Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 19 2024 20:31:08 FOUS30 KWBC 192031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 PM EST Fri Jan 19 2024 Day 1 Valid 16Z Fri Jan 19 2024 - 12Z Sat Jan 20 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Otto Day 2 Valid 12Z Sat Jan 20 2024 - 12Z Sun Jan 21 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA AND SOUTHWEST OREGON... 21Z update: There's good model agreement with the deamplifying closed 500 mb low/trough moving into the Pacific Northwest early Saturday morning and areas of moderate to heavy rain ongoing across the northern California coast at 12Z Saturday. Rainfall rates of 0.25 to 0.50 (localized 0.5+ in/hr rates) are expected to impact portions of the northern California coast (with a focus from northern Sonoma County to Cape Mendocino) through 00Z as heavier rainfall shifts northward into far northern CA and southwestern Oregon. While precipitable water values from northwestern California into southwestern Oregon may struggle to reach 1 inch, persistent rainfall through the period is expected to allow for localized 3-5 inch totals for the 24 hour period ending 12Z Sunday. Given 200 to 400 percent of average rainfall for much of the region over the past week, some flooding concerns may be realized with a continuation of the previous Slight Risk with no significant changes. Otto Guidance has remained consistently wet now over the past few runs across portions of northern California and southwestern Oregon.=20 Precipitation is expected to develop across much of the region prior to Day 2 as a negatively-tilted mid-to-upper level trough and its associated frontal band move into the region Friday night. The models have been consistent in showing a weakening IVT plume as it moves onshore late Friday into early Saturday. However, westerly flow south of the parent low is expected to tap a secondary surge of deeper moisture and direct into the region as it lifts northeast along the northern California to southern Oregon coasts on Saturday. This moisture along with sufficient large-scale lift is expected to support an increase in rainfall rates with moderate to heavy amounts expected beginning Saturday morning. The general consensus of the deterministic models over past few runs suggests amounts of 1-3 inches, with locally heavier accumulations are likely across the region, with the heaviest amounts most likely in the upslope regions of the coastal ranges and the northern Sierra and southern Shasta foothills. HREF probabilities suggest that much of this may occur in the first 12 hours, showing high neighborhood probabilities for accumulations of 2 inches or more from 12Z Sat - 00Z Sun across parts of the area. Given the wet antecedent conditions and the move in the models toward a wetter solution, opted to upgrade this area to a Slight Risk.=20=20=20 Pereira Day 3 Valid 12Z Sun Jan 21 2024 - 12Z Mon Jan 22 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... 21Z update: ....California and southwest Oregon... While detail differences remain in the latest guidance, there is general agreement for westerly mid-level flow through the period with an amplifying shortwave to approach the northern California coast by 12Z Monday. Precipitable water values remain weakly anomalous until closer to 12Z Monday as the shortwave approaches. Periods of light to moderate rain look to affect the West Coast through much of the period but increasing in intensity prior to 12Z Monday. Given some lingering detail differences with the approaching shortwave and antecedent rainfall leading up to the Day 3 period, the Slight Risk was left in place given continued concerns for flooding from additional rainfall across the region. ....Southern and central Texas... Previous synopsis looks good from below, although precipitable water values look to increase above 1.5 inches for parts of the lower Texas coast after 00Z Monday. Adjusted the Marginal Risk northward into portions of central Texas where 12Z guidance indicates some elevated instability (though 12Z NAM is by far the most bullish with 1000+ J/kg MUCAPE in some locations). Increasing ascent/instability and some repeating/training of cells from south to north will be possible. Otto ....California and southwest Oregon... An unsettled period is forecast to continue with widespread precipitation and the potential for locally heavy amounts continuing as a broad upper trough with a series of embedded shortwaves moves across the region. Given what is significant model QPF spread, confidence in the details is limited, however opted to maintain a Slight Risk across an area similar to Day 2, where some models indicate the redevelopment of heavier rates ahead of an amplifying shortwave approaching Northern California early Monday. In contrast to Day 2, the overall model QPF trend has been lighter. Should this trend continue, the Slight Risk may be removed at some point. However, given the potential for at least locally heavy amounts across the same area where heavy accumulations are expected to occur in the prior period, opted to maintain the Slight Risk for now. .....Southern and central Texas... As a broad upper trough moves across the Southwest into the southern Rockies and High Plains, southerly low level inflow is expected to increase across southern to eastern Texas. PWs are forecast to increase to 1.25-1.5 along a 40-50 kt low level jet extending from the northwestern Gulf of Mexico through eastern Texas. This moisture, interacting with increasing ascent afforded in part by favorable upper jet forcing, will support the development of moderate to heavy rainfall across portions of southern to central Texas. While several models indicate heavy amounts, they differ on the magnitude and location. And while they show ample moisture and lift, rainfall rates may be limited by the lack of instability apart from South Texas and along the Gulf Coast. Therefore, opted to maintain the Marginal Risk. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-8rSu5g-eeFfBVjrRE7n1uoTe8jA6D2pgMuJ2q3A4b9V= uMQRezm0XLy_JTXBH2BzBhvR7TV19B2uzzs-dlrsuQVFagM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-8rSu5g-eeFfBVjrRE7n1uoTe8jA6D2pgMuJ2q3A4b9V= uMQRezm0XLy_JTXBH2BzBhvR7TV19B2uzzs-dlrsQbMw1Dk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-8rSu5g-eeFfBVjrRE7n1uoTe8jA6D2pgMuJ2q3A4b9V= uMQRezm0XLy_JTXBH2BzBhvR7TV19B2uzzs-dlrsYUYZ_HM$=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. 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