Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1041 am CDT Monday Jul 6 2015
issued at 1041 am CDT Monday Jul 6 2015
A messy forecast for the rest of today.
Showers and thunderstorms developed in the 7-9 am time period
from western WI into eastern Iowa...along the nose of 925-850mb
moisture transport. The strongest convection was located in
Iowa...co-located with the greater instability. This area of
convection was not handled well by any model to this point...and
the net result has been to stabilize conditions more than
expected. Other showers persist across Iowa into Minnesota and
northwest WI...associated with a broad area of low to middle level
frontogenesis ahead of troughing in western Minnesota...and right
entrance region forcing from an upper jet streak in northern Minnesota.
Plentiful moisture in place as well to aid in that shower
generation with precipitable water in the 1.5-2 inch range.
As the upper trough over western Minnesota continues its March
east...so does the broad area of low to middle level frontogenesis.
With ample moisture in place...we should see clouds and showers
continue to generate. Thus...a wet day remains on tap.
However...there are plenty of negatives for deeper convection /
severe weather and heavy rainfall...
1. The right entrance region helping to force current activity is
prognosticated to lift to the northeast...so we lose that forcing.
2. With plenty of clouds and showers...instability will be hard to
build ahead of the cold front marching eastward...currently
from Minneapolis to Farmington Minnesota. Lesser instability will both
impact potential for deeper convection and heavy rain.
3. 700-500mb lapse rates are mostly moist adiabatic...not
favorable for deep convection/severe weather.
4. The ongoing convection in eastern Iowa into eastern WI stabilized
conditions ahead of the front...which shortens the time window for
destabilization prior to the front arrival.
The NCEP hrrr and gsd/Boulder hrrr model runs have been about the
only models having a decent handle on ongoing activity. They both
suggest mainly a showery day with some thunderstorms
possible...particularly in northeast Iowa and far southwest WI where
an opportunity exists for some cape to build prior to the frontal
arrival late today. Have cut back thunderstorm potential overall.
After coordination with Storm Prediction Center...the plan is for most if not all of
the slight risk to be removed on the 1630z day 1 update. Have
begun tailoring services to decrease the threat for severe and
Short term...(today and tonight)
issued at 255 am CDT Monday Jul 6 2015
The forecast questions for today are not if it will rain /it will/
but rather how strong the storms could get...how big is the severe
risk...and what is the heavy rain outlook.
No change in the models...which have been consistent/insistent in
taking an upper level trough west-east across the Canadian/US border
today...driving its associated cold front across the upper
Mississippi River as it does. Strong frontogenetic response with the
boundary - shown in time/height x-sections. Bulk of the shortwave
energy holds north...but 300 mb jet right entrance region likely to
aid in the upward motions. Plenty of lift for a line/area of showers
and storms - evidenced by the ongoing convection to the west.
Morning line with continue its trek east...with mesoscale models pointing
to a lull in the chances before refiring of showers/storms along the
front move in for the afternoon. Will try and paint the rain chances
There continues to be a disconnect between the best wind shear and
the cape axis - a limiting factor to severe potential. An axis of
upwards of 2500 j/kg of MUCAPE develops over western WI by
18z...shifting southeast by 00z. Again - the NAM remains more robust
than the GFS. The deeper...stronger shear lies behind the
front...but there is some overlap around the boundary...especially
in the 0-3 km layer where the NAM and GFS suggest 30-40 kts are
possible. But...clouds and any lingering showers/storms this morning
into the afternoon will cut into that potential instability. Strong
storms seem like a certainty - but the severe possibilities will
depend greatly on whether instability can build. As this
moment...the higher threat would be from late afternoon into early
evening - roughly 21-01z. Locations would be from northeast Iowa into
For threats...the high 0-3 km shear supports damaging wind potential
if a storm can orientate itself normal to the vector -
east/northeast movers. Not much of a dry sub cloud layer per BUFKIT
soundings...so no help for winds there. Think large hail is going to
have a hard time developing. Warm cloud depth is very high. BUFKIT
soundings have rather skinny / long cape. Believe a lot of small
hail will be possible...but in excess of 1 inch could have a
difficult time. Significant rainfall remains high...and more on that
in the hydrology section below.
One last thing...see some isolated tornado potential...moreso for
southwest WI per latest models. NAM/GFS peg 20+ kts of 0-1 shear by
late afternoon over northeast Iowa/southwest WI...with 0-1km cape
upwards of 200 j/kg. Mesoscale models place a surface low over central Iowa by
00z this evening...with the cold front and potentially other surface
boundaries in the area. Doesn/T appear to be enough shear to support
supercells...but if a storm can attach itself to a boundary...the
potential to ingest enough helicity to spin something up is
there. It not a great threat...but it bears watching.
Long term...(tuesday through sunday)
issued at 255 am CDT Monday Jul 6 2015
Models continue to suggest the zonal upper level flow shifts to
broad ridging across the central U.S. By the upcoming weekend.
Trends favor keeping most ripples in the flow north or south of the
local area...with perhaps a shortwave topping the ridge Friday
night/Sat. Not a lot of consistency in this Stead though...with the
GFS/Gem/ec all showing varying degrees of flip flopping with placing
small chance here and there. The likelihood of a dry work week after
Monday gets an uptick as a result. Consensus solution throwing in
some probability of precipitation for Friday-sun with GFS/ec suggesting ridge runners bringing
a shower/storm threat. Confidence not high...but will stay with
consensus for now.
With the anticipated shift to ridging a loft by the weekend...look
for temperatures to return or rise above the seasonable normals. It could
get rather steamy too as the GFS and ec suggest middle 60s dewpoints.
Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 631 am CDT Monday Jul 6 2015
A cold front will push showers and thunderstorms through the area
today. Look for prevalent rain showers/ts at klse and krst through this
morning with perhaps a bit of a lull later this morning into early
this afternoon. Looking for redevelopment of storms along the cold
front in the 20-21z time frame...then clearing the area after 00z.
Otherwise...prevalent MVFR conditions expected today with bouts of
IFR. Look for VFR conditions after 23z at krst and 04z at klse as
drier Canadian air filters into the region behind the cold front.
issued at 255 am CDT Monday Jul 6 2015
Models stay the course with painting excellent conditions for
efficient rain producers and a heavy rain potential today. Precipitable waters
exceed 2 inches...warm cloud depth of 4000+ M...and a steady influx
of warm southerly air rich in moisture. Ample lift along and Post
the cold front to put this moisture to good use. Naefs anomalies
still right around +3 too. Rainfall rates will be high in the
stronger storms - key for urbanized flooding. Thankfully...the
northeast-southwest orientation and general progressive trend of the
showers/storms with the system should limit the potential for
training storms. But if they would occur...flooding quickly becomes
a concern. The higher threat should be this afternoon/evening.
Downward trend in storm intensity as the current storms to the west
move in will get a bump up for the afternoon as instability builds
ahead of the line. Overall...localized 2-3 inch amounts are easily