Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 161811

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
111 PM CDT Wed Aug 16 2017

.SHORT TERM...(Today and tonight)
Issued at 350 AM CDT Wed Aug 16 2017

Surface analysis early this morning shows a low pressure center
over northwest Kansas/southwest Nebraska with a warm front
extending nearly due east to southern Ohio and a trailing cold
front extending southwest through the TX/OK Panhandles into
central New Mexico. Aloft, the Upper Mississippi River Valley is
caught between a ridge axis extending south southeast from western
Hudson Bay into the southeast CONUS and a longwave trough over
the lee of the Rockies. A poignant shortwave axis curls from west-
central Saskatchewan province into eastern North Dakota then into
eastern Wyoming. In advance of the surface features, the deep
southwesterly flow aloft is tapping moisture from the subtropical
jet, pushing PWATs to between 1.50-2.00 inches over the WFO MPX
coverage area this morning. A band of light-moderate showers
associated with a smaller shortwave aloft will continue to push
northeast through eastern MN into western WI. These showers will
be only the beginning of what will be a rather wet 30 hours over
the coverage area. The larger trough axis aloft will help drag the
surface low east northeast, taking a path that will bring it to
near KFSD by midday today then over far southern MN tonight to
around KMSP by daybreak Thursday morning. This is a slower
progression than models have depicted in previous days so have
adjusted the precip chances as such. After the showers this
morning, there will be a relative break in the action for eastern
MN into western WI through early afternoon while western MN sees
an increase in rain coverage in advance of the surface low. In the
meantime, the trough features to the west will pivot and allow a
cutoff low to develop within in, offset to the west of the surface
low. As this scenario develops, precipitation coverage should
increase steadily this afternoon through this evening, with
rainfall potentially heavy at times over central MN. With the deep
moisture remaining in place as the surface low moves across,
rainfall totals of 1-2" will be commonplace with amounts as high
as 3" not unreasonable. Given the recent rainfall over portions of
western and central MN recently, isolated flooding issues are
possible but the progressive nature of the storm system is not
expected to be conducive to training storms. There is also a small
risk of severe storms given the enhanced jetting but instability
will be meager at best and with little insolation expected and
warm sectoring to remain to the south of the area, the risk for
severe weather is quite low. The threat for heavy rain will
continue into tonight with the slow progression of the surface low
but rain coverage and intensity is expected to diminish during
the early morning hours into daybreak Thursday.

As for temperatures, have continued the trend of advertising
slightly cooler temperatures from the previous forecast. As such,
will look for highs in the lower 70s in central MN and in the mid
70s in southern MN into western WI. Lows tonight will be similar
to this morning, ranging from the upper 50s to mid 60s.

.LONG TERM...(Thursday through Tuesday)
Issued at 350 AM CDT Wed Aug 16 2017

The longer term concerns remain timing of the exit of the upper
trough Thursday. Then timing of the next short wave through the
eastern CONUS trough will occur Friday with the next frontal
passage for late in the weekend/early next week.

The trough exits to the east of the area through Thursday
afternoon. Will taper PoPs and overall thunder chances from west
to east. Temperatures should remain on the cool side with ample
cloud cover remaining over much of the area through late

The deterministic models trend another short wave moving through
the area Friday/Friday night with attendant weak instability. We
should see some drier conditions through Saturday and into Sunday
across the region as flow aloft becomes more zonal with slowly
rising heights. This should allow warmer temperatures ahead of
the next frontal system which is progged to move across the area
Sunday night through Monday night. Temperatures are expected to
warm through the lower and mid 80s through the weekend with
increasing dewpoints ahead of the front. Passage of the front
could produce locally heavy rain, especially if it moves slower
than progged. Severe weather threat is possible be as the front
moves through Monday/Monday night as well.

Drier and somewhat cooler conditions will develop in the wake of
the front through midweek.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1255 PM CDT Wed Aug 16 2017

Surface low down by Omaha will deepen overnight as it moves into
southern MN. For us this means heavy rain and low cigs. Short term
models are deeper with the low than models were overnight, which
is slowing down arrival of precip and delayed heaviest precip
(around 00z) an hour or two from previous TAF. With the low moving
over/near STC/MKT/MSP, cigs will get quite low tonight and can`t
argue much with what the LAMP has. With the deepening of the low,
also boosted wind speeds quite a bit for when NW directions arrive
at site.

KMSP...Like the idea of the hrrr, but based on where convection is
now in central IA, think it is to slow in brining storms with
heavy rain into MSP, hence the timing in the current TAF that is a
couple hours ahead of the HRRR. Surface low looks like it will
nearly go overhead tomorrow morning so have high confidence in IFR
cigs being in place for the morning push Thursday.

Fri...VFR. Chance afternoon/evening TSRA. Wind W 5 kts.
Sat...VFR. Wind W 5 kts.
Sun...VFR. Chance afternoon/evening TSRA. Wind S 5-10 kts.




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