Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 181818
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
118 PM CDT Tue Jul 18 2017

.UPDATE...
Issued at 1142 AM CDT Tue Jul 18 2017

Severe threat today really looks to be on a downward trend. The
front at 11am does stretch from about Jackson, MN through the
Twin Cities, but he combination of light precip and cloud cover
out ahead of it will limit the amount of instability we develop.
In addition, we are weakly capped right now, so once we build up a
little instability, the atmosphere quickly does away with it by
bubbling up some short-lived showers/storms. The other change we
have made for today/tonight is to speed up the improvement
process, with activity looking to largely be out of around area by
10pm.

However, as one days severe risk is on the downward trend, the 12z
NAM and GFS point to our severe risk heading the other direction
for Wednesday evening. It is looking increasingly likely that we
will see a strong MCS come out of the SD/ND/MN region and streak
toward SW WI/NW IL Wednesday night. We have been in coordination
with the SPC this morning on changes to the Day 2 outlook and
there will be an upgrade out of the marginal risk for the Day 2
update due out around noon.

&&

.SHORT TERM...(Today and tonight)
Issued at 315 AM CDT Tue Jul 18 2017

Surface analysis shows a NE-SW oriented cold front from a low
pressure center over Hudson Bay, extending through NW Minnesota
into western South Dakotas into SE Wyoming. Outflow boundaries
from earlier convection are scattered across the area (which will
contribute to additional convection later today). Aloft, broad
upper level ridging remains centered from the Four Corners region
into the Deep South with zonal flow across the northern tier
states from the northern Rockies through to the Great Lakes. With
pronounced south to southwest flow in place in advance of the cold
front, a deep plume of moisture continues to surge into the upper
Midwest from the deep South, contributing to PWATs around 2
inches for much of the area and expected to last through to this
evening. Also of note are dewpoints around 70 even in the
overnight hours for much of central MN while the rest of the WFO
MPX coverage area has dewpoints in the 60s. These dewpoints will
only increase today while the front out west takes its time moving
through the region. Given the presence of multiple outflow
boundaries and the approach of the cold front, it will not take
much for more widespread convection to once again develop later on
today. MUCAPEs will again climb to 3000-4000 j/kg while bulk
shear hits the 30-35kt range. The deeply moist environment with
little-to-no capping will result in clusters of storms developing
by late morning in western and central MN which will slowly but
steadily progress southeast. In terms of severe weather, nothing
overly organized is expected but some storms could contain
strong/severe winds in a downburst fashion and potentially
marginally severe large hail. The cold front is expected to drop
into southern MN and western WI by late this afternoon then drop
south of the WFO MPX coverage area altogether overnight tonight,
bringing an end to the precipitation across the area. QPF totals
through this evening may commonly range between 1.0-1.5 inches
with locally higher amounts certainly possible. Have added heavy
rain mention into the grids as there remains the Slight Risk for
Excessive Rainfall. As for temperatures, the added cloud cover
will hold back highs from the 90s experienced yesterday for much
of the MN portion of the coverage area. Max temps today will
remain in the low-mid 80s but there is a shot that highs could hit
the upper 80s close to the Iowa border. Lows tonight will run
about 5-10 degrees cooler, mainly due to the passage of the cold
front, resulting in slight cold air advection and incoming drier
low level air.

.LONG TERM...(Wednesday through Monday)
Issued at 315 AM CDT Tue Jul 18 2017

The longer term remains active through the end of the week with
the area remaining on the edge of the mid level cap/surface front.
Periodic bouts of thunderstorms will continue with severe weather
possible along with torrential rain, mainly Wednesday night,
thursday night into possibly Friday night.

Much of Wednesday will remained capped for convective development
with mid level temperatures warming to +14c into the southwest.
An 100kt upper 300mb level jet rides across northern MN and the
right entrance region will move into MN Wednesday evening. Very
unstable airmass will be in place along with PW`s over 2 inches.
The GFS has been consistent in developing an MCS and moving it
along the instability gradient and mid level cap the last several
runs. This continues to favor about the southern third of MN. WPC
paints a slight risk of excessive rainfall during this period
which is definitely possible. We should see rainfall rates of 2-3
inches an hour once convection matures into the late
evening/overnight. The convection should be moving however/forward
propagating with damaging winds the main severe weather threat.

The area continues to remain in the favored region for convective
development again later Thursday, Thursday night into Friday. The
southern third, perhaps a bit farther south if Wednesday nights
convection moves the surface boundary farther south, looks like
the area of concern. Again torrential rain and severe storms will
be possible. Large hail and damaging winds being the main
threats.

Finally, the deterministic models drive a cold front through the
area Saturday and should end the convective threat lasting into
early next week. Temperatures and dewpoints will drop off as well
as high pressure builds south over the Great Lakes.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 117 PM CDT Tue Jul 18 2017

Our convection threat the rest of the night is diminishing. We
have a wave moving into western MN at the start of the period and
we anticipate showers and storms to bubble up and down as this
wave tracks across the area. Have started to back off on thunder
mention in tafs, pulling back tempo tsra groups to just prevailing
shra with a vcts. Timed the shra groups to what the HRRR has,
though do think the HRRR is overdoing the strength of storms we
will see locally, some stronger storms look better for southeast
MN where some sunny skies and better instability reside. We did
speed up the clearing of skies and only real concern tonight will
be for fog development over central MN.

KMSP...Expect on and off shra with occasional thunder through
about 22z. Wind directions will be problematic today. There is a
remnant outflow boundary moving through now, which has NW winds to
start the TAF, but out in west central MN, winds go back to the
southeast before you get variable winds along the boundary before
going northeast in its wake. That frontal boundary and variable
winds looks to arrive near or just after sunset.

/OUTLOOK FOR KMSP/
Wed night...Strong-severe TSRA possible in evening.
Thu...VFR. NE 5 kts.
Fri...Mainly VFR. -SHRA/TSRA likely. Winds SE 5-10 kts.
Sat...Mainly VFR. Chc MVFR in -SHRA/TSRA. Winds lgt and vrbl.


&&

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

$$

UPDATE...MPG
SHORT TERM...JPC
LONG TERM...DWE
AVIATION...MPG



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