Sunrise 6:12 AM   Sunset 7:50 PM Moonrise 2:47 PM   Moonset 3:41 AM

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FXUS63 KARX 191040 AFDARX

Area Forecast Discussion National Weather Service La Crosse WI 540 AM CDT Fri Apr 19 2024

KEY MESSAGES

- Blustery with westerly gusts to 35 mph today. Small snow shower chances (20-30%) north of I-90

- Cold weekend with frost or freezing conditions expected Sat/Sun morning

- Next shot for widespread precipitation (rain) comes Monday into Tuesday

DISCUSSION
Issued at 200 AM CDT Fri Apr 19 2024

* BLUSTERY TODAY - small shower potential (mostly snow) north of I- 90 late afternoon/evening

The surface pressure gradient is tightening ahead of an upper level shortwave trough (dropping southeast out of Canada) and winds will be on the increase as a result. Mixing in the lower layers of the atmosphere will promote gusts upwards of 35 mph (per RAP/HRRR bufkit soundings).

In addition, cold air advection will increase near sfc lapse rates today, resulting in instability and additional lift for potential showers. CAMS models - moreso the RAP and HRRR - produce spotty to scattered convection for the afternoon into early evening along/north of I-90. RH fields and soundings show enough saturation for at least aggravated CU, but the sub cloud layer is rather dry.
Precipitation falling through this layer will have to work against the drying while also promoting cooling. A couple things from that: 1) anything falling out of the cu could turn out to be just virga.
Never reaching the sfc but locally enhancing wind gusts. 2) pcpn could reach the surface, and because of the appreciable cooling of that dry layer, it falls as snow. For now, the lift and saturation looks enough to warrant at least low end chances (20-30%). Despite expecting highs to top 40 degrees in all areas, for pcpn to reach the surface the cooling required would make snow showers more likely. Won't amount to much if realized.

* RELATIVELY CHILLY WEEKEND - frost and/or freeze for many/most

Cold air settles in for the weekend with 850 mb temps falling to as cold as -10 C by 12z Sat morning. A consistent signal from the past several days in the long range guidance, continuing now into the short term. Latest HRRR holds any chance to warm above 50 across parts of northeast IA/southwest WI today (and perhaps some valley locations) with virtually a 0% to exceed 50 Sat for highs. EPS and GEFS ensemble suites show the same with only a few degree spread in the warmest 75% of their members - which hold 50 or colder. Same HRRR says "everyone" outside of valley locations will experience freezing temps Sat morning with something similar Sun morning.

Frost and/or a freeze is likely for most across the forecast area Sat/Sun mornings. Questions arise on whether a widespread frost will be realized or mostly "freeze". Clouds and winds can act as deterrents to frost formation by limiting diurnal cooling of the near sfc/micro layer. The clouds by insolation and the winds by mixing. With the influx of cold air, temps still progged to fall into the upper 20s/lower 30s - so perhaps a moot point. However, sfc dewpoint spreads expected to range from 5 to 10 degrees, and mixing will work against moisture (dew) formation on grasses/vegetation tonight (winds expected around 10 mph or greater). It will still be cold Sat morning, but not necessarily widespread "white". Sunday might have a better shot with lighter winds. All in all, even if the "frost" isn't widespread, the expected freezing temps could/would cause damage to sensitive vegetation and early season planters should take action to protect these plants from the cold.

* NEXT CHANCE TO GET WET COMES MON/TUE

The next precipitation maker looks to come out of the PAC NW as a shortwave trough shifts east Sat night, dropping across the northern plains/upper mississippi river valley Monday. Cyclogenesis occurs as it moves out of the lee of the Rockies, hanging a sfc cold front southwestward of its attendant sfc low (which looks to track west- east across portions of MN/WI). Currently, the EPS and GEFS are suggesting two main pcpn regions with the available lift - one is the shortwave itself, with current trends favoring northern parts of the region. Second, along the cold front as low level moisture transport/jet helps convection fire.
This could be south and east of the local area. Still looks like most (if not all) of the forecast area could/would see some rain...but outlay of the higher amount will be high dependent on the track/positioning of these key variables..which isn't clear at this time. The model blend currently paints a 40-70% for at least 1/4" of rain. Westward handing trough/secondary cold front will likely continue pcpn chances through Tue.

* GRADUAL WARMING NEXT WEEK - more rain chances on the horizon for the weekend

The long range guidance remains in good agreement with amplifying an upper level ridge as it shifts from the west coast to across the plains by Thu night. Milder/warmer air will return to the region as it pools under the ridge. Trends in the EPS and GEFS suggest more of a gradual warm up with the upper 75% of their ensemble members warming back into the 60s as we move into next weekend. 10-20% shot for 70+. CPC outlooks out 14 days also show an increasing likelihood for warming - venturing into above normal - for the rest of the month.

Meanwhile, there has been a steady signal for a shortwave trough to lift out of the desert southwest and meander across the upper mississippi river valley Fri-Sat, the last weekend of April. Some differences in timing/locations of the pertinent features, but ample support from both models' ensemble members for a good shot to get wet.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/
Issued at 530 AM CDT Fri Apr 19 2024

VFR conditions are forecast through the TAF period. Breezy today with westerly winds blowing 15-25 knots, gusting 25-35 knots.
Mid-level scattered/broken cloud deck likely this afternoon.
Isolated to scattered snow showers cannot be ruled out north of I-90 today. Very low (<20%) chance for impacts at the TAF sites.



ARX WATCHES/WARNINGS/ADVISORIES
WI...Frost Advisory until 8 AM CDT this morning for WIZ032-033-041- 053>055-061.
MN...Freeze Warning until 8 AM CDT this morning for MNZ086-087-094- 095.
Frost Advisory until 8 AM CDT this morning for MNZ079-088-096.
IA...Freeze Warning until 8 AM CDT this morning for IAZ008>010-018- 019-029.
Frost Advisory until 8 AM CDT this morning for IAZ011-030.

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