation in that surge occurs, or if a faster solution such as
the NAM suggests were to occur. However, as the deformation zone
sets up later Sunday into Monday all of the guidance drops
temperatures through the column, as would be expected given dynamic
cooling from the stronger vertical motion and some downward
transport of colder temperatures owing to precipitation drag. Even
the ECMWF, which is by far the warmest with 925-700 mb temperatures,
cools them dramatically in the deformation zone precipitation axis
Sunday night. So, where that forcing and associated precipitation
sets up will likely see things transition to all snow for a period
of time when precipitation is heaviest, and significant
accumulations look like a possibility. The problem is, there is a
wide variation in the guidance on exactly where that will setup,
making it difficult to go with a prolonged period of just snow for
any particular area at this point in time. A look at GEFS snowfall
probabilities shows a southwest to northeast oriented swath of
greater than 70% potential for 4+" of snow from southwest Minnesota
into northwest Wisconsin, with some probabilities still near 50% for
greater than 10" (it should be noted, however, that a 13:1 ratio is
used, which would likely not be attained).

A comparison of local latent heat sensitivity model runs from the
WRF-ARW adds some additional food for thought. The solution that
takes latent heating into account, which is the appropriate thing to
do, is farther east and weaker than the non-latent heat run. This
would suggest that the impacts of latent heat release may be acting
to keep the system more south/east, which would argue against the
ECMWF solution. However, the divergence in these sensitivity runs
occurs early in the forecast cycle, by Friday evening, when things
are just starting to kick out of the southwest U.S., making it
difficult to interpret the impact of subsequent differences.
However, this might at least suggest that we will see solutions come
into better agreement when we get to the 00Z Saturday cycle. After
the Sunday/Monday system we'll see cool conditions along with some
rain/snow showers into Tuesday. High pressure then looks to build in
from the west with moderating temperatures from Wednesday into
Thursday, along with some chances for light rain in the return flow
on Thursday.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 656 PM CDT Thu Apr 27 2017

MVFR cigs continue early this evening area wide, but some
improvements are noted to the west. Tough to say when things will
trend VFR, but expecting that improvement shortly at AXN/RWF and
the other TAF locations late this evening or even overnight.

KMSP...Pushed back the timing of VFR cigs several hours, and it
may not be long enough depending of things trend slower in the
next couple hours. The rising cigs across western MN leave some
hope they may reach MSP later this evening.

Sat...VFR. Winds NE 10-15 kts.
Sun...MVFR/IFR with RA or SN, possibly becoming LIFR/+SN Sunday
night. Winds NE 15-20G30kts.
Mon...IFR/SN ending late. Winds N 15-20g30kts.




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