Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 302336

Area Forecast Discussion
National Weather Service La Crosse WI
636 PM CDT Sun Apr 30 2017

.SHORT TERM...(This evening through Tuesday night)
Issued at 153 PM CDT Sun Apr 30 2017

Early this afternoon, water vapor satellite imagery and surface
analysis showed a large, almost vertically stacked system over
southern Kansas. Local and regional radars showing two main bands
of precipitation, one from northwest Wisconsin into eastern
Nebraska and the other from central Illinois across southern Iowa
and northern Missouri into Nebraska and Kansas. In between these
two bands, not much was happening other than some occasional very
light showers or sprinkles.

The 30.12Z models are in agreement in maintaining the upper level
low as a closed system as it lifts northeast across the Great
Lakes and into southern Ontario late Monday night into Tuesday
morning. This will take the surface low from northeast Kansas
across eastern Iowa to northeast Wisconsin through Monday
afternoon. Several short wave troughs will rotate around the upper
level low and are expected to bring moderate to at times strong pv
advection in the 500-300 mb layer across the area overnight into
Monday morning. The best low level moisture transport will occur
this evening when this will be aimed right into the forecast area.
This will then move off to the east overnight but start to bend
back to the northwest around the upper level low and feed into the
deformation band. The frontogenesis with this system will be deep,
extending from the surface up to 300 mb, and be moderate to at
times strong in strength. This will start to move in late this
afternoon and then remain over the region tonight. The upper level
portion of this will move off to the northeast Monday while the
low level, 1000-700 mb layer, will be weak but linger over the
area through Monday. On the 290K isentropic surface, the up glide
will arrive in conjunction with the frontogenesis and should be
on the order of 2 to 5 ubar/s this evening. This will diminish
overnight into the 1 to 2 ubar/s range overnight and then continue
into Monday morning as the trowal wraps around the upper level
system tonight and then moves across the region. With all this
forcing, expecting the band of showers across Iowa and Missouri
to lift into the area this evening and then move across the area
through the night. Even though the forcing will be much weaker
Monday, still expecting an prolonged period of light rain as the
frontogenesis and trowal/isentropic up glide moves across the
area, but there may not be a lot of QPF with amounts generally
under a tenth of an inch. There may still be a little bit of
lingering precipitation into Monday night when enough cold air
works in that some snow could mix in with the light rain. Not
expecting any accumulation as amounts will be very light. All the
precipitation will then end Tuesday as the system moves off into

.LONG TERM...(Wednesday through Sunday)
Issued at 153 PM CDT Sun Apr 30 2017

Northwest flow will be established behind the current system. This
will allow a short wave trough topping the western upper level
ridge to move over the Missouri River Valley Wednesday. This will
then form another upper level low over the middle Mississippi
River Valley Wednesday night that then lifts northeast through the
Ohio River Valley Thursday and Thursday night. Some showers from
this system could impact the area from Wednesday afternoon into
Thursday before the forcing shifts well away from the area. The
rest of the week then looks dry as the upper level ridge shifts
east over the Midwest.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 636 PM CDT Sun Apr 30 2017

Strong low pressure system continues to work toward the area, and
will bring with it a period of increased rainfall through late
evening and early overnight. During the 03-06Z time frame,
isolated thunderstorms are also possible at both RST and LSE, with
current MVFR conditions expected to decrease to IFR levels
overnight and into Monday morning. However, we should see ceilings
rise back up to MVFR at LSE as the surface low tracks just west of
that location, while RST likely stays IFR through Monday evening.
Behind the departing main batch of heaviest rainfall through 06Z,
more sporadic showers are expected, increasing once again in
coverage toward and after midday Monday. Winds from the northeast
will remain gusty a few more hours, becoming much lighter
overnight as the low approaches before shifting west to northwest
later Monday with the passage of that feature.




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