Friday Night: Rain will begin in the evening hours. Low: 38
Saturday: Heavy rain, rumbles of thunder possible. High: 56
Saturday Night: Partly cloudy. High: 39
Sunday: Possibility of light rain in the morning. Low: 59
Sunday Night: Partly cloudy, clearing into Monday. High: 38
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Discussion:
This weekend will be a rainy one, starting Friday evening through Saturday. Energy is present for some thunder, but not enough for any strong thunderstorms. There will be a short dry period Saturday night, then a second wave of light rain will arrive on Sunday.
-Sausen
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Forecasters: Thomas, Sausen
Date Issued: 03/24/2023 7:23PM CDT
Technical Discussion:
Satellite imagery shows a line of rapidly growing convection from the ArkLaTex region up north to southeast Missouri, whilst the cloud shield for the parent system continues to overspread the forecast region. A few stray showers are present on radar nearby, however returns are light and high-altitude, leading me to believe that Columbia will see trace amounts at best with the initial conditions. As significant weather appears likely in the very near-term and synoptic model ramp-up may degrade the usefulness of the GFS and NAM, I’ve chosen to use the RAP model for Friday night’s forecast into early Saturday morning. Additionally, I’ve chosen the GFS model for the rest of the period as GEFS members have significantly less temperature spread than the SREF model for today’s highs, leading me to believe the GFS system as a whole will have a better handle on what’s going on. In a general sense, most model systems have a significant amount of disagreement due to ongoing convection this afternoon, so I believe that any particular synoptic model may not hold much merit for Friday night.
At the 250mb level, an upper level trough is situated over the Rockies all the way into the Four Corners region, whilst a jet streak is noted between eastern Wisconsin into Maine offshore, whilst a second lobe of higher upper level winds is situated between Oklahoma and Kansas, leaving largely southwest flow in the fcst area. This trough should tilt negatively as it progresses into the Midwest. Overnight, a brief upper and lower correlated level jet max is to form over Arkansas in correlation with linear convection overnight, which may help enhance convection overnight as it advects northward into Missouri. At the 500mb level, vorticity maxima associated with convective storms should traverse the southern half of our state, before the main lobe of precipitation moves through with its’ associated vorticity maxima around 9pm, and moves out by Saturday morning. At the 700mb level, vertical velocity is maximized over the ArkLaTex area into the Paducah WFO, and maxima look to sit in two areas of Missouri, splitting the forecast area. Deep layer moisture builds into Missouri around 4pm, but sharply dries out at 11pm between the two areas of forcing. Afterwards, very moist wraparound precipitation takes over Saturday morning, though the southern extent of this moisture plume is questionable as to whether or not it makes it to the forecast area. At the 850mb layer, a lower level jet minima is strongly pronounced directly over the forecast area for most of the overnight period.
While the ingredients for significant rainfall are nearby, I think the combination of the strong LLJ minima as well as critical moisture being quickly advected away from the fcst area during the period of greatest forcing will significantly limit our QPF totals overnight, despite having two separate waves of precipitation and plenty of time for totals to build. However, CPC calculated soil moisture content indicates that the surface is already rather saturated. As a result, minor flooding does appear possible in the forecast area. As for severe convection, while a significant amount of 0-6km bulk shear is noted (around 105kts) over our forecast area during the evening, MUCAPE values are significantly lacking, as well as lapse rates until late tonight. HREF derived thunderstorm chances are close to zero during the entire period save for between 9pm and midnight tonight. I can’t rule out the possibility of an isolated rumble of thunder or two, but beyond that severe potential is slim to none.
One additional concern for the near term forecast period is the potential for wintry precipitation Saturday morning. HREF ensemble mean places the rain/snow line a few counties to our north at 18 hours out, which leads me to believe that a change in 1 or 2 degrees from the forecast could give us different outcomes than currently forecast. RAP soundings show a uniformly saturated layer up to 700mb for the time of concern, however the DGZ sits above the saturated layer, and the surface should remain above freezing, therefore I presume that our wintry precip chances are slim to none.
As for the rest of the forecast period, upper level winds shift to a zonal flow pattern with the passage of the large-scale trough, accompanied by jet maxima on the north side of the streak between Colorado all the way into Pennsylvania. By Saturday night, a new set of moisture return will quickly make its way across the area with a shortwave, although the moisture quality isn’t ideal for precipitation judging from PWATs of around 0.3 inches and limited moisture depth, and I believe nothing more than cloudiness will come of this initial lobe of moisture.
The second system of concern will be Sunday night into Monday morning. 500mb deterministic output places a vorticity maxima over the majority of the state Sunday evening into the overnight hours. 700mb vertical velocity is very supportive during the period, with a large maxima centered over Missouri as a whole Sunday night, with a notable shortwave leading it. 850mb winds are decently high, and moisture should stick around between 6pm and midnight on Sunday. Sounding analysis from both the NAM and the GFS demonstrates a very deeply saturated layer Saturday evening into the overnight hours, with largely below-freezing temperatures until the lowest 1km of the atmosphere, and a temperature in the upper 30s at the surface. At 63 hours out with a saturated DGZ, I wouldn’t entirely rule out wintry precipitation with this system should a slightly colder than currently predicted trend emerge, and with the current large-scale system affecting the forecast area at the time of writing this AFD, uncertainty remains relatively high, and computer models as a whole have been in disagreement. As for timing on this event, the GEFS has an unusually tight clustering around 8pm on Sunday, and the majority of SREF ensemble members that do show precipitation during the event period place timing at 8pm as well. With the wide spread on temps at the beginning of the utilized SREF run indicating to me that ensemble spread is high, I believe the confidence on timing for this event is rather high. As for QPFs, GEFS outputs average a PWAT of .6 inches during the period, and estimated precip totals sit around 7 tenths of an inch, with a low collective model spread on QPFs. Given the cumulative rainfall total during the forecast period, flooding is once again a concern, though the extent thereof is yet to be realized.
For temperatures, highs around 60 can be expected on Saturday and Sunday, which sits comfortably next to the climatological means for those days in our area.
-Thomas
Thomas, Sausen
