Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 181102

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
602 AM CDT Mon Sep 18 2017

.SHORT TERM...(Today and tonight)
Issued at 422 AM CDT Mon Sep 18 2017

Lead WAA advection has brought ample mid level cloud cover to the
area overnight, with enough lift to generate the odd sprinkle/light
rain shower, but the wave of interest is the one moving up out of
Nebraska. This wave is looking a bit stronger than what our previous
forecast had and our expected QPF is certainly on the upward trend.

As this wave moves into southern MN, models are consistent with
showing a jet streak strengthening from southwest MN up to Lake
Superior. This will place a good right entrance region centered over
the southeast third of MN. This will provide us with ample upper
divergence for the wave in Nebraska to work with as it gets here.
Both the NAM/GFS are showing a strong band of h7-h6 fgen moving up
across south central MN. Instability will be limited (less than 500
j/kg of MUCAPE), likely not enough to really get much lightning
going, but enough to boost lift and precip rates. Put all of this
together and you get an overachieving wave, with many of the short
term models showing total rainfall amounts of around an inch falling
from about St. James, up through Mankato and over toward Red Wing.

For temperatures, there will not be much warming with rain and
clouds across eastern MN and western WI and there`s a chance current
forecast of highs in the low 60s may be too warm.  For far western
MN the question becomes how much sun do they see, but there is a top
end for highs out toward Madison and Morris in the mid 70s.

Tonight, we will be dry as deep moisture slides off to the east, but
persistent southeast winds will spell trouble of a different
variety, stratus. Good model consensus that the western half of MN
will have stratus, with the question being how far east does the
stratus setup. At the same time, those southeast winds will be
brining some dry air up from the southeast, with some clearing
possible for southeast MN. For now significantly increased cloud
cover from western through central MN, with the main issue being how
low will ceilings be for aviation purposes.

.LONG TERM...(Tuesday through Sunday)
Issued at 422 AM CDT Mon Sep 18 2017

It appears much of this week will be quite busy with the potential
for multiple hazards, beginning with a severe weather risk
Tuesday. A surface anticyclone will continue shifting eastward
toward the eastern Great Lakes Tuesday, while an approaching
trough crosses the northern Plains. Return flow beneath steep
lapse rates will allow a moderately unstable airmass to develop
over the Dakotas during the afternoon. A potent negatively tilted
mid level wave will rotate northeastward across the Dakotas during
the evening with a 60-70 kt jet streak at 500 mb. This forcing and
convergence on the front should overcome the cap and get
convection developing. The shear profiles will likely organize
this activity into a squall line before it propagates eastward
into Minnesota during the evening. Very impressive low level wind
fields, exceeding 50-60 kt from 900-800 mb should maintain
at least elevated storm longevity into the late evening hours.
Severe weather risk would increase markedly with a bit higher
ThetaE air than currently forecast. Storms will weaken overnight
before reaching I-35.

A deep trough will develop over the western U.S. again by late
week. Meridional flow across the Plains and Mississippi Valley
will bring very warm and humid air northward again. Scattered storms
may accompany the surface warm front Thursday night. By Friday,
gusty south winds will bring hot temperatures back with
temperatures likely headed back toward 90 over much of the area as
thicknesses approach 580 dm, 850 mb temps reach 23C and 925 mb
temps reach 25C. High ThetaE air in the warm sector will keep
temperatures at night very warm as well, with many areas remaining
in the upper 60s or warmer both Thursday and Friday nights.

The meridional flow will slow the eastward progression of the
trough to a crawl through the weekend. Southwest flow parallel to
a nearly stalled surface boundary and pwats around 2 inches will
foster torrential rainfall potential where ever this front sets
up. Several inches of rain are likely near this front through the


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 602 AM CDT Mon Sep 18 2017

Biggest change to these TAFs was to speed up arrival of MVFR cigs
in western MN, which the LAV and HRRR have coming in faster. The
HRRR has also been spreading rain farther to the northwest this
morning, so went with a prevailing rain at STC and added a VCSH at
AXN. Big question rest of the period is how far east does
MVFR/IFR stratus setup tonight. Continued to follow blended short
term guidance for cigs, which looks similar to the LAV, with
socking in AXN/RWF/STC, leaving MKT on the edge and MSP/RNH/EAU
east of the IFR/MVFR stratus. The NAM is much more aggressive (as
is usually the case) and spreads the stratus across the entire
area. SREF probs for MVFR cigs tonight looks similar to the
GFS/LAV, so continued to favor that idea for cigs.

KMSP...Kept MSP VFR, though we could see MVFR cigs for a time this
afternoon if we get into heavy enough rain, though the heavies
rain look to be south of MSP. Is a threat for MVFR or lower cigs
with stratus developing to the west, but continued to trend toward
the GFS with cigs at MSP hanging out between 4k and 6k feet.

Tue...Mainly VFR. Chance MVFR ceilings. Winds SE 10-15 kts.
Wed...Mainly VFR. Chance TSRA/MVFR early. Winds W 5-10 kts.
Thu...VFR. Winds SE 10-15 kts.




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