Mesoscale Precipitation Discussion 0723
NWS Weather Prediction Center College Park MD
427 PM EDT Mon Aug 27 2018

Areas affected...ERN MN...NRN WI...NRN MI...FAR NRN IA

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

Valid 272026Z - 280226Z

Summary...Flash flooding likely, especially in areas saturated by
recent rainfall denoted on the discussion graphic in Wisconsin and
Minnesota.

Discussion...At 20Z convection was increasing in coverage
immediately ahead of a pronounced shortwave trough that was
evident in GOES-16 water vapor imagery over northwest Iowa to
southwest Minnesota. A separate batch of strong convection was
expanding along I-35 north of Minneapolis over into northern
Wisconsin. This area is of particular concern owing to recent
rains that had saturated the soil and lowered Flash Flood Guidance
values to 1.00-1.50 inches in one hour or 1.50-2.00 inches in 3
hours. Estimated rain rates from KMPX at 20Z were already around
1.50 inches in an hour in spots.

Synoptically, the event was well defined, and the convection
extending into northern Wisconsin represented a warm advection
wing north of the re-enforced effective warm front and roughly
downstream from an effective triple-point near Moose Lake. Storms
forming in this regime are typically a little slower moving,
experiencing some backing of flow in advance of the approaching
shortwave, and are oriented in a manner to allow for training.
Mesoscale guidance is practically unanimous in producing at least
some 2 to 3 inch rainfall totals here from eastern MN into
northern WI this afternoon and early evening. With precipitable
water values some 2.5 standard deviations above climatology and
storms becoming increasingly surface based (noting erosion of
inhibition for mixed-layer parcels marked by >2000 J/kg CAPE per
the SPC Mesoanalysis), some intense short term rain rates and
isolated new rainfall totals over 4 inches appears possible. The
NSSL WRF (QPF shown in graphic) and many other hi-res models place
this event right over the lowest FFG.

Meanwhile farther south, a progressive MCS is forecast to occur
ahead of the wave moving into IA/MN and eventually western
Wisconsin. A similarly moist and unstable environment throughout
the region will support rapid accumulation of up to 2 inches of
rain wherever cell mergers occur - such as had the potential to
occur near the MN/IA border per 20Z radar loops. The RAP depicts
continued low level mass convergence in this region amid ongoing
deep layer height falls. As such, the initial round of convection
will likely be followed by continued growth of new storms
occurring north of outflows well into the evening. This will
become more of a problem downstream into Wisconsin where heavy
rain fell last night, and there are indications that training
could become more focused there beyond about 01-02Z.

Burke/Weiss

ATTN...WFO...ARX...DLH...DMX...FSD...GRB...MPX...MQT...

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

LAT...LON   47658765 46978751 44499056 43079566 43899598
            45549526 46849295 47329122 47099046 46958988
            47498860