Forecast Discussion: 1.25” to 1.5” of precipitable water, February 19, 2017

by
IEM AFD from NWS STO

417

FXUS66 KSTO 191324

AFDSTO

Area Forecast Discussion

National Weather Service Sacramento CA

524 AM PST Sun Feb 19 2017

.Synopsis...

Wet weather with renewed flooding concerns into the middle of next week with wettest storm of the bunch expected for Monday through Tuesday. Gusty winds likely again Monday. Mountain travel will also be impacted at times due to heavy snow. Valley thunderstorms possible Tuesday and Wednesday.

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. Discussion...

Large upper level trough in the northeast Pacific will continue to impact norcal weather over the next several days. A shortwave disturbance pivoting through central California this morning in southwest flow ahead of the trough is bringing light rain to the Sacramento and Delta area this morning. A 6 mb surface gradient from Redding to Sacramento is bringing breezy winds to the northern Sacramento valley this morning with sustained winds between 20 and 25 mph north of about Chico. Have issued wind advisory through today for this area as model guidance keeps this level of winds going through the day. Snow levels this morning range from about 3500 feet northern Shasta county to 5000 feet over the northern Sierra. Snowfall amounts should be relatively light today although chain controls are still in affect for the major Sierra passes. More light showers expected this afternoon as another shortwave disturbance pivots through...this time a little farther north.

Things begin to ramp up this evening as a much stronger and wetter Pacific storm system moves onshore. Widespread moderate to heavy rain and snow are expected with gusty winds as this atmospheric river weather system moves through. The blended total precipitable water satellite product shows PW values of and inch and a half within the river with locally even higher values. This is a return interval in the 5-10 year range. Precipitation amounts max out during the day on Monday. Between tonight and Monday night...1 1/2 to 2 1/2 inches of rain are forecast for the valley and orographically favored areas of the Sierra west slopes could see up to 8 inches of rainfall during this time. Snow levels during this time will climb to about 7000 feet but are still likely to be low enough to continue to impact pass level travel over the Sierra. Rainfall will likely create even more flood issues during and after Monday. A flood warning will remain in place for interior Norcal through much of the week. Winter storm warnings have been issued for the Sierra Cascade range. As if this is not enough...evening stronger winds are expected Monday evening as the main frontal band shifts through with 925 nb progs from the NAM showing over 55 mph winds. This will likely produce another round of power outages as soggy ground allows even more trees to topple on powerlines. Focus of heaviest rainfall continues to shift southward. Heaviest rainfall still likely over the American river basin and now southward so needed to increase QPF south of the 80 corridor from previous forecasts.