Scientific Forecaster Discussion
fxus63 karx 202310
Area forecast discussion
National Weather Service La Crosse WI
610 PM CDT Wed Sep 20 2017
Short term...(this evening through Thursday night)
issued at 230 PM CDT Wed Sep 20 2017
Monitoring the cold front as it shifts east this afternoon and
evening. A very Stout cap exists at about 800mb ahead of the front
and convective initiation even on the front will battle this
somewhat. MLCAPE values are growing into the 2000 j/kg range ahead
of the front as dewpoints rise to 70f. Wind shear has become a
bit more impressive now as 50kt flags at 3km are showing up on
karx radar and 0-3km bulk shear is in the 40 kt range...pretty
favorable for bowing segments and damaging winds.
Bottom line is convective development is expected on the front.
Severe storm window is 4-8 PM, mainly south and east of La
Crosse, with damaging winds the main threat. A slightly later
time may occur with stronger cap in place. Some discussion about
the front slowing and lagging in the southern forecast area did
occur, although cams over past couple hours have become a bit more
The cold front will only make it south to about Interstate 80
tonight before the model consensus beings it north again as a
warm front as ridge builds. Early morning stratus may be found
near the front. There is moderate low-level moisture transport
convergence working north Thursday, however, it appears this will
work to moisten the layer, but below a sizable mid-level cap
around 700mb. Have decided to keep it dry, following model
consensus on Thursday as front shifts through.
Long term...(thursday night through wednesday)
issued at 230 PM CDT Wed Sep 20 2017
The low-level moisture transport convergence continues to migrate
north through the evening Thursday, into nrn WI and nrn Minnesota. The
entire forecast area should be back in dewpoints in the 65-70f
range with southerly flow through at least Saturday night.
Friday through Saturday...the heat is on. Have kept close to the
previous forecast highs for temperatures Friday-Saturday. This
will be a hot weekend for later September. The forecast highs and
low-level temperatures are very similar to those seen on Sep
14/15. Current forecast has heat index values in the 95-100f
range on Friday and in the middle 90s for Saturday. Will increase
the heat messaging across our services to raise awareness.
There will a great amount of convective activity to the west as the
synoptic front stalls over central Minnesota. It is potentially a Maddox
synoptic frontal flash flood scenario with multiple rounds of
training rain there starting Saturday night. It appears this will
mainly stay to the west of the forecast area.
Saturday night through Wednesday...
the warm weather will continue for the beginning of this period,
with 925 mb temperatures in the 24-26c range leading to highs in
the upper 80s for much of the area. Guidance suggests some
showers/storms associated with a surface boundary draped southwest
to northeast across Minnesota could clip the very northwest edge
of the forecast area on Sunday. However, models have trended
slower with the eastward advancement of this front, so precip
could hold off until Sunday night into Monday. The boundary and a
low pressure system will slowly move through the area, providing
shower/storm chances through at least Tuesday. The 20.12z GFS is a
bit faster, ending the precip by Tuesday evening, while the
20.00z European model (ecmwf) keeps precip in the area until Wednesday morning.
Both models are then in general agreement with a trough axis and
much cooler air moving into the area at the end of the period,
with highs in the 60s for Wednesday.
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 610 PM CDT Wed Sep 20 2017
A cold front has passed east of both taf airfields as of 20.23z
with the thunderstorm threat having ended. Clearing skies will be
the rule for the rest of the evening/overnight with light
northwest wind gradually turning to the north-northeast.
Aforementioned front stalls overnight from southern WI into
central Iowa and then lifts back northward on Thursday as a warm
front. Some indications of low MVFR stratus developing along this
boundary and lifting back north across taf airfields Thursday
morning. Something to watch, but not enough confidence at this
time to include in 00z tafs. If lower clouds do develop, they may
hang tough for much of the day. Otherwise, expect periods of VFR
clouds and light east-southeast wind through 22.00z.