Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 07 2024 07:18:41 FOUS30 KWBC 070718 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 217 AM EST Sun Jan 07 2024 Day 1 Valid 12Z Sun Jan 07 2024 - 12Z Mon Jan 08 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth Day 2 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...=20 An anomalously deep cyclone for early to mid January -- near record for January at the surface near the AR/MO/TN border junction -- moves across the Southern Plains towards the Lower to Mid Mississippi Valley Monday into early Tuesday. A deep layer flow becomes increasingly out of the Gulf of Mexico with time.=20 =46rom a model diagnostics standpoint, the 00z NAM was a bit of an outlier with the cyclone mass field-wise (still too far to the north) but even so, it's not terribly dissimilar from a QPF perspective. As this system is quite strong, surface based instability should make inroads a bit inland, north of I-10/I-12, with sufficient MUCAPE values -- 500-1000 J/kg -- for a decent period of time across portions of southeast TX, LA, MS, southern AL, and portions of the FL Panhandle. The amount of low-level convergence/confluence and its strength (at 850 hPa) is impressive, rising to 50-80 kts over a broad area. Precipitable water values rise to 1.5-1.75" over a fairly wide area. IVT values are significant, approaching 1400 kg/m/s, and the broad moisture plume emerging from the Gulf can be identified as an atmospheric river. There's little question that the atmosphere will be saturated or that any convection that forms could become organized over a 3-6 hour period at any particular location -- the degree of effective bulk shear and instability should support the formation of mesocyclones regionally and a progressive convective line closer to the cold front. Because of the breadth of the moisture and instability near the Gulf Coast, there is a concern for cell mergers between organized and disorganized convection as well as any progressive squall line with any pre-squall convection, and with the increasingly deep layer south to southwest flow, concerns for short periods of cell training as well. Hourly rain totals to 2.5" with local amounts to 6" still seem a conservative bet. While the 00z Canadian Regional is much wetter than the remainder of the guidance suite, its local maxima can't be ruled out, given the above ingredients. More problematic is the day-to-day trends in the other guidance slowly in the direction of the strength of the Canadian Regional's signal.=20 Outside of the north-central Gulf coast, portions of the northwestern and northeastern Gulf Coast have pockets where 300%+ of their average seven day rain has fallen, making those areas more sensitive. Urban centers are also expected to be more sensitive to possible flash flooding, as usual. The increasing risk of higher totals led to the introduction of a Moderate Risk across southeast LA, southern MS, and southern AL. The broad ERO risk pattern was coordinated with the LCH/Lake Charles LA, LIX/Slidell LA, JAN/Jackson MS, MOB/Mobile AL, and TAE/Tallahassee FL forecast offices.=20 =20 =20 Pacific Northwest...=20 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 Monday 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.=20 =20 Roth Day 3 Valid 12Z Tue Jan 09 2024 - 12Z Wed Jan 10 2024 ....A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE NORTHERN MID-ATLANTIC STATES... A deep layer cyclone, on the strong side of January climatology, will be intensifying as it moves northeast through the Midwest and Great Lakes, leaving the East within a divergent/diffluent flow pattern aloft. Inflow from the Atlantic on its east side at 850 hPa approaches 100 kts on the guidance -- there will be no lack of effective bulk shear to help organize convection. Precipitable water values of 1.5-1.75" surge northeast from the northeast Gulf Coast through the VA Capes, with 1.25"+ values making inroads as far northeast as southern New England. IVT values are forecast to peak close to 1600 kg/m/s, which is exceedingly high/unusual.=20 Unlike the last system, some surface based CAPE could exist as far north as PA per 21z SREF output. The 00z NAM, while not the ideal solution from a mass field perspective, indicates pockets of 250+ J/kg of MU CAPE near and east of the Fall Line across the Eastern Seaboard as far north as New York City -- this may trend upward in later cycles. When combined with ample low- to mid-level frontogenesis, hourly rain totals to 1" should be possible. For northern portions of the risk areas from PA into southern New England, heavy rainfall will fall upon snow pack, causing melting, which is still unfolding at this hour. Some additional snowfall is also possible in northern areas just prior to the transition to heavy rain with this system during the day 3 period/Tuesday. When combined with the possibility of strong onshore flow causing coastal flood issues, backwater flooding will be possible on rivers/bays that drain into the Atlantic, spelling a dire situation from a river flood perspective and possibly aiding flash flooding as well since runoff may be more difficult to channel than usual. Flash flood guidance is quite low right now, and even with some recovery, won't likely rise enough between now and Tuesday. Before today's rainfall, NASA SPORT imagery showed 60-65% saturation down to 100 cm -- those numbers are likely to jump once the recent rains are accounted for. The Moderate Risk remains a good idea, and on this cycle has been expanded southwest based upon the multi-faceted flood concerns mentioned above, which was done in coordination with the Mid-Atlantic RFC and the PHI/Mount Holly NJ forecast office. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5A4Oszi3eHvMlYwXnrv44mfchKYzubpbQSuHaZmRaAmL= B8DGgbGtTWk7Ud91eHihe2wALY1ZEo0iFffAwF4gaT6KM0g$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5A4Oszi3eHvMlYwXnrv44mfchKYzubpbQSuHaZmRaAmL= B8DGgbGtTWk7Ud91eHihe2wALY1ZEo0iFffAwF4gfjJFIOM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5A4Oszi3eHvMlYwXnrv44mfchKYzubpbQSuHaZmRaAmL= B8DGgbGtTWk7Ud91eHihe2wALY1ZEo0iFffAwF4gydZ2k8g$=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) .