Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 282049
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
349 PM CDT Wed Jun 28 2017

.SHORT TERM...(This evening through Thursday)
Issued at 349 PM CDT Wed Jun 28 2017

In the wake of morning convection, we saw a seasonably strong
stratospheric intrusion with the 1.5 PV surface getting down to
about 400 mb. This has shown up well on all 3 water vapor channels.
Just before 3pm, this upper feature reached the cold front near the
I-35 corridor, with storms initiating over Dakota county. The
cells that went up over Wright county over to the north metro are on
the northern edge of this PV feature and were more elevated, at
least initially.  We have had great low level shear most of the day
with backed surface winds. However, as these storms have developed,
our surface winds have really started to veer and our shear values
have plummeted, to the point where in MN, we have only weak speed
shear. As a result, the storms we have seen near the metro to this
point have been more pulse in nature and have not been able to
sustain strong updrafts long enough to get anything severe going.
Effective shear values over 30 kts reside where the SPC has a slight
risk in the Day 1 outlook and our thinking on the severe threat is
pretty similar, with it being confined to southeast MN toward
central WI where the better shear resides and the potential for
better organization exists.  So we`ll have to keep an eye on this
acivity as it moves into this region.

Tonight, these storms will exit our east by midnight as we lose
diurnal heat and the front continues its march into WI. The main
question for tonight is how extensive does low stratus become as we
get into the backside of the low. The NAM/RAP/HRRR all bring it down
to about the US-12 corridor. This may be a bit far south, but did
increase sky grids quite a bit across central MN into western WI for
tonight.

Thursday will feature ridging over the upper MS Valley, but another
trough will be working across the Dakotas in the afternoon. The
front working through tonight will be washing out across IA. All of
this puts the MPX area in a bit of a lull when it comes to forcing,
with thunderstorm activity through the day Thursday expected to be
mainly down in Iowa near the remnant boundary and to the west in the
Dakotas with the upper wave. Our precip chances look to increase
Thursday evening (start of the long term) as the Dakotas trough
moves into MN.

.LONG TERM...(Thursday night through Wednesday)
Issued at 349 PM CDT Wed Jun 28 2017

Expect periodic chances for showers and thunderstorms, but
overall not a complete washout for the upcoming weekend. The first
half of the weekend will be cooler and cloudy with light winds and a
few scattered showers or thunderstorms. High pressure will briefly
move in for Sunday and Monday which will dry things out north of I-
90 and bring temperatures closer to normal. Looking ahead, southerly
flow will return early next week and set the stage for more
widespread showers and thunderstorms for the middle to end of next
week.

An upper level trough will drive a couple of cold fronts through the
region over the weekend. The synoptic forcing ahead of this trough,
together with the small, thin CAPE profiles support scattered
showers and thunderstorms on Friday, similar to what occurred this
past Saturday. Marginal CAPE and Shear profiles limit the risk of
severe weather, so at this time only expect perhaps some small hail
with a few of the stronger cells. Saturday will have less of a
chance for showers and thunderstorms, but can`t go completely dry.

On Sunday a secondary cold front will move through the region as a
shortwave trough sweeps across the Boundary Waters to the north.
Models try to produce some precip along this front, but it seems
like the large-scale subsidence should win out, so went drier than
the model guidance for Sunday. The GFS tries to hang up a boundary
across the south with repeated chances for showers and thunderstorms
early next week, but these should be more favored across the south
near IA/IL. Looking ahead, another shortwave trough should bring
showers and thunderstorms across the Upper Midwest for the
middle/end of next week, with warmer more humid weather to follow
after that.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 133 PM CDT Wed Jun 28 2017

Two concerns this period are thunderstorm potential this afternoon
and threat for IFR or lower cigs to drop out of central/northern
MN tonight. For storms, clearing south of I-94 and west of I-35
has resulted in instability growing in this region, with a warm
front extending from Albert Lea up toward Long Prairie and the
cold front sitting from Alexandria down to Marshall. Like the
idea of the HRRR with a broken line of storms developing along the
boundary at about 21z. Just think the HRRR is too far west with
its development based on where the front is now. This puts MSP
and especially RNH/EAU at risk for another round of storms.
Tonight, as the surface low currently north of Alexandria shifts
into northern WI, the NAM/RAP/HRRR show IFR and lower CIGS
dropping south out of northern MN. Expect these clouds down to
about I-94, with MSP likely being near the southern edge of these
clouds. The HRRR looks to provide a general idea of how these
clouds will evolve overnight, though is likely a bit too far south
with where the low clouds are by 10z tonight.

KMSP...Based on where the front is now, MSP looks to be very near
where afternoon initialization occurs, so storms will either
go up overhead or just east of MSP. 22z to 00z will be our prime
time for any storm activity. For clouds tonight, the RAP would
slam the low clouds in after 9z, but the NAM keeps this enhanced
low level moisture to the north and the SREF probs for MVFR or
less cigs are confined to central MN, so kept MSP VFR for now.

/OUTLOOK FOR KMSP/
Fri...VFR. Chance MVFR with likely TSRA. Winds variable 5 kts.
Sat...VFR. Winds WNW 5-10 kts.
Sun...VFR. Chance of TSRA/SHRA. Winds light and variable.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
&&

$$

SHORT TERM...MPG
LONG TERM...JRB
AVIATION...MPG



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