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Area forecast discussion
National Weather Service La Crosse WI
624 am CDT Thu Sep 19 2019

Update...
issued at 355 am CDT Thu Sep 19 2019

Based on radar trends, expanded the Flash Flood Watch to include
Houston, Fillmore, and Mower counties in southeast Minnesota. Storms have
been focused along the Minnesota/Iowa border with rainfall estimates of
1-2"/hr via mrms. Rainfall totals will add up quickly where storms
train over the same areas.

&&

Short term...(today through friday)
issued at 302 am CDT Thu Sep 19 2019

Heavy rains remain the main concern, especially through this
morning.

Anomalously high precipitable water values up to around 1.75 inches
will remain in place with persistent low-level southerly flow
associated with expansive mid-level ridging ahead of a western US
trough. Early this morning, expect some uptick in elevated
showers/storms per high res model consensus in proximity to the
low-level boundary as a weak shortwave trough approaches. The high
pwats, plentiful elevated instability, and deep warm cloud layer
will support high rainfall rates. A heightened flood risk will
exist in northeast Iowa/southwest WI from heavy rainfall due to the
wet antecedent conditions. No changes are planned to the Flash
Flood Watch at this time, although will have to monitor trends
over the next few hours. With steep mid-level lapse rates,
stronger updrafts may produce some hail for a time early today. As
the weak wave/possible mesoscale convective vortex passes east and convective overturning
reduces the elevated instability, expect a gradual decrease in
showers/storms through the morning. In this pattern, though,
couldn't rule out a few showers/storms through the day, though.

Potential for showers or storms will continue tonight into Friday
in the vicinity of a low-level boundary that will begin to
retreat slowly north as a warm front. Confidence is fairly low on
the evolution of convection tonight with weak forcing underneath
the mid-level ridge, but the elevated instability axis is forecast
to extend from the Dakotas southeast through Iowa/southern Minnesota and
southwest WI. The push of moisture transport looks to be mainly
west of the MS river tonight, with cams favoring the western half
of the area for highest rain chances. The environment will remain
favorable for heavy rainfall with any storms, although coverage of
storms looks lower tonight. Low stratus/fog is possible overnight
tonight into Friday outside of rainfall given the very weak flow.
A warm, humid airmass will persist, with highs in the mid 70s to
mid 80s through Friday.

Long term...(friday night through wednesday)
issued at 302 am CDT Thu Sep 19 2019

The western trough will eject across the northern plains into the
upper Great Lakes Saturday into Sunday. Strong northward mass
response/moisture transport ahead of the trough will maintain very
high pwats ahead of the associated cold front with continued chances
for storms. Still expect locally heavy rain to be a threat given
the very moist background environment. Deep layer winds do finally
begin to increase as the upper trough approaches, so at least a
low-end severe weather threat could develop on Saturday
afternoon/evening, depending on how much instability can be
realized. With the strong moisture feed into the area, low clouds
may hinder surface-based destabilization. 0-3 km shear looks to be
mostly in the 25-35 kt range with higher 0-6 km shear along or
lagging the front, so not expecting a widespread strong/severe
event.

A pattern shift will occur late in the weekend into next week as a
drier airmass works into the area behind the cold front along with
more seasonable temps. Some showers/storms may linger into Sunday
across northeast Iowa/southwest WI before exiting south and east
with a few showers possible through the afternoon as the mid-
level trough swings through. Into early next week, surface high
pressure should provide a few dry days with a split upper flow
pattern dominated by a stronger quasi-zonal northern stream
branch over the area.

&&

Aviation...(for the 12z tafs through 12z Friday morning)
issued at 624 am CDT Thu Sep 19 2019

Widespread showers and storms continue to slowly advance east
through the area. As of 1120z, they have cleared rst and will
clear lse by 13z based on the latest trends. Outside of heavier
rainfall and MVFR/IFR visibility, conditions are VFR and should
remain that way for much of the day, as thicker mid level clouds
scatter out and some diurnal cumulus develops by afternoon. By
tonight, there are plentiful questions about how clouds will
behave, with some hint that stratus and/or fog might develop late
tonight, but also with some potential for additional scattered
thunderstorms to develop. Confidence at this point remains low
enough to hold off on any widespread lower ceiling mention and see
where later trends take US.

&&

Hydrology...
issued at 302 am CDT Thu Sep 19 2019

A Flash Flood Watch remains in effect for northeast Iowa/southwest
WI/far southeast Minnesota this morning. High rainfall rates with storms
this morning will pose a flash flood risk with rises possible on
some rivers. In addition, this area is susceptible to flooding
given the antecedent wet conditions. Additional locally heavy
rainfall is possible into the start of the weekend, especially
Saturday/Saturday evening.

&&

Arx watches/warnings/advisories...
WI...Flash Flood Watch until noon CDT today for wiz053>055-061.

Minnesota...Flash Flood Watch until noon CDT today for mnz094>096.

Iowa...Flash Flood Watch until noon CDT today for iaz008>011-018-019-
029-030.

&&

$$

Update...jm

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