Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 02 2023 00:50:32 FOUS30 KWBC 020050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 750 PM EST Wed Mar 01 2023 Day 1 Valid 01Z Thu Mar 02 2023 - 12Z Thu Mar 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN PARTS OF THE TENNESSEE VALLEY AND NEARBY OHIO VALLEY SOUTHWESTWARD INTO ARKANSAS... ....01Z Excessive Rainfall Outlook... Expanded the Slight Risk across portions of the Tennessee Valley and a small portion of Ohio Valley based on short term radar imagery that showed a line of convection that was producing locally heavy rainfall. The convection was aligned roughly parallel to the mean flow ahead of an approaching cold front...which was setting up the potential for some training of cells or repeat convection. The HRRR was focusing one axis of heavier amounts here where it was depicting modest surface based instability and southerly winds which were starting to draw air with dewpoints in the 50s to near 60 across parts of Kentucky.=20 Back where moisture was deeper...pockets of 1.25 to 1.50 inches per hour rates were depicted by MRMS. The expectation is that the coverage of rainfall moving out of Arkansas and southwest Tennessee will be fed by strengthening low level jet later this evening that results in an expanding coverage of moderate to locally heavy rainfall more focused over the Tennessee Valley and far northern Mississippi/northwest Alabama that persists into the overnight/early morning hours One offsetting factor is that individual cells may be progressive enough but the potential for repeat convection suggests that not all of the rainfall will be absorbed. Bann Day 2 Valid 12Z Thu Mar 02 2023 - 12Z Fri Mar 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN OKLAHOMA INTO ARKANSAS AND THE MID-MISSISSIPPI VALLEY...=20 =20 ....ArkLaTex to Mid-South and Lower Ohio Valley...=20 As a strong closed low moves from the Southwest into MO this period, a surface cyclone will rapidly strengthen in the Southern Plains, tracking toward the Midwest early Friday morning. The upper trough axis is forecast to transition from a positive to negative tilt Thursday night into Friday, allowing for significant divergence/difluence aloft to develop downstream. Ahead of the related surface low/cold front and along the eastward extending warm front, much of AR into the Lower MS Valley remains within the 1.25-1.75" precipitable water value range per the 12Z model consensus for an extended period of time (correlating to standardized anomalies of 2 to 3+ for early March). MUCAPE rises into the 500-1500 J/kg range (locally higher) for upstream locations across northeast TX and southeast OK. Somewhat convergent inflow at 850 hPa with speeds increasing through the 30-60 kt range and mid-level height falls will support a rapid expansion in the coverage of thunderstorms from TX into OK between 18-00Z. Low level convergence axes aligned with the the mean 850-400 hPa flow will allow training and repeating of heavy rain, with peak hourly totals in the 2-3 inch range, especially with efficient rainfall tied to organized supercells embedded within the borader convective shield. The 12Z guidance advertises local amounts in the 3-6" range, but more is possible as the low-level low backs as convection initially develops -- there's a potentially higher ceiling closer to 8 or 9" on a localized basis before forward propagation occurs later Thursday night/early Friday. There is some overlap of the outlook with recent heavy rainfall extending from AR into TN and northern MS/AL where soil moisture values are higher than average per recent NASA SPORT imagery, including a narrow stripe of 2 to 4+ inches of rain which fell across Little Rock AR at the end of last week. Considering all of the above, the threat level remains Moderate Risk, although some notable differences remain in the QPF in the 12Z models. The consensus of the latest guidance however, supported a southwestward nudge to the Slight and Moderate Risk areas into the eastern TX/OK border, while trimming back some of the Marginal and Slight Risk areas that were in place over portions of the Ohio Valley on the previous outlook. =20 Otto/Roth=20 Day 3 Valid 12Z Fri Mar 03 2023 - 12Z Sat Mar 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR INDIANA AND OHIO... Aa a deep layer cyclone moves from near the MO/AR border into the Northeast between Friday morning and Saturday morning, precipitable water values are expected to rise above 1.25 inches from the Upper Ohio Valley into portions of the Northeast. 850 mb inflow of 30-60 kt out of the south-southwest will aid the northward surge in moisture ahead of the accompanying cold front. A significant player in the rainfall efficiency across the Midwest/Ohio Valley will be the overlapping low- to mid-level frontogenesis, with WNW to ESE banding lifting northward wtih time. Some instability is expected to be available, especially early in the period from IL into portions of IN and KY compared to later in the period as 850 mb dewpoints struggle to reach 12 C beyond 18Z Friday for the Upper Ohio Valley and into the northern Mid-Atlantic. MUCAPE for much of the Friday afternoon into Saturday may not get above 500 J/kg for appreciable areas but hourly rainfall near 1 inch will still be possible at times. Northern areas of the new Slight Risk area have seen 600% of their average rainfall during the past week, which has led to 65-80% saturation in the top 10 cm of soil per NASA SPORT imagery. The guidance suggests a general soaking, with local 2-3" amounts expected, but that will depend on the degree of instability that is available which this far out is an open question as the guidance generally underplays MUCAPE forecasts and could trend higher as the event draws near. No significant changes were made to the Marginal or Slight Risk areas compared to the previous outlook but the farther west/north 12Z NAM was given little weight in the overall forecast of excessive rainfall. The 12Z GFS was also thought to be a bit too fast with more weight given to the ECMWF and UKMET with probabilities of higher QPF. Otto/Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6S0bMuRxsLzkU-_EhWgiUqvNRFiruEJ2lsR8RjA0Qrc5= oEL9N4UY_JATEXNhO5eJdpd2WF4HoD4c_vNLFj4cc8WhsyQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6S0bMuRxsLzkU-_EhWgiUqvNRFiruEJ2lsR8RjA0Qrc5= oEL9N4UY_JATEXNhO5eJdpd2WF4HoD4c_vNLFj4c30LhX9I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6S0bMuRxsLzkU-_EhWgiUqvNRFiruEJ2lsR8RjA0Qrc5= oEL9N4UY_JATEXNhO5eJdpd2WF4HoD4c_vNLFj4cMxoIKDU$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .