Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 20 2023 20:23:29 FOUS30 KWBC 202023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 322 PM EST Mon Feb 20 2023 Day 1 Valid 16Z Mon Feb 20 2023 - 12Z Tue Feb 21 2023 ....16Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Tue Feb 21 2023 - 12Z Wed Feb 22 2023 ....2030Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Wed Feb 22 2023 - 12Z Thu Feb 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEASTERN MISSOURI TO SOUTHWESTERN MICHIGAN... ....2030Z Update... A few minor adjustments were made with this afternoon's update to Wednesday's ERO. The run-to-run trends in most of the guidance show a slow but steady drift in the axis of heaviest rainfall north and west with time. Thus, both the Slight and Marginal risk areas were moved about a row of counties northwest with this update. In coordination with LOT/Chicago, IL forecast office, this now puts most of Chicago in the Slight Risk area. There remain several points of uncertainty. Besides where the heaviest rain sets up, the placement of the rain/snow line with this storm continues to be uncertain. To be more precise for purposes of the ERO, it's really the rain/freezing rain-mixed precipitation line that remains uncertain. Further, soil moisture throughout this portion of the Midwest remains incredibly dry, with much of the Slight Risk area from central IL to the IN/MI border below 2% of climatological normals based on NASA SPoRT soil moisture imagery. Note that much of the rainfall in the Slight Risk region and points east will fall in about 6-9 hours, and of course localized heavier rainfall is possible with any convective banding, which is probable given the strength of the parent low and the plentiful moisture to work with. As for the rain/ice line, if the colder air can extend further south the inclusion of the southernmost counties of MI would be unnecessary, but much of the guidance suggests the rain/ice line will be a bit further north. Thus, there remains plenty of opportunity for refinement as the event draws closer. Wegman ....Previous Discussion... ....Midwest from Northern Missouri to Southern MI/Northern Ohio.... A strong dampening shortwave races out of the Desert Southwest and accelerates through the Southern Plains into the Ohio Valley/Great Lakes by the end of the period. The shortwave will be driven by the digging northern stream kicker wave and nose of expected 170kt jet streak across the Central Plains. This will spur very strong cyclogenesis across the Central High Plains into the Missouri Valley, which will accelerate the low level jet with highly anomalous moisture flux across the broad warm sector. At the start of the period 12z Wednesday, the sharpening warm front should extend from near Kansas City across central MO toward the Ohio River. Broad 20-25kts of sfc to boundary layer warm advection should drive the warm front northward throughout the day. As the low takes shape by early afternoon, very strong moisture flux convergence near and downstream of the surface cyclone combined with weak available instability should allow for focused convective development. The convection will become more scattered in nature further downstream across IL/IN/OH given weaker low level flow and instability. Isallobaric wind flow should back the strengthening low level flow while the LLJ strengthens to over 50-60kts. The LLJ will advect deeper moisture with total PWat of 1.5" to 1.6" northward as ECENS and GEFS ensembles show 3.5 to 4 standard anomaly values across the Mississippi River Valley for the low level moisture flux parameter. Solid northeasterly flow north of the front should sharpen and steepen the warm front. With directional convergence, convection should further expand across western IL toward Chicagoland by evening hours. With the backed flow due to the strong pressure falls, the convergence axis will broaden generally parallel with the motion of the cyclone and deeper layer steering flow. This will allow for favorable training profiles. While storm cell motions will be very fast, limiting rainfall duration...the length of this convergence axis should support an axis of 2-3" with isolated pockets of higher totals possible. Soil conditions are anomalously dry per NASA SPoRT relative soil moisture fields through 40cm depth. Nearly 30-40% of the area is in the 1st to 5th percentile. Given the rates and hard, dormant ground conditions much of the rainfall at this intensity is likely to run off. There is good confidence that the short duration should result in scattered incidents of excessive rainfall and possible flash flooding. As such, the Slight Risk of Excessive Rainfall has been maintained from the Day 4 period into the short range. Global models seem to have honed in on an axis from north to northeastern MO across western IL toward northwestern IN. Peak rain rates and duration should be over north-central IL and may glance the southern portions of Chicagoland, with a northward trend potentially further worsening conditions in this direction. Another consideration is the proximity to the sharpening cold air, which may support a broad streak of freezing rain conditions north of the frontal zone across southern WI and into the Lower Peninsula of MI. As such the bounding Marginal Risk area to the north is a bit tight and more uncertain given ice formation. As the cyclone races into the Great Lakes, the LLJ will veer and become less orthogonal for the ascent pattern across NE IN into Northern Ohio, but should still be high enough to suggest still some isolated flooding concerns. The Marginal Risk areas was expanded as far east as the PA/OH border.=20 ....Southern Plains, Ozarks to the Lower Ohio River Valley... Strong height-falls associated with the shortwave will drive the associated cold front quickly through the Central Plains. The greatest potential for intense rainfall rates may occur with quick initiation with slow cell motions across northeast TX, eastern OK and southeast KS before the cold front presses too quickly for a prolonged duration for extreme rainfall totals. As such a broad Marginal Risk of Excessive Rainfall remains highlighted across Northern Texas through much of central MO including western AR and eastern OK. Orthogonal moisture flux into the Boston and Ouachita ranges of AR may allow for earlier shallow cell development ahead of the main line suggesting a bit higher risk for enhanced rainfall totals, but there is not sufficient evidence of consistency to delineate a higher risk category at this time. We will continue to monitor this trend, especially as the event enters the Hi-Res CAM window.=20 Gallina/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!858zbGyOJaO73GU_-sktsiG0f-tyd7LbIIjuXGZCV1RN= wH82_l2q7oN9cmlAy2HglkDXcDpH4-BsLWhBf4GZjoeBANM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!858zbGyOJaO73GU_-sktsiG0f-tyd7LbIIjuXGZCV1RN= wH82_l2q7oN9cmlAy2HglkDXcDpH4-BsLWhBf4GZ4t1bLqg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!858zbGyOJaO73GU_-sktsiG0f-tyd7LbIIjuXGZCV1RN= wH82_l2q7oN9cmlAy2HglkDXcDpH4-BsLWhBf4GZ5UY36lA$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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