Area forecast discussion
National Weather Service La Crosse WI
647 am CDT sun may 19 2013
Short term...(today and tonight)
issued at 318 am CDT sun may 19 2013
It still appears ample instability will be around by afternoon
over the forecast area....however a clear trigger and focus for
thunderstorms and severe weather is still not clear.
At 08z the High Plains convection continues to shift east across
Iowa...being fueled by a maritime tropical air mass surge as
pressures fall in response to the initial shortwave trough
ejecting north from the long wave trough over the intermountain.
Currently the trough is over northestern Colorado per GOES water vapor imagery.
Another piece of energy is over the western Texas Panhandle and will
arrive this evening. 00z radiosonde observation data indicates that maritime
tropical air mass well...with an axis of 1.25 to 1.50 inch
precipitable water values from Omaha to Springfield /170 percent
normal/. This is compared to 1.00 to 1.25 inches msp-dvn. This air
mass arrives over the forecast area today with surface dewpoints
the highest of the year...in the middle to upper 60s. 88d wind
profilers showing an increasing low- level jet across
Iowa...converging into Minnesota. This trajectory is not lifting
adiabatically however...with little temperature gradient through
that area. The convergence and instability is fueling the storms.
A few warnings have been issued recently for hail. Wind shear is
weaker so isolated large hail is really all we would expect.
Have been monitoring the hrrr runs all evening and until the
19.03z run...a very consistent signal was present of bringing the
convection into the western forecast area around 12z. Have slowly
stepped up the rain chances based on the low-level jet and
moisture transport convergence in concert with increasing cape as
that moist air mass begins its influence. Currently MUCAPES of
around 1000 j/kg are west of the miss river...but this is likely
to increase overnight and probably be in the 1300 j/kg range by
dawn.
The moisture surge and transport shift northward and begin what
appears to be a longer term focus of convergence northwest of the
forecast area. This afternoon it appears the model guidance is
consistent in a fairly uniform southeast flow over the forecast
area...with convergence across northern WI to central Minnesota...along the
stationary boundary. With very high moisture in place and some
heating...we will build some higher MLCAPES around 1800 j/kg
quickly /maybe 3000 j/kg SBCAPE/. This using a 67f dewpoint.
The wind shear over the area has been consistently forecast below
supercell thresholds this afternoon with most of the shear a
moderate 30-35kts from 0-3km and little increase above. This would
favor cold pool systems and bowing wind segments. The 19.00z NAM
is a bit of an outlier suggesting stronger winds aloft further
north...in the supercell range. Have stayed the course with a
weaker gradient aloft with the low center still far west. So the
threats continue to be damaging wind and large hail. Flash
flooding could also be a problem...see hydrology section. An
isolated tornado is possible too.
The main problem right now seems to be the lack of a good trigger
this afternoon. The boundary layer flow is a uniform southeast and
little convergence is seen in the area. Thus...any boundary that
does present itself could trigger initiation as there will be no
cap in place. With such vigorous convection in KS/OK...upstream of
the area...we could see a convective vorticity maximum appear in the
flow and possibly become an initiator. The hrrr and hi res nmm seem
to be hinting at this solution later afternoon.
This evening...as strong energy rotates northeast...currently in
West Texas...the low-level jet in the 19.00z NAM excites and a nice
convergent low-level jet develops convection over Iowa and moves
it northward. The 19.00z GFS does not agree on this with its focus
north again on the northwestern WI-central Minnesota boundary. But the hi res 00z
runs...using the NAM as initialization and boundary
conditions...bring widespread convection in from Iowa. There is
better jet dynamics and transport in the NAM..and this seems
reasonable...thus..have continued to hit the evening
hard...evolving the weather north by morning. The 19.00z Gem and European model (ecmwf)
agree well with this scenario. So...looking for a wet overnight.
There is still instability overnight...2k MUCAPE...and a bit
better shear.
Bottom line is that the daytime hours may be pretty clean because
of lack of a thunderstorm trigger. If an mesoscale convective vortex from OK/Kansas can
develop thunderstorms and rain and the thunderstorms and rain can root itself in the boundary layer
over Iowa...or some boundary exists...chances increase for afternoon
severe weather...as the air mass will be weakly capped. This
evening seems to promote increasing rain and thunderstorms with
still some severe potential until the cape is used up...but not
quite as widespread as an afternoon event.
Long term...(monday through saturday)
issued at 318 am CDT sun may 19 2013
As the low draws closer Monday...shear increases to supercell
strength...but again it may be difficult to identify a trigger
with little surface boundary action in the area. The stationary
boundary remains to the northwest with the best convergence. However...with
the surface low expected to be near kfsd...and ample MLCAPE again
near 2000 j/kg...storms look to form to the west and move into the
area Tuesday evening. However...this pattern is so uncapped and
unstable...we found it hard to get detailed on timing anywhere.
Monday still has the potential for severe weather...and it looks
like the evening hours. With increased wind shear...rotating
storms are possible and thus large hail and wind. An isolated
tornado cannot be ruled out but the wind environment doesnt look
overly favorable.
The low continues to shift slowly east and provide rain threats
that are difficult to target through Thursday. Some drier air begins to
shift into the area on southwest flow off The Rockies on
Tuesday...and somewhat cooler air. This will end the severe
weather threat.
&&
Aviation...(for the 12z tafs through 12z Monday morning)
issued at 646 am CDT sun may 19 2013
A weakening line of showers and a few thunderstorms will move into
western Wisconsin around 19.12z. This line is not causing any
visibility restrictions or a wind shift. In addition...the
ceilings will remain VFR. Mesoscale models suggest that several lines
of showers and thunderstorms will move through the taf sites this
afternoon and tonight. The only time period that seems to be
consistent in timing is the one that moves through between 19.18z
and 19... included some thunderstorms during this time
period. A few of these storms may even become severe with large
hail and damaging winds being the primary concern. Beyond this
time period...the timing is very inconsistent...so just went with
showers for now.
&&
Hydrology...
issued at 318 am CDT sun may 19 2013
Hydrology...the probabilities have tipped toward a Flash Flood Watch
for 3 PM today through Monday. There are too many variables in
play that suggest heavy rainfall and flooding potential including
soil moisture centered over southeastern Minnesota abnormally high...fridays
rainfall having higher end flooding outcomes in southeastern
Minnesota...precipitable water values over 150 percent normal moving
in...SBCAPE building during the day to near 3k /ml near 1800/
based on 67f dew point...and forcing increasing during the evening
with stronger moisture transport convergence and shear moving into
the area.
This Flash Flood Watch may be expanded and possibly extended into
Monday night should details suggest the flooding threat could
remain. There is some suggestion that the very high precipitable
water moist conveyor belt would shift into WI as a drier rockies air
mass advects in on southwesterly flow.
Rivers in the watch area would also see rapid rises aside from the
flash flooding effects.
&&
Arx watches/warnings/advisories...
WI...none.
Minnesota...Flash Flood Watch from 3 PM CDT this afternoon through Monday
evening for mnz079-086-087-094-095.
Iowa...Flash Flood Watch from 3 PM CDT this afternoon through Monday
evening for iaz008.
&&
$$
Short term...baumgardt
long term...baumgardt
aviation...boyne
hydrology...baumgardt
