Graphic for MPD #0763

Mesoscale Precipitation Discussion 0763
NWS Weather Prediction Center College Park MD
248 PM EDT Tue Sep 04 2018

Areas affected...Srn Minnesota; NW/WC Wisconsin; Nrn/Wrn Iowa

Concerning...Heavy rainfall...Flash flooding likely

Valid 041846Z - 050045Z

Summary...A significant threat of flash flooding will begin to
ramp up this afternoon across portions of the Upper Midwest.
Thunderstorms will become more numerous in northwest Iowa shortly,
and then develop and move northeast into portions of Minnesota and
Wisconsin late in the afternoon and into the evening. Hourly rain
rates of 1-2 in/hr (locally higher) appear likely, and these rates
may persist in a given location for several hours. This would be
likely to lead to a band of very heavy rainfall and flash
flooding. The threat is likely to persist beyond the expiration
time of this particular discussion, and may actually be highest in
the evening or overnight hours.

Discussion...We are likely seeing the beginnings of a significant
flash flood threat developing across the Upper Midwest early this
afternoon. Scattered convection was becoming more widespread
across portions of western and north-central Iowa as of 1830Z,
with regional radars detecting a number of new updrafts. This is
expected to continue over the next few hours ahead of an
approaching mid-upper level vort max (currently over northeast
Kansas, evident on GOES-16 satellite imagery and radar mosaics).
An associated developing low-level cyclone is also expected to
lift north into northwest Iowa by 00Z, with a prominent LLJ just
to the east. Significant northward moisture flux is expected with
this LLJ, and an increasingly focused area of convection is
expected near the nose of the strongest 925-850mb moisture
transport and in the region of PVA aloft. In the 21-00Z time
frame, this is likely to be centered near the central portion of
the MN-IA border, perhaps just into south-central Minnesota. RAP
forecasts show substantial PWATs near 2.2 to 2.3 inches, which
would be near record values for early to mid September in this
region. On the 12Z soundings, the LMN sounding had a 2.27 inch
PWAT, and that is relatively close to the 500-1500m AGL air mass
source region using backward trajectory analysis on the RAP
forecast. Therefore, the RAP forecasts seem reasonable, and this
notable slug of low-mid level moisture should support significant
rainfall.

The 12Z HREF indicated relatively high probabilities of exceeding
2 in/hr rain rates in the 21-00Z window near the MN-IA border,
with lower (but non-zero) probabilities extending northeast along
the instability gradient into SE MN and WC WI. Individual hi-res
models suggest some localized areas could see in excess of 3 to 4
inches of total rainfall by 00Z. Therefore, the threat of flash
flooding should begin to markedly increase in the next few hours,
although the threat is likely to continue to increase into the
evening hours across the region. And the most favored location for
flash flooding in the next six hours is in northern Iowa and
southern Minnesota, and it should expand northeast from there
beyond the scope of this discussion. There is another band of
moderate to occasionally heavy rain developing in central
Minnesota, associated with a 850-700mb deformation zone. The
threat of flooding is lower in this area given the combination of
generally lower rain rates and higher flash flood guidance, but
cannot be ruled out given slow or nearly stationary movement of
the band.

Portions of south-central and southeast Minnesota, and
west-central into northern Wisconsin, have been upgraded to a High
Risk on the Day 1 Excessive Rainfall Outlook. The threat should
extent into the overnight hours.

Lamers

ATTN...WFO...ABR...ARX...DLH...DMX...FSD...MPX...OAX...

ATTN...RFC...MBRFC...NCRFC...

LAT...LON   45929226 45769132 45259033 44139047 42789277
            42099390 41009523 41409662 42389678 43389682
            44129744 45059598 45779401


Last Updated: 248 PM EDT Tue Sep 04 2018