Monday, May 6, 2019

Monday - Mostly cloudy skies. A stray afternoon thunderstorm is possible. High: 72-76

Monday Night - Skies becoming cloudy. Thunderstorms developing after midnight. Storms may produce heavy rain. Low: 60-64

Tuesday - Cloudy. Widespread thunderstormsduring the morning and early afternoon becoming scattered during the late afternoon. Storms may produce heavy rainfall. High: 70-74

Tuesday Night - Cloudy. Scattered thunderstorms during the evening becoming more widespread after midnight. Storms may produce heavy rain. Low: 60-64

Wednesday - Cloudy. Thunderstorms likely during the morning, becoming scattered after noon. A few storms may be severe. High: 70-74   

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Better have the umbrella or rain jacket handy for the next few days, because thunderstorm activity will dominate this forecast period. Narrowing down the timing of these individual storm events will be tough. Tuesday into Tuesday night could see widespread heavy rainfall. Wednesday, we see the potential for strong to severe storms. The main threat with Wednesday will come in the form of damaging wind and hail.

Forecasters: Ritter, Travis, and Hirsch
Issued: 10:30 a.m.; 6 May 2019

Technical Discussion (The nerdy stuff we are discussing in class!)
With active weather in store over the next couple of days, the issues of this forecast period will be timing of thunderstorms as well as associated severe and flooding threats. Unfortunately, this active period is not being handled well by the models, with each model handling each system over the CONUS differently. Therefore, we have proceeded with caution using the GFS, supplementing its guidance with the HREF for this evening's storms and the SREF for storms Tuesday morning through Wednesday.

The overall synoptic scale setup advertised by the GFS is quite suggestive of active weather during the early weak this week. Beginning at 250 hPa, a jet streak will develop over the Great Lakes region this afternoon and evening, strengthening throughout the day tomorrow. Meanwhile, another 90+ kt jet streak will be moving out of the Desert Southwest and into the Central Plains Tuesday evening into Wednesday. The development of the jet to our northeast this evening will put central Missouri in an area of enhanced divergence at 250 hPa between 0600 and 1800 UTC Tuesday, while the interaction of the northern and southern jet will put central Missouri under a region of enhanced divergence at 250 hPa between 0600 and 1800 UTC Wednesday. Stepping down to 500 hPa, the GFS suggests zonal flow over Missouri through roughly 0000 UTC Wednesday, transitioning to more amplified flow afterwards as a shortwave trough and associated vort max approach from the west. Moving on to 700 hPa, the GFS shows plenty of moisture and lift over central Missouri between 0600 UTC Tuesday and 1800 UTC Wednesday. At 850 hPa, the GFS suggests weak southwesterly flow through 0600 UTC Wednesday. Between 0600 and 2100 UTC Wednesday, the GFS develops a 50 kt LLJ over Oklahoma that moves northeast during that period. At the surface, the GFS moves a surface front south from Nebraska and Iowa into Missouri throughout the day today. This front will stall around 0000 UTC Tuesday parallel to but just north of Interstate 70, and it will move very little through 0600 UTC Wednesday. After that period, the GFS moves the front north toward the Missouri-Iowa border by 0000 UTC Thursday. Menawhile, a surface low will develop over the panhandles of Oklahoma and Texas, progressing very little during the forecast period.

For today, it does not appear as though precipitation is likely. GFS Skew-T's show a stout dry layer between 850 hPa and 200 hPa through 2100 UTC today, with the dry layer extending to the surface between 0000 and 0600 UTC Tuesday. It also appears as though any lift in the column does not develop until after 0600 UTC Tuesday. However, there is more than sufficient CAPE (>1500 J/kg generated by the GFS and HREF members) as well as Lifted Index values around -5 and Total Totals values between 52 and 54. Therefore, we have opted to suggest a stray thunderstorm is possible this afternoon. As for temperatures, with increased cloud cover between yesterday and today, maximum afternoon tempratures today should be a few degrees cooler than those of yesterday.

The chance for storms ramps up tonight into midday Tuesday. GFS Skew-T's again show plenty of lift in the column at 0600 and 0900 UTC, with saturation of the column occurring sometime between the two timesteps. Precipitable water values also jump to around 1.5 inches during this period, suggesting that there will be a heavy rainfall threat with the storms tonight into tomorrow morning. Moreover, the NWS in St. Louis has put Boone county in a Flash Flood Watch beginning at 0000 UTC Tuesday, effective through the remainder of the forecast period. HREF ensemble reflectivity shows an MCS developing over eastern Kansas and western Missouri between 0000 and 0300 UTC, progressing eastward and eventually making it to central Missouri between 0600 and 0900 UTC. Given the pre-storm environment mentioned earlier, the leading edge of this MCS may contain a few severe thunderstorms. As this is the case, the SPC has highlighted central Missouri in a slight risk of severe thunderstorms in its day 1 outlook. With little in the way of shear (<100 m2/s2 of 0-3 km SRH), tornadoes aren't a significant concern. Also, mid-level lapse rates less than 7 C/km generated by the GFS suggest hail won't be that large a threat either. Therefore, expect the main concern with these storms to be damaging winds along with heavy rainfall. Widespread, steady rain and a few thunderstorms, some producing heavy rain, should continue through roughly 1800 UTC Tuesday, after which storms will become more scattered in nature due to separation between the temperature and dewpoint profiles on GFS Skew-T's. Lows Monday night should be a few degrees warmer than those of last night due to cloud cover, with highs Tuesday being slightly cooler than today's due to rainfall during the morning.

After a brief reprieve in widespread rainfall Tuesday afternoon and evening, activity ramps back up between 0600 and 1800 UTC Wednesday. Once again, the column saturates and lift redevelops, providing the ingredients for storms. The GFS suggests the Tuesday night pre-storm environment will not be as conducive for severe as Monday night's; therefore, we expect general thunderstorms overnight Tuesday into Wednesday morning. However, precipitable water values will continue to sit in the 1.4-1.5 inch range, suggesting any stoms will have the potential to produce heavy rainfall. By 2100 UTC Wednesday, the column dries out significantly between 850 and 200 hPa, with PWATs dropping to 1.2 inches. However, by 0000 UTC Thursday, CAPE rises above 1000 J/kg, the Lifted Index falls to -6, Total Totals rise to around 52, theta-e consistently falls with height, and mid-level lapse rates rise to near 8 C/km. Therefore, we have opted to include scattered thunderstorms with the potential for severe Wednesday afternoon, although it appears the best chance for severe weather will hold off until Wednesday night. Therefore, will defer to subsequent forecast shifts to nail down timing and severity of Wednesday night's storms. As for temperatures, considering very similar weather conditions to Tuesday's will continue Wednesday, overnight lows Tuesday should be very near those of Monday night, and afternoon highs on Wednesday should be similar to those of Tuesday.

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