Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 181751
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1251 PM CDT SUN SEP 18 2016

.SHORT TERM...(Today and tonight)
Issued at 350 AM CDT Sun Sep 18 2016

The main forecast concern is timing and coverage of thunderstorms
later this afternoon, and into the overnight hours. Secondary
concerns are higher temperatures along the Buffalo Ridge in western
Minnesota.

Early this morning temperatures have fallen into the low to mid 40s
across central and east central Minnesota, as well as west central
Wisconsin. Some fog had also formed, but regional satellite imagery
and local observation indicated that most of the fog was localized
to rivers, lakes, and the typical areas of west central Wisconsin.

Along the Buffalo Ridge in west central Minnesota, gusty downslope
winds developed and caused temperatures to rise slightly overnight.
As of 3 am, Canby was in the mid 60s, where a few miles to the east,
temperatures fall back into the 50s. These higher temperatures to
start out with will likely bring a few mid 80s in the typical areas
of west central Minnesota. The only concern for higher temperatures
this afternoon is the increasing cloud cover by mid/late morning.

Several of the local CAMS have most of this afternoon dry with a few
sprinkles possible as mid level moisture and lift begins to expand
across the region. Not until 00z-03z do any of the models develop
simulated "Higher" reflectivity returns along a front across
Minnesota. Due to the weak instability axis along this front and
weak surface convergence, most of the activity will be isolated-
scattered. The best potential of more thunderstorms will occur in
western Wisconsin as moisture depth increases late in the evening.

Mid level lapse rates and shear values indicated some of the more
organized storms could produce large hail and gusty winds. Latest
SPC day 1 outlook as parts of east central Minnesota and west
central Wisconsin under a marginal risk of severe thunderstorms.

.LONG TERM...(Monday through Saturday)
Issued at 350 AM CDT Sun Sep 18 2016

The long term period will begin with quiet weather, but the pattern
will become more active locally Tuesday night.  We are still
concerned about the heavy rain threat as a frontal boundary across
northern IA/southern MN will be the main focus of heavy rainfall
potential.  Severe weather will also be possible.

On Monday, we may start out with some cloud cover around, as an east
to west band of mid level moisture passes by in the upper level
zonal flow regime. But overall it should be a partly cloudy day for
most, and dew points will be quite a bit lower behind the cold front
with a dry area of high pressure pushing in from the west.
Meanwhile, our next feature of interest will be sitting off the
coast southwest of San Diego, CA.  Monday night will be quiet as
well with temepratures bottoming out in the upper 40s to lower 50s.

On Tuesday morning, that system off the coast of California will
start moving quickly northeast as it gets phased into the flow of a
large scale upper trough off in the Pacific Northwest region. It
will accelerate northeast Tuesday morning and as this occurs, the
response at the surface will be an area of low pressure developing
on the lee of the Rockies.  By Tuesday night, the warm front
associate with this low will advance north into southern Minnesota.
Very strong theta-e advection into southern MN demonstrates the warm
and moist air being advected in by 30-45 knot southwesterly winds at
850mb.  With a warm front draped across the area and plenty of broad
lift owing to that low level WAA and the strong jet to our north,
the next question is just how moist the atmosphere will be and how
unstable.  Precipitable water values increase Tuesday night as a
low level jet develops across Kansas and Nebraska, aimed at southern
MN and parts of Iowa.  PWATS of around 1.75" will be nearly off the
charts for this time of year, and that low level jet will enhance
low level convergence along the front.  In terms of instability,
MLCAPE of around 1,000-2,000 j/kg is likely.  Shear looks to be
limited, but is uni-directional which would indicate a heavy rain
threat as storms could backbuild/train along the nose of the low
level jet. A deep saturated warm layer is evident on the model
soundings as freezing levels look to be near 13-14k feet. So, it`s
all about where that front ends up Tuesday night/Wednesday morning.
The NAM would like to indicate a southern shift, but for now we felt
more confident sticking with a GFS/EC blend.

This setup for Tuesday Night matches well with local Heavy Rain
Climatology research we`ve done with the Archetype we refer to as
Warm Front 1. This Archetype matched the September 23, 2010
extreme rainfall event in southern MN which has been indicated as
one of the top analogs to Tuesday Night. So, this is certainly
something to watch as we get close to Tuesday night and can sort
out some of the details.

On Wednesday, the main upper level shortwave will move in which will
help drive afternoon thunderstorm activity across most of eastern
Minnesota and western WI.  The heavy rain threat Wednesday night
looks to be from southeast MN through west central WI.  Again,
severe weather chances look low.  Thursday looks to be our best
potential for a dry day but even so, there will enough instability
and moisture around to warrant chance POPs for thunderstorms
activity for now.  Earlier in the discussion I mentioned a trough
over the Pacific Northwest.  Well, by Friday that trough will be
quite deep over the Rockies and advancing east toward the Upper
Midwest for next weekend.  Warm and moisture advection ahead of it
and shortwave ripples breaking off will give us continued
opportunities for showers and thunderstorms which explains why there
are POPs in every period right through the end of the long term
forecast.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1250 PM CDT Sun Sep 18 2016

VFR is expected through the TAF period. A few more high clouds and drier
southwest winds at the surface should keep morning fog out of the
area tonight and tomorrow morning. There is a front passing
through this evening and a few showers (possibly a rumble of
thunder) can not be completely ruled out, but we think the chance
of these impacting the TAF sites in Minnesota are pretty low - a
little better chance in western Wisconsin.

KMSP...
A stray vicinity shower can`t completely be ruled out this
evening, but the airport is looking at dry weather for the most
part with VFR conditions. Winds will gradually veer from south to
west this evening through tomorrow morning - becoming lighter with
time.

/OUTLOOK FOR KMSP/
Tue...VFR. SW winds 5 kt.
Wed...VFR/Chc MVFR. TSRA likely. SE wind 5-15 kts.
Thu...MVFR/IFR. Chc of TSRA. NE wind 5-10 kts.

&&

.MPX Watches/Warnings/Advisories...
WI...None.
MN...None.
&&

$$

SHORT TERM...JLT
LONG TERM...SPD
AVIATION...CLF



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