Flash Flood Guidance
Issued by NWS
Issued by NWS
367 AWUS01 KWNH 220401 FFGMPD CAZ000-ORZ000-221600- Mesoscale Precipitation Discussion 1164 NWS Weather Prediction Center College Park MD 1100 PM EST Thu Nov 21 2024 Areas affected...Northern California...Southwest Oregon Concerning...Heavy rainfall...Flash flooding possible Valid 220400Z - 221600Z SUMMARY...Strong atmospheric river activity will continue to impact areas of northern CA overnight and into early Friday while gradually refocusing farther north back into portions of southwest OR. Widespread areal flooding concerns will continue, and there will also still be a possibility for some localized burn scar flash flooding impacts. DISCUSSION...The latest GOES-W GeoColor satellite imagery is showing the rapid deepening of a new area of low pressure offshore of the West Coast as strong shortwave energy rounds the base of the persistent larger scale upper-level trough. This low pressure center will strengthen over the next 6 to 12 hours and move to a position southwest of Vancouver Island by later Friday morning. As this occurs, the long duration, multi-day atmospheric river that continues to impact northern CA will advance a bit farther north and edge back into areas of southwest OR which will be the result of a backing of the deeper layer flow across the region and the northward advance of a warm front. By very early Friday morning, a cold front will then begin to arrive across coastal areas of southwest OR and northwest CA. Enhanced low to mid-level southwest flow out ahead of this cold front will continue to drive areas of persistently heavy rain given the level of moisture transport and warm air advection coupled with upslope flow/orographic ascent over the higher terrain. The 850/700 mb moisture flux anomalies are forecast to peak between 3 and 5 standard deviations above normal by around 12Z (4AM PST) across coastal areas of northwest CA and southwest OR and the IVT magnitudes are forecast to locally peak in between 750 and 1000 kg/m/s. While the highest values of IVT will be along the immediate coast, some of these elevated IVT magnitudes are forecast to spread inland including the northern Sacramento Valley. The 00Z HREF guidance suggests rainfall rates will occasionally reach 0.50" to 0.75"/hour, with potentially some spotty rates in the 0.75" to 1.0"/hour late tonight as peak IVT magnitudes arrive ahead of the cold front. Additional rainfall totals going through early Friday morning are expected to reach 3 to 5 inches, with isolated heavier amounts not out of the question. Given the persistence of the rainfall, and level of soil saturation and high streamflows already from this multi-day atmospheric river event, there will continue to be concerns for widespread flooding going through Friday morning. This may again include concerns for some burn scar flash flooding impacts at least locally. Orrison ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...EKA...MFR...MTR...REV...STO... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 43542413 43432352 42482323 41812272 41672224 41692162 41402136 40982140 40402082 39812057 39322086 38962179 38232215 38022260 38282320 39062390 39812426 40672459 42002459 43102467