Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 191138

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
638 AM CDT MON SEP 19 2016

.UPDATE...For 12z Aviation discussion below


.SHORT TERM...(Today and tonight)
Issued at 400 AM CDT Mon Sep 19 2016

An area of showers is making progress across the northern half of
South Dakota early this morning. Said activity corresponds well with
700-600mb 2-D frontogenesis off of the RAP. This narrow ribbon of
lift is forecast to slide across the heart of Minnesota this
morning, primarily along and north of Highway 12 in Minnesota and
Interstate 94 in Wisconsin. The main caveat with this activity is
the lack of moisture below 10Kft. This will limit rainfall amounts,
with a couple hundredths of an inch the maximum amount most areas
will receive today.

This afternoon skies clear out from west to east, and temperatures
while not as warm as Sunday, will still climb 5-8 degrees above
normal. Expect highs in the 70s.

Tonight skies stay mostly clear and winds, while southwesterly, will
be pretty light around 3-4kts. Temperatures will lower into the
upper 40s to mid 50s range, which will mean the potential for patchy
fog primarily over west central Wisconsin.

.LONG TERM...(Tuesday through Sunday)
Issued at 400 AM CDT Mon Sep 19 2016

Heavy rainfall and subsequent flooding is still the primary threat
Tuesday night through Friday as multiple rounds of thunderstorms are
possible over the area.  Wednesday and Wednesday night are the
highest risk periods.  The highest threat area will be in the
vicinity of the warm front, across southern MN into WI.

By Tuesday morning, low level southwesterly flow will develop across
southern MN ahead of a developing surface low on the lee of the
rockies.  The upper level shortwave will advance northeast from
southern California over the four corners region during the day.  A
surface warm front will lift north through the midwest during the
day, and eventually into far southern MN Tuesday night.

The latest guidance for this event suggests that Wednesday and
Wednesday night are the more concerning periods for excessive
rainfall.  This doesn`t mean there won`t be storms that could
produce copious rainfall amounts Tuesday night, but it doesn`t
appear as likely as previous forecasts.  One reason for this is the
strength of the low level jet.  We`ve been talking for days about
how the September 22nd-23rd, 2010 heavy rainfall event was the top
analog. This is no longer the case and one of the main differences
is at 850mb.  The low level jet Tuesday night looks weaker than
previous model runs indicated.  The LLJ will be about 30-35 kts into
far southern MN Tuesday night.  In comparison, the LLJ in the
September 22-23, 2010 event was much stronger with 50+ kts at 850mb
into southern MN south of the warm front.  With a weaker LLJ on
Tuesday, the continued of feed of moisture rich air is simply not as
strong which is needed to sustain extremely high rainfall rates.
With that said, we still have a warm front in the area and 1,000-
2,000 j/kg of ML CAPE and nearly uni-directional shear.  But with
the 850mb winds not all that strong, the odds of anchoring storms on
the nose of it are less likely given the increasing speeds with
height above that level. Ultimately, the weaker LLJ limits the
vertical moisture flux as moisture transport and moisture
convergence are not as high with a weaker LLJ.  Still, expect
thunderstorm activity across southern MN and with PWATs nearing 1.75-
2", there is still a risk of flooding, even if slightly less than
previously thought.  The Weather Prediction Center has highlighted
this area in a Marginal Risk of excessive rainfall.

For Wednesday and Wednesday night, the WPC has highlighted parts of
southeastern MN and western WI in a moderate risk of excessive
rainfall and includes part of the Twin Cities, with almost all of
the rest of southern and central MN in a slight risk of excessive
rainfall. The warm front will be quasi-stationary through this
period and that LLJ will become a little stronger by Wednesday night
and focused in SE MN. In addition, the upper level shortwave will
arrive from the southwest by Wednesday night which will add
additional forcing to aiding in thunderstorm development.  PWAT
values will remain high and instability will be marginally stronger
Wednesday evening.  Very heavy rainfall rates are likely and
thunderstorms with the tendency to train along the front are

The front will finally sink south of the area Thursday and provide a
reprieve from the stormy activity.  But, POPs increase again
Thursday night through the weekend as a deep low pressure system
lifts northeast into the Dakotas with warm air and moisture
advection out ahead of it.  Showers and thunderstorms along the warm
front Saturday and the cold front Sunday appear possible.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 639 AM CDT Mon Sep 19 2016

An area of light showers will affect central MN TAF sites this
morning, while mid-level clouds spread across the area. Despite
the dry low-level layer models have been showing on forecast
soundings, the forcing has been sufficient for rain to reach the
ground. Precip & clouds clear this afternoon, with mostly clear
skies through the remainder of the period. West northwest winds
today (with gusts to around 20 kts at KAXN) back to west/southwest
this afternoon/eve.

A brief period of light rain is possible between 14z-19z. Conditions
are expected to remain VFR however.

Wed...VFR/Chc MVFR. TSRA likely. SE wind 5-15 kts.
Thu...MVFR/IFR. Chc of TSRA. NE wind 5-10 kts.
Fri...MVFR. Chc of TSRA. E wind 7-10 kts.


.MPX Watches/Warnings/Advisories...


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