Area Forecast Discussion
Issued by NWS Twin Cities, MN
FXUS63 KMPX 191803
Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
103 PM 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
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
.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1258 PM CDT Tue Jul 19 2016
Upper level ridging is building north and east and will influence
the convective forecast over western areas of the forecast area.
Tricky forecast in terms of convective initiation tonight. Bottom
line is any convection north and west of KMSP will be very
isolated hence nothing in the terminals. Depending on what hi-res
solution we view there are various outcomes, but most favor very
isolated or no convection at all. The 12Z GFS and is on board with
convection south and east of KMSP and the 15z HRRRx supports that
through the early evening. It appears H700 temperatures are just
a tad to warm to have organized convection such as a MCS that was previously
discussed. Not overly comfortable making such a change, but see no
good reason continue chance TSRA. H700 temperatures are cooler to
the south east and will keep a mention as in PROB30 in the TAFs.
We certainly have plenty of moisture advecting northward but at
this time it appears the main convection will be located in Iowa.
At any rate the forecast for the next 12 hours bears watching
closely for any subtle changes to the atmosphere that would
encourage organized convection. If condition did develop expetc
heavy rain in the vicinity of TRWs.
Once we get past 15z Wednesday expect a fair amount of SKC with a
few puffs of cirrus noted and hot and humid.
KMSP...May see a few gusts this afternoon in the upper teens range
with a few cumulus. At this point in time I am doubtful we will
see any convection at the MSP terminal, but will keep a PROB30
for a few hours...just before daybreak. Otherwise SKC most of the
/OUTLOOK FOR KMSP/
Thu...VFR. Wind SSW at 5-10kts.
Fri...VFR. Chc -TSRA late. Wind SE 5 kt.
Sat...VFR. Chc -TSRA late. Wind S-SE 5-10 kt.
WI...Flash Flood Watch from 7 PM CDT 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
Flash Flood Watch from 7 PM CDT this evening through Wednesday
morning for MNZ051>053-059>063-066>070-075>078-083>085-092-
Excessive Heat Watch from Wednesday afternoon through Friday
evening for MNZ041>045-050-052-053-077-078-084-085-091>093.