Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 210254
Area Forecast Discussion
National Weather Service La Crosse WI
954 PM CDT WED JUL 20 2016
Issued at 954 PM CDT Wed Jul 20 2016
Well...it wouldn`t be a summer ridge pattern without some threat
for convective issues. We have lots of those late this evening,
with a notable complex continuing to develop across central North
Dakota, featuring quite a few supercells beneath a corridor of
stronger effective shear but those increasingly trying to organize
into more of a linear structure. The big question is how that
stuff behaves the rest of the night and into Thursday morning. For
one thing, we will have some increasing capping working north and
eastward across the area, with the strongest deep layer moisture
transport/convergence progged to lie north of the I-94 corridor
late tonight, coincident with a weakish low level jet feature.
Current activity is being aided by a shortwave progressing through
western North Dakota, with that feature looking to ride the ridge
top and end up near western Lake Superior after sunrise. As such,
and per Corfidi vector analysis, would think the northern
component of the convection will work in that same direction,
generally crossing northern Minnesota and Wisconsin.
However, what about the southern/outflow component? Should we be
able to organize a better cold pool, it is possible that a
southward drift will initiate on the edge of strongest
warmth/capping aloft, intercepting the better moisture advection
to perhaps increase convective coverage south toward I-94 or
maybe even a little farther south. Also, storm motion vectors get
rather small from 11Z-16Z, suggestive that outflow could dominate.
All told, this remains a very low confidence scenario overnight,
but it is not at all impossible that convection could end up
working even a bit more southward, in line with recent HRRR and
Hop-WRF runs, which suggest a linear feature simply marching
through most of the CWA into Thursday morning. Not ready to bite
on that just yet, but it bears watching, as any such outcome could
severely disrupt our heating for at least a time tomorrow.
.SHORT TERM...(Tonight through Thursday Night)
Issued at 236 PM CDT Wed Jul 20 2016
Concern through the period continues to be increasing heat and
some storm chances.
GOES Water Vapor satellite imagery/RAP 500mb analysis showing an
expansive ridge of high pressure over the central conus with
monsoonal plume of moisture pushing into the region. Embedded/
nondescript shortwave troughs within this flow sparking occasional
rounds of convection over the region. Once such area of shra/ts was
pushing east out of the area into southeast MN/northern IL. Some
showers also burbling along the MN/IA border. This seems to be
firing ahead of a weaker mid-level trough moving across eastern
Models continue to show increasing 850mb moisture transport through
tonight ahead of low pressure over the Northern Plains. This, and
the possibility of another nondescript mid-level wave in the
aforementioned monsoonal tap, will produce a chance of showers and
storms mainly east of the Mississippi River or on the eastern
periphery of strengthening 850-700mb cap. Otherwise, looks like a
fairly uncomfortable night ahead as lows only dip into the
lower/middle 70s with dew points in the 70s as well.
The heat is on for Thursday as 925mb temperatures surge to around
30C. Some question as to how much ongoing elevated convection will
be going on or amount of convective debris cloud will be overhead.
Current thinking is that surface temperatures will respond to the
warm air advection...topping off in the 90s. This pushes heat
indices into the 100-115 range during the afternoon. As
such...Excessive Heat Warning goes into effect at Noon
Thursday...dropping off at 10 pm for Clark/Taylor counties but
continuing through Friday for the rest of the area as heat/humidity
continues. Please see our latest Excessive Heat Warning products for
Otherwise, looks like a frontal boundary drops through Thursday
night. However, NAM Bufkit sounding shows a stout cap in place. Will
carry a 20-30 pop for now based on this. Overnight lows Thursday
night only falling into the lower/middle 70s again.
.LONG TERM...(Friday through Wednesday)
Issued at 236 PM CDT Wed Jul 20 2016
As mentioned above...Excessive Heat Warning continues through Friday
for most of the area with the exception of Clark/Taylor counties in
north central Wisconsin where some cooling takes place. Will have to
watch for a continued chance of thunderstorms mainly south of I-90
in the vicinity of the surface frontal boundary.
Thunderstorm chances return to the area late Friday night, then
becoming likely Saturday into Saturday night as a warm front pushes
into the area ahead of low pressure approaching from the Northern
Plains. Will have to watch for severe threat given ample CAPE and
decent 0-3km Bulk Shear.
There will be a slight chance of storms Sunday, but looks like most
of this should be pushed east with cold front.
High pressure builds in Monday for a brief dry period, but
shower/thunder chances move in again Tuesday night into Wednesday as
both the ECMWF/GFS bring a trough through the region.
.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 639 PM CDT Wed Jul 20 2016
Looking like a hot/humid period of weather upcoming for the TAF
sites, but with some very small (and very low confidence) chances
for showers and storms somewhere nearby. Persistent MVFR cloud
deck around KRST is starting to dissipate, but may briefly pass
back over that site and perhaps even KLSE into late evening as
the low level flow slowly shifts more southwesterly.
Otherwise, we will be closely monitoring upstream convection over
the Dakotas. If (that`s a big `if`) storms can get a little more
organized up that way tonight, it`s not totally out of the
question they could work southeastward and toward the area by
around sunrise, though honestly it appears the better chance will
reside north of KLSE/KRST. Looking like not much for the day on
Thursday itself, with a weak cold front approaching toward 00Z and
maybe spreading in additional shower/storm chances. Too early to
introduce any mention given the low probability but something to
watch as any storms will have quite a bit of instability with
which to work (AKA - they could be on the stronger side).
WI...Excessive Heat Warning from noon Thursday to 7 PM CDT Friday for
Excessive Heat Warning from noon to 10 PM CDT Thursday for
MN...Excessive Heat Warning from noon Thursday to 7 PM CDT Friday for
IA...Excessive Heat Warning from noon Thursday to 7 PM CDT Friday for