Unsettled Week Ahead

Unsettled Week Ahead, Convective Events Possible

MARCH 23, 2014

Have you enjoyed the last couple weeks of dry, warm, spring-ish weather? Well, you've got Monday left before things change for the better for us active weather lovers (at least for a couple days).

Monday, a deep low in the Gulf of Alaska begins extend southeast, with a secondary low within the larger-scale upper level low rotating off the Pacific Northwest coast by Tuesday. This low will send a cold front through northern California Tuesday, bringing the first precipitation in more than 2 weeks. This front will have a narrow moisture plume, thus making for a narrow precipitation band. 700mb vertical velocities are modeled to be moderate to strong (30 – 40 ub/s), with precipitable water values of .9 – 1″, which isn't very significant, but combined with decent lift should make for a burst of light to moderate rainfall over a duration of a couple hours.

18z NAM’s forecast 500mb winds and MSLP, valid Tuesday afternoon.

Currently, the NAM/GFS/ECMWF indicate the timing of frontal passage in the early to mid-morning along the coast, late morning in the northern/central Sacramento valley, and early afternoon for the southern Sacramento valley. If this timing is correct, the post-frontal cold pool may slide over the central/northern Sacramento valley in-time to allow for some convective activity, if clearing were to occur behind the front. If the front were to slip through a few hours faster, stronger diurnal heating beneath a -20c 500mb cold pool would lead to steeper lapse rates and better convective potential. Instability lingers into the overnight Tuesday, thus continuing a chance of convection through the overnight.

Overnight Tuesday into Wednesday, the low off the Pacific Northwest coast rotates a stronger shortwave around the low into northern California by the early to mid-morning hours. This secondary shortwave will not have a great moisture source to work with, limiting precipitable water values to .5 – .7″, however, this shortwave trough takes on more of a negative-tilt as a strong mid/upper level jet streak (100 – 120kts at 300mb, and 80 – 90kts at 500mb) rotates around the base of the trough. These dynamic forces will greatly make up for the lack of moisture, in addition to much colder air aloft and increasing instability.

18z NAM’s forecast 500mb winds and MSLP, valid Wednesday afternoon. Note the band of strong jet stream level winds along the west side of the trough, blasting southward digging the trough deeper into California.

This second shortwave will swing through bringing a widespread increase showers through the morning and afternoon Wednesday as lift and forcing increases. -25 to -28c 500mb temps aloft Wednesday will make for even steeper lapse rates compared to those seen Tuesday, and while surface temperatures may be limited to the 50s due to significant cloud cover and shower coverage, cold air aloft combined with decent low-level moisture should be enough to promote between 100 – 300 j/kg surface-based CAPE values from the coast to the valley. If any pockets of clearing were to present themselves during the day Tuesday to locally boost buoyancy enough to get some deep convection going. Shear during the day Wednesday will be quite decent given the tilt of the trough and the position of the jet streak, supportive of some weak rotation with stronger storms. However, given a some uncertainty as to the coverage of cloud cover and precipitation Wednesday, thunderstorm potential is tough to tell at the moment.

Forecast 0 – 3km storm relative helicity, as well as 10-meter and 500mb wind arrows. Note that the (near) surface winds are out of the south, while the mid/upper level winds are jetting from the west, indicative of great directional shear.

Apart from thunderstorms/convection, between Tuesday and Wednesday the lower elevations of northern California could pick up anywhere from a quarter to half an inch of rain, locally up to three quarters of an inch depending on convective coverage Wednesday. Snow levels Tuesday are expected to be around 4500 – 5000ft in the sierra, perhaps dropping off a bit more Wednesday as the secondary shortwave comes through, possibly to 4000ft. 1 – 3″ of liquid precipitation in the foothills up into the highest peaks would translate to 10 – 16″ of snow above around 5000ft, locally up to 2ft.

18z NAM’s forecast snowfall through late Wednesday. 12km tiles may be masking higher totals, though.

Wind wise, given a ~990mb low off the Pacific Northwest coast, breezy winds are possibly Tuesday as a band of 30 – 40kt 850mb winds slides through with the cold front, and again Wednesday as the second, stronger shortwave rotates inland. Currently, these low-level winds would support some 20 – 30mph surface gusts in the valley and along the coast. Tuesday, in the mountains, a band of 50 – 70kt 700mb winds slides over the sierra/western Nevada, which combined with dynamics with the cold front could lead to 40 – 50mph gusts in favored terrain, locally higher as energy falls down the east slope of the sierra into western Nevada.

The trough exists Thursday, with warming into Saturday. Uncertainty exists beyond Saturday, as models have been trying to indicate another trough getting in here by Sunday or early next week, but consistency and agreement is very poor, so we’ll have to wait and see.

I’ll have another update on the for the Tuesday or Wednesday system depending how each one looks convective wise. If Tuesday looks to be convectively active, I’ll surely post something late Monday or before the afternoon Tuesday, and if Wednesday looks active as well, I’ll update Tuesday night or early Monday. Stay tuned, and enjoy the active weather while we’ve got it!