textproduct: La Crosse

KEY MESSAGES

- Freezing drizzle threat has started to manifest itself this morning and a winter weather advisory has been issued with slick roads and slideoffs being reported.

- Periods of drizzle/showers today into midday Friday with more widespread rain Friday afternoon and night. Storm total rainfall amounts could exceed 1/2 of an inch. Widespread fog, likely dense, during this period.

- Warmer weather for the end of the week and the weekend with highs in the low to mid 40s, cooling off steady back to seasonal values by the middle of next week.

- Cannot rule out snow/rain Monday into early Tuesday.

UPDATE

Issued at 405 AM CST Thu Dec 26 2024

No sooner had this morning's discussion gone out the door, the freezing drizzle picked up again on radar and at the office, producing a glazing of ice on the parking lot and cars. A few traffic slowdowns have shown up and at least one slideoffs reported on Interstate 39. Given that the main impacts are expected over the next 6-8 hours, did go ahead and issue a winter weather advisory for far southeast MN/northeast Iowa and much of southwestern Wisconsin through noon.

DISCUSSION

Issued at 325 AM CST Thu Dec 26 2024

This Morning: Freezing Drizzle Potential

The Christmas miracle that gave us the unexpected break in the clouds yesterday afternoon may have been a grinch in disguise as air and road temperatures in the clearing fell 5-10 degrees before stratus moved back in later in the evening. These temperatures have been slow to recover overnight with readings in the mid to upper 20s as of 3 am, whereas areas under the clouds further south and west stayed in the low to mid-30s. Deeper low-level moisture has been slowly spreading northward overnight within a region of weak isentropic ascent from 280-290K. The HREF members have been overestimating the aerial extent of the drizzle coverage so far tonight, with the hourly runs of the RAP/HRRR pulling the threat area for drizzle slightly farther east with each successive run.

This does not instill confidence in the coverage or impacts of any freezing drizzle through the mid-morning hours. One band of freezing drizzle did lift through southwestern Wisconsin between midnight and 2am, but impacts were negligible and no glazing was observed at the office here in La Crosse. There is a ribbon of stronger ascent that lifts northward along the 290K surface this morning, focused mainly east of the Mississippi River between 6 am and noon. The manifestation of this lift may be starting to appear on the DMX/DVN radar scopes in central and eastern Iowa, though there has been little for drizzle reported with it.

The MRMS sub-freezing pavement temperature probabilities have been slowly eroding northward and this mirrors the observations from the southern forecast area where wet bulb and road temperatures have risen above freezing. By midday, the threat for freezing drizzle should be over with the warmer air having settled in place.

This Afternoon - Sat Morning: Stretch of Wet Weather and Fog

A weakening meridional mid to upper level tropospheric trough slides out of the Great Plains and into the Mid-Mississippi River valley by Friday morning. Given its filling nature, synoptic lift will be limited and the threat for any showers driven by transient mesoscale forcing. The low level moisture will be in place for continued drizzle or light showers and the convective models are emphasizing the scattered nature of any precipitation in their PoP/composite reflectivity progs. Didn't lower PoPs too much given that fact that most folks should see precipitation, but did lean into the scattered/shower wording in the worded forecast to highlight this point.

A deeper, negatively-tilted wave drives into the backside of this weakening trough by midday Friday and ejects over the region Friday night into early Saturday morning. The arrival of this wave helps reinvigorate the modest surface low that formed with the lead wave over Texas, pulling the low northward and deepening it slightly in the process. This broad low tracks over the region Friday afternoon and night with the accompanying deeper moisture and isentropic/ kinematic lift driving moderate rainfall with its passage. Roughly 40 to 60 percent of the LREF members have a storm total QPF of over 1/2 of an inch by Saturday morning, an increase from what we were seeing yesterday.

