textproduct: Des Moines

KEY MESSAGES

- Areas of dense fog and drizzle through this morning. Drizzle or light rain into this afternoon.

- Potential for more dense fog tonight and precipitation chances increase. Wet into Friday evening.

- Warm this weekend.

DISCUSSION

Issued at 400 AM CST Thu Dec 26 2024

Low level moisture continues to slowly lift into Iowa early this morning and is most evident over northern Iowa where temperatures and dew points have been slowly rising towards 32 degrees. Conditions have been more steady over central and south central iowa with readings mainly in the mid 30s. This moisture which is situated under an inversion located around 5 kft above the ground, is resulting in widespread fog with much of it becoming dense along and south of Highway 20. Therefore, have expanded the Dense Fog Advisory north and west and will likely need to expand even more this morning. The deeper low level moisture is over southeast Iowa and some vertical ascent and resultant collision coalescence has led to an area of drizzle to development over the southeast third of the area including moving into the southeast portion of the Des Moines metro. Temperatures and road temperatures are above freezing so not expecting any icing.

The moisture advection will continue to move into Iowa today with dew points gradually rising and this moisture will be driving the high temperatures today. Have highs today near the NBM 25th percentile and fairly close to the expected dew point values which will rise through the 30s north and into the 40s south. A deepening moisture profile may eventually lead to light rain as opposed to drizzle especially by this afternoon and tonight. Overall the fog conditions will improve diurnally today but some areas may be shrouded in the fog with visibilities remaining below 1 mile at times and it is possible that a time extension for the Dense Fog Advisory is needed over parts of the area. The fog/dense fog is expected to return tonight.

The precipitation chances will be maximized on Friday as the upper level short wave arrives. Expect widespread rain/showers over the state into Friday night. The moisture will begin to erode late Friday night into Saturday and the precipitation will end. There is a chance there could be a few peaks at the sun on Saturday after several days socked in low stratus, fog and precipitation. Sunday should also have some sunshine and will be warm with highs in the 40s and 50s.

The next system will arrive Monday and Monday night. Currently, the progged setup is an upper level system arriving with surface cyclogenesis occurring over Kansas and Oklahoma with the low then moving east into Missouri. The upper level thermal profiles will be cooling during this time but it does appear a surface warm layer will hang around across Iowa that would result in any precipitation beginning as rain. A transition to snow would occur from northwest to southeast as the surface warm layer is eroded. Not many details beyond that at this point and a lot can chance. Beyond that, conditions are favorable for colder to perhaps much colder temperatures to arrive by mid next week for New Years as the upper flow becomes northerly though at this point this colder airmass does not appear to have Arctic properties.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 539 AM CST Thu Dec 26 2024

LIFR cigs will persist through the period with cigs in the 200-300 ft AGL range. Fog will also persist through much of the forecast period with vsbys starting at 1sm or below with some improvement during the day. The fog will again become dense at times tonight. Expect periods of drizzle today with rain developing tonight. The wind will remain mainly light and from the southeast at less than 10 kt.

DMX WATCHES/WARNINGS/ADVISORIES

Dense Fog Advisory until 10 AM CST this morning for IAZ004>007- 015>017-023>028-033>039-044>050-057>062-071>075-082>086- 094>097.