Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 10 2025 20:21:52 FOUS30 KWBC 102021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 321 PM EST Mon Feb 10 2025 Day 1 Valid 16Z Mon Feb 10 2025 - 12Z Tue Feb 11 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Arklatex/Lower MS Valley into portions of TN/KY... Strengthening warm air advection/isentropic ascent associated with gradually backing mid-level flow and an increasing southwest low- level jet (reaching 30 to 40+ kts) will facilitate the development and expansion of moderate to locally heavy shower activity tonight through Tuesday morning along an axis from northeast TX/southeast OK northeastward through central and northern AR and into areas of central/western TN and southern KY. Some return of weak elevated instability will support some localized convection that may yield some hourly rainfall totals of a 0.25" to 0.50" based off the 12Z HREF guidance, but generally the rainfall totals tonight going through 12Z/Tuesday should be in the 1 to 2 inch range. Trends will continue to be monitored, but these modest rainfall rates/totals at least through early Tuesday are not likely to result in any FFG exceedance concerns. Any runoff problems are expected to be isolated at best. Orrison Day 2 Valid 12Z Tue Feb 11 2025 - 12Z Wed Feb 12 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE MID-SOUTH AND LOWER MISSISSIPPI VALLEY INTO THE TENNESSEE VALLEY... 20z Update: Main adjustment was to expand the Slight risk to the=20 southwest into more of eastern TX. We often see convection get=20 organized further southwest than expected in events like this, and=20 there is growing QPF signal in the 12z ECMWF, UKMET and Gem Reg=20 over this area. Will also note that the CSU GEFS based machine=20 learning guidance also indicates higher probabilities of excessive=20 rainfall further to the southwest into more of eastern TX and=20 southwestern LA. The northeastern extent of the Slight risk was=20 trimmed back a bit over eastern TN and northern GA. Instability=20 will initially be lacking here which should delay the flash flood=20 risk...expect the flash flood risk to pick up more over this area=20 as we head into day 3. Chenard ....Previous Discussion... ....ArkLaTex-ArkLaMis into the Mid-South and Tennessee Valley... Increasing coverage of showers and thunderstorms along and ahead of a well defined cold front trailing a developing area of low pressure taking shape over the Tennessee Valley will lead to an increasing threat of excessive rainfall Wednesday into early Thursday from easter Texas and parts of Arkansas and Louisiana into the west aspect of the Appalachians of Tennessee and southeast Kentucky. At the synoptic scale, a broad trough over the western U.S. will amplify, thereby enhancing the deep-layer ascent and southwesterly flow/moisture transport over this part of the country due to a increasing low level flow. 850-700 mb moisture flux anomalies still look to increase to 2-3 standard deviations above normal Tuesday/Tuesday night while Precipitable Water values climb to 1.5-1.75" (around +2 standard deviation above normal). The strengthening dynamical and thermodynamical support will make for efficient rainfall rates despite the lack of deep- layer instability and the exit region of a 125 to 150 kt upper level jet should aid deep layer vertical ascent. Much of the guidance generally depicts a widespread swath of 1.5-3" of rain within the outlook areas, with the most aggressive model being the ECMWF with maximum values exceeding 4 inches. Storm motions nearly parallel to the WSW-ENE oriented surface boundary will boost the potential for training despite meager deep- layer elevated instability (MUCAPEs generally 250-500 J/Kg). With 2.00"+/3hr rainfall rates still being anticipated where the cells do train, again most likely within the Slight Risk area, which could lead to localized- scattered instances of flash flooding. Bann Day 3 Valid 12Z Wed Feb 12 2025 - 12Z Thu Feb 13 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE TENNESSEE VALLEY INTO THE MID-SOUTH... 20z Update: The prolonged heavy rainfall event will continue=20 Wednesday into Wednesday night over the Southeast. The extended=20 duration of this event will lead to storm total rainfall into the=20 3-5" range, with locally higher totals possible. As stronger=20 forcing ejects eastward on Wednesday, an increase in the low level jet should aid in both increasing moisture convergence and=20 advecting instability northward. Thus higher rainfall rates are=20 possible...and these higher rates moving over what should be=20 saturating ground conditions could result in an increased flash=20 flood risk by later in the period. The best chance of this=20 occurring is currently over portions of northern AL/GA into far=20 southeastern TN and the far western Carolinas. Do consider this a=20 higher end Slight risk over these areas. Can not rule out an=20 eventual MDT risk upgrade pending model trends over the coming days and how effective the extended duration of rain is at saturating=20 what will initially be dry soil and streamflow conditions. Chenard ....Previous Discussion...=20 ....Tennessee Valley into the Mid-South... The threat of excessive rainfall will continue on Wednesday and early Thursday ahead of the same system which brought the excessive rainfall potential on Tuesday into early Wednesday as a cold front makes its way eastward. Broad scale ascent will continue to aided by the southern entrance region of a 135 to 155 kt upper level jet...while moisture convergence is maximized along a well defined surface cold front which intercepts a plume of moisture being pulled northward by a 40 to 50 knot low level jet. There is still spread in the guidance about placement of where the heaviest rainfall will occur but amounts generally in the 1 to 3 inch range...with locally higher amount where any enhanced rainfall rates set up due to banding/training. Given antecedent conditions...feel that a Slight risk should suffice. ....California Coast... The plume of moisture from the Pacific Ocean towards the end of Day 2 will start to bring rain to the coastal areas during the Day 3 period...beginning in northern and central portions of California on Wednesday that propagates southward during Wednesday night and Thursday. Present indications are that the heaviest amounts remain off shore...but a general uptick in amounts was noted over the past 24 hours in this area. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!883TzQjPq24cdhpHmAXDtkOBuBvwzPdJWvqUPkUjDwuX= ixgBYFwIcAu2gUQX0HrInwC08u8ejVUY-P2-I_keg5cZIjo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!883TzQjPq24cdhpHmAXDtkOBuBvwzPdJWvqUPkUjDwuX= ixgBYFwIcAu2gUQX0HrInwC08u8ejVUY-P2-I_keOZe1_mE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!883TzQjPq24cdhpHmAXDtkOBuBvwzPdJWvqUPkUjDwuX= ixgBYFwIcAu2gUQX0HrInwC08u8ejVUY-P2-I_kefcLB_-U$=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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