Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 29 2024 19:56:38 FOUS30 KWBC 291956 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 255 PM EST Thu Feb 29 2024 Day 1 Valid 16Z Thu Feb 29 2024 - 12Z Fri Mar 01 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF COASTAL OREGON AND NORTHERN CALIFORNIA, AS WELL AS THE LOWER ELEVATIONS AT THE FOOT OF THE SIERRA NEVADA... 16z update... Lower Mississippi River Valley... Fast moving weakly amplified shortwave is starting to emerge into the Southern Plains currently resulting in low level cyclogenesis across the Lower Mississippi Valley and modest moisture return across Eastern Texas and Louisiana. Guidance remains uncertain on timing/placement of the moisture/instability within the confluent flow out of the western Gulf of Mexico and return flow from the Caribbean/southeastern Gulf merging between Galveston bay to the mouth of the Mississippi today into early tomorrow (Friday) morning. Faster, earlier arriving moisture suggest elevated convection across portions of E TX, central LA into south-central MS, cells may have a tendency to train, but are likely to be more limited in moisture availability and updraft strength limiting overall totals to between 1.5-2.5". This is supported by Hi-res CAMs of the 12z ARW and lesser so the 12z HRRR solution, with other guidance trending away from the higher rainfall values. In addition, very dry soils per NASA SPoRT LIS 0-40cm are values less than 50% and are in the 5-10th percentile for the time of year, any flash flooding risk is contingent on the short-term rates which do not appear to near the required values to pose a risk at this time. However, slower arriving moisture with greater/stronger flux convergence upstream well offshore will have much greater deeper layer moisture (1.5-1.75" total PWATs) and higher unstable Theta-E air resulting in nearer surface based convection initiating much later in the forecast time period. However, deep layer steering will support transport of stronger cells toward the central and southeastern LA coast, perhaps with similar training profiles to allow for enhanced rainfall totals. Most 12z Hi-Res guidance is trending toward this solution lead by the FV3CAM and older ECMWF solutions and given the deeper vertical depth of the cells, rates in the 2"/hr range suggest a few spots of 2-4" are possible mainly over the swamps of southern LA. Strong surface/boundary layer flux with greater unstable air to the south suggests a favorable back-building environment for prolonged training may be overcome by southward propagation of convective line that may form over night on Friday morning. 12z HREF probabilities remain bearish with less than 25% of 2"/3hrs by 12z or 3" totals remaining offshore. As such, confidence in placement/totals crossing areas prone to this level of rainfall may still be limited for risk of flash flooding (minus urban center of New Orleans); and as such will remain below categorized risk for Excessive Rainfall at this time. Given timing after 00z, will continue to watch guidance trend closely for any potential upgrade at the 01z ERO issuance. =20 Southwest Oregon/Northern California... Regional RADAR and GOES-W trends show main core of Atmospheric River continues to press southward through the OR/CA coast as the height-falls associated with the broad/large 2.5-3 standard deviation closed low reaches the Pacific Northwest. Cold air cumulus showers and shallow convective activity will remain north across OR/WA but given fast moving pace and limited overall totals with any shower, the Marginal Risk has been trimmed southward to account mainly for the AR axis. While moisture is not very anomalous, the strength of the flux into the terrain remains supportive of 300-500 kg/m/s IVT values supporting an hour or two of .33 to .5"/hr rates along the supportive terrain. Freezing levels are relatively high (3000-4500Kft) in the warm advective scheme of the AR, but rapidly crash to 1500-3000Kft thereafter.=20 As such, the Siskiyou, Trinity Ranges will quickly turn to snow and the risk areas will be limited to the Coastal Range rainforests. So will maintain a Marginal Risk, expanding a bit further south into northern Central California coast, and similar latitudes along the lower foothills of the Northern Sierra Nevada Range.=20=20 Gallina ---Prior Discussion--- Current WV satellite imagery shows a large, closed upper-low off the Pacific Northwest with a funnel of moisture aimed into coastal WA and OR with a persistent southwest to northeast orientation. Rainfall totals of 2-4" with locally up to 5" have been seen across the Olympic Range in northwest WA down into the northwest corner of OR where the atmospheric plume has since settled over the past several hrs. We'll continue to see a steady southward progression of the moisture plume with flow generally perpendicular to the coastal plain of southwest OR into northern CA with the highest precip totals shifting to those areas through the forecast cycle. A modest surge is anticipated in the initial stages of the precip this morning with 1-3" likely falling across coastal OR and the adjacent ranges just inland over a 6-8 hr period. This will advance