Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 08 2024 08:30:02 FOUS30 KWBC 080829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 AM EST Mon Jan 08 2024 Day 1 Valid 12Z Mon Jan 08 2024 - 12Z Tue Jan 09 2024 ....A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR THE CENTRAL GULF COAST... Southeast/Gulf Coast... Maturing upper level pattern across the central and southern plains will allow for a deep moisture advection pattern downstream of the mean trough with the central Gulf coast in the crosshairs for the most prevalent low-level moisture and accompanying instability thanks to the rich, Gulf air being tapped in the setup. Intense cyclogenesis pattern over the southern plains in tandem with backside flow from a 1030+ mb high pressure east of the Mississippi will create a strong south to southeast low-level wind pattern that will advect tropical air northward allowing PWAT anomalies to grow up to 2-3 deviations above normal within parts of the southeast up into the southern reaches of the Midwest. This amplified pattern will aid in period of very heavy rainfall to initiate over the southeast with the highest probability for heavy rainfall and flooding concerns over the central Gulf coast. A couple of major factors at play that will enhance the opportunity for flash flooding, as well as areal extent of the potential. The first is the highly anomalous setup at both the surface and mid-upper levels where elevated integrated vapor transport is all but certain as IVT values of over 1000 kg/ms are forecast off the latest deterministic guidance, a prolific 6-8 sigma above normal characteristic indicative of the tropical moisture source that will be available within the confines of the Gulf coast. This allows for PWAT indices to reach upwards of 1.6-1.8", something you see more of in the warm season comparatively which is genuinely when flash flooding concerns begin to rise. Considering the airmass, theta-E advection regime will be co-located within the right entrance of a strengthening mid-level jet pivoting around the base of the upstream trough. Instability will rise through the afternoon with 1000-1500 J/kg CAPE signatures being forecast within the zone from Lake Charles over into southern AL and as far north as Jackson, MS and Montgomery, AL. Within this area will be the highest risk for flash flooding concerns given the expected convective signatures both ahead of and along an advancing cold front that will quickly propagate eastward on the southern edge of the surface low to the northwest. A strong low-level convergence pattern will develop across the south with an axis of thunderstorms and very heavy rainfall likely to move from south-central LA through southern MS and AL by the end of the period. 00z HREF neighborhood 1"/hr rate probabilities are over 80% for a large area within the aforementioned area with 2"/hr rate probabilities solidly between 25-40% between the hours of 00-12z with emphasis over southern MS. HREF neighborhood probabilities of at least 5" during the period from 00-12z Tuesday are over 80% between Baton Rouge through the Mississippi Delta with 8" probabilities of 20-25% showing up in the probability fields. This is a significant flood potential from both the means and probability fields and that doesn't even touch the local maximums from deterministic output. Have maintained the previous Moderate risk area, but expanded to the east over southern AL given the latest guidance expanding the heavy rain QPF footprint and correlated probability increases within the zone. Slight risk was also expanded further north into northern AL to account for both guidance and analog output for the incoming event. Kleebauer =20 =20 Pacific Northwest...=20 No changes to the previous forecast as the overall synoptic evolution and expected rainfall remains on track. Precipitable water values of 0.75"+ move ashore on the heels of 50 kts of 850 hPa inflow out of the west to the south of a strengthening cyclone heading towards Vancouver Island. Two bouts of heavier precipitation are expected...a warm air advection precipitation pattern early on and then the main moisture plume near the cold front late in the period when some instability moves in from the Pacific. Between the degree of inflow, expected boundary layer moisture convergence, and moisture availability, hourly rain totals of 0.5"+ should be possible at times today into early Tuesday. An old rule of thumb based on the sustained onshore flow suggest local maxima in the 4" neighborhood, not far from the 00z Canadian Regional guidance. Enough was there to maintain the Marginal Risk and continuity was generally maintained. Roth/Bann Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5x8QDthgSZZeXHb8Dabsa3Ln0NooZecwJ13Y6FcjAajG= T_wFM-EaWnM7sbTErz1p7hwvaS6ng8z-qg8Lx8lwTZNREI0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5x8QDthgSZZeXHb8Dabsa3Ln0NooZecwJ13Y6FcjAajG= T_wFM-EaWnM7sbTErz1p7hwvaS6ng8z-qg8Lx8lweAOfqGc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5x8QDthgSZZeXHb8Dabsa3Ln0NooZecwJ13Y6FcjAajG= T_wFM-EaWnM7sbTErz1p7hwvaS6ng8z-qg8Lx8lwRmAM4EA$=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) .