Wednesday Night: Rain and scattered thunderstorms. Low: 61
Thursday: Cloudy with light rain. Dry time during the afternoon. High: 65
Thursday Night: Scattered showers. Low: 41
Friday: Rain. High: 46
Friday Night: Scattered showers. Low: 40
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Discussion:
Rain chances persist through the end of the week with scattered thunderstorms expected overnight. Thursday will morning rain with plenty of afternoon dry time. Be prepared for rain throughout the day on Friday with temperatures staying in the lower to mid 40s.
-Chirpich
Forecasters: Cook and Chirpich
Date Issued: 03/22/2023 5:00PM CDT
With Spring finally here, rain is dominating our forecast period. There exists a possibility for thunderstorms Wednesday night into Thursday morning. Rain is also likely Thursday evening, continuing through Friday and into Saturday. The rain will be associated with a low-pressure system that will pass over our CWA Friday night. The 12Z run of the GFS was used for this forecast, due to the forecast temperatures of the GFS more closely resembling the current conditions observed at Sanborn Field at the time of forecasting.
The aforementioned low-pressure system, currently located over Colorado, is digging a trough over the western half of the CONUS. This has caused a ridge to build in over mid-Missouri that will quickly be forced east as the trough moves closer to Missouri. Ahead of the western low-pressure system, there is a surface low-pressure system that will be the driving force behind the storm potential for Wednesday night into Thursday morning. On Thursday morning, there is limited divergence modeled at 250mb, so it will not be the main factor supplying the forcing for this system. Rather, a FROPA associated with the low-pressure system is the predicted source of lift needed to support thunderstorm development. However, the FROPA isn't expected until late morning. Model soundings indicate that the mid-level lapse rates are greater than 6.5°C/km for all of Thursday morning, indicating that the atmosphere is particularly unstable for this time period. Moisture, another ingredient required for possible storms, is lacking Wednesday night at 700mb. However, moisture is seen at 850mb. The LLJ is also notably strong overnight on Wednesday, allowing increased moisture transport into the forecast area.
After consulting soundings, a clearer picture is formed regarding the incoming system. On Wednesday night, CAPE values near 900 J/Kg are accompanied by CIN values ranging from 0-100 J/Kg. This signals a weak cap, not allowing much energy to build up within a thunderstorm. However, various severe weather indices exhibit a possibility for severe weather, with a maximum Lifted Index of -4 signaling marginal instability and a maximum Total Totals value of 53 pointing to widely scattered severe storms. SWEAT values are also increased, further supporting the possibility of a severe thunderstorm. But, omega values are low throughout the entire passage of the system, leaving the cold front as the main source of lift for the system. There is also a significant dry layer in the mid-levels that never fully disappears, hindering deep saturation of the atmosphere.
Due to the above indices having varying levels of disagreement on the certainty of severe weather, a cap that is basically nonexistent, and lack of substantial moisture and lift, severe weather is not expected for tonight. There likely won’t be any issues, but it cannot be ruled out. Forecast confidence regarding severe weather is moderate, as a change in the timing of the FROPA could cause enough lift to aid in convection if it passes earlier in the morning. But, at this time, severe weather is not expected.
After the FROPA Thursday morning, the area will be left with light rain as the mid-level dry layer persists. The passage of the cold front Thursday will hinder any temperature warm up on Thursday, and allow for much cooler temperatures for Friday. Model soundings show low level saturation from midday Thursday through midday Friday, at which the saturation is modeled to deepen, suggesting an increase in the rate of rainfall. This deepened saturation persists, into the afternoon, and subsides in the early evening Friday. after this, low level saturation persists throughout the remainder of the forecast period, suggesting light rain or drizzly conditions for Friday night into Saturday morning. Precipitation totals for the entire system are forecast to range from 0.5” to 1” of rain, with locally higher amounts possible.
The passage of the low pressure system Thursday night into Friday morning will be causing issues for states to our south, such as Arkansas, Tennessee, and Mississippi. With a screaming LLJ providing ample moisture, Significant divergence signatures at 250mb along with strong vorticity and vertical velocity signatures at 500mb and 700mb respectively, and an unstable atmosphere shown by model soundings, all modes of severe weather are possible for these states on Friday. If the track of this low shifts northward, these issues could become our issues. Future forecast shifts should continue to monitor the track of the low as it approaches our CWA later this week.
-Chirpich, Cook
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