Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 04 2024 15:58:39 FOUS30 KWBC 041558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1057 AM EST Thu Jan 04 2024 Day 1 Valid 16Z Thu Jan 04 2024 - 12Z Fri Jan 05 2024 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS NEAR THE MIDDLE TEXAS COAST... ....Middle Texas Coast... Maintained the previously issued Marginal Risk area along the Middle Texas coast due to the models continuing to suggest some heavy rainfall mainly after 05/06Z in and near sections of the Middle TX coast between Houston and Refugio. Some instability will be available (250-500 J/kg MU CAPE) with precipitable water values forecast to increase into the 1.25-1.5 inch range. Hourly rain totals of 1-2" are possible here given the ingredients. The 04/12Z HREF probabilities for 1 inch of rainfall in 1 hour decreased a bit compared with the 04/00Z run but still high enough to support the on-going Marginal Risk area. That was reinforced by the 12Z HREF maintaining low-end chances for 2 inches of rain in an hour in a region where flash flood guidance was on the order of 2.5 inches in three hours. Low-level inflow (850 hPa) should be decent, close to 40 kts, which along with the related effective bulk shear should try to organize convection. The biggest issue appears system progression as locations don't have that long to rain heavily before the inflow veers to southwest and moisture erodes. However, rainfall has been 150-300% of average along the Texas coast over the past seven days. Enough is there overall for a Marginal Risk, which was added this cycle. Bann Day 2 Valid 12Z Fri Jan 05 2024 - 12Z Sat Jan 06 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN GULF COAST...=20 =20 ....Gulf Coast and Southeast...=20=20=20 A strong and fast-moving system will track northeastward from Texas to the Southeast by Saturday morning. The result will be an increasingly favorable divergence pattern aloft associated with the shortwave itself, which will support deepening of a surface low that will track across the Southeast near and ahead of the shortwave. The low will take advantage of increasing Gulf moisture to result in a line of showers and storms that will from from the low center south to the Gulf. The storms will be capable of heavy rainfall, as precipitable water values increase to as high as 1.75". Inflow at 850 hPa and effective bulk shear build to 50+ kts and some areas could see MU CAPE rise to 500+ J/kg. Decent low- to mid-level frontogenesis should also exist. The ingredients should allow for hourly rain totals to 2" and local amounts to 4". The fast forward speed of the storms potentially limits how much rain any one area sees, so if an organized line forms, some mesoscale waves will need to form along it or some cell mergers would need to occur to create issues. Soils all across the Marginal Risk area are either drier or much drier than normal, so the rivers and streams could take on some rainfall.=20 The result will be any isolated flash flooding most likely occurring in flood-sensitive urban and poor-drainage areas.=20 Changes made to the risk area were predicated on the new guidance. =20 =20 ....Olympic Mountains of Washington State...=20 A strong front and attendant shortwave will dig southeastward out of the North Pacific and into coastal Washington Friday night. The period of heaviest rainfall into the Olympic Mountains is likely to be after 06Z Saturday, but due to the strong forcing, rainfall totals of 0.5"+ in an hour are possible, resulting in pockets of 2" of rain overall at lower elevations. Heavy rainfall rates may result in isolated flash flooding in the flood-sensitive valleys draining the west facing slopes of the Olympic Mountains, but the area impacted appears rather small, too small to depict using an excessive rainfall area, so removed the Marginal Risk area from continuity. The heavy rains will quickly move down the coast and the Cascades on Saturday, so the heaviest rains will be short-duration, which in turn will limit flash flooding. Any heavy rainfall will be south of the Olympics by 12Z Saturday. Soil moisture is around average for this time of year, but based on NASA Sport relative soil moisture guidance is between 65-70% saturated. Heavy rainfall should further saturate the soils in this region.=20 =20 Roth=20 Day 3 Valid 12Z Sat Jan 06 2024 - 12Z Sun Jan 07 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE EASTERN CAROLINAS AND EASTERN PORTIONS OF THE MID-ATLANTIC STATES... Carolinas/Mid-Atlantic... A pair of mid-level shortwaves should allow for diffluence aloft near the East Coast. Down the Carolinas, instability is available near the coast along with precipitable water values of 1.25-1.5" in an environment with 1000-500 hPa thickness values in the 5500-5600 meter range to potentially lead to efficient heavy rainfall across the Carolinas within a moist atmosphere. Inflow of ~50 kts at 850 hPa is expected off the Atlantic. In the Carolinas, hourly rain totals of 1.5" are possible which would be problematic in urban areas. Low- to mid-level frontogenesis is likely to do the heavy lifting north of the Carolinas where instability will be near zero, which should be able to yield hourly rain totals of 0.5"+ at times. The signal for heavy rainfall exists, but the guidance could show more agreement and 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. Overall, thought there was enough there for the Marginal Risk to carry forward, but some adjustments were made due to the newly available guidance. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9KyjMQE8bIobUknNWQOaA-zJC4frBFZcVaXX9PJWNS0E= KG7eFdAhJHgsIGGMWZ5mjuu1SIeBrX4wm4Xtc5IrKZTeYnE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9KyjMQE8bIobUknNWQOaA-zJC4frBFZcVaXX9PJWNS0E= KG7eFdAhJHgsIGGMWZ5mjuu1SIeBrX4wm4Xtc5IrYtKBcA8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9KyjMQE8bIobUknNWQOaA-zJC4frBFZcVaXX9PJWNS0E= KG7eFdAhJHgsIGGMWZ5mjuu1SIeBrX4wm4Xtc5IrkfRnKg4$=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) .