Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 07 2024 20:24:23 FOUS30 KWBC 072024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 323 PM EST Sun Jan 07 2024 Day 1 Valid 16Z Sun Jan 07 2024 - 12Z Mon Jan 08 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann 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... The models still advertise the development of an anomalously deep cyclone over portions of the Southern Plains on Monday which induces an impressive response that includes 850 mb winds accelerating into the 50 kt to 80 kt range...northward moisture transport/moisture flux off the Gulf of Mexico in the 1000 to 850 mb range that exceeds 5 standardized anomalies and draws an airmass with precipitable water values approaching 1.75 inches across the area...pronounced upper level divergence of coupled jets which aid direct circulation as well as supporting broad-scale vertical motion aloft. These ingredients should support convection capable of producing very intense rainfall rates with an associated risk of excessive rainfall (which looks to be most over portions of eastern Louisiana into southern Mississippi). =46rom a model diagnostics perspective...the 12Z NAM remained a bit of a northern outlier while there was a general westward shift in the placement of the QPF since the 12Z model run on Saturday.=20 Nudged the corresponding risk areas a bit westward but thinking that the need for a Moderate risk area remained just as strong as before. Surface based instability should make inroads a bit inland, north of I-10/I-12, with sufficient MUCAPE values -- 500-1000 J/kg -- given the strength of the low level flow that develops for a decent period of time across portions of southeast Texas, Louisiana, Mississippi, southern Alabama and portions of the Florida Panhandle. The amount of low-level convergence/confluence and its strength (at 850 hPa) is impressive given the low level flow of 50-80 kts over a broad area.=20 Precipitable water values rise to 1.5-1.75 inches...between 2.5 and 3.0 standard anomalies above climatology for this time of year over a fairly wide area. The resulting 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.=20 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.=20 Successive runs of the Canadian GEM Regional continued to be a wet outlier compared with other global, ensemble and convective-allowing guidance for several days. Today's 12Z run of the Canadian GEM continued to up the ante rather than back down on its maximum rainfall amounts by showing 2 separate areas of 10 inches or greater in the 24-hour period ending at 12Z on Tuesday.=20 More problematic is the day-to-day trends in the other guidance slowly in the direction of the strength of the Canadian Regional signal. 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. =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/Bann 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... ....2030Z Excessive Rainfall Discussion... Much of the guidance has remained consistent with the mass-fields and QPF of a deep layer cyclone that moves northeast into the Great Lakes. There were still some signals for low-end amounts of instability becoming available which potentially enhances rainfall rates on a fairly local scale. Overall...though...it is the combination of deep moisture being collocated with broad upper level difluence to produce as much as a couple of inches of rain.=20 The concern for excessive rainfall is as much about how much rain falls on the snowpack left by this weekend's snowstorm and how much that additional water will contribute to runoff/ponding concern...especially where flash flood guidance remains low. The overall forecast reason remains similar to the 0830Z Excessive Rainfall Discussion. Bann ....0830Z Excessive Rainfall Discussion... 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 mulch-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!6Ni_JdL27MTgh1jTX08KQkt23UgaCLCtIVdNe2h47dy8= iBdMjSkzxiCOqbHCg6Ha1bLckYVHYKidPOzY8hZn4ly4RTM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Ni_JdL27MTgh1jTX08KQkt23UgaCLCtIVdNe2h47dy8= iBdMjSkzxiCOqbHCg6Ha1bLckYVHYKidPOzY8hZn5nZ-ryc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Ni_JdL27MTgh1jTX08KQkt23UgaCLCtIVdNe2h47dy8= iBdMjSkzxiCOqbHCg6Ha1bLckYVHYKidPOzY8hZnTGVzQ0o$=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) .