Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 161136
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
636 AM CDT Tue May 16 2017

.SHORT TERM...(Today and tonight)
Issued at 337 AM CDT Tue May 16 2017

Main fcst concerns this period are SHRA/TSRA chances this morning
and tonight, very warm temperatures today.

Data analysis at 06z had a broad area of low pressure in the
central/northern plains with a warm front east across northern IA.
Low level jet and feed of a 1.5 inch PW airmass northward and over
this boundary producing a band of SHRA/TSRA across the area from
near the Twin Cities to KLSE to KDLL. TSRA have been rather slow
moving, producing locally heavy rains. Warm, summery airmass in
place south of the warm front with early morning temps in the mid
60s to mid 70s, while a more typical mid May airmass with temps in
the 40s and 50s was over the north half of the region.

No issues noted with 16.00z model initializations. Solutions quite
similar, at last at 500mb as the mid level low over southern CA/NV
early this morning lifts into the central plains by 12z Wed,
keeping deep S/SW flow over the region. Short-term fcst confidence
is good this cycle.

In the short term, shortwave energy over the plains lifts north
of the area today as the mid level ridging amplifies over the
mid MS valley, ahead of the low/troughing coming thru the southern
Rockies. One sfc-850mb low/wave lifting into northern MN lifts the
warm front north of the area this morning, taking the bulk of the
lower level thermo-dynamic forcing and theta-e convergence with it.
South of this front are rather flat 925-850mb thermal fields.
Model soundings indicating some 100 to 300+ J/KG of sfc based CIN
over most of the area this afternoon under an inversion near 850mb,
a rather classic warm sector. Some SHRA/TSRA to continue lifting
across mainly the north parts of the fcst area this morning, with
the warm/capped airmass S of the warm front to spread N across the
area thru the late morning/afternoon hours. A few of the TSRA this
morning could still be strong with small hail and brief heavy
rains. 925mb temps in the warm sector airmass in the 21c-24c range,
with S-SW gradient winds of 15-25mph an gusty for plenty of mixing.
Model do show some decrease of PW over the area this afternoon,
which should allow for at least partial sunshine over much of the
area. Consensus highs today mostly in the 80s, perhaps a 90 or two
in the MS/WI river valleys quite reasonable.

The sfc cold front approaches tonight with the next sfc low
deepening in the central plains in response to the shortwave lifting
out of the southern Rockies. Moisture return increases into the area
thru tonight with PW values in the 1.5+ inch range by later tonight.
This with a MUCAPE axis of 500-1000 j/kg ahead of the cold front
remaining over the area thru the night. Consensus SHRA/TSRA chances
increasing (highest across the NW half) and spreading east across
the fcst area tonight look good. CAPE and deep shear parameters
indicate a few TSRA across the NW half of the fcst area could be
strong to severe Tue evening before the CAPE wanes for the night.
See SWODY1 for more details.

.LONG TERM...(Wednesday through Monday)
Issued at 337 AM CDT Tue May 16 2017

For Wednesday thru Thursday night: main fcst concerns this period
are SHRA/TSRA chances and rain amounts Wed/Wed night and cooling
temperatures.

Model runs of 16.00z in decent agreement lifting mid level low in
the central plains at 12z Wed into norhtern WI by 12z Thu, then
building shortwave ridging over the region Thu/Thu night as this low
departs and the next deep low digs thru the central Rockies. Fcst
confidence in this time period is generally good this cycle.

Approaching/passing sfc-mid level low spreads deep layered forcing/
lift across the area Wed/Wed night. This with what is expected to be
a 1.25 to 1.75 inch PW airmass in place across most of the Upper
Midwest.  MUCAPES in the range of at least 500-1500 j/kg over much
of WI/IA/southern MN Wed/Wed evening. Deep layered shear ahead of
the sfc-500mb low in the 40+kt range as well. SWODY2 with a marginal
risk of strong/severe TSRA across the area mainly Wed afternoon/
evening looks well trended. Consensus SHRA/TSRA chances in the 60-
80% range across the entire area for Wed afternoon/evening looking
quite good for now. Threat of locally heavy rains exists as
well, especially across the NW half of the fcst area in what would
be a deformation band/longer duration rainfall near/NW of the sfc-
mid level low track. Additional rain amounts of 1 to 2 inches from
tonight thru Wed night west/north of KLSE appear reasonable. Precip
chances diminish rather quickly later Wed night/Thu morning with
passage of the sfc-mid level low and drier mid level air wrapping in
from the west and south behind the departing low. CAPE wanes rather
quickly after midnight as the column cools under the mid level low,
with any precip after midnight Wed night lingering into Thu morning
as SHRA. Much cooler by Thu as low level NW flow and cold advection
spread across the region. Trend of highs below normal by Thu looks
good.

For Friday thru Monday (days 4-7): main fcst concerns this period
are more rain chances spreading across the area Fri thru Sat as the
next mid level low lifts out of the Rockies and across the Upper
Midwest. Given a tightening model consensus, fcst confidence in the
Fri/Sat time period is above average this cycle.

The Fri/Sat system is set to lift NE across the region with PWs in
the inflow airmass again in the 1.5 inch range. One thing minimal or
missing from this next system for Fri/Sat is the CAPE as models/
model consensus trends to keep the bulk of the instability south of
the fcst area until perhaps Sat afternoon. Consensus precip chances
increasing back into the 40-70% range for much of Fri afternoon thru
Sat appear reasonable. Potential for heavy rains with this system as
well, but without the CAPE, it would be more a heavy rain threat
from the extended duration of the event vs. short-term TSRA. Thru
all this, temps continue to cool. Consensus highs well below normal
for Fri thru Sun look good as the next mid level low approaches/
passes and mostly cloudy to cloudy skies prevail. A cool low level
airmass remains over the region Mon but some temp recovery to start
the next work week reasonable as a weak ridge of high pressure looks
to bring some sunshine to the area.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 636 AM CDT Tue May 16 2017

VFR conditions are expected today into early this evening across
the area, with any remaining showers and storms now north of
LSE/RST. We will have to watch a weak convectively-induced system
lifting out of Nebraska for some potential of redevelopment
briefly this morning, though confidence in that remains on the
lower side. Instead, much of the afternoon/early evening should
remain dry and breezy, with southwest winds gusting 25-35 knots,
before additional shower and storm chances return late this
evening and overnight. Placement and exact timing of those also
remains lower confidence this far out, but a period of MVFR or
even IFR is possible, especially for RST.

&&

.HYDROLOGY...(tonight thru Wed night and again Fri thru Sat)
Issued at 337 AM CDT Tue May 16 2017

With one round of locally heavy rains in the past 12 hrs under
our belts, attention turns to the potential for more locally
heavy rains in the next few days. The system set to lift NE
across the region tonight into Wed night has potential to produce
1 to 2 inches of rain across roughly the NW half of the fcst area,
near/NW of the sfc thru mid level low track. Another strong/moist
system then looks to lift NE across the region Fri thru Sat, with
potential for another widespread 1 to 2 inches of rain across the
area. With ET across the landscape not at full-steam yet (row
crop field bare and trees just leafed out) most soils remain
rather moist. The locally heavy rains overnight have already
created significant within bank rises on a few rivers and streams
in the area. An additional 1 to 2 inches tonight-Wed night,
locally more if TSRA repeat across the same area, is likely to
cause additional rises. Then there would be little recovery time
before the round of rain Fri/Sat. flood watches/warnings may be
needed for parts of the area as the week wears on and details of
the additional heavy rain threats become clearer or additional
heavy rains fall.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...Lawrence
HYDROLOGY....RRS



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