Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 300755
Area Forecast Discussion
National Weather Service La Crosse WI
255 AM CDT TUE AUG 30 2016
.SHORT TERM...(Today and Tonight)
Issued at 255 AM CDT Tue Aug 30 2016
Area of convection from central MN into northern WI getting a hand
from an upper level shortwave trough pushing into northern MN, sfc
cold front running from central MN northeast, and a fetch of low
level moisture transport. Satellite trends point to the shortwave
moving mostly due east, while the nose of the moisture transport
continues to push east. That said, radar trends continue to show the
storms backbuilding into a moisture plume evident on latest
watervapor satellite imagery. This region also looks like its
starting to feel the southern portions of that shortwave. Meso
models having a really hard time with this.
Question becomes how far south the showers/storms will make it
before they start to wane/dissipate. Considering where the main
players for initiation are expected to move, believe there should be
a weakening trend in the convection as it sinks slowly southward
this morning. Could hold intensity a bit longer over southern MN
where that moisture plume lies...ala the ARW solution.
Another item of interest is an outflow dropping south ahead of the
main convection. We`ll have to watch that as a source for lift
through the day - if it holds together, pcpn could pop up where it
might not be expected (ala, with the front).
Meanwhile, showers have developed over northeast IA/southwest WI.
SPC RAP meso analysis places weak CAPE/effective wind shear in this
location, and it might be tied to a left over outflow boundary from
the south yesterday. Recent HRRR runs get a bit aggressive with
shower/storm development in this region, pushing it south/east by
15z. HOPWRF more showery, and quicker to exit.
Will follow radar/satellite trends to carve out the early morning
chances, with some weight toward the HRRR/Hopwrf.
The cold front continues to sink southward through the day today,
but again, with such a saggy/baggy pressure gradient, the
convergence with the boundary is weak. It will work into an area of
increasing instability though, thanks to a relatively warm/moist
airmass that moves in yesterday. Would expect some showers/storms to
fire as the day wears on. Little wind shear to play with, so storms
should be pulsy in nature. Some potential for small hail, with
associated gusts, but not much more than that - and a very minimal
(if any) severe threat. As for heavy rain, warm cloud layer only 3-
3.5 kft with pws across the south near 1.5 inches. Nowhere near the
setup we have had for recent heavy rain events. However, when the
storms develop, they aren`t moving all that fast with light steering
currents. So, while they may not be the most efficient rain
producers, they will be capable of heavy downpours that could linger
at any given location. Not expecting much of a flash flood threat,
but this is a scenario to watch through the day into tonight.
So how does this play out? Going to lean the forecast toward the
scenario of a weakening area of pcpn working from the north, minimal
(maybe even just dry) chances around the I-90 corridor, and then
then an uptick in chances mid afternoon over northeast IA/southwest
WI. Any convection that does get going later today looks scattered
in nature, and exits south/east by 06z. We`ll also have to watch
cold pool interaction for storms that do get going later today -they
will be another source for storm initiation.
.LONG TERM...(Wednesday through Monday)
Issued at 255 AM CDT Tue Aug 30 2016
Wed-Fri still setting up to be some real nice days with high
pressure at the surface and an amplified upper level ridge axis
meandering across the Upper Mississippi River Valley. Highs in the
70s with lows in the 50s. Thu/Fri mornings looking like a good bet
for fog in area river valleys.
Unsettled weather looks to return to the region for the weekend as
the sfc high exits east and upper level ridge breaks down/bends
eastward. Avenue of low level moisture sets up to the west saturday,
gradually pressing eastward over the local area for sun/mon. While
an upper level trough is progged via the GFS/EC to ride the exiting
ridge into southern Canada, bypassing the local area, bits of energy
may spin across the region, and perhaps more importantly, there
looks to be a north-south running front shifting across the area
Sun/Mon (associated with the upper level trough). Areas of
showers/storms would result. Timing and where the higher chances
will be aren/t certain yet, some variability in the models, but all
favor a rain threat.
.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1141 PM CDT Mon Aug 29 2016
Very moist airmass remains in place across the area, with current
temp/dew point spreads quite low, on the order of 1-3 degrees.
Given continued light flow and development of visibility reductions
already across many of the river valleys, confidence remains higher
that LSE has a chance to see a period of LIFR visibility in some
dense fog through sunrise. Still, don`t really like seeing all of
those thicker high clouds overhead and recent guidance trends of
slightly stronger above-inversion flow to around 10-15 knots, but
with some shreds of stratus already showing up on our local valley
webcam, have to believe there will be at least a temporary period of
some thicker fog. If not at the airport, it will likely be very
close. For RST, should again see some periods of IFR with fog
development, though not sure just how dense it will become.
Any fog will likely lift into a stratus/stratocumulus deck through
the morning in advance of an approaching cold front. That boundary
right now is providing storms to the north, though those are showing
signs of weakening and don`t envision them impacting RST or LSE the
rest of tonight. Additional storms are possible as the front arrives
today, though it appears the best risk will be south of the TAF
sites. Behind the front, much drier air arriving should clear out
any remaining lower clouds, with just some cirrus into the evening.