The steady fetch of low-level moisture ahead of this system has resulted in the formation of dense fog across much of central and eastern Iowa early this morning. This deeper moisture advances over the area today and should only be amplified by the melting of our current snowpack. Did issue a dense fog advisory for parts of northeast Iowa and far southwestern Wisconsin for this round of fog this morning, but in all likelihood we will need a larger fog advisory later today with the 26.00Z HREF having probabilities of 80- 100 percent of visibilities of less than a quarter of a mile after 2 to 4 pm today. This fog may linger well into Friday and only lift when the heavier rainfall arrives in the afternoon.

Warm Weekend Ahead, Cooling Off Next Week

The increase in low-level moisture will bring with it warmer temperatures. Despite the clouds and fog, temperatures are expected to top 40 degrees for most locales for Friday and Saturday with lows only falling a few degrees each night under the dense cloud cover. There a signal in the Extreme Forecast Index that we could see record warm lows with roughly 60 to 70 percent of the EPS members exceeding the model climatology and the current NBM forecast for Rochester and La Crosse meeting or exceeding the daily records on Saturday. Even with the departure of the shortwave on Saturday, flow remains from the southwest ahead of a secondary shortwave embedded within the mean longwave trough, which does not clear the area until Monday. Therefore, temperatures for Sunday and Monday will be similar to Friday and Saturday. The flows turns northwesterly and cooler air slides southeastward late Monday through the remainder of the week, with lower tropospheric temperatures slowly falling through the period from +1 C on Monday to -15 C by Thursday. As a result, surface temperatures should fall to or below average by midweek.

Monday - Early Tuesday: Rain/Snow Clipping the Region

A 150-kt jet streak ushers a compact vort lobe across the Rockies and into the central CONUS Sunday into early Monday. Given that the energy lobe is still out over the Aleutians and lingers over the Pacific until midday Sunday, it is not surprising that there is modest spread in the track of the surface low response across the Mid-Missouri River Valley. There has been a slight northward shift in the track in the LREF progs over the last 24 hours, driven heavily by the GEFS members that are starting to show the surface low recurving to the northeast sooner than the EPS solutions. This bears watching as such a shift does bring precipitation farther northward into the forecast area, likely in the form of snow by Monday evening with nearly all LREF/NBM members depicting surface temperatures below the mid-30s by sunset. It remains too early to speculate on amounts, but such systems can often deliver a modest amount of snow in a hurry if conditions line up.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 551 AM CST Thu Dec 26 2024

IFR/LIFR conditions are expected throughout the TAF period with low- stratus overhead. Ongoing freezing drizzle this morning will continue until around 15z-18z when surface temperatures will warm above freezing and drizzle will no longer pose any aviation impacts. Additionally, already noting fog overspreading much of the local area this morning with KRST reporting IFR vsbys as of recent observations with warmer, moisture rich air moving over the present snowpack. As we continue throughout the morning, expecting visibilities to continue to drop to 1/2SM with some spots falling even lower. Overall, guidance has been on the slower side with this fog based on observational trends so have introduced 1/2SM fog as early as 14z at KRST and this could be on the later side. Probabilities for 1/4SM are very high (70-100%) past sunset for much of the area outside of river valleys in the 26.00z HREF so have opted to introduce 1/4SM at KRST by 23z. However, cannot rule out some periods of 1/4SM earlier than this. Fog will likely linger through much of the overnight with showers beginning to increase in coverage towards Friday morning ahead of an incoming weather system. The fog will likely disperse once more widespread rainfall moves into the region on Friday.

ARX WATCHES/WARNINGS/ADVISORIES

WI...Dense Fog Advisory until 9 AM CST this morning for WIZ043-044- 053>055-061. Winter Weather Advisory until noon CST today for WIZ017-029- 033-034-041>044-053>055. MN...Dense Fog Advisory until 9 AM CST this morning for MNZ094>096. Winter Weather Advisory until noon CST today for MNZ088-096. IA...Dense Fog Advisory until 9 AM CST this morning for IAZ008>011- 018-019-029-030. Winter Weather Advisory until noon CST today for IAZ010-011.