Sunrise 5:49 AM   Sunset 8:31 PM Moonrise 2:52 PM   Moonset 2:32 AM

  Print   HELP   Reset   Save   Recall  New

FXUS63 KDMX 180937 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 437 AM CDT Sat May 18 2024

KEY MESSAGES

- Although there will be a few breaks, the pattern will generally be active through at least Tuesday with several periods of storms possible

- Strong to severe storms and locally heavy rains possible at times, especially Tuesday

- Outside of warm temps today, a fairly seasonal temperature regime in place through next week

DISCUSSION
Issued at 436 AM CDT Sat May 18 2024

Our weather pattern isn't expected to change a whole lot through the weekend and through the next week with progressive mainly lower amplitude flow resulting in systems of varied strengths every few days. The first of which will affect mainly eastern sections later today. At onset this morning short waves exiting the Dakotas into MN, and the parent wave moving from MT into ND have already spread decent QG forcing into the Siouxland area this morning. Moisture return is limited however with little sensible reflection here and the better warm/theta-e advection farther north resulting in only scattered convection near the Twin Cities. This forcing, and a noted cold front aloft, will push into IA later today along with the upper wave's attendant cold front during peak heating and highs well into the 80s, likely the warmest day of the forecast period.
Moisture pooling along and ahead of the front is expected to result in 1500+ J/kg uncapped MLCAPEs by late afternoon. Some models such as the RAP are more aggressive, with surface dewpoints to near 70F, but that seems overdone considering upstream conditions per the 08z surface analysis and the current state of evapotranspiration. Guidance varies with respect to coverage, but at least a few storms are expected to develop along the front east during the late afternoon and evening hours. Deep shear is weak keeping any severe potential on the low side, but the degree of instability and especially higher cloud bases and four figure DCAPEs could result in some strong, gusty winds until the front exits by late evening leading to dry conditions and surface high pressure overnight.

Chances for showers and storms will increase once again west to east Sunday however with weak short waves crossing the Rockies into the Plains and moisture return behind the departing 1-3km ridge. While the best instability and severe potential will remain to our west tomorrow and tomorrow night, sufficient instability and just enough deep shear may be present for a few stronger storms. Any surface based storms to our south and west may transition to more elevated and continue to be fueled by a 40kt low level jet and moisture transport into the night. While the environment doesn't appear too extreme, seasonally high precipitable water and specific humidity values, and 3.5km warm cloud depths, may result in some locally heavy rainfall with isolated 1-2" amounts.

By Monday what looks to be the strongest system of the period will begin with enhanced southwest flow aloft ahead of the maturing western trough. Instability will again build into the MO Valley with the western fringe of a warm front into IA.
Plains convection may again develop during peak heating and sustain itself into IA through the night with another cycle of deep moisture convergence on the nose of a 35-40kt low level jet focused into the south. Deep shear may just be sufficient at >30kts for organized storms and some wind and hail potential.
The heavy rain parameter space pushes well through elevated climatological percentiles resulting more potential for isolated heavy rains.

The best potential for severe weather will be Tuesday when synoptic forcing, instability, and deep shear all increase during peak heating as the surface wave deepens and moves into IA, coincident with a strong mid level speed max. 0-6km shear increases to near 50kts with 2000-3000 J/kg MLCAPEs and reduced CINH. Discrete supercells are possible at onset of development with deep shear vectors normal to the advancing boundary.
However a more linear structure may evolve with time as the 0-6km shear backs as the storms move eastward. With this in mind, the new SPC Day 4 outlook has increased severe probabilities to 30 percent central and southeast, with a notable signal in the latest CSU machine learning guidance. As mentioned in the previous discussion, previous MCSs have the potential to contaminate the environment at times early next week, so repetitive rounds of convection and questionable thermodynamic recovery will need to be monitored late Sun into Tue before details of any particular event become more clear in the coming days.

Although the signal is much weaker for the rest of the forecast, active progressive flow will result in at least some chances for additional precipitation off and on through the end of next week. The 00z GFS has a fairly strong, faster short wave and resultant surface system through the Siouxland Friday, but as if often the case this may be a bit fast with much of the other deterministic, ensemble, and AI guidance suggesting a slower progression.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/
Issued at 1136 PM CDT Fri May 17 2024

VFR conditions continue into Saturday, with clear skies across much of the state. High clouds are expected to stream across the state later on this morning as a boundary passes through, which may also come with a few rain showers until mid morning before ending. As the front reaches into eastern Iowa around 00z, new development of showers and thunderstorms are possible, mainly looking to impact KOTM, though uncertainty remains on the higher end at this time. Winds will increase through the morning, gradually shifting northwesterly as the aforementioned front passes across the state.

DMX WATCHES/WARNINGS/ADVISORIES
None.

Full Color Weather Map

Link to Loop


Other links: GEOS-West     Contential US     Full GOES-East

Des Moines, IA,