into northern CA by the late-morning and afternoon time frame with the main target now focused in-of the Klamath Range out to Mount Shasta and the Northern Sierra's. Snow levels will take a tumble through the period as the first cold front advances inland and changes the higher elevations quickly over to snow with the lower hills and coastal plain leftover as the primary rain beneficiaries. Totals of 2-4" with locally up to 5" are forecast within all of southwestern OR and northern CA with the highest totals mainly within the Sierra where snow will be the primary weather focus. Marginal Risk remains in place from previous forecast issuance, but the primary concern will not be about rates, but the prolonged moderate to heavy rainfall regime thanks to the stout moisture flux pattern associated with our closed upper-level circulation. Localized flooding concerns will be highest within the complex terrain situated below 3000ft MSL, especially within the windward confines of the coastal ranges just inland of OR and CA. Kleebauer Day 2 Valid 12Z Fri Mar 01 2024 - 12Z Sat Mar 02 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Oregon/California Coastal Ranges... The core shortwave swings around the base of the broader and anomalously deep (2.5-3 standard deviation) closed low reaches the central Oregon coast about midday Friday strengthening overall deep layer flow to re-enhance atmospheric river values up to 400 kg/m/s generally between 00z and 12z Saturday. DPVA from the shortwave and cold air aloft will allow for modest instability and convective clusters with the flow to support short-bursts of rainfall (hail and graupel) in the coastal ranges and areas generally below very low freezing levels (2000-3000Kft). Given depth of cold air, there is very little deep layer moisture below ..75", total areas likely to be affected by significant rainfall resulting above normal runoff appear diminished enough to remove the Marginal Risk at this time. ....Central Gulf Coast... As noted in the Day 1 period, there is low confidence in the latitude of ongoing convective activity across southeast LA into southern MS at the start of Day 2 (01.12z Sat). A Marginal Risk may need to be introduced IF this activity reaches levels required on the end of the Day 1 period. At this time, guidance is not consistent enough or reaches high enough totals in a 3-6hr period to exceed the high FFG values of the swampy/sandy areas in the coastal regions of the central Gulf. If trends are a bit further north toward NOLA, Gulfport into Mobile urban centers, a Marginal Risk may be introduced with subsequent updates, but at this time no categorical risk area is denoted. ....Georgia/South Carolina coast... Progressive shortwave and associated surface/low level cyclone will be progressing through the Tennessee Valley with convective cells with southwesterly return moisture flow out of the Gulf of Mexico across S MS, AL into GA throughout the morning into the afternoon. Concurrently, strengthening surface to boundary layer flow across the Sargasso Sea will result in strong frontogenetical forcing across the Gulf Stream connecting up the the rising warm front across GA. Strong moisture flux along 15-25kt confluent flow, will result in moisture values reaching 1.25 to 1.5" throughout the evening. While instability is limited to below 500 J/kg, strong deep layer bulk shear may allow for some rotating updrafts further strengthening moisture flux toward thunderstorms that may develop near/along the northeast GA and SC coastline, resulting in enhanced rainfall production. While strongest cells are likely to be within the warmer air above the Gulf Stream, a few may get close enough to support 1.5"/hr rates. Slow northward motion may also support redevelopment and/or training along the coastal zone as far north as Wilmington, NC by Sunday morning. Overall, given the area is generally swampy/sandy and has been relatively dry with soil saturation ratio below 50% should be able to handle the 2-4" totals that the most aggressive Hi-Res CAMs are suggesting. HREF 2"/3hr reaching near 40% with 3" total probabilities of 75-90%, but 5" less than 35% in proximity suggests some problems may arise but only for the few prone urban centers, especially if near high-tide given stronger than normal onshore flow may limit any outflow of the near coast swamps/estuaries. There remains enough run to run inconsistencies in placement (particularly favoring off-shore corridor of strongest cells and therefore heaviest rainfall), to rise to a Marginal Risk at this time, but any westward shift may result in an upgrade to Marginal Risk in subsequent updates. Gallina Day 3 Valid 12Z Sat Mar 02 2024 - 12Z Sun Mar 03 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5I-3uUn6MCx8RoUxv6T48M9ve6GvOe6GC9N8MspcaV5X= qvbP0UbFaxJbAbCVeImvP0KSR0mVlisCTN5RR3uivYVPgmY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5I-3uUn6MCx8RoUxv6T48M9ve6GvOe6GC9N8MspcaV5X= qvbP0UbFaxJbAbCVeImvP0KSR0mVlisCTN5RR3uif57EevA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5I-3uUn6MCx8RoUxv6T48M9ve6GvOe6GC9N8MspcaV5X= qvbP0UbFaxJbAbCVeImvP0KSR0mVlisCTN5RR3uiOu2rB8A$=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) .