Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 261056
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
556 AM CDT FRI AUG 26 2016

.SHORT TERM...(Today and tonight)
Issued at 402 AM CDT Fri Aug 26 2016

It will be another spectacular weather day today as high pressure
currently over southern MN slowly slides toward the Great Lakes.
There is strong model consensus on highs in the low to mid 70s, with
humidity levels remaining comfortable.

All of this changes though tonight as the front and associated
moisture plume down from Kansas into central Illinois lifts north in
conjunction with the arrival of a shortwave currently over CO that
was generated in SW monsoon convection Thursday and a northern
stream trough that will be working across the Dakotas. We are still
seeing a good deal of spread with models with how much precip we
will see locally, with the GFS/Canadian keeping the MPX area largely
dry while most of the CAMs and the NAM/ECMWF bring precip up through
most of the MPX area, though have the heaviest precip off to the
southeast.

For the NAM/GFS difference, the biggest reason for the spread is how
they handle the upper jet streak. The NAM holds it back across
eastern ND/northwest MN tonight, which allows the 925-850 moisture
transport plume to get pulled west within its right entrance region.
The GFS has the jet streak more over the arrowhead, with the 925-850
moisture transport plume remaining south/east of the MPX area. The
ECMWF cuts some sort of a middle ground between the two and given
the large majority of CAMs showing a more westward track of precip
shield, maintained higher pops we had going for tonight, though did
slow down their northern push across the area. Instability will be
marginal and combine that with best moisture sliding up from
northwest MO into southern WI and like the idea the ECMWF gives with
heaviest precip remaining southeast of the MPX area, though we will
have lighter QPF working north across southern and eastern MN as the
wave currently down in CO works through.

.LONG TERM...(Saturday through Thursday)
Issued at 402 AM CDT Fri Aug 26 2016

Concerns in the long term are focused on Saturday and Sunday with
regards to precipitation chances and severe weather.

As we close in on the weekend, problems are beginning to surface
between the deterministic models and the CAMS for precipitation
placement and coverage for Saturday. The upper trough approaching
from the west is made up of both a northern and southern stream
feature. This has been well advertised over the past few days.
It`s the southern wave that forces much of the precipitation on
Saturday. The deterministic solutions are trying to split the
precipitation into two areas, one to the northwest of our FA
associated with the northern wave with the other across southeast
MN and southern WI with the southern wave. This leaves a good
portion of the FA basically dry on Saturday. This is just the
opposite of what the CAMS show with quite an area of showers and
embedded thunderstorms pushing across much of the FA Saturday
morning. Both the NSSL and NCAR ensemble WRFs have plenty of
members with widespread showers, although the NCAR is faster with
both the onset and ending of the precipitation. The decision was
made to stay with continuity with the gridded pop forecast with
likely pops for a time Saturday morning.

Another concern on Saturday is the threat for a few severe storms
in the afternoon and evening as we cool off aloft as the northern
stream short wave moves in from the Dakotas. How much
destabilization occurs will be tied to the morning showers and how
quickly cloudiness can decrease for a time in the early afternoon.
Several members of the NCAR ensemble WRF show clusters of storms
developing around mid afternoon in west central MN and advancing
through the Twin Cities by evening. SPC has indicated a marginal
risk at this time for much of our FA.

Sunday is another day of interest, primarily for severe weather,
as a warm front lifts slowly northward into central MN and
adjoining areas of WI by afternoon. Plenty of CAPE and deep layer
shear are indicated by the NAM and GFS. CWASP values reach 60-70
percent by mid afternoon along and north of the I-94 corridor.
There is even some low probabilities (10-20 percent) for
tornadoes indicated by equation 2 of the Conditional Probability
of a Significant Tornado. In fact, checking the CIPS analogs for
Sunday reveals that the number 1 analog is September 11th 1990.
This day had several tornadoes occur in Barron county WI with a
paper published in Weather and Forecasting about the event. This
particular event had a warm front lifting north through the area
in the afternoon along with a weak H5 short wave moving in from
Dakotas during the day which is very similar to what we are seeing
in the forecast. Again, SPC has a marginal outlook at this point
and this will need to be watched closely going forward.

Small pops dominate the forecast from Monday through Thursday of
next week as a boundary pushes in from the Dakotas early in the
week, becomes stationary across the local area Tuesday into
Wednesday, before lifting back north as a warm front late in the
week. Although high temperatures will be only a little above
normal, it looks like dew points will be well into the 60s with
even some 70s, making for uncomfortable humidity.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 556 AM CDT Fri Aug 26 2016

Patchy fog out there this morning will quickly burn off, with
nothing more than increasing high clouds expected today. For
tonight there is a good deal of spread in the models, with the GFS
still going dry, while the NAM and most of the CAMs bring showers
up across MN and into WI late tonight. Favored this forecast
toward a NAM/CAM solution. With the rain will also come the threat
for MVFR cigs, but played that threat conservative for now with
SREF probabilities for CIGS under 3k ft not picking up until you
get behind the potential showers.

KMSP...We have been advertising the threat for showers at MSP now
for the last few days, so stuck with continuity in our forecast
with bringing -ra in late tonight with a timing in line with the
NAM, though we could very well end up getting split by rain to the
southeast and northwest as the GFS shows. Instability is minimal
with this batch of precip which is why there is no mention of TS.
For the MVFR cigs Saturday morning, they will likely be
conditional on seeing some rain, but even if there is no rain, the
GFS shows low level moisture increasing enough in southeast winds
overnight to support a threat for MVFR cigs.

/OUTLOOK FOR KMSP/
Sat...MVFR psbl. Chance TSRA. Wind E-SE 5-10 kts.
Sun...VFR. Wind S-SW 5-10 kts.
Mon...VFR. Wind S 5-10 kts.

&&

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

$$

SHORT TERM...MPG
LONG TERM...RAH
AVIATION...MPG



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