Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1137 PM CDT Friday Aug 1 2014
Short term...(tonight through Saturday night)
issued at 310 PM CDT Friday Aug 1 2014
Main forecast concerns...shra/tsra chances much of the period as
weak shortwaves continue to rotate southeast across the region.
18z data analysis had weak low pressure over northeast WI with a
weak trough/boundary westward roughly along Highway 29 and into east
central Minnesota. Diurnal warming...1k-2k j/kg SBCAPE/no capping...
convergence near the trough/boundary and weak shortwaves aloft
resulting in scattered rain showers/thunderstorms and rain from central Minnesota to east central WI. Few
of the stronger thunderstorms and rain over the east half of WI producing up to nickel
sized hail. Early afternoon temperatures again in the upper 70s/low 80s
across much of MN/IA/WI.
No major errors noted with 01.12z model initializations. GFS with
its usual odd/biased surface dew point analysis across the upper
Midwest...which looks to impact its model output across the area
already by 18z. Solutions quite similar for tonight through Sat night
as the region remains under northwest-north flow aloft and the surface
high pressure ridge axis slowly drifts south into the middle MS valley.
Tighter 500mb consensus for one shortwave trough/axis to pass late
this afternoon...heights to rise a bit tonight into Sat then lower Sat
night as another shortwave drops through the northwest flow. Short term
forecast confidence would normally be quite good...but surface to 500mb
gradients remain rather weak across the area through Sat night. The
weak gradients/weak features combined with diurnally warmed
soundings indicating 1k-2k j/kg of sb cape and no cin produce
uncertainty around convection potential. Short-term forecast confidence
average to good this cycle.
For the short term...weak shortwaves will slide across the forecast area
this afternoon...with the weak surface trough/front slowly sagging south
across the forecast area through the evening. Still 1k-2k j/kg of sb/mu
cape over the area 00z. Expect some rain showers/thunderstorms and rain to linger into and
wane through the evening as the cape wanes/airmass slowly stabilizes.
Would expect little in the way of forcing/lift with weak shortwave
ridging/rising heights building in tonight. However...850mb ridge axis
is nudged a bit southward tonight with models showing some weak 850-
700mb warm advection/moisture transport and lift into the area
overnight. Some model soundings showing 850mb parcels with around
500 j/kg cape and no cin at midnight tonight. Will continue the
downward trend of rain showers/thunderstorms and rain through the late evening with a generally
dry forecast overnight however would not be surprised to see isolated/
widely scattered rain showers/thunderstorms and rain to continue well past midnight. With mostly
clear skies for stronger radiational cooling and the light winds
left the late night patchy to areas of fog in grids for the valleys/
low laying areas along/east of the MS river.
Weak 850-700mb warm advection/moisture transport continues over the
area Sat...with an increasing model signal for a surface trough roughly
along the MS river. Model soundings again showing diurnal warming
produces 500-1000 j/kg SBCAPE Sat afternoon with little or no
capping. Honored all this with a 20 percent rain showers/ thunderstorms and rain chance across
much of the forecast area Sat afternoon lingering into Sat evening. Next
stronger northwest flow shortwave pushes a little stronger surface-700mb
trough into central Minnesota/northwest WI late Sat night. Bit stronger 925-
700mb warm advection/moisture transport spreads into the north end
of the forecast area late Sat night and continued the small rain showers/thunderstorms and rain
chance there. Overall little change of the airmass over the area
through Sat night...with consensus lows in the middle 50s-low 60s and
highs in the upper 70s-low 80s looking good.
Long term...(sunday through friday)
issued at 310 PM CDT Friday Aug 1 2014
Main concerns for Sunday through Monday night...shra/tsra chances
through the period...temperatures.
Good consensus among the 01.12z models for heights to continue to fall
sun/Sun night as several pieces of generally weak shortwave energy
drop across Minnesota/IA/WI and the western Great Lakes. Area remains under
northwest flow yet Monday/Monday night but models begin to differ on the
timing/strength of the next round of shortwave energy to drop
southeast toward/into the region. Forecast confidence on the good side
sun/Sun night then average for Monday/Monday night.
Passing shortwave sun/Sun night pushes the surface-850mb trough slowly
southward and into the forecast area. What weak lower level Thermo-
dynamic forcing there is slowly translates south across the forecast
area. Model soundings showing 500-1500 j/kg sb/mu cape and minimal
capping centered on Sun afternoon...with with around 500 j/kg of
cape and minimal cin when lifting 850mb parcels Sun night. Given the
weak shortwaves/falling heights aloft and precipitable water values in the 1.5 inch
range by 00z Monday...20-50 percent rain showers/thunderstorms and rain chances slowly spreading
south across the forecast area reasonable for sun/Sun night. Lower level
trough/boundary is in/near the south 1/2 of the forecast area Monday. This
with diurnal warming producing 1k-2k j/kg sb/mu cape and minimal cin
by Monday afternoon. 30-50 percent rain showers/thunderstorms and rain chances Monday look good.
Boundary to push south of the area Monday night...but with potential
for it to slow/linger left the consensus 20-40 percent rain showers/thunderstorms and rain
chances Monday night for now. Used a blend of the numerical guidance
highs/lows for sun through Monday night.
For tonight through Monday night 0-6km shear generally remains on the weak
side...with risk of organized severe storms quite low. See swody1...
swody2 and swody3 for more details. However most thunderstorms and rain will be slow
moving and capable of producing localized heavy rains.
For Tuesday through Friday.../days 4 to 7/...
Forecast concerns remain rain showers/thunderstorms and rain chances through the period.
01.00z/01.12z medium range models in general agreement for rising of
heights over eastern noam and falling heights over western noam next week.
Much less agreement both between models and run-to-run on the
shortwave details as the flow would slowly transition to quasi-zonal
/and active/ across the northern Continental U.S. By Thu/Fri. Given the
shortwave detail/timing differences and impacts they would have on
the sensible weather...forecast confidence in the day 4-7 period is
average at best.
Reasonable consensus for the area to remain under northwest flow
aloft Tuesday with yet another shortwave to be dropping southeast across
the region. However...at the surface the frontal boundary is pushed well
south of the area with can high pressure building toward the u.P. Of
Michigan. Latest consensus now has less troughing over the area Wednesday with
the surface high pressure ridging over the Great Lakes. Consensus trend
of lower rain showers/thunderstorms and rain chances Tuesday/Wednesday looks good at this point. If
present trends hold Wednesday night may well end up dry across the forecast
area. By Thursday shortwave energy in the more westerly flow moves across
the north-central with this trough into upper Midwest for Friday. Surface
low/troughing in the Central Plains is slowly lifted northeast into
the upper Midwest...along with an increase of moisture/instability/
lift. 20-40 percent rain showers/thunderstorms and rain chances Thursday/Friday reasonable for now...
but given the timing/detail differences and less than desired run-to-
run consistency...confidence to deviate much from the model
consensus is lacking. The more zonal flow next week would mean temperatures
near normal...but appears model consensus with slightly below normal
highs is being biased by clouds and the rain showers/thunderstorms and rain chances. Model/
ensemble consensus highs/lows on days 4-7 reasonable until shortwave
timing detail is clearer.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1123 PM CDT Friday Aug 1 2014
Confidence in valley fog at klse not as great with 04 UTC
temperature/dew point spread still 12 degrees. Latest GFS/NAM MOS
guidance also not as low in its visibility forecast. With no rain
from this evening/S thunderstorms and little change in moisture
overnight...will not completely remove fog mention from taf...but
increase the 09 to 12 UTC tempo group to 2sm in mist with VFR
ceilings. Winds to remain light and variable through the night.
Some thermal cumulus expected Saturday afternoon in the 5000 to 6000
feet layer. Isolated showers and thunderstorms possible...but
coverage and timing uncertainty precludes mention in current tafs.