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Area forecast discussion
National Weather Service La Crosse WI
335 PM CDT Fri Jul 19 2019

Short term...(tonight)
issued at 335 PM CDT Fri Jul 19 2019

Early afternoon analysis showed a surface boundary draped west to
east across northern portions of the forecast area, with a surface
low near the Minnesota/South Dakota border. The low will make
progress east/northeastward and the idea is that this provides
enough convergence along the front to get some showers and storms to
fire across western/central Minnesota. The 18z mpx sounding
indicates 5000+ j/kg of MUCAPE and 50+ kts of deep layer shear with
a large, looping hodograph, so would certainly expect supercells to
develop with all severe weather hazards possible. Cells are
eventually expected to merge into a bowing mesoscale convective system with a widespread
damaging wind, tornado, and heavy rain threat. However, confidence
remains low in exactly how everything will play out, as a decaying
mesoscale convective system moving out of the Dakotas and across Minnesota most of the
morning has led to more cloud cover and rain-cooled air than
initially expected, slowing the boundary's northward progression. In
addition, strong capping remains in place, with a 700 mb temperature
near 14 c per the kmpx 18z sounding, so storms could have some
difficulties getting going at first. Bottom line, expect
supercellular storms northwest of the forecast area to merge into a
line as they approach Taylor and Clark counties this evening.

Following those storms, a cold front will slide into the area and
perhaps provide another focus for some overnight convection near and
north of the I-90 corridor. Strong moisture transport looks to be
oriented more parallel to the front, so not an extremely favorable
setup for flash flooding. However, the environment remains conducive
to efficient rain-producing storms and given the very wet soil, it
wouldn't take much to cause more flooding, especially as the
boundary looks to stall overnight...something to monitor very
closely as we go through the night.

Long term...(saturday through friday)
issued at 335 PM CDT Fri Jul 19 2019

On Saturday, the flow will become westerly across the as the exit
region of a 700 to 500 mb level jet moves eastward across the
region. At the same time, a surface cold front will advance
south and southeast across the upper Mississippi River valley. The
airmass ahead of this front will remain very moist with surface
dewpoints in the 70s. As surface heating occurs, ml convective available potential energy will
climb to around 3,500 j/kg. With still strong capping preceding
this front, the initial shower and storm development will be
elevated on the north side of the boundary. As the heating
continues expect surface-based storms to develop during the mid to
late afternoon across northeast Iowa and southwest Wisconsin.
With very steep low-level lapse rates and moderate 0-6 km layer
shear, a few supercell storms may develop. These storms may evolve
into a damaging wind threat as their cold pool coalesce. In
addition with precipitable water values of 2 to 2.25 inches and
warm cloud layer depths of 4-4.5 km, there will be a threat for
heavy rain. May need a Flash Flood Watch if later models suggest
training will occur for Saturday and Saturday night.

&&

Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1256 PM CDT Fri Jul 19 2019

Mainly VFR conditions expected through the afternoon with some scattered
passing clouds in the 1500 to 2500 ft level. Winds will remain
southerly/southeasterly around 10 kts, with some gusts just over
20 kts at times at rst. Showers and storms are then expected to
develop across central Minnesota and move east/southeast into
Wisconsin. There is a chance some storms could reach as far south
as rst and lse as a cold front moves in and stalls, but
confidence remains somewhat low, so going to keep a thunderstorms in the vicinity mention.
Expect a break in the precip during the early morning hours, but
additional storms may develop by mid morning along the stalled
front. With any of the storms overnight and Saturday, MVFR or IFR
would be possible and should storms not develop this far south
tonight, some guidance hints at some lower MVFR ceilings along the
cold front. Otherwise, expect northerly or northwesterly winds
around 10 kts behind the front.



&&

Arx watches/warnings/advisories...
WI...excessive heat warning until 7 PM CDT Saturday for wiz054-061.

Excessive heat warning until 7 PM CDT this evening for wiz029-
032>034-041>044-053-055.

Heat advisory until 7 PM CDT this evening for wiz017.

Minnesota...excessive heat warning until 7 PM CDT this evening for mnz079-
086>088-094>096.

Iowa...excessive heat warning until 7 PM CDT Saturday for iaz011-018-
019-029-030.

Excessive heat warning until 7 PM CDT this evening for
iaz008>010.

&&

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