Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 292246

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
546 PM CDT Mon May 29 2017

.UPDATE...For 00z Aviation discussion below


.SHORT TERM...(This evening through Tuesday)
Issued at 255 PM CDT Mon May 29 2017

Latest analysis indicates the large low pressure system in control
of our weather is centered over western Ontario.  Widespread
stratocu has enveloped the area and diurnally-driven scattered
showers have developed across the region.  Strong northwest flow will
continue but the surface speeds will come down quite a bit at night.

The low will slowly lift northeast through the period, but tomorrow
will see similarly cool temperatures to today.  Lapse rates will not
be as steep though, and moisture not as deep.  Hence, do not expect
the same degree of shower activity that we are seeing this afternoon
for tomorrow, but isolated showers will be possible.  Do expect a
similar looking in day in terms of widespread cumulus and breezy
northwest winds.

With highs being held 10 to 15 degrees below normal both today and
tomorrow, the month will end below normal.  For the Twin Cities,
this will be the first below normal month since August, 2015.

.LONG TERM...(Tuesday night through Monday)
Issued at 255 PM CDT Mon May 29 2017

Unfortunately, the confidence in the forecast after after Wednesday
night decreases quite a bit. The problem at hand is the evolution of
the sprawling upper low over Ontario and how it impacts how far
north a warm front returns late week into the weekend. Of note for
this forecast, the one model that we tried to avoid as much as
possible was the Canadian, which develop an unrealistically strong
surface low in the upper MS Valley over the weekend, likely due to
convective feedback issues. But even ignoring the Canadian, we have
pretty good spread through the weekend between the GFS/ECMWF/MPAS.

We`ll start the long term off with an absolutely gorgeous day on
Wednesday as a surface ridge works across, we`ll have light winds,
sunny skies, low humidity, and highs in the lower 70s. In other
words, perfect weather to end the month of May.

However, things start to change Wednesday night. We`ll see a warm
front quickly working north across the Plains as we get into return
flow on the backside of Wednesday`s high pressure. Guidance
continues to show an elevated cluster of thunderstorms north of the
warm front Wednesday night, though the GFS/ECMWF have shifted this
cluster south down toward the KC area, with the MPAS holding the
line on a more northward solution in NW IA for the cluster. However,
with the bulk of guidance shifting south, we did bump out PoPs down

After Wednesday night, the question becomes how far north does the
warm front make it. The MPAS/GFS have the Ontario low farther west,
with a strong Hudson high dropping down from the northeast.  This
stops the northward push of the warm front at the IA border, with
our weekend going off mild and dry under the influence of the Hudson
High. The ECMWF is not as strong with the Ontario upper low, so it`s
not as aggressive with the Hudson high. This allows the ECMWF to
drive the warm front up to about I-90, with a washout more likely
for Saturday. This is a pretty big northward shift for the ECMWF on
Saturday, which looked more like the current GFS/MPAS with its 00z
run. At this point, given the spread, stuck close to a blended
forecast with low chance PoPs, but we either look to be dry or
pretty wet. At this point, would favor the drier solutions as
confidence is high in the thunderstorm complex to our southwest
Wednesday night. The low level flow is not all that strong on
Thursday/Friday in the wake of this activity and think cloud
cover/rain cooled air in the wake of the convection Wednesday night
will help check up the warm front further south going into the

Finally, one other area where some changes may be needed in the
future are with highs Thursday/Friday. We`ll be getting into an h85
warm nose these days with h85 pushing up to around 16c.  Most of the
bias corrected guidance has highs both days pushing into the low
80s, so highs 3-5 degrees warmer than what we currently have are
possible both days.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 546 PM CDT Mon May 29 2017

More of the same this TAF period, as isolated showers and wind
gusts diminish around sunset, and then MVFR stratus clouds build
in again around daybreak Tuesday. West/northwest winds gusts
increase to between 22 and 25 kts around 15z Tuesday, with
isolated showers developing again for the daytime hours.

No variations from the above discussion anticipated.

Wed...VFR. Wind WNW 5-10 kts.
Thu...VFR. Wind S 5 kts.
Fri...VFR. Wind E 5-10 kts.




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