Area forecast discussion
National Weather Service La Crosse WI
645 am CDT Sat may 18 2013
Short term...(today and tonight)
issued at 400 am CDT Sat may 18 2013
Upper level ridging taking place across the upper Midwest today as
the western Continental U.S. Trough deepens. Plenty of convection firing up off
the 850 mb low level jet in conjunction with a passing shortwave across
central Minnesota back into the Dakotas. This activity will mostly
remain north/northwest of the forecast area...however could clip
portions of north central Wisconsin by middle to late morning. Latest
runs of the hrrr and rap show convection waning but possibly
impacting Taylor and Clark counties in Wisconsin so have maintained
shower/storm chances there. Moist easterly flow and decaying
cirrus will keep skies mostly cloudy to overcast this
morning...but as the mesoscale convective system lifts north...it will pull the warm front
through the area. Partly to mostly sunny skies will most likely be
seen across northeast Iowa where the front is able to clear
through the most.
Decent instability expected today in the warm sector...and MUCAPE
values range from 1000 to 2000 j/kg in a weakly capped environment.
Lack of surface convergence however precludes higher precipitation
chances but most of the deterministic and probabilistic guidance
hints that central Wisconsin could see a stray shower or
thunderstorm this afternoon. Have maintained precipitation chances
there. Otherwise...a warm day on tap with lower to middle 80s expected
south of I-94.
Relatively quiet conditions tonight as convection is expected to
remain well to the west and north. Mild night with southerly
flow...and overnight lows in the upper 50s to lower 60s.
Long term...(sunday through friday)
issued at 400 am CDT Sat may 18 2013
Focus is on severe weather chances Sunday and Monday as active
weather pattern sets up across much of the Central Plains.
Upper level negative tilted shortwave trough ejects into the Central
Plains on Sunday...meanwhile at the surface a few waves of low pressure
should be developing across Oklahoma into Nebraska. This will pull
north ample Gulf of Mexico moisture...with precipitable waters increasing from
around 1 inch...to nearly 1.75 inches...or 175 percent of normal.
Surface dewpoints will rise into the low to middle 60s. There should
be convection across western Iowa into southwest Minnesota early
Sunday morning...which will be feeding off the 850 mb low level jet. This
should weaken as it approaches the western forecast area but good
consensus that showers and thunderstorms will be around in the
morning hours. Then...the big question becomes how much clearing
we would see to aid in destabilization. Assuming we clear
out...0-3km MUCAPE is expected to increase to 1500-2000
j/kg...highest to the south and west. Overall the shear profiles
are less impressive...but still show 0-3km bulk shear of nearly 30
kts. The atmosphere does remain uncapped so expecting good chances
of convective initiation with organized storms in the form of
supercells or possibly bowing segments. Strong to severe storms
would develop across western into central Iowa...and track
east/northeast through the evening hours. The primary threats are
large hail and damaging winds. The environment still supports an
isolated tornado...however depending where a warm front lays is
more uncertain. 18.00z GFS has this boundary across central
Iowa...but 18.00z European model (ecmwf)/NAM hint this could be further south.
The whole system edges eastward Monday...with the both the
surface and upper level low becoming stacked near Sioux Falls South Dakota
by 12z Monday. Pieces of energy will be rotating through southern
Bend of this trough...lifting through the area during the day. The
synoptic setup is slightly more favorable for severe weather for
parts of the area. The shear profiles are more impressive with
0-6km bulk shear values 40 to 50 kts across eastern Iowa into
southern Wisconsin. The more limiting factor this day is how much
destabilization will occur. There could be scattered showers and
plenty of clouds around to limit instability. If we can break out
and fully destabilize...then 0-3km MUCAPE may reach 1500 to 2000
j/kg. The environment should support discrete storm cells...with
the threats again large hail and damaging winds.
Tuesday and beyond...the upper low remains nearly stationary across
the upper Midwest. This gives periodic chances for showers and
thunderstorms across the entire area Tuesday-Thursday. Model
consensus paints 30 to 60 percent chances which is appropriate at
this time. Feel that much of next week could be unsettled but see
little opportunity to provide more details in what periods would be
more active...since timing pieces of rotating around the upper low
is difficult this far out. The bigger story would be the heightened
risk for excessive rainfall and localized flooding if repeated
rounds of convection do impact the area. By mid-week...soils would
become even more saturated...and flash guidance further reduced. See
hydrology section below for more details on that risk.
Temperature-wise...warmest days look to be Sunday and Monday before
a gradual cool down Tuesday...then to below normal readings
Wednesday through Thursday as the upper low drifts overhead.
&&
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 644 am CDT Sat may 18 2013
With the best forcing remaining north and west of the forecast
area through this evening...kept the taf sites dry with ceilings
above 12k feet. The mesoscale models are in good agreement that a short
wave trough will move northeast through the area. This will bring
showers and thunderstorms to northeast Iowa and southeast
Minnesota after 19.09z. Despite this precipitation...soundings
suggest that the ceilings will remain VFR. While there may be
some brief visibilities in the IFR/MVFR range...timing is very
uncertain...so left them VFR.
&&
Hydrology...Sunday and into next week
issued at 400 am CDT Sat may 18 2013
Heavy rainfall /2 to 2.5 inches/ fell Friday across portions of
southeast Minnesota and northeast Iowa...and combined with a record
wet meteorological Spring...results in an increased risk for
localized flash flooding as we head into the latter part of the
weekend and much of next week. Periods of showers and thunderstorms
look to impact much of southeast Minnesota and northeast beginning
Sunday afternoon. An additional 1 to 3 inches is possible...and if
this falls in a short period...localized flash flooding would occur.
The 1 and 3 hour flash guidance remains well below normal...and is
generally less than 1 and 2 inches...respectively. Wpc continues the
slight risk of excessive rainfall and flash flooding early next week
across much of central Minnesota into northwest Wisconsin. A Flash
Flood Watch may be necessary for portions of the area.
Streams...creeks...and rivers have responded to the recent
rainfall...mostly within bank rises. However...repeated rounds of
heavy rain could push some of the more flashy basins across
southeast Minnesota and northeast Iowa to near flood stage.
&&
Arx watches/warnings/advisories...
WI...none.
Minnesota...none.
Iowa...none.
&&
$$
Short term...zt
long term...zt
aviation...boyne
hydrology...zt
