Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 06 2024 20:25:46 FOUS30 KWBC 062025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 324 PM EST Sat Jan 06 2024 Day 1 Valid 16Z Sat Jan 06 2024 - 12Z Sun Jan 07 2024 ....16Z Excessive Rainfall Discussion... No big changes were needed to the previously issued Marginal Risk area. Neighborhood probabilities from the 12Z HREF still point to some low-end risk of rainfall exceeding flash flood guidance...moreso in the 3- and 6-hour time frames later in the afternoon and evening across parts of the Mid-Atlantic and points northeast. Spaghetti plots of the SREF/GEFS QPF showed better chance for 2+ inch amounts being confined in the immediate vicinity of the coast and an inch or more along the urban corridor. So the QPF signal itself remained meager...but felt the Marginal risk area still captures the essence of isolated ponding and run off problems in urban areas and areas of poor drainage (especially in areas that received locally heavy rainfall during the previous week or two).=20 Bann ....0830Z Excessive Rainfall Discussion... A pair of mid-level shortwaves should allow for diffluence aloft near the East Coast. Inflow of 50-60 kts at 850 hPa from the southeast is expected off the Atlantic. Low- to mid-level frontogenesis is likely to do the heavy lifting as instability will be quite low -- at or below 250 J/kg -- which should be able to yield hourly rain totals of 0.5"+ at times, an idea supported by the 00z HREF. The signal for heavy rainfall exists, but the guidance is not overly strong with the QPF signal as the system overall should remain progressive. Portions of this region experienced heavy rainfall within the past week, with pockets receiving 300%+ of their average weekly rainfall for this time of the year, which is the main reason the Marginal Risk area has been kept from continuity. Overall, thought there was just enough there for the Marginal Risk to carry forward, but some adjustments were made based on newly available guidance QPF-wise and to better account for the precipitation type expected with a little bit of overlap into the expected snow area (in case the guidance edges farther northwest on the expected rain/snow line as the system moves through general southwest flow aloft downstream of a more significant deep cyclone).=20 =20=20 Roth=20 Day 2 Valid 12Z Sun Jan 07 2024 - 12Z Mon Jan 08 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 =20 Bann Day 3 Valid 12Z Mon Jan 08 2024 - 12Z Tue Jan 09 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ALONG PORTIONS OF THE GULF COAST... 2030Z Excessive Rainfall Discussion... The system that deepens over the Southern Plains remains the primary concern in the Day 3 period. Latest model runs still depict an deep-layered low forming over parts of Texas and Oklahoma...with heights greater than 3 standardized anomalies from 850 mb through at least 200 mb by 09/00Z. As a result...low level flow off the Gulf of Mexico accelerates to between 50 kts and 70 knots at 850 mb and results in moisture flux values of nearly 5 standardized anomalies from Louisiana and southeast Arkansas into portions of Mississippi and Alabama Monday night into early Tuesday morning. With precipitable water values increasing into the 1.5 to 1.75 inch range and a strong feed of moisture and the direct circulation supported by the upper level jet...storms should be capable of supporting 1.5 to 2.5 inch per hour rates with localized amounts of 4 to 6 inches although the the 12Z run of the Canadian Regional continued to be most aggressive in generating isolated maximum values of 7+ inches amounts in portions of Alabama and southwest Georgia. Certainly the ingredients for intense downpours and some heavy rainfall amounts look to be in place by the Day 3 period...maintained the higher-end Slight Risk area. Over the Pacific Northwest...the mass-fields and associated precipitation footprint changed little from the overnight run.=20 The only change was to trim out the area out of the higher elevations of the Cascades where a majority of precipitation is expected to fall as snow. Bann ....0830Z Excessive Rainfall Discussion... Southeast... 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, but even so, it's not terribly dissimilar from a QPF perspective, perhaps too narrow with the heavy rain signal along the Gulf Coast and too poleward with 1.5-2.5" amounts across northern AL and central TN. As this system is quite a bit stronger than the one that just passed through, surface based instability should make inroads along the immediate coast as far east as Florida with sufficient MUCAPE values -- 500-1000 J/kg -- for a decent period of time north of I-10, in LA northward past I-20 on more limited time scales. The amount of low-level convergence and its strength (at 850 hPa) is impressive, rising to 50-75 kts over a broad area. Precipitable water values rise to 1.5-1.75" over a fairly wide area. IVT values are significant, rising to 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.=20 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, 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" seems 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. Rainfall over the past 24 hours should have brought the past week's rainfall closer to average, particularly in southern MS/southern AL and southwest LA, but other locations near the Gulf Coast may end up still being below average over the past week; urban centers are expected to be more sensitive to possible flash flooding, as usual. The Slight Risk depicted is a higher end one, given the ongoing ingredients and the 00z Canadian output. Continuity was generally maintained. Pacific Northwest... 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. 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 with local amounts of 3"+ in the mountains. The 00z Canadian Regional was exceedingly wet compared to the other guidance. However, old rules of thumb based on the sustained onshore flow suggest local maxima in the 4-5" neighborhood are more likely; less than what the 00z Canadian is advertising yet more than seen on the other guidance. Enough was there to maintain the Marginal Risk, but simplified the shape based upon the neighborhood probability paradigm of the excessive rainfall product. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_LRDOAhF77NYa2VjGKXpQ9GzKyWgUIaXZRbbQX-q7V1g= 980Rr2hlzOxsrNG5IL4uA3X4nr-DONhk3USA719nhJSZ4wo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_LRDOAhF77NYa2VjGKXpQ9GzKyWgUIaXZRbbQX-q7V1g= 980Rr2hlzOxsrNG5IL4uA3X4nr-DONhk3USA719n7whAMxk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_LRDOAhF77NYa2VjGKXpQ9GzKyWgUIaXZRbbQX-q7V1g= 980Rr2hlzOxsrNG5IL4uA3X4nr-DONhk3USA719nC0tIpos$=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) .