Area Forecast Discussion
Issued by NWS Twin Cities, MN

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139
FXUS63 KMPX 191213 AAA
AFDMPX

Area Forecast Discussion...UPDATED
National Weather Service Twin Cities/Chanhassen MN
713 AM CDT TUE JUL 19 2016

.SHORT TERM...(Today and tonight)
Issued at 400 AM CDT Tue Jul 19 2016

A rather active short term period lies ahead with a couple rounds of
convection expected.

The first round, now in Nebraska, is associated with the first of
two short waves lifting northeast around the building ridge centered
over the central Plains. This cluster of showers and storms have
emitted a large outflow boundary which has since developed
additional convection over southeastern South Dakota, around the
Yankton area. With the ridge building north and the moisture
advection beginning to focus over eastern South Dakota and far
southwestern Minnesota, believe additional storms will develop in
those areas during the next few hours and shift east across southern
Minnesota through the early afternoon before diving back southeast
on the east side of the ridge. Pops were kept low over Wisconsin
through the evening, but were raised to about 50 percent across
Minnesota. Further adjustments will be needed as convection
develops. Don`t anticipate these storms becoming severe with modest
dew points keeping CAPE values low, but pwats increasing to
1.5 inches could bring locally heavy rain.

Of greater concern is the second round tonight. A warm front will
work east into Minnesota as another mid level short wave and
developing LLJ push overhead as well. General mass fields and
conceptual model would point to portions of east central MN and
southeast as a zone for thunderstorm activity developing mid to late
evening, persisting for much of the overnight hours before the LLJ
veers easterly toward dawn. Pwats increasing to 2 to 2.25 inches and
a warm cloud layer nearly 13,000 ft deep will strongly favor heavy
rainfall rates of 2 to 3 inches per hour. Believe the GFS and ECMWF
show this potential the best of all models, with the CAMs struggling
a bit which isn`t unusual for this range. They may be impacted by
low level capping. The WRFNMM does show a narrow corridor of
torrential rain along the Mississippi River (7 inch maxima) which
matches well with the GFS/EC although a bit further east. After
extensive coordination with WPC and the surrounding WFOs, think the
potential is high enough to justify a Flash Flood Watch for portions
of eastern Minnesota and far western Wisconsin. Given the expected
rates, it won`t take long for several inches to accumulate in spots
and it only takes about 1.5 inches within an hour to produce flash
flooding in urban areas.

.LONG TERM...(Wednesday through Monday)
Issued at 400 AM CDT Tue Jul 19 2016

Heat wave remains on track beginning tomorrow. Overnight
convection will likely linger into tomorrow mainly in eastern MN
and western WI, but during the afternoon clearing skies and
increasing temperatures and dew points will lead to heat index
values well over 100 degrees across western MN through the Twin
Cities Metro. This will be the beginning of a heatwave that will
last through Friday, with the worst of it being on Thursday. Made
little change to the going forecast for temperatures and dew
points. While the actual air temperatures may not be extreme for
heat wave standards, its the extremely high dew points that really
push the heat indices up in the 100 to 110 degree range. Dew
points tomorrow will only be exacerbated with the morning
convection. Continued widespread upper 70s for dew points, but
those may actually be too low tomorrow afternoon across part of
the area. Both the GFS and ECMWF indicate a pool of surface dew
points in excess of 80 degrees both tomorrow and Thursday. Another
big concern with this event is how warm the overnight lows will
be. We could be flirting with record warm overnight lows both
Wednesday night and Thursday night in the Twin Cities, which are
both in the upper 70s. So, the night time will provide little
relief from the heat. This is a dangerous situation for those with
no air conditioning.

Decided to issue the Excessive Heat Warning for much of western MN
through the metro, these areas will be dangerously hot and humid
tomorrow afternoon. Decided to hold off farther east as heat
indices won`t be quite as warm tomorrow, especially in WI due to
lingering precipitation chances and certainly cloud cover. Those
areas will likely need a Warning for Thursday. Unfortunately, the
latest guidance slowed down the cold front for the weekend, which
means Saturday will be warm and humid with temps in the upper 80s
and dew points around 70, but heat indices won`t be quite as high as
tomorrow through Friday. The GFS and EC now bring the cold front
through from late Saturday night through Sunday morning. Dew points
will fall to more comfortable levels by Sunday.

In terms of precipitation through the long term period, we`ll
start out tomorrow dealing with thunderstorms in eastern MN and
western WI from the activity left over from the overnight period.
Once that precip moves out, the thermal ridge moving in should
effectively cap the atmosphere, keeping the precip to our north
where temperatures aloft aren`t quite as warm.  The next good precip
chance will come along and ahead of the cold front this weekend.
Timing on when that moves through is difficult to nail down at this
time.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 714 AM CDT Tue Jul 19 2016

Some MVFR cigs are pushing into southwest MN ahead of a narrow
band of showers and thunderstorms. This activity should continue
expanding a bit this morning before diminishing late this morning
and early this afternoon. RWF and MSP are the locations most at
risk for these showers/storms. A period of dry weather expected
this afternoon and evening before more showers and thunderstorms
develop across eastern MN and western WI overnight. Still some
uncertainty as to where they will form and track, but all taf
sites except RWF have some type of TSRA mention tonight.

KMSP...The showers and storms will likely be in their waning phase
by late this morning as they pass through. VFR conditions are
expected through the period generally, with lower conditions
possible in and near thunderstorms.

/OUTLOOK FOR KMSP/
Wed...VFR. Chc morning -TSRA. Wind S at 10g20kt.
Thu...VFR. Wind SSW at 5-10kts.
Fri...VFR. Chc -TSRA late. Wind SE 5 kt.

&&

.MPX Watches/Warnings/Advisories...
WI...Flash Flood Watch from this evening through Wednesday morning
     for WIZ023-024-026.

     Excessive Heat Watch from Thursday morning through Friday
     evening for WIZ014>016-023>028.

MN...Excessive Heat Warning from 1 PM Wednesday to 7 PM CDT Friday
     for MNZ047>049-051-054>070-073>076-082-083.

     Flash Flood Watch from this evening through Wednesday morning
     for MNZ051>053-059>063-066>070-075>078-083>085-092-093.

     Excessive Heat Watch from Wednesday afternoon through Friday
     evening for MNZ041>045-050-052-053-077-078-084-085-091>093.

&&

$$

SHORT TERM...BORGHOFF
LONG TERM...SPD
AVIATION...BORGHOFF



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