Current Conditions at Sanborn Field

Monday Night: Mostly clear. Low: 71
 

 

Tuesday: Partly cloudy in the afternoon. High: 88
 

 

Tuesday Night: Becoming cloudy. Rain likely. Scattered thunderstorms possible. Low: 65
 

 

Wednesday: Becoming partly cloudy in the afternoon. Rain ending around sunrise. High: 80

 

 

Wednesday Night: Skies clearing. Low: 63

 

 

 

 

 

 

Thanks to IconArchive.com for the icons! 

=====================================================================

Discussion:

Monday night, a lingering haze from the wildfires to the northwest paired with southerly flowing winds leads to an above average overnight low just above 70. Cloud cover remains clear as winds begin to decrease to 5 mph from the previous 10-15 mph we saw Sunday night. Tuesday morning, sky conditions begin clear with southwesterly flowing winds. Throughout the early afternoon hours, cloud cover begins to move in from the northwest out ahead of an approaching cold front. Tuesday afternoon's high tops out in the upper '80s. Early Tuesday night, scattered showers and storms begin to shift southeast toward Central Missouri. The biggest threats with these showers and storms will be brief downpours and lightning. Throughout the late evening hours, the majority of possible storms begin to diminish into widespread showers. The overnight cold front allows for temperatures to decrease to the mid-'60s. Around sunrise showers begin to push to the southeast leaving behind partly cloudy skies and patchy fog located near river valleys. With partly cloudy skies and a passed cold front, the afternoon high tops out at 80. Skies continue to clear with winds out of the north/ northwest leading to an overnight low in the lower '60s.

=====================================================================

 Forecasters: Alexander, Gromada, Gotsch, Travis

Date Issued: 09/13/21 5:30 PM CST

Technical discussion:

After an extended period of quiet weather, the main focus of this forecast period is a weak cold front expected to blow through early Wednesday morning bringing with it chances for rain. The GFS was mainly used as the NAM was far too cool for daytime highs and overnight lows. GFS MOS was consulted for temperatures as well as the GEFS to judge model performance. 

A strong upper-level ridge over the southern CONUS has kept large-scale, upper-level flow over the Mid-Mississippi River Valley zonal for several days. GFS 250-hPa winds show the jet stream and the associated storm track remaining in southern Canada for the remainder of Monday and into Tuesday. Tuesday night into Wednesday, a shortwave can be seen in the 250-hPa height field (much less so at 500 hPa). A weak jet streak forms over the Great Lakes region putting the Missouri, Illinois region in its poleward entrance region of enhanced divergence. At 500 hPa this disturbance shows up in the absolute vorticity field, but is more subtle highlight its weak nature. 

GFS plots of 700-hPa RH depicts the landfall of Tropical Storm Nicholas in TX. This system should stay well to the south due to the aforementioned shortwave's propagation southeast. Tuesday night into Wednesday morning, saturation over Missouri increases with the approaching system. This saturation collocated with rising motion dynamically supports overnight showers. This is supported by GFS soundings with near complete saturation of the column overnight. Model trends have increased instability between the 06z and 12z runs. Where 06z GFS soundings had no surface based CAPE, the 12z soundings have ~500 J/kg of surface based CAPE at the time of precipitation. MUCAPE increased as well between model runs. A brief look at SREF dprog/dt for MUCAPE also indicated the general model trend of increased instability. As such, decided to go with scattered thunderstorms possible.

MSLP and surface winds place the timing of frontal passage near 06z Wednesday. GFS 850-hPa and surface maps of height and MSLP respectively, shows the quick build in of high pressure behind the front. This will push rain out of the area by sunrise on Wednesday and allow skies to clear partially by the afternoon. Latest GFS model soundings point to the possibility of patchy fog early Wednesday morning. As rain from earlier in the evening help saturate the near surface, a 5 knot wind at the surface may hinder large scale fog development. More likely that any fog that does form will stick to low-lying valleys. Post-frontal temperatures for Wednesday will be noticeably cooler than the summer-like low 90s we've seen these past few